WO1989009221A1 - Oligonucleotide n-alkylphosphoramidates - Google Patents

Abstract

Oligonucleotide N-alkylphosphoramidates useful for combatting diseases by biochemical intervention at the RNA and DNA level are disclosed.

Description

Description

Oligonucleotide N-Alkylphosphoramidates

Technical Field

The invention relates to substances which have the ability to bind to polynucleotides, e.g. DNA or RNA.

Background Art

There is a need for a class of compounds which are capable of transport across cellular membrane, which are resistant to in vivo degradation, and which are capable of selectively binding polynucleotides (both DNA and RNA) both extracellularly and intracellularly. Such compounds would be useful in combating diseases by biochemical intervention at the DNA or RNA level.

Disclosure of the Invention

Accordingly, it is an object of the invention to provide a class of compounds capable of transport across cellular membranes.

It is another object to provide a class of compounds which are resistant to in vivo degradation.

It is another object to provide a class of compounds capable of selectively and strongly binding a polynucleotide target.

It is another object of this invention to provide a class of compounds capable of selectively and strongly binding a DNA target.

It is another object of this invention to provide a class of compounds capable of selectively and strongly binding a RNA target.

It is another object of this invention to provide a class of compounds capable of selectively and strongly binding a polynucleotide target in vivo.

It is another object of this invention to provide a class of compounds capable of selectively and strongly binding DNA in vivo.

It is another object of this invention to provide a class of compounds capable of selectively and strongly binding RNA in vivo.

It is another object of this invention to provide a class of compounds capable of selectively neutralizing DNA in vivo.

It is another object of this invention to provide a class of compounds capable of selectively neutralizing RNA in vivo.

It is another object of this invention to provide a class of compounds capable of combatting diseases by biochemical intervention at the DNA level.

It is another object of this invention to provide a class of compounds capable of combatting diseases by biochemical intervention at the RNA level.

It is another object of this invention to provide a class of compounds capable of combatting viral diseases.

It is another object of this invention to provide a class of compounds capable of combatting genetically based diseases.

These objects of the invention and others which will become apparent from the description of the invention provided below have now surprisingly been discovered to be satisfied by the inventor's discovery of the following novel class of compounds of formula (I), or a salt thereof:

R¹ and R² are each independently H; C_1-15 saturated or unsaturated acyl; C_1-15 ether; C_1-15 acetal; C_1-15 linear, branched or cyclic, saturated or unsaturated alkyl; phosphate; or sulfate; wherein when said R¹ and R² are a carbon-containing group, said carbon-containing group may be substituted by at least one member selected from the group consisting of halogen atoms (i.e., fluorine, chlorine, bromine or iodine), mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkyl sulfites wherein each alkyl group contain independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl groups and heterocyclic substituents, i.e., 4 to 16-membered cyclic or bicyclic compounds containing at least one nitrogen, oxygen and/or sulfur atom(s). Any R¹ or R² which is a carbon-containing group can also be substituted by a C_1-6 alkyl substituent interrupted by one carbonyl functionality, or a C_1-6 alkyl substituent terminated by an aldehyde group or any group R¹ and R² can itself be terminated by an aldehyde group.

R¹ and R² can also be each, independently, substituents which facilitate or control DNA or RNA binding by "classical" mechanisms. Thus R¹ and R² can be a polyamine or a polypyrrole (see, for example, Goodfall et al., J. Med. Chem., vol. 29, pp. 727-733 (1986); Agmley, Molec. and Cell. Biochem., vol. 43, pp. 167-181 (1982); or Feigon, J. Med. Chem., vol. 27, pp. 450-465 (1984)) or an intercalator (see, for example, G. Dougherty and J. R. Tilbrow, Int'l J. of Biochem., vol. 16, p. 1179 (1984)) as defined in the references noted which are hereby incorporated by reference.

B¹ and B² are, independently of any other group B¹ or B² in said compound, a purine or pyrimidine such as an adenine, uracil, guanine, cytosine or thymine moiety, or. any other heterocyclic moiety capable of hydrogen-bonding with DNA or RNA. Examples of such heterocyclic moieties include hypoxanthine, xanthine, 6-thioguanine, purine, 6-thiopurine, pyrimidine, 2-thiouracil, 4-thiouracil and other heterocycles capable of forming Watson-Crick base pairs with normal constituent bases of RNA and DNA, as well as Hoogsteen base pairs and base triplets.

X is, independently of any other group X in said compound, a group NR³R⁴ or a group R⁵.

R³ and R⁴ are each, independently of any other R³ or R⁴ in the compound, a hydrogen atom, or a C_1-16 alkyl group which can be linear, cyclic, branched, saturated or unsaturated. All of these groups R³ and R⁴ can either be unsubstituted or substituted by at least one member selected from the group consisting of halogen atoms (i.e., fluorine, chlorine, bromine or iodine, mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkyl sulfites wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl groups and heterocyclic substituents, i.e. 4 to 16-membered cyclic or bicyclic compounds containing at least one nitrogen, oxygen and/or sulfur atom(s). These groups R³ and R⁴ can also be substituted by at least one C_1-6 alkyl group interrupted by a carbonyl functionality or they can be substituted by at least one C_1-6 alkyl group terminated by an aldehyde functionality, or any group R³ and R⁴ can also be terminated by an aldehyde group. Examples of R³ and R⁴ include octyl, decyl, pentadecyl, 10-cyclopentyldecyl, or derivatives of these containing phenyl, thiophene, pyrrole, furan, aldehyde, keto, thio, amino or imino groups or double bonds.

R³ and R⁴ can also be each, independently of any other R³ or R⁴ in said compound, H; C_1-16 linear, branched or cyclic, saturated or unsaturated alkyl; C_1-16 linear, branched or cyclic, saturated or unsaturated alkyl containing at least one member selected from the group consisting of halogens, phenyl, thiophene, furan, pyrrole, keto groups, aldehyde groups, thiol groups, amino groups, imino groups, double bonds, triple bonds, an oxygen atom, and a sulfur atom.

R³ and R⁴ can be each, independently of any other group R³ or R⁴ in the compound, a C_1-16 haloalkyl group which can be linear, cyclic, branched, saturated or unsaturated, wherein the halogen atoms of the haloalkyl substituent are fluorine, chlorine, bromine or iodine, for example, mono-, di- and trichlorodecyl, mono-, di- and tribromooctyl, and mono- or di-triiododecyl, a C_1-16 ether or a C_1-16 thioether.

Also included within the scope of this invention are groups NR³R⁴ in which R³ or R⁴ each independently contain appended substituents, for example polyamines, intercalators, groove binders, that can further augment or control polynucleotide binding by the probe molecule.

R⁵ is, independently of any other group R⁵ in the molecule, a C_2-20 alkyl group which can be linear, cyclic, branched, saturated or unsaturated. Any one of these groups R⁵ can be substituted by at least one member selected from the group consisting of halogen atoms (i.e. fluorine, chlorine, bromine or iodine), mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio. groups, dialkyl sulfites wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl groups, and heterocyclic substituents, i.e. 4 to 16-membered cyclic or bicyclic compounds containing at least one nitrogen, oxygen and/or sulfur atom(s). Any of these groups R⁵ can further be substituted by a C_1-6 alkyl substituent interrupted by one carbonyl functionality or they can be substituted by C_1-6 alkyl substituent terminated by an aldehyde functionality, or any one of these groups R⁵ can be terminated by an aldehyde functionality. In addition, R⁵ can be any group in accordance with the definitions of R³ and R⁴ given in this document.

X can also be, independently of any other group X, -O-(C_2-16 alkyl) or -S-(C_2-16 alkyl). Examples of the substituents include N-octylthio, N-butyloxy, 5-cis-octenylthio, 10-phenyldecyloxy, etc.

The compounds of this invention are as defined above with the proviso that (1) only one group R³ or R⁴ can be hydrogen on any one group NR³R⁴ in the molecule, and (2) if the compound is by itself and if one variable X is a group selected from the group consisting of -OCH₂CCl₃, -OC(CH₃)₂CCl₃, -NHCH₂CH₂NH₂, -NH₂, and -OCH₂CCl₃, then the compound possesses at least two different groups X. In pharmaceutical composition and in the use of the compound of this invention proviso (2) does not apply.

Z is, independently of any other Z in the molecule, H, OH or SH.

n, m and p are each independently an integer of from 1 to 20.

The sum of n, m and p < 20. Brief Description of the Drawings

A more complete appreciation of the invention and many of its advantages will be obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying figures.

Figure 1 illustrates the separation of diastereomeric dinucleoside n-octylphosphoramidates 19a. The mixture of diastereomers was applied to a 25 × 4.6 cm Alltech C₁₈ column (10μ) and washed with

35% CH₃CN in 0.1N ammonium formate at a flow rate of 1.0 mL/min. The column was monitored at 254 nm.

Figure 2 illustrates the separation of diastereomeric DMTr-22 on reverse phase HPLC. The diastereomeric mixture was separated preparatively on a 25 × 1.0 cm C₁₈ column (10μ) which was washed with 35% CH₃CN in 0.02 N triethylammonium acetate, pH 6.9, at a flow rate of 6 mL/min.

Figure 3 provides the circular dichroic spectra d (ApA) ( . .. . . ) , dAp(NMe₂ ) dA ( 19f ) ( - ) , and dAp(NHC₄H₉)dA (19c) (▬ ) in 0.01 M Tris-HCl (pH 7.5)-0.01 M-MgCr₂ at 25°C. The nucleotide concentration was 6×10^-5M.

Figure 4 provides the circular dichroic spectra of d(ApApA) (- ) and d(Ap(NHC₁₂H₂₅)ApA (▬ ) in 0.01

M Tris-HCl (pH 7.5)-0.01 M MgCl₂ at 25°C. The nucleotide concentration was 6 × 10^-5M.

Figure 5 provides a mixing profile of dinucleoside n-butylphosphoramidate 19c and poly(thymidylic acid). The complex was formed in 10 mM Tris-HCl, pH 7.5, containing 10 mM MgCl₂. Equimolar stock solutions of 19c and poly (T) (6×10^-5 M nucleotide concentration) were mixed in different ratios and allowed to reach equilibrium at 0°C, after which A₂₆₀ was recorded.

Best Mode for Carrying Out the Invention

The oligonucleotides N-alkylphosphoramidates of the present invention are compounds having the structure or formula (I) provided above. In this formula, Z is H, OH or SH. When Z = OH, one gets (1) more stable duplexes, (2) an altered sugar pucker/conformation, (3) a general base to facilitate certain reactions leading to polynucleotide strand scission, and (4) a group onto which other chemical groups, e.g., nucleic acid cleavers, binding adjuvants, can be appended. However, when Z = OH one also gets diminished chemical and enzymatic stability.

The groups Z in the molecule can either be all the same, or they can each be independently selected from the group of H, OH and SH. When the compound possesses one or more groups Z = OH and the remaining variables Z are H, one obtains probes which possess the following advantages: ( 1 ) inclusion of structural adjuvants that can bind or degrade target RNA or DNA is facilitated; and (2) turnover of the probe, enhancing its pharmacokinetic behavior, is also facilitated.

Variables n, m and p are each, independently of any other variable n, m or p in the compound, an integer of from 1 to 20, with the proviso that the sum of variables n, m and p does not exceed 20. Shorter probes, probes in which the sum of n, m and p falls in the range of 8 to 12 can be used in most cases where the target is single-stranded and biochemically accessible, binding selectivity is not absolutely critical, and target destruction will suffice therapeutically. These types of probes can be used, for example, in antiviral therapy.

Larger probes, probes in which the sum of variables n, m and p is 12 or greater, are required to dissect events within the human genome due to access/selectivity problems.

The compounds of the present invention can be designed to complex to specific segments of either DNA or RNA. In one embodiment of this invention, the compounds of the present invention can be designed to selectively bind to DNA by choosing variables B¹ and B² from adenine, thymine (uracil), guanine, and cytosine.

It should be recognized that the present invention describes a generic, sequence-neutral form of DNA or RNA affinity and that other binding adjuvants can also be included within the structures of the probe molecules to enhance affinity. Of special interest are binding adjuvants, such as intercalators (e.g., ethidium, methidium, proflavine), groove binders (distamycin, netropsin, pyrrolobenzodiazepines) and electrostatic binders (spermine, spermidine, agmatine) and probes containing modified heterσcycles (e.g. xanthine, 6-thioguanine, purine) that have special H-binding properties and permit modulation of probe-target interaction.

The oligonucleotides provided by the present invention will associate with their target via Watson- Crick, Hoogsteen or base-triple interaction, and that the binding adjuvants (in the form of N-alkyl substituents, as defined herein) will increase this intrinsic source of affinity. The binding per se renders the target oligonucleotide biochemically dysfunctional, or "tags" the target for destruction by cellular nucleases (e.g. ribonuclease H) .

Variable X in formula (I) is a group NR³R⁴ or a group R⁵. When any of these groups is an alkyl group, this definition is intended to include simple hydrocarbon groups which may be linear, cyclic, branched, saturated or unsaturated. These groups may also be halogenated with at least one chlorine, bromine, or iodine atom, or this group containing from 1 to 16 carbon atoms may be interrupted by oxygen atoms or sulphur atoms or a combination of these.

When either group R³ or R⁴ or R⁵ is an unsaturated hydrocarbon, these unsaturated hydrocarbons include alkenes and alkynes. The alkenes are preferably cis-alkenes, although both trans-alkenes and mixed cis-/trans-alkenes can also be used.

In a preferred embodiment of this invention, the compound contains no more than three phosphoramidate groups in a single probe molecule, and all of these phosphoramidate groups can be either the same or different. The phosphoramidate structures are evenly spaced within the molecule, or they can be situated at the middle of the molecule and on one end thereof.

The compounds of the invention can be administered via any administration route known. For example, they can be administered intravenously or intramuscularly in solution. They can be administered intranasally as aerosols or drops, and they can be administered with carriers, for example liposomes or nanoparticles or as suppositories. These compounds are administered to a patient in amounts of up to 50 mg per administration until the therapeutic end point is reached.

The probes of the present invention can be used to target messenger RNA of various agents causing diseases. These agents can be infective agents of a bacterial, viral or protozoal nature. Viral agents include AIDS, influenza or herpes. Protozoan agents include, e.g., trypanosomes or malaria.

The compounds provided by the present invention can also be used to target genetic diseases. They can. be used to target DNA responsible for these genetic diseases or control regions of DNA, or RNA to block/regulate gene expression.

The compounds of the present invention can be present in their free form or as any of well known physiologically acceptable salt. These salts include mono- and divalent cations commonly used therapeutically, e.g., sodium, potassium, calcium, lithium and magnesium salts, and alkylamine salts, such as polyamine salts.

The oligonucleotide alkylphosphoramidates of this invention can be made by automated solid phase synthesis. For each of these oligonucleotides, it is generally possible to separate the diastereomeric species by reverse phase hplc. Individual isomers bind to their complementary polynucleotides, but with somewhat different affinities.

The nucleoside N-alkylphosphoramidates can be prepared by a few different procedures, but preferably by utilizing the observations of Nemer and Ogilvie (1980), who demonstrated that oxidation of an intermediate dinucleoside trichloroethyl phosphite with iodine in the presence of an alkylamine establishes the requisite N-alkylphosphoramidate linkage. As illustrated in Scheme III, when protected dinucleoside O-methylphosphites were treated with alkylamines, the desired (protected) dinucleoside N-alkylphosphoramidate derivatives were obtained in moderate to good yields, and could be deprotected readily to afford the desired dinucleoside N-phosphoramidates.

Oligonucleotide N-alkylphosphoramidates are also accessible by solid phase synthesis. This is achieved by modification of the method of Matteucci and Caruthers (1981), which involved substitution of I₂-alkylamine for I₂-H₂O in the oxidative step of the coupling cycle(s) in which the N-alkylphosphoramidate linkage was to be introduced.

Other features of the invention will become apparent in the course of the following descriptions of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof. In the discussion below the inventor provides plausible theoretical explanations of how his novel compound might be understood to work. These explanations are not provided to limit, this invention in any manner, but are simply provided for the benefit of the reader. In the examples section provided below, the inventor discusses a few different methods for the preparation of oligonucleotide N-alkylphosphoramidates which were compared directly. One of these, which involves the use of protected nucleoside phosphites as building blocks, provided the requisite N-alkylphosphoramidates via oxidation of the intermediate dinucleoside methyl phosphites with iodine in the presence of the appropriate alkylamine.

This method was found to have several attractive features, including the use of building blocks identical with those employed for the synthesis of DNA, and compatibility with procedures and instruments employed for the stepwise synthesis of oligonucleotides by solution and solid phase methods. This procedure was used to make several di-, tri- and tetranucleotide N-alkylphosphoramidates derived from deoxyadenosine and thymidine; alkyl substituents included N,N-dimethyl, N-butyl, N-octyl, N-dodecyl and N-(5-aminopentyl). The aminoalkyl derivative of TpT (24) was used to demonstrate the feasibility of introducing an intercalative agent to the alkylphosphoramidate moiety of such derivatives.

The oligonucleotide N-alkylphosphoramidates were separated into their component diastereomers and characterized structurally by a number of techniques including circular dichroism, high field ¹H-NMR spectroscopy, FAB mass spectrometry and enzymatic digestion to authentic nucleosides and nucleotides. Physicochemical characterization of several di- and trinucleotide alkylphosphoramidates revealed that the adenine nucleotide analogs formed stable complexes with poly(thymidylic acid). The stabilities of these complexes were found to increase with increasing chain length of the N- alkylphosphoramidate substituents. The N-alkylphosphoramidate substituents enhance the binding of certain oligonucleotides to their complementary polynucleotides, providing a novel source of polynucleotide affinity.

This invention is due in part to the discovery of a polynucleotide binding adjuvant that works by a novel principle. The inventor has found that in addition to the known types of DNA binders (intercalators, groove binders, polycations), there are a number of highly lipophilic natural products that associate with DNA or RNA in a sequence-neutral fashion and with good affinity.

Presumably, the source of the affinity derives from the fact that the interior of the DNA or RNA duplex is the most lipophilic component of an aqueous solution containing dissolved DNA or RNA. Admixture of a lipophilic molecule to this aqueous DNA solution would then result in association of the lipophilic probe with the most lipophilic portion of the DNA.

This principle was then extended to the preparation of dinucleotides of type II, i.e., analogs of the Letsinger-Schott dinucleotide 1.

where B = adenine or thymine, and R = C₄H₉, C₈H₁₇ or C₁₂H₂₅.

When the adenine dinucleotides of type 2 were studied as binders of poly T, interesting results were obtained. As shown in Table 1, the presence of the alkyl groups resulted in an increase in the Tm (i.e., melting temperature) values. The extent of increase in Tm was also proportional to the length of the alkyl substituent. The observation of an increase in Tm has also been made for oligonucleotide phosphoramidates containing intercalators rather than as alkyl group (Letsinger and Schott, 1981; Asseline et al., 1984; Helene et al., 1985). This evidences that the lipophilic alkylphosphoraraidate groups of the present invention promoted the binding of these oligonucleotides to poly T.

The hypochromicity values are also shown in Table I for some of the dinucleotides studied. Unlike the results typically obtained with intercalators, where increased affinity correlates with increased base stacking (and hence with the increased hypochromicity), dinucleotides 2 exhibited an optimum for % hypochromicity. This may reflect a disordering of the formed duplex by the lipophilic alkyl chain as the latter extends beyond a certain size.

Aside from their ability to stablize a duplex with their Watson-Crick complementary sequences, the nucleotide phosphoramidates provided by this invention were found to have the following two additional important features. They are surprising lipophilic, making them capable of transport across cellular membranes and they are refractory to nucleolytic degradation at the phosphoramidate linkages.

The preparation of oligonucleotides containing modified phosphodiester linkages is of current interest as a source of sequence-specific nucleic acid probes (Miller et al., 1981; Letsinger & Schott, 1981; Asseline et al., 1984; Chu & Orgel, 1985; Dreyer & Dervan, 1985; Thuong et al., 1987). Nucleoside phosphoramites have been prepared previously by several procedures involving both tri- and pentavalent phosphorous intermediates. These have included the condensation of nucleoside phosphate diesters with amines in the presence of triphenylphosphine-CCl₄ (Appel, 1975; Stec, 1983), nucleophilic substitution of nucleoside phosphate triesters with alkylamines (Meyer et al., 1973; Juodka & Smrt, 1974; Letsinger et al., 1986), addition of alkyl and aryl azides to phosphites (Cramer et al., 1972; Letsinger & Schott, 1981), as well as the oxidation of intermediate nucleoside phosphites (Nemer & Ogilvie, 1980 a,b) or nucleoside H-phosphonate diesters (Froehler, 1986) with iodine in the presence of alkylamines.

Experiments are described below which illustrate such transformations for the preparation of oligonucleotide phosphoramides, both by solution and solid phase techniques. Also described is chromatographic resolution and analysis of the formed diastereomers, and their analysis by spectral and degradative techniques.

The association of a series of diadenosine N-alkylphosphoramidates with poly(thymidylie acid) was characterized by measurement of Tm and hypochromicity values, as well as by determination of the stoichiometry of association. These measurements indicated that the N-alkyl groups promoted the binding of the diadenosine N-alkylphosphoramidate derivatives to poly T, and the affinity increased with increasing alkyl chain length. The implications of this novel source of affinity for the design of sequence-specific nucleic acid probes is discussed.

Because there has been no direct comparison of the several methods available for the preparation of oligodeoxynucleoside N-alkylphosphoramidates, the study was initiated by comparison of a few promising methods.

The condensation of nucleoside phosphate diesters with amines in the presence of triphenylphosphine-CCl₄ (Appel, 1975) has been used previously for the preparation of nucleoside phosphoranilidate derivatives (Stec, 1983) with the desired products being obtained in moderate yield. Application of this method for the synthesis of deoxynucleoside N-alkylphosphoramidates was attempted using fully protected thymidylyl(3'→5')- thymidine derivative 2 (Scheme I).

Scheme I: Reagents: (a) C₆H₅SH, Et₃N, dioxane; (b) (C₆H₅)₃P, CCl₄; (c) n-C₄H₉NH₂; (d) t-C₄H₉NH₂,

CH₃OH.

Successive treatments of 2 with ~3 equivalents of triphenylphosphine-CCl₄ in CH₃CN-pyridine, and then with an excess of n-butylamine, afforded dinucleoside n-butylphosphoramidate 3 in 24% isolated yield. The structures of 3, and its debenzoylated derivative 4, were consistent with the behavior of each on silica gel TLC, and with their measured UV and high field ¹H-NMR spectra (Table II). Table (II): Preparation of Dinucleoside N-.Alkylphosphoramidates 17 and 19^a

Compound Base R R' Yield ( % ) R_f Value^b

3 T MTr n-C₄H₉ 48

17a T MTr n-C₈H₁₇ 76

17b T MTr n-C₁₂H₂₅ 56

17c A^Bz DMTr n-C₄H₉ 54 17d A^Bz DMTr n-C₈H₁₇ 83

17e A^Bz DMTr n-C₁₂H₂₅ 79

17f A^Bz DMTr (CH₃)₂c 87

19g T H n-C₄H₉ 58 0.45

19a T H n-C₈H₁₇ 77 0.60

19b T H n-C₁₂H₂₅ 100 0.66

19c A H n-C₄H₉ 81 0.26

19d A H n-C₈H₁₇ 59 0..38

19e A H n-C₁₂H₂₅ 77 0.50

19f A ^: H (CH₃)₂c 94 0.26

^a See Experimental Procedures for methods of preparation. b Silica gel TLC, development with 5:1 CH₂Cl₂-CH₃OH. c M,N-Dimethylphosphoramidate derivative. Dinucleoside n-butylphosphoramidate 4 was also synthesized on a polymeric support by modification of the method of Matteucci and Caruthers (1981) (Scheme II).

Scheme II: Reagents: (a) C₆H₅SH, Et₃N, dioxane; (b) (C₆H₅)₃P, CCl₄; (c) n-C₄H₉NH₂; (d) t-C₄H₉NH₂,

CH₃OH.

Compound 4 was obtained in ~70% overall yield from the resin-bound precursor 5, and was shown to be identical in all respects with the product obtained by solution phase synthesis.

The preparation of nucleoside N-alkylphosphoramidates via treatment of nucleoside phenylphosphite derivatives 8 with n-hexylazide (25°C, 4 days) was also studied (Scheme III).

Scheme III .

The use of 8a and 8b, containing phenyl and 4- chlorophenyl phosphites, respectively, led only to solvolysis and other decomposition products. On the other hand, nucleoside 8£ provided the desired nucleoside n-hexylphosphoramide derivative 9 in 54% yield, along with quantities of nucleoside H- phosphonate 10 and 5'-0-(methoxytrityl) thymidine.

For comparative purposes, the reaction of 8c with methyl azidoacetate, a transformation more closely analogous to that reported by Letsinger & Schott (1981) was also studied (Scheme IV).

Scheme IV.

Not surprisingly this reaction proceeded with greater facility, providing compound 11 in 68% isolated yield after 2 days at 25°C.

The procedure reported by Nemer and Ogilvie (1980), involving oxidation of an intermediate dinucleoside trichloroethyl phosphite with iodine in the presence of an alkylamine, appeared particularly attractive as the oxidation was rapid and essentially quantitative, and the reported conditions appeared amenable for adaptation to solid phase synthesis. Because the formation of an N-alkylphosphoramidates by this procedure must involve the loss of one of the original phosphite substituents, it was first sought to determine the extent to which this step might proceed selectively. Accordingly, deoxynucleoside 3'-phosphite 12 was prepared and treated with a slight excess of I₂ in dry tetrahydrofuran-n-butylamine. Work-up of the reaction mixture provided ethyl 5'-0- (dimethoxytrityl) thymidine 3'-(N-n-butyl)- phosphoramidate (13) in 45% isolated yield; very little (<10%) of the putative O-methyl analog 14 could be detected (Scheme V).

Scheme V.

The selective loss of the methyl substituent suggested that this approach might well afford the requisite oligonucleotide N-alkylphosphoramidates. Further, as O-methyl protection is frequently employed for phosphate esters during solid phase oligonucleotide synthesis (Matteucci & Caruthers, 1981), the protected nucleoside 3'-(O-methyl), N,N-diisopropylamino)- phosphoramidites employed as building blocks in such schemes could potentially be employed as precursors both for phosphate ester and phosphoramidate linkages.

As outlined in Scheme (VI), two different dinucleoside O-methyl phosphites were prepared and each was treated with a few different alkylamines in the presence of I₂.

Scheme (VI): Reagents: (a) tetrazole, CH₃CN; (b) I₂, R'NH₂ THF; (c) t-C₄H₉NH₂, CH₃OH, 45°C; (d) aq HOAc.

The resulting fully protected dinucleoside N- alkylphosphoramidates (3, 17a - 17f) were purified by chromatography on silica gel; the yields of each (48-87%) are given in Table II. Successive debenzoylation (t-BuNH₂, CH₃OH) and detritylation (80% aq CH₃COOH) afforded the respective dinucleoside N-alkylphosp-hora midates 19a - 19g in isolated yields of 58-100%, as shown in Table II supra.

Products were characterized by silica gel TLC and reverse phase HPLC, as well as by UV and 360 MHz ¹H-NMR spectroscopy (see Table III and Table IV, infra).

TABLE IV

360 MHz Ml-NMR Spectra of Dinucleoside Alkylphosphoramidate Derivatives 19^a

Chemical Shift Values (δ)

Proton

19g 19a 19b 19c 19d 19e 19f^b

H-6 7.66.7.23 7. 68,7,41.7.32 7.73, 7.44.7.37

H-2 8,01,8.00 8.02,8.00 8.18,8.17 8.18,8.16 7.97 7.99.7.97 8.14 8.14,8.13

H-8 7,87,7.85 7.88,7.82 8.00,7.97 8.06.8.02 7.82,7.76 7.81,7.77 7.89,7.85 7.88,7.94

H-1' 6.18-6.13 6.3 6,3 6.2-6.1 6.2,6.0 6.4-5.9 6.33,6.30

H-3'(pN) 4.86 5.08 5.05 4.9 4.9 5.1 5.04

H-3'(Np) 4.3-4.2 4.45 4.4 4.4 4.4 4.5 4.5

4.2-4.0 4.2-4.1

H-4', 11-5' 4.0-3.6 3.8,3.6 4.1.3.8 4.2-3.4 4.2-3.4 4.3-3,2 4.3-3.7

NHCH₂. H-2' 2.8-2.7 2.9-2.B c c c 2.4-2.0 2.9-2.1 2,5-2.0 3.0-2.4

CH₃(T) 1.78 1.92,1.90 1.92,1.88

1.5-1.4 1.5-1.4 (CH₂)_n 1.35-1.1 1.5-1.3 1.4-1.1 1.2-1,0 1.2-1.0 1,3-1,0 CH ₃ 0.8-0.7 0.87 0.90 0.7 0.6-0.7 0.8-0.7 2.62,2.59 ^aRecorded in CDCl,-DMCO-d₆. ^bRecorded in CDCl₃-CD₃OD. ^cNot assigned

Dinucleoside N-alkylphosphoramidates 19a, 19b and 19g were also characterized by the appearance of the respective molecular ions in their FAB mass spectra (positive and negative ion; glycerol matrix). Each of the tritylated and deprotected dinucleoside N- alkylphosphoramidates (18 and 19, respectively) could be separated by reverse phase HPLC (CH₃CN-H₂O mixtures) into two components present in approximately equal amounts (see, e.g., Figure 1). In each case, the high field ¹H-NMR spectra of the constituents were consistent with their formulation as diastereomers. In some cases the two diastereomers were evaluated separately for their ability to associate with the complementary single stranded nucleic acid (vide infra).

To evaluate the applicability of this approach for the elaboration of oligodeoxynucleotide N-alkylphosphoramidates, a few different tetranucleotides were prepared by solid phase synthesis. Each of these contained a single N-alkylphosphoramidate linkage at one of the two terminal positions. Synthesis of the oligonucleotides was carried out by the method of Matteucci and Caruthers (1981), with the exception that introduction of the alkylphosphoramidate linkages were accomplished by substitution of I₂-alkylamine treatment for I₂-H₂O in the oxidative step of the coupling cycle. Following partial deblocking and removal from the column (t-butylamine-CH₃OH), the tritylated tetranucleotides were purified by preparative HPLC on a C₁₈ reverse phase column. Elution with 0.02-0.04 M tetraethylammonium acetate, pH 6.9, containing an appropriate amount of acetonitrile effected purification of the oligonucleotides and separation of the diastereomers, as illustrated in Figure 2 for tetranucleotide DMTr-22.

Because the method of solid phase synthesis employed here proceeds from the 3'-end of the nascent oligonucleotide, the successful synthesis of tetranucleotide analog 22 (Scheme VII) demonstrated the stability of the N-alkylphosphoramidate bond during the subsequent condensation, oxidation and detritylation procedures. In addition, during the synthesis of 22 a small amount of the solid support was removed after the first condensation-oxidation cycle and subjected to complete deprotection. This procedure yielded a diastereomeric mixture identical in all respects (¹H-NMR, silica gel TLC, reverse phase HPLC) with 19g prepared by solution phase synthesis.

Scheme VII. Characterization of tetrathymidylate analogs 20 to 22 included enzymatic digestion of each with nuclease Pl and alkaline phosphatase (Connolly et al., 1984). Dinucleoside N-alkylphosphoramidate 19a was shown to be resistant to nuclease Pl under conditions that led to complete hydrolysis of TpT. Treatment of a single diastereomer of 22 with nuclease Pl gave three products, including equal amounts of pT and T. Further treatment with calf intestine alkaline phosphatase coverted the product mixture to a mixture of thymidine and a single diastereomer of 19g. Analogous digestion of 20 and 21 with nuclease PI resulted in the formation of 19g and 19b, respectively, plus pT. In each case (20 to 22) , digestion of the diastereomic mixture of tetranucleotides was found to produce both diastereomers of its respective dinucleoside N-alkylphosphoramidate 19. Each of the diastereomers of the tetranucleotide was shown to lead exclusively to one of the two diastereomers of 19.

In recent years, numerous reports have described the synthesis of oligonucleotides modified to contain adjuvants useful in DNA binding (Letsinger & Schott, 1981; Asseline et al., 1984; Helene et al., 1985; Thuong et al., 1987), site-selective alkylation (Vlassov et al., 1985; Knorre et al., 1985ab; Zarytova et al., 1986; Iverson & Dervan, 1987) or cleavage (Boutorin et al., 1984; Chu & Orgel, 1985; Dreyer & Dervan, 1985; Le Doan et al., 1986; Boidot-Forget et al., 1986, 1987).

The chemistry described here is applicable for the synthesis of such modified oligonucleotides, and it was attempted to prepare a representative dinucleoside phosphoramidate 26. As outlined in Scheme (VIII), key intermediate 24 was prepared by I₂-diaminopentane oxidation of the dinucleoside O-methyl phosphite derived from 15 + 23.

Scheme (VIII) : Reagents: (a) tetrazole, CH₃CN; (b) I₂, NH₂ (CH₂) ₅NH₂ ; THF; (c) 6-chloro- 9-(p-chlorophenoxy)-2-methoxyacridine; (d) 2% CF₃COOH in CH₂Cl₂

Individual diastereomers of 24 were obtained by silica gel flash chromatography. Each was treated separately with 6-chloro-9-(p-chlorophenoxy)-2-methoxyacridine at 60°C overnight to produce 25. Following detritylation (2% CF₃COOH in CH₂Cl₂, 30 min) each of the diastereomers of 26 was isolated by precipitation of its trifluoroacetate salt from a large volume of ether. The less and more polar isomers were obtained in overall yields from 24 of 64% and 54%, respectively. Compound 26 was also prepared by solid phase synthesis in analogy with the synthesis of 20-22.

The ultraviolet hypochromicities and circular dichroism (CD) spectra of the individual oligoadenylate N-alkylphosphoramidates indicated a diminution of base stacking relative to d(ApA) and d(ApApA). The magnitude of the hypochromic effect was not affected by the nature of the N-alkyl group (Table V), in contrast to earlier reports concerning the hypochromicities of alkyl phosphotriesters (Miller et al., 1971; Letsinger et al., 1986).

The lack of dependence of alkyl chain length was also apparent in the CD spectra. As can be seen in Figure 3, for example, the spectra of 19c and 19f were very similar. This was also true for the CD spectrum of 19d. The wavelengths of their maximum and minimum ellipticity in these spectra matched those of d(ApA), but their amplitudes were greatly reduced, as was seen with alkyl phosphotriesters (Miller et al., 1971).

A comparison of the CD spectrum of d(ApApA) with that of d(Ap(NHC₁₂H₂₅)ApA) was also carried out. As anticipated, both spectra exhibited maximum and minimum ellipticity at the same wavelengths, but the spectrum of d(Ap(NHC₁₂H₂₅)ApA) was diminished in amplitude (Figure 4). Both the GO and hypochromicity results are consistent with decreased interaction between adjacent bases (Kondo et al., 1970; Miller et al., 1971) in the N-alkylphosphoramidates.

Oligonucleotide Binding of Adenine Nucleoside N-Alkylphosphoramidates

The interaction of individual oligonucleotide N-alkylphosphoramidates was studied initially by the use of absorbance mixing curves. These experiments readily demonstrated the formation of complexes between oligoadenylate N-alkylphosphoramidates and poly(thymidylic acid), but not between oligothymidylate 21 and poly(dA). Accordingly, the former were studied further. As shown in Figure 5, when measured in 10 mM Tris-HCl, pH 7.5, containing 10 mM MgCl₂, dinucleotide analog 19c gave an absorption minimum at 66 mole % of poly (T), corresponding to a dA:T nucleotide stoichiometry of 1:2. Since triple helix formation by polymers of adenine and thymine(uracil) nucleotides is well documented (Stevens & Felsenfeld, 1964; Davies, 1967; Tazawa et al., 1970; Miller et al., 1971; Arnott et al., 1976, Miller et al., 1981; Letsinger et al., 1986), this result was not unexpected. Essentially the same result was obtained with the other tested oligoadenylate-derived phosphoramidates .

A number of oligodenylate N-alkylphosphoramidate analogs were employed for measurements of hypochromicity and melting temperature (Table IV) . The Tm values and hypochromicities in the presence of complementary oligonucleotides were determined simultaneously. In the case of 19e, 15% methanol was included to effect dissolution of the nucleotide analog.

The Tm values were determined both by heating of the formed oligonucleotide - poly (T) complexes, and by slow cooling of solutions initially maintained above the Tm. As shown in the table, the introduction of an n-butylphosphoramidate moiety in place of the phosphate ester (i.e., 19c vs. d(ApA)) resulted in a species whose binding to poly(thymidylic acid) was altered, as judged by a substantial increase in % hypochromicity and Tm. Precisely the same effect was obtained in the presence of 15% CH₃OH, although the absolute values for % hypochromicity and Tm were slightly lower.

The enhanced binding of 19c to poly (T) was consistent with the observations of Letsinger et al. (1986), who found that the unsubstituted phosphoramidate of ApA also exhibited an enhanced affinity for poly (T). These authors attributed the increased binding to the absence of charge repulsion by the uncharged phosphoramidate -NH₂ moiety and its ability to form H bonds in aqueous media.

Also apparent in Table (V) is the effect of increasing alkyl chain length on the affinity of the oligoadenylate N-alkylphosphoramidates for poly (T). Direct comparison of d(ApA), 19c and 19e (in 15% CH₃OH) revealed an increase in Tm with increasing chain length. This was true for both methods of Tm determination. A significant increase in Tm was also observed when one of the phosphate esters in d(ApApA) was replaced by an N-dodecylphosphoramidate. This increase in affinity is entirely consistent with the results of Letsinger et al. (1986) who found that trichloroethyl and 1,1-dimethyltrichloroethyl esters of d(ApA) and d(ApApA) bound to poly (T) with significantly enhanced affinity. The present results suggest that lipophilic substituents may act more generally to stabilize DNA helix structure. It is worthy of note that both diastereomers of 19e had essentially the same Tm, whether measured by heating or cooling.

One interesting facet of the measurements made in the presence of poly(thymidylic acid) involved the change in % hypochromicity observed with increasing alkyl chain length (Table V). Following an initial increase in % hypochromicity as the N-alkylphosphoramidate substituent was introduced, further increases in alkyl chain length actually resulted in a decrease of the measured hypochromicity for both the di- and trinucleotide analogs studied. While the magnitude of the hypochromicity obviously reflects a complex variety of factors, the observed pattern is intriguing. Another effect was observed in parallel with this decrease in % hypochromicity. As illustrated in Table (V), for most of the nucleotide analogs studied the Tm values were essentially identical whether determined by heating or cooling. However, for the most lipophilic di- and trinucleotide derivatives studied, the Tm's measured by heating were consistently several degrees higher than those measured by cooling. This was true for both diastereomers of 19e and for d(Ap(NHC₁₂H₂₅)ApA); the effect was unaltered by reasonable changes in the rate of heating or cooling. These observations suggest that the larger lipophilic substituents may cause disruption of the base stacking normally associated with polynucleotides and form duplexes that are structurally altered to permit accommodation of the liophilic substituents within the least polar regions of the formed complexes, thereby avoiding the interaction of the lipophilic groups with the polar aqueous medium. A relatively slow rate of formation and dissociation of such "disordered" oligonucleotide complexes could well be consistent with the observed differences in Tm according to the method of measurement employed.

The observed increase in affinity for a polynucleotide by its complementary oligonucleotide bearing a lipophilic alkyl substituent suggests the existence of a novel source of polynucleotide affinity. It also suggests that the alkyl groups sometimes used to covalently tether more classical DNA binding agents to polynucleotides could well be participants in the overall process by which polynucleotide binding is enhanced.

It is important to note that the present measurements were carried out with oligonucleotides of a type known form base triples readily (Stevens & Felsenfeld, 1964; Davies, 1967; Tazawa, 1970; Miller et al., 1971; Arnott et al., 1976; Miller et al., 1981; Letsinger et al., 1986); indeed the mixing profile obtained for 19c and poly (T) is suggestive of a triple strand structure. It may be the case that the novel source of affinity noted here is most readily apparent where the potential for formation of a triple strand structure exists. Indeed, this might offer some interesting opportunities for manipulation of nucleic acid structure (Moser & Dervan, 1987).

In the context of the design of sequence-specific nucleic acid probes, the oligonucleotide N-alkyl- phosphoramidates described here are of interest for different reasons. These include resistance to nuclease degradation, as observed upon incubation with Penicillium citrinum nuclease P1, and the possibility that the presence of a lipophilic alkylphosphoramidate moiety may render mammalian cells permeable to the oligonucleotide (see, e.g., Miller et al., 1985). Further, these data show that the oligonucleotide N-alkylphosphoramidate derivatives should exhibit unusual properties in systems that model DNA assembly (Behr, 1986) and encapsulation within lipid membranes (Jay & Gilbert, 1987).

Experimental Procedures:

Thymidine and tetrazole were obtained from Aldrich Chemicals. 2'-Deoxyadenosine and 3'-O-benzoylthymidine were purchased from Sigma Chemicals as were poly(dA) and poly(T). 5'-Dimethoxytrityl-2'-deoxyadenosine was obtained from Bachem. Anhydrous acetonitrile was distilled from calcium hydride and stored over 4Å molecular sieves. Anhydrous tetrahydrofuran was heated at reflux over lithium aluminum hydride for 1 h prior to use. Oligonucleotide synthesis was carried out on a manual polynucleotide synthesizer (Matteucci &

Caruthers, 1981) using Vydac TP-20 spherical silica gel (The Separations Group) as a solid support. Nuclease P1 (Penicillium citrinum; one unit catalyzes the hydrolysis of one μmol of phosphodiester linkages in yeast RNA in 1 min at 37°C) and alkaline phosphatase (calf intestine; one unit catalyzes the hydrolysis of 1 ymol of p-nitrophenyl phosphate in 1 min at pH 10.4 (glycine buffer) and 37°C) were obtained from Boehringer-Mannheim. Chromatographic separations were carried out on silica gel columns (Merck silica gel 60, 70-230 mesh (230-400 mesh for flash chromatography)) or TLC¹ plates (Merck silica gel 60, F-254, 0.2 or 2 mm thickness). HPLC analysis was carried out on an Alltech 10μ C₁₈ column (250 x 4.6 mm) using CH₃CN-H₂O mixtures (either 40 mM triethylammonium acetate, pH 6.0, or 0.1 M ammonium formate buffers). For preparative isolations, the appropriate fractions were collected and concentrated under diminished pressure, then diluted with water and desalted on a Bond Elut C₁₈ cartridge (Analytichem International).

NMR spectra were obtained on Varian EM-390 or Nicolet NT-360 spectrometers. Ultraviolet spectra were obtained on Gary 15 or 17 spectrophotometers. Circular dichroism spectra were obtained on a Jasco J-500C spectrometer. Melting profiles were obtained on a Perkin-Elmer Lambda 5 spectrophotometer. 5'-O-(Methoxytrityl) thymidine 3'-(O-methyl, N,N-disopropylamino)phosphoramidite (15, R=MTr)

A solution containing 772 mg (1.50 mmol) of 5'-O-(p-methoxytrityl) thymidine (Schaller et al., 1963) in 8 mL of CH₂Cl₂ was treated with 1.75 mL (10.0 mmol) of N,N-diisopropylethylamine. Methyl (N,N- diisopropylamino)phosphorochloridite (400 μL; 2.25 mmol) was added slowly and the combined solution was maintained at 25°C for 20 min prior to addition of 100 mL of CH₂Cl₂. The reaction mixture was extracted with 150 mL of saturated aqueous NaHC0₃ and the organic phase was dried (Na₂SO₄) and concentrated. The resulting foam was purified by silica gel flash column chromatography (Still et al., 1978); elution was with 6:4 CH-Cl₂-hexane. The appropriate fractions were combined and concentrated under diminished pressure to afford the desired product as a gum. The product was dissolved in 6 mL of toluene and added dropwise to 200 mL of cold (-78°C) hexane, which effected precipitation of the product as a white powder. Filtration afforded 979 mg (97%) of the activated nucleoside phosphoramidite, silica gel TLC R_f 0.85 (ethyl acetate).

5'-O-(Methoxytrityl)thymidylyl ( 3 '→5 ' ) 3 ' -O-benzoyl- thymidine ( 2 )

A solution containing 338 mg (0.5 mmol) of 5 ' -O- (p-methoxytrityl) thymidine 3'-(O-methyl, N,N-diisopropylamino)phosphoramidite in 2 mL of dry CH₃CN was treated with 140 mg (2.0 mmol) of..tetrazole and 115 mg (0.33 mmol) of 3'-O-benzoylthymidine (de Rooji et al., 1979) under N₂ at 25°C for 15 min. The phosphite intermediate was oxidized by addition of an aqueous pyridine solution containing 0.1 M iodine, and the reaction mixture was partitioned between CH₂Cl₂ and H₂O. The dried- ((Na₂SO₄) organic phase was concentrated and the product was purified by flash chromatography on silica gel; elution was with mixtures of CH₂Cl₂ and ethyl acetate. Dinucleoside monophosphate 1 was isolated as a colorless foam (-310 mg) and was employed directly for the preparation of nucleoside 2.

Demethylation of 1 was carried out by treatment with 4 mL of dioxane, 2 mL of triethylamine and 2 mL of thiophenol at 25°C for 3 h. The reaction mixture was concentrated under diminished pressure and the residue was triturated with hexane. The residue was purified by flash chromatography on silica gel; elution was with mixtures of CH₂Cl₂ and CH₃OH containing 1% triethylamine. The appropriate fractions were combined and concentrated, then applied to a 10-mL Amberlite IR- 120 column (pyridinium form) as a methanolic solution. Elution with methanol afforded dinucleoside monophosphate 1 in quantitative yield as the pyridinium salt. The product was isolated as a powder following precipitation from a large volume of n-hexane-ether, silica gel TLC R_f O.48 (100:10:2 CH₂C1₂-CH₃OH- triethylamine).

P-(n-Butylamino), P-deoxy-5'-O-(methoxytrityl)-thymidylyl(3'-*5') (3'-O-benzoylthymidine) (3)

A mixture of 68 mg (-70 μmol) pyridinium dinucleoside monophosphate 2 and 52 mg (0.20 mmol) of triphenylphosphine was rendered anhydrous by repeated evaporations of portions of dry acetonitrile. The mixture was then dissolved in 1.5 mL of acetonitrile and 0.2 ml of dry pyridine and treated with 17 μL (27 mg; 0.18 mmol) of CCl₄. The reaction mixture was stirred at 25°C for 4 h, then treated with 150 μL (110 mg; 1.5 mmol) of n-butylamine and maintained at 25°C for an additional 30 min. The reaction mixture was concentrated under diminished pressure and the residue was purified by preparative silica gel TLC; development was with 10:1 CH₂Cl₂~CH₃0H. The product was isolated as a yellow powder, yield 16 mg (24%); ¹H-NMR (Table III).

P-(n-Butylamino), P-deoxy-5'-O-(methoxytrityl)- thymidylyl(3'→5')thymidine (4)

Dinucleoside monophosphate 3 (20 mg; 20 μmol) was dissolved in 2 mL of 1:1 CH₃OH-t-butylamine. The reaction mixture was maintained at 40-45°C for 2 days, then concentrated under diminished pressure. The residue was purified by preparative silica gel TLC; development was with 9:1 CH₂Cl₂-CH₃OH. Dinucleoside monophosphate 4 was isolated as an off-white gummy solid following lyophilization, yield 15 mg (84%); silica gel TLC R_f 0.50 (10:1 CH₂Cl₂-CH₃OH); ¹H-NMR (CDCl₃, (CH₃)₄Si) & 0.8-0.9 (m), 1.2-1.35 (m), 1.32 (s), 1.81 (s), 2.1-2.9 (m), 3.00 (m), 3.4-4.3 (m), 3.71 (s), 4.4 (m), 4.6 (m), 5.04 (m), 6.07 (t), 6.35 (m), 6.78 (d), 7.2-7.4 (m), 7.50 (s) and 8.46 (br s).

Synthesis of P-(n-Butylamino), P-deoxy-5'-O-(methoxytrityl) thymidylyl(3' 5')thymidine (4) on a Solid Support A 100-mg sample of Vydac TP-20 silica gel was derivatized with ~3 μmol of fully protected dinucleoside monophosphate 5 by the method of Matteucci & Caruthers (1981). Demethylation of the phosphate ester was accomplished by treatment of the derivatized support with 2 mL of 1:2:1 triethylamine-dioxane- thiophenol. The mixture was shaken at 25°C for 3 h, then filtered and washed successively with dioxane, pyridine, methanol and ether prior to overnight drying.

The silica gel containing putative 6_ was then treated with 52 mg (0.20 mmol) of triphenylphosphine, 17 μL (27 mg; 0.18 mmol) of CCl₄, 8 μL (8 mg; 0.1 mmol) of dry pyridine and enough dry acetonitrile to permit the mixture to be shaken at 25°C for 3 h. The mixture was then treated with 100 μL (73 mg; 1.0 mmol) of n- butylamine and maintained at 25°C for an additional 2 h. Putative dinucleoside monophosphate 7 was treated with 3 mL of 1:1 CH₃OH-t-butylamine at 40°C for 16 h. The silica gel was filtered and washed extensively with methanol. The filtrate was concentrated and the residue was purified by preparative silica gel TLC. The product (2.1 μmol; ~ 70% yield) was isolated as a solid max (pH 7) 268 nm. This material was shown to be identical with dinucleoside monophosphate 4 prepared via solution phase synthesis (vide supra).

Ethyl 5'-O-(Methoxytrityl) thymidine 3'-(N-n-Hexyl)- phosphoramidate (9)

A solution containing 619 mg (1.20 mmol) of 5'-O- (methyoxytrityl) thymidine (Schaller et al., 1963) in 3 mL of dry tetrahydrofuran was added dropwise to a reaction vessel (N₂, -78°C) containing 220 μL (343 mg; 1.30 mmol) of 2,4-dichlorophenyl phosphorodichloridite (Tolkmith, 1958), 280 μL (274 mg; 3.5 mmol) of dry pyridine and 5 mL of tetrahydrofuran. The reaction mixture was stirred at -78°C for 10 min, then treated with 100 μL (79 mg; 1.70 mmol) of absolute ethanol and allowed to warm to room temperature. The reaction mixture was partitioned between CH₂Cl₂ and water, and the organic extract was dried (Na₂SO₄) and concentrated under diminished pressure.

Putative nucleoside 8£ was dissolved in 15 mL of dry tetrahydrofuran and treated with 1.5 mL of n- hexylazide (Grundman, 1965) at 25°C for 4 days. The reaction mixture was concentrated under diminished pressure and the residue was triturated with 30 mL of n-hexane. Purification of the crude product was effected by flash chromatography on silica gel (Still et al., 1978); elution was with increasing amounts of ethyl acetate in CH₂Cl₂, then with 10:1 CH₂Cl₂-CH₃OH. Ethyl 5'-O-(methoxytrityl) thymidine 3'-(N-n- hexyl) phosphoramidate (9) was isolated as a white powder, yield 373 mg (54%); silica gel TLC R_f 0.47 (ethyl acetate), 0.53 (10:1 CH₂Cl₂-CH₃OH); ¹H-NMR (CDCl₃, (CH₃)₄Si) δ 0.79 (m, 3), 1.1-1.3 (m, 11), 1.33 (s, 3), 2.2-2.9 (m, 4), 3.33 (br s, 2), 3.66 (s, 3), 3.94 (q, 2 ) , 4.181m, 1), 5.02 (m, 1), 6.35 (dd, 1), 6.7-7.4 (m, 14), 7.45 (s, 1) and 9.35 (br s, 1).

Ethyl 5'-O-(Methoxytrityl) thymidine 3'-[Phosphoramidato -2"-N-(glycine methyl ester)] (11)

A solution containing 129 mg (0.25.mmol) of 5'-O-(methoxytrityl) thymidine (Schaller et al., 1963) in

1.5 mL of dry tetrahydrofuran was added dropwise over a period of 5 min to a reaction vessel (N₂, -78°C) containing 47 μL (73 mg; 0.28 mmol) of 2,4-dichlorophenyl phosphorodichloridite (Tolkmith, 1958), 64 μL (63 mg; 0.80 mmol) of dry pyridine and 2 mL of dry tetrahydrofuran. The reaction mixture was stirred at -78°C for 10 min, then treated with 24 μL (19 mg; 0.40 mmol) of absolute ethanol and allowed to warm to room temperature. The reaction mixture was partitioned between CH₂Cl₂ and water, and the organic phase was dried (Na₂SO₄) and concentrated.

Putative 8£ was dissolved in 3 mL of dry tetrahydrofuran and treated with 375 μL of methyl azidoacetate (Grundman, 1965) at 25°C for 2 days. The mixture was concentrated under diminished pressure and the residue was purified by silica gel flash column chromatography; elution was with CH₂Cl₂ containing increasing amounts of CH₃OH (up to 10%). Ethyl 5'-O- (methyltrityl) thymidine 3'-[phosphoramidato-2"-N-(glycine methyl ester)] (11) was isolated as a white powder, yield 118 mg (68%); silica gel TLC R_f 0.36 (ethyl acetate), 0.50 (10:1 CH-Cl₂-CH₃Oh); ¹H-NMR (CDCl₃, (CH₃)₄Si) δ 1.15 (t, 3), 1.28 (s, 3), 2.0-2.6 (m, 2), 3.3-3.7 (m, 10), 3.96 (q, 2), 4.17 (m, 1), 5.09 (m, 1), 6.36 (dd, 1), 6.7-7.4 (m, 14), 7.47 (s, 1) and 9.39 (br s, 1).

Ethyl 5'-O-(Diemthoxytrityl) thymidine 3'-(N-n- Butyl)phosphoramidate (13)

A solution containing 338 mg (0.62 mmol) of 5'-O- (dimethoxytrityl) thymidine (Schaller et-al., 1963) in l„8 mL of dry tetrahydrofuran was added dropwise to a reaction vessel (N₂, -30°C) containing 57 μL (80 mg; 0.60 mmol) of methyl phosphorodichoridite (Martin & Pizzolato, 1950), 160 μL (158 mg; 2.0 mmol) of dry pyridine and 4 mL of tetrahydrofuran. The reaction mixture was stirred at -30°C for 5 min, then treated with 100 μL (79 mg; 1.70 mmol) of absolute ethanol. The reaction mixture was allowed to warm to 0°C, then added to ice water and extracted with portions of CHCl₃. The combined CHCl₃ extract was dried (Na₂SO₄) and concentrated. Putative nucleoside phosphite derivative 12 was dried carefully by coevaporation of portions of dry toluene and tetrahydrofuran, then treated with a solution containing 200 mg (0.8 mmol) of iodine in 2 mL of dry tetrahydrofuran + 1 mL of n-butylamine. The reaction mixture was stirred at 25°C for 5 min, then partitioned between CHCl₃ and aqueous NaHSO₃. The organic extract was dried (Na₂SO₄) and concentrated under diminished pressure. The residue was purified by silica gel flash chromatography (20-g column); elution was with 0-2% CH₃OH in ethyl acetate. Ethyl 5'-O-(dimethoxytrityl) thymidine 3'-(N-n-butyl phosphoramidate (L3) was obtained as a white solid by precipitation from a large volume of hexane, yield 196 mg (45%); silica gel TLC R_f 0.26 (ethyl acetate), 0.50 (20:1 CHCl₃-CH₃OH); ¹H-NMR (CDCl₃, (CH₃)₄Si) 0.8 (t, 3), 1.05-1.2 (m, 10), 2.15-2.9

(m, 4), 3.32 (br S, 2), 3.67 (s, 6), 3.88 (q, 2), 4.15 (m, 1), 5.00 (m, 1), 6.35 (dd, 1), 6.65-7.3 (m, 13), 7.49 (s, 1) and 9.03 (s, 1).

Nucleoside phosphoramidate 13 was characterized further by ¹H-NMR following deprotection (81% yield) with 80% aqueous HOAc. General Procedure for the Preparation of Dinucleoside Phosphoramidates (17)

In a typical experiment, 0.3 mmol of 5'-hydroxy-3'-O-benzoylated nucleoside (16i) (Eckstein, 1967; Ogilvie, 1973) was treated with 0.40-0.45 mmol of nucleoside 3'-(O-methyl, N,N-diisopropylamino)-phosphoramidite (1J5) in 1.5 mL of dry acetonitrile containing 1.6-2.0 mmol of tetrazole. The reaction mixture was maintained at 25°C for 20-30 min, then partitioned between CH₂Cl₂ and water. The CH₂Cl₂ layer was dried (Na₂SO₄) and concentrated under diminished pressure. After codistillation of several portions of dry tetrahydrofuran from the residue, the resulting foam was dissolved in 2 mL of dry tetrahydrofuran and treated with 0.40-0.45 mmol of iodine in 2 mL of tetrahydrofuran and 1 mL of alkylamine. After 5 min, the reaction mixture was poured into aqueous sodium bisulfite and extracted with portions of CH₂Cl₂. The CH₂Cl₂ extract was dried (Na₂SO₄) and concentrated. The residue was purified by silica gel flash chromatography (Still et al., 1978) or by preparative silica gel TLC using CH₂Cl₂-CH₃OH mixtures.

In this fashion, 0.45 mmol (304 mg) of 5 '-O-(methoxytrityl) thymidine 3'-(O-methyl, N,N- diisopropylamino)phosphoramidite and 0.30 mmol (114 mg) of 3'-O-benzoylthymidine afforded P-deoxy, P-(n- octylamino)-5'-(0-methoxytrityl)thymidylyl(3'→5') (3'-0- benzoylthymidine (17a) as a white solid following chromatographic purification and precipitation from a large volume of 2:1 hexane-ether, yield -237 mg (76%); silica gel TLC R_f 0.36 and 0.43 (ethyl acetate), 0.32 and 0.36 (10:1 CH₂Cl₂-CH₃OH); ¹H-NMR (CDCl₃, (CH₃)₄Si) θ 0.87 (t), 1.2-1.3 (m), 1.37 (s), 1.5 (m), 1.92 (s), 1.94 (s), 2.2-3.0 (m), 3.3-3.6 (m), 3.77 (s), 3.79 (s), 4.2-4.4 (m), 5.15 (m), 5.47 (d), 5.56 (d), 6.3-6.5 (m), 6.8-6.9 (2d), 7.2-7.6 (m), 7.53 (s), 8.0 (2d) and 8.4 (br s). The yields of the other dinucleoside phosphoramidates prepared is given in Table I; ¹H-NMR values for these compounds is also provided (Table IV) supra.

Virtually identical yields of these products were obtained in one-pot procedures that omitted isolation of the intermediate dinucleoside phosphites.

General Procedure for Deprotection of Dinucleoside Phosphoramidates (17)

In a typical experiment, 0.1 mmol of the fully protected dinucleoside phosphoramidate was debenzoylated to afford 18 by stirring overnight with 10 mL of 1:1 t-butylamine-methanol at 45-50°C. The solvent was removed under diminished pressure and the residue was purified by silica gel flash chromatography or by preparative silica gel TLC using CH₂Cl₂-CH₃OH mixtures. Detritylation was effected by treatment with 2 mL of 80% aqueous acetic acid at 25°C for 1 h (dimethoxytrityl derivatives) or 16 h

(monomethoxytrityl derivatives), affording deprotected dinucleoside phosphoramidate 19. Following removal of acetic acid under diminished pressure the residue was co-evaporated with portions of toluene and then purified by preparative silica gel TLC (development with CH₂Cl₂-CH₃OH mixtures).

In this fashion, 100 μmol (104 mg) of P-deoxy, P-(n-octylamino)-5'-0- (methoxytrityl)thymidylyl(3'→5')(3'-O-benzoylthymidine) (17a) was deprotected to provide P-deoxy, P-(n-octylamino)thymidylyl(3'→5')thymidine (19a) as a white solid following preparative silica gel TLC (10:1 CH₂Cl₂-CH₃OH) and lyophilization from dioxane, yield 53 mg (77%); silica gel TLC R_f 0.07 (10:1 CH₂Cl₂-CH₃OH), 0.60 (5:1 CH₂Cl₂-CH₃OH); ¹H-NMR (CDCl₃, (CH₃)₄Si) δ 0.87 (t), 1.3 (m), 1.5 (m), 1.90 (s), 1.92 (s), 2.1-2.9 (m), 3.6 (m), 3.8 (m), 4.0-4.2 (m), 4.45 (m), 5.08 (m), 6.30 (dd), 7.32 (s), 7.41 (s) and 7.68 (s). The yields of the other dinucleoside phosphoramidates prepared is given in Table I; ¹H-NMR values for these compounds is also summarized (Table IV).

Synthesis of TpTpTpT Phosphoramidates 20-22

The requisite tetrathymidylate derivatives were prepared on a manual polynucleotide synthesizer as described by Matteucci and Caruthers, (1981). Derivatized silica gel (71 μmol DMTr groups/g silica gel) was used in 400-mg columns. Individual couplings were carried out as described (Matteucci s Caruthers, 1981). using 5'-O-(dimethoxytrityl) thymidine 3'-(O-methyl, N,N-diisopropylamino)phosphoramidite + tetrazole in -tetrahydrofuran. Oxidation of individual phosphite bonds was also carried out as described using 0.1 M I₂ in 2:1:1 tetrahydrofuran-H₂O-lutidine, except that introduction of the N-alkylphosphoramidate linkage was accomplished by substitution of 0.1 M I₂in 2:1 tetrahydrofuran-alkylamine for introduction of the appropriate internucleotide bond.

After completion of each tetranucleotide synthesis the silica gel containing the fully protected tetranucleotide was treated with 2 mL of 2:1:1 dioxane- thiophenol-triethylamine at 25°C for 90 min. The silica gel was washed successively with dioxane, CH₃OH and CH₂Cl₂, after which the tetranucleotide was hydrolyzed from the support by treatment overnight with 2 mL of 1:1 t-butylamine-CH₃OH at 45°C and the tritylated oligomers were purified by preparative HPLC on a 10μ C₁₈ column; elution was effected with 0.04 M triethylammonium acetate, pH 6.9, containing 35% (for DMTr - 22 ) , 38% (for DMTr - 20 ) or 51% (for DMTr -21) acetonitrile. Purification by HPLC also effected separation of the diastereomers (retention times: 10.9 and 15.8 min for DMTr - 20; 9 _'7 and 12.5 min for DMTr - 21 ; 10.3 and 14.0 min. for DMTr - 22) . Approximately 20% of each crude tetranucleotide preparation was purified by HPLC, yielding at each case -8 A₂₆₀ units of the individual diastereomers.

Deprotection of individual diastereomers of DMTr - 20 - 22 was carried out with 1 mL of 80% aqueous HOAc at 25°C overnight. The solvent was concentrated under diminished pressure and the residue was co-evaporated with portions of toluene and triturated with ether. Purification was effected by C₁₈ reverse phase HPLC using CH₃CN in 0.04 M triethylammonium acetate, pH 6.9. Approximately 5 A₂₆₀ units of each tetranucleotide was obtained.

Enzymatic Digestion of TpTpTpT Phosphoramidates 20 - 22

Digestion of tetranucleotides 20 - 22 (0.1-0.2 A₂₆₀ unit scale) was carried out in 100 μL of 0.25 M

Tris'HCl, pH 7.0, containing 6-9 units of Pencillium citrinium nuclease Pl. The digestions were carried out at 37°C for 4 h, and one half of the reaction mixtures were analyzed by HPLC. The remaining half of each reaction mixture was combined with 2 units of calf intestine alkaline phosphatase and incubated for an additional 30 min prior to HPLC analysis.

Also analyzed in the same fashion was a diastereomeric mixture of P-(n-butylamino), P-deoxy-thymidylyl(3'→5')thymidine.

6-Chloro-9-(p-chlorophenoxy)-2-methoxyacridine

A reaction mixture consisting of 1.39 g (5.0 mmol) of 6,9-dichloro-2-methoxyacridine and 3.0 g (23 mmol) of p-chlorophenol was heated at 80°C for 4 h. The hot reaction mixture was poured into hot 5% aqueous NaOH. The combined solution was stirred until a fine yellow crystalline precipitate appeared. The crystals were filtered, washed to neutrality with water and dried at 70°C in vacuo. Recrystallization from pyridine afforded 6-chloro-9-(p-chlorophenoxy)-2-methoxyacridine as yellow microcrystals, yield 1.06 g (57%), mp 151.5-153°C; ^λmax (CH₃OH) 400, 381, 351, 333, 318 (sh) and 262 nm; ¹H-NMR (CDCl₃, (CH₃)₄Si) δ 3.76 (s, 3) 6.7-6.9 (m, 2) 7.1--7.6 (m, 5) and 7.8-8.2 (m, 3).

P-(5-Aminopentylamino), P-deoxy-5'-O-(methoxytrityl)-thymidylyl(3'→5') (3'-O-methoxytritylthymidine) (24)

A solution containing 370 mg (0.55 mmol) of 5'-O-(methoxytrityl) thymidine 3'-O-methyl, N,N- diisopropylamino)phosphoramidite (15, R = MTr) in 2 mL of dry acetonitrile was treated with 178 mg (2.5 mmol) of tetrazole and 232 mg (0.45 mmol) of 3'- methoxytritylthymidine (Ogilvie & Letsinger, 1967, Matteucci & Caruthers, 1980). The reaction mixture was maintained at 25°C for 20 min, then treated with 125 mg (0.49 mmol) of iodine in 3 mL of 2:1 tetrahydrofuran - 1,5-diaminopentane. After 5 min at 25°C, the reaction mixture was concentrated under diminished pressure and the residue was partitioned between CH₂Cl₂ and 50% aqueous methanol. The organic phase was dried (Na₂SO₄) and concentrated. The residue was purified by silica gel flash chromatography (30-g column); elution was carried out using CH₂Cl₂ containing increasing amounts of methanol. In this fashion each of the diastereomers of P-(5-aminopentylamino), P-deoxy-5'-O-(methoxy-trityl)thymidylyl(3'-5)(3'-O-methoxytritylthymidine) (24) was obtained as an off-white powder in chromatography homogeneous form, yield 125 mg (24%) of the less polar isomer (silica gel TLC R_f 0.48 (5:1 CH₂Cl₂-CH₃OH)) and 166 mg (31%) of the more polar isomer (R_f 0.43); ¹H-NMR (less polar isomer) (CDCl₃, (CH₃)₄Si) δ 1.38 (s), 1.78 (s), 1.2-2.0 (m), 2.2-2.8 (m), 3.2-3.7 (m), 3.75 (s), 3.78 (s), 3.9-4.3 (m), 5.03 (br t), 6.17 (t), 6.33 (dd), 6.83 (dd), 7.2-7.5 (m), 7.50 (s) and 8.38 (s). ¹H-NMR (more polar isomer) δ 1.34 (s), 1.77 (s), 1.2-1.8 (m), 2.2-2.9 (m), 3.3-3.7 (m), 3.75 (s), 3.77 (s), 3.9-4.2 (m), 5.08 (br t), 6.22 (t), 6.33 (dd), 6.8-6.9 (m), 7.2-7.4 (m), 7.54 (s), and 8.42 (s).

P-(5-(6'-Chloro-2'-methoxyacridin-9'-yl)aminopentyl- amino), P-deoxy-thymidylyl ( 3' → 5') thymidine (26)

To solutions of 118 mg (0.10 mmol) of each of the diastereomers of 24 in 400 μL of pyridine was added 167 mg (0.45 mmol) of 6-chloro-9-(p-chlorophenoxy)-2-methoxyacridine and the reaction mixtures were heated overnight at 60°C. Each reaction mixture was diluted with CH₃OH, filtered to remove insoluble material and concentrated under diminished pressure. The yellow solid product obtained from each was purified by preparative silica gel TLC, development with 9:1 CH₂Cl₂-CH₃OH. The individual diastereomers (putative 25) were treated separately with 6 mL of 2% trifluoroacetic acid in CH₂Cl₂ (30 min, 25°C) to effect ditritylation. The individual diastereomers were isolated as yellow powders following precipitation of each from a large volume of ether as the presumed trifluoroacetate salt, yield 63 mg (64%) of the less polar isomer; 53 mg (54%) of the more polar isomer; ¹H-NMR (less polar isomer) (CDCl₃, DMSO-d₆)δ 1.2-1.4 (m), 1.78 (s), 2.0-2.8 (m), 3.65 (s), 3.84 (t), 3.92 (s), 4.0-4.1 (m), 4.28 (m), 4.5 (br s), 4.95 (t, 1), 6.22 (q, 2), 7.6-7.7 (m), 7.79 (d, 1), 7.84 (s) and 8.23 (d). ¹H-NMR (more polar isomer) δ 1.2-1.6 (m), 1.90 (S), 2.0-2.6 (m), 3.8-4.2 (m), 3.92 (s), 4.4 (br s), 4.6 (br s), 5.05 (m), 6.3 (m), 7.6-7.7 (m), 7.79 (d), 7.84 (s) and 8.23 (d).

Compound 26 was also prepared on a solid support in analogy with "the synthesis of n- alkylphosphoramidates 20 - 22.

Determination of Melting Temperatures for Complexes of Oligoadenylate Analogs and Poly (T)

Solutions of both the oligonucleotides and poly(thymidylic acid) were prepared at nucleotide concentrations of 6×10^-5 M, and mixed in a nucleotide ratio of 1:2 (dA : T) in 10 mM Tris-HCl, pH 7.5, containing 10 mM MgCl₂. In some cases, 15% CH₃OH was added to facilitate dissolution of oligonucleotide N- alkylphosphoramidates. The spectroscopic behavior upon cooling or heating was monitored by UV at 260 nm. Hypochromicities of individual oligonucleotide analogs were determined as described (Tazawa et al., 1970; Miller et al., 1981). Molar extinction coefficients of 8.52 × 10³ (264 nm) for poly (T) and 9.39 × 10³ (257 nm) for poly(dA) were used.

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Shishkin, G. V. (1986) Bioorg. Khim 12, 911-920. Zarytova, V. F., Kutyavin, I. V., Sil'nikov, V. N. and Shishkin, G. V. (1986) Bioorg. Khim. 12, 911.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

Claims

1. A compound of the formula:

wherein:

R¹ and R² are each indepently H; C_1-15 acyl; C_1-15 ether; C_1-15 acetal; C_1-15 linear, branched or cyclic, saturated or unsaturated alkyl; phosphate; sulfate; a polyamine; a polypyrrole; or an intercalator; or

R¹ and R² are each independently a moiety selected from the group cpnsisting of C_1-15 acyl; C_1-15 acetal, and C_1-15 linear, branched or cyclic, saturated or unsaturated alkyl groups all substituted by at least one member selected from the group consisting of halogen atoms, mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkyl sulfites wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl group, 4 to 16-membered cyclic and bicyclic heterocycles containing at least one nitrogen, oxygen or sulfur atom, C_1-6 alkyls interrupted by a carboxyl function, C_1-6 alkyl terminated by an aldehyde function;

B¹ and B² are each, independently of any other B¹ or B² in said compound, a purine base, a pyrimidine base or another heterocycle capable of hydrogen-bonding with DNA or RNA;

X is, independently of any other group X in said compound, a group NR³R⁴ or a group R⁵, wherein:

R³ and R⁴ are each, independently of any other R³ or R⁴ in saidi compound, H; C_1-15-alkyl-CHO; C _{1- 16} linear or cyclic or branched, saturated or unsaturated alkyl; C_1-16 linear or cyclic or branched, saturated or unsaturated alkyl substituted by at least one member selected from the group consisting of halogen atoms, mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkylsulfite groups wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl groups, heterocyclic groups, C_1-6 alkyl interrupted by carbonyl, C_1-6 alkyl terminated by aldehyde, phenyl and imino groups; C_1-15-alkyl-CHO substituted by at least one member selected from the group consisting of halogen atoms, mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkylsulfite groups wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl groups, heterocyclic groups, C_1-6 alkyl interrupted by carbonyl, C_1-6 alkyl terminated by aldehyde, phenyl and imino groups; -O(C_2-6 alkyl) or -S(C_2-6 alkyl); R⁵ is, indepently of any other group R⁵ in said compound, a group R³ or R⁴;

Z is, independently of any other group Z in said compound, H, OH, or SH; and

n, m and p are each independently an integer of from 1 to 20, with the sum of n, m and p ≤ 20;

with the proviso that (1) only one group R³ or R⁴ can be hydrogen on any one group NR³R⁴, and (2) when one X in said compound is a member selected from the group consisting of -OCH₂CCl₃, -OC(CH₃)₂CCl₃,

-NHCH₂CH₂NH₂, and -NH₂, said compounds possess at least 2 different groups X.

2. The compound of Claim 1, wherein:

B¹ or B² is, independently of any other B¹ or B² in said compound, one member selected from the group consisting of hypoxanthine, xanthine, 6-thioguanine, purine, 6-thiopurine, pyrinidine, 2-thiouracil, and 4-thiouracil.

3. The compound of Claim 1, wherein:

R³ or R⁴ is, independently of any other R³ or R⁴ in said compound, octyl, decyl, pentadecyl, or 10-cyclopentyl decyl.

4. The compound of Claim 1, wherein:

R³ or R⁴ is, independently of any other R³ or R⁴ in said compound, a C_1-16 linear, branched or cyclic, saturated or unsaturated haloalkyl, a C_1-16 linear. branched or cyclic, saturated or unsaturated ether or a C_1-16 linear, branched or cyclic, saturated or unsaturated thioether.

5. The compound of Claim 1, wherein X is R .

6. The compound of Claim 1, wherein X is NR³R .

7. A pharmaceutical composition, comprising, in association with a pharmaceutically acceptable excipient or carrier, a compound of the formula:

wherein:

R¹ and R² are each indepently H; C_1-15 acyl; C_1-15 ether; C_1-15 acetal; C_1-15 linear, branched or cyclic, saturated or unsaturated alkyl; phosphate; sulfate; a polyamine; a polypyrrole; or an intercalator; or

R¹ and R² are each independently a moiety selected from the group consisting of C_1-15 acyl; C_1-15 acetal, and C_1-15 linear, branched or cyclic, saturated or unsaturated alkyl groups all substituted by at least one member selected from the group consisting of halogen atoms, mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkyl sulfites wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl group, 4 to 16-membered cyclic and bicyclic heterocycles containing at least one nitrogen, oxygen or sulfur atom, C₁_₆ alkyls interrupted by a carboxyl function, C_1-6 alkyl terminated by an aldehyde function;

B¹ and B² are each, independently of any other B¹ or B² in said compound, a purine base, a pyrimidine base or another heterocycle capable of hydrogen-bonding with DNA or RNA;

X is, independently of any other group X in said compound, a group NR³R⁴ or a group R⁵, wherein:

R³ and R⁴ are each, independently of any other R³ or R⁴ in saidi compound, H; C_1-15-alkyl-CHO; C_1-16 linear or cyclic or branched, saturated or unsaturated alkyl; C_1-16 linear or cyclic or branched, saturated or unsaturated alkyl substituted by at least one member selected from the group consisting of halogen atoms, mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkylsulfite groups wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl groups, heterocyclic groups, C_1-6 alkyl interrupted by carbonyl, C_1-6 alkyl terminated by aldehyde, phenyl and imino groups; C_1-15-alkyl-CHO substituted by at least one member selected from the group consisting of halogen atoms. mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkylsulfite groups wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl groups, heterocyclic groups, C_1-6 alkyl interrupted by carbonyl, C_1-6 alkyl terminated by aldehyde, phenyl and imino groups; -O(C_2-6 alkyl) or -S(C_2-6 alkyl);

R⁵ is, indepently of any other group R⁵ in said compound, a group R³ or R⁴;

Z is, independently of any other group Z in said compound, H, OH, or SH; and

n, m and p are each independently an integer of from 1 to 20, with the sum of n, m and p ≤ 20;

with the proviso that only one group R³ or R⁴ can be hydrogen on any one group NR³R⁴.

8. The composition of Claim 7, wherein:

B¹ or B² is, independently of any other B¹ or B² in said compound, one member selected from the group consisting of hypoxanthine, xanthine, 6-thioguanine, purine, 6-thiopu-rine, pyrinidine, 2-thiouracil, and 4-thiouracil.

9. The composition of Claim 7, wherein:

R³ or R⁴ is, independently of any other R³ or R⁴ in said compound, octyl, decyl, pentadecyl, or 10-cyclopentyl decyl.

10. The composition of Claim 7, wherein: R³ or R⁴ is, independently of any other R³ or R⁴ in said compound, a C_1-16 linear, branched or cyclic, saturated or unsaturated haloalkyl, a C_1-16 linear, branched or cyclic, saturated or unsaturated ether or C_1-16 linear, branched or cyclic, saturated or unsaturated thioether.

11. The composition of Claim 7, wherein X is R⁵.

12. The composition of Claim 7, wherein X is

NR³R⁴.

13. A method for treating a patient of a disease, comprising administering to said patient a compound of the formula:

wherein:

R¹ and R² are each indepently H; C_1-15 acyl; C_1-15 ether; C_1-15 acetal; C_1-15 linear, branched or cyclic, saturated or unsaturated alkyl; phosphate; sulfate; a polyamine; a polypyrrole; or an intercalator; or R¹ and R² are each independently a moiety selected from the group consisting of C_1-15 acyl; C_1-15 acetal, and C_1-15 linear, branched or cyclic, saturated or unsaturated alkyl groups substituted by at least one member selected from the group consisting of halogen atoms, sulfo groups, amino groups, nitro groups, carboxyl group, 4 to 16-membered cyclic and bicyclic heterocycles containing at least one nitrogen, oxygen or sulfur atom, C_1-6 alkyls interrupted by a carboxyl function, C_1-6 alkyl terminated by an aldehyde function;

B¹ and B² are each, independently of any other B¹ or B² in said compound, a purine base, a pyrimidine base of another heterocycle capable of hydrogen-bonding with DNA or RNA;

X is, independently of any other group X in said compound, a group NR³R⁴ or a group R⁵, wherein:

R³ and R⁴ are each, independently of any other R³ or R⁴ in saidi compound, H; C_1-15-alkyl-CHO; C_1-16 linear or cyclic or branched, saturated or unsaturated alkyl; C_1-16 linear or cyclic or branched, saturated or unsaturated alkyl substituted by at least one member selected from the group consisting of halogen atoms, mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkylsulfite groups wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl groups, heterocyclic groups, C_1-6 alkyl interrupted by carbpnyl, C_1-6 alkyl terminated by aldehyde, phenyl and imino groups; C_1-15-alkyl-CHO substituted by at least one member selected from the group consisting of halogen atoms. mercaptan groups containing from 1 to 5 carbon atoms, phosphate groups, sulfate groups, thio groups, dialkylsulfite groups wherein each alkyl group contains independently 1 to 5 carbon atoms, amino groups, nitro groups, carboxyl groups, heterocyclic groups, C_1-6 alkyl interrupted by carbonyl, C_1-6 alkyl terminated by aldehyde, phenyl and imino groups; -O(C_2-6 alkyl) or -S(C_2-6 alkyl);

R⁵ is, indepently of any other group R⁵ in said compound, a group R³ or R⁴;

Z is, independently of any other group Z in said compound, H, OH, or SH; and

n, m and p are each independently an integer of from 1 to 20, with the sum of n, m and p ≤ 20;

with the proviso that only one group R³ or R⁴ can be hydrogen on any one group NR³R⁴.

14. The method of Claim 13, wherein:

B¹ or B² is, independently of any other B¹ or B² in said compound, one member selected from the group consisting of hypoxanthine, xanthine, 6-thioguanine, purine, 6-thiopurine, pyrinidine, 2-thiouracil, and 4-thiouracil.

15. The compound of Claim 13, wherein:

R³ or R⁴ is, independently of any_. other R³ or R⁴ in said compound, octyl, decyl, pentadecyl, or 10-cyclopentyl decyl.

16. The method of Claim 13, wherein:

R³ or R⁴ is, independently of any other R³ or R⁴ in said compound, a C_1-16 linear, branched or cyclic, saturated or unsaturated haloalkyl, a C_1-16 linear, branched or cyclic, saturated or unsaturated ether or a C_1-16 linear, branched or cyclic, saturated or unsaturated thioether.

17. The method of Claim 13, wherein X is R⁵.

18. The method of Claim 13, wherein X is NR³R⁴.