WO2026013665A1

WO2026013665A1 - Purification of cis-2-alkenoic acids

Info

Publication number: WO2026013665A1
Application number: PCT/IL2025/050574
Authority: WO
Inventors: Ari Ayalon; Ronit YAHALOMI SEGUI; Batya KAUFMAN; Michal RODENSKY
Original assignee: Bromine Compounds Ltd
Current assignee: Bromine Compounds Ltd
Priority date: 2024-07-07
Filing date: 2025-07-06
Publication date: 2026-01-15
Anticipated expiration: 2027-01-07

Abstract

A method comprising reacting crude cis-2-alkenoic acid with an amine base and isolating the resultant ammonium salt of cis-2- alkenoic acid in a solid form, from an organic solvent or a mixture of organic solvents, is provided by the invention. The method can be used to purify crude cis-2-decenoic acid. The crystalline cyclohexylammonium salt of cis-2-decenoic acid forms another aspect of the invention.

Description

Purification of cis-2-alkenoic acids

The invention relates to the purification of long chain cis-a, p- unsaturated acids of the formula R-CH=CH-COOH, i.e., cis-2- alkenoic acids, where R indicates an alkyl residue (linear or branched) consisting of not less than four carbon atoms, specifically cis-2-decenoic acid:

It has been reported (see WO 2008/143889 and Davies et al, Journal of Bacteriology 191:1393-1403 (2009) ) that cis-2- decenoic acid (abbreviated CDA) , produced by the bacterium Pseudomonas aeruginosa _r is a bio-dispersant, capable of inducing P. aeruginosa and other gram-negative and gram-positive bacteria and fungi to undergo a physiologically-mediated dispersion response, resulting in the dis-aggregation of surface-associated microbial populations and communities known as biofilms. In coassigned WO 2020/240559, it was shown that CDA can act as an effective adjunctive to bromine-containing biocides in the treatment of biofilm and planktonic bacteria in water systems and on surfaces in contact with the water, to achieve significant enhancement in the killing of bacteria in both pure and mixed cultures typically found in industrial and natural waters, relative to treatment with the brominated biocides alone.

Cis-2-alkenoic acids can be prepared by a two-step process consisting of brominating 2-alkanone to give 1 , 3-dibromo-2- alkanone as the main product, followed by rearrangement of the 1 , 3-dibromo-2-alkanone to the cis-2-alkenoic acids: where R’ is alkyl, e.g., C2H5, C₃H₇, C₄H₉, CsHn and C₆HI₃. The abovementioned two-step synthesis was first described by Rappe et al. [Acta Chemica Scandinavica (1965) , Vol. 19, p. 383- 389] . The rearrangement took place in an alkaline environment, using alkali carbonates or alkali bicarbonates as a base. A similar approach was reported by the same research group in Organic Syntheses (1973) , Vol . 53, p.123-127 and in US 8,748,486, with alkali hydroxide as a base.

The two-step synthesis shown above was further modified in a coassigned WO 2022/118309 by the addition of a catalytically effective amount of an alkali salt of ODA at the beginning of the rearrangement reaction, to advance the rearrangement reaction of the 1 , 3-dibromo-2-alkanone in an effective and manageable manner.

ODA is an oily substance with a high boiling point: Cahiez et al. ("Stereospecific syntheses of alkenyl lithium reagents from alkenyl iodides", Synthesis, 1976, 4, 245-8) reported a boiling point of 102-103°C/0.5 torr. The crude ODA obtained through the synthetic pathway discussed above or by other methods usually has an assay of ~60-70% measured by high- performance liquid chromatography (HPLC) as absolute quantification based on calibration with a commercially available external standard (>97%) .

Unfortunately, crude CDA cannot be purified easily to remove synthetic by-products. The recently published co-assigned WO 2023/238135 shows the results obtained with a few purification methods, such as silica gel column chromatography (achieving high purity but unsatisfactory low yield (<40%) ) and thermal separation methods. The problem with the latter approach is that purification of cis-2-decenoic acid by thermal separation methods can lead to isomerization of the cis isomer into the trans isomer, as was shown in WO 2023/238135. If thermal separation by evaporation/distillation is not managed properly, the amount of the trans isomer can exceed 1.0% (HPLC area) . But despite the thermal lability of CDA, it was shown in WO 2023/238135 that CDA can be purified efficiently by wiped film evaporation (WFE) to achieve high assay and good purification yield .

The range of impurities accompanying crude CDA is determined by the synthetic pathway, but it appears that the presence of organic acid impurities is unavoidable, e.g., trans-2-decenoic acid and in the case of the synthetic pathway involving the rearrangement of the 1 , 3-dibromo-2-decanone to CDA, also 2- bromomethylidene nonanoic acid (BMNA) . The amount of BMNA formed as a by-product is not insignificant (WO 2022/118309) .

Fairly little was reported on ammonium salts of CDA. Nakayama et al. [The Effect of Carboxylate Anions on the Formation of Clathrate Hydrates of Tetrabutylammonium Carboxylates] . In: Atwood, J.L., Davies, J.E.D., Osa, T. (eds) Clathrate Compounds, Molecular Inclusion Phenomena, and Cyclodextrins. Advances in Inclusion Science, vol 3. Springer, Dordrecht. http :// oi.org/lC.10C7/978-~94-009-5376-5 27 prepared the tetrabutylammonium salt of CDA, starting with neutralization of the free acid with sodium hydroxide in water. Then the aqueous solution of the sodium salt was reacted with silver nitrate to form the silver salt of CDA. On reaction between the silver salt and tetrabutylammonium iodide in water, ions were exchanged to give the tetrabutylammonium salt of CDA and silver iodide. The tetrabutylammonium salt of CDA was characterized as a hydrate with a low melting point. JP 59-199606 shows salts of the formula (R²) (R¹) -C=CHCOON⁺H2R³R⁴ and mentions, albeit without showing preparation, salts of 2-decenoic acid with a few amine bases (butylamine, hexylamine, 2-ethylhexylamine, n-octylamine and dihexylamine) . We have now found that purification of cis-2-alkenoic acids can be achieved by selective precipitation/crystallization of organic ammonium salts of the cis-2-alkenoic acid whilst leaving acidic impurities in the solution. Experimental results reported below show that attempts to separate a water-soluble alkali metal salt of cis-2-alkenoic acid (e.g., potassium salt) from an aqueous solution by the addition of an antisolvent did not result in the formation of a precipitate. However, a neutralization reaction of crude cis-2-alkenoic acid with some amine bases (mainly cyclic amines and medium-chain acyclic amines) , taking place in a suitable organic solvent, or in a mixture of organic solvents, results in the precipitation/crystallization of the corresponding ammonium salt from the organic solution in a selective manner. That is, with a proper choice of a precipitation reagent, namely, the amine base, and an organic reaction solvent, it is possible to induce preferentially the precipitation/crystallization of an insoluble ammonium salt of, e.g., CDA, while keeping the concentrations of salts of acidic impurities below their saturation limit in the organic solution, such that they remain in solution, thus achieving the desired purification effect. By selective precipitation/crystallization of an ammonium salt of cis-2-alkenoic acid, it is meant an enrichment of the cis-2-alkenoic acid content in the isolated salt, compared to the crude acid before neutralization with the amine base, indicated by a > 5%, > 10%, > 15% increase in the HPLC assay of the salt compared to the HPLC assay of the crude cis-2-alkenoic acid (e.g., from 50-70% to 80-95%) . Purification of cis-2-alkenoic acids by crystallization in the form of ammonium salts is amenable to large scale production. For example, it is cheaper than chromatography and thermal separation methods and it usually requires less sophisticated equipment leading to reduction of costs. Accordingly, the invention is primarily directed to a method comprising reacting crude cis-2-alkenoic acid with an amine base and isolating the resultant ammonium salt of cis-2-alkenoic acid in a solid form, from an organic solvent or a mixture of organic solvents .

Suitable amine bases include cyclic amines and medium-chain acyclic amines. The resultant ammonium salt of the cis-2- alkenoic acid is a useful intermediate that enables the purification of the crude acid to a high degree of purity on a large scale. Some ammonium salts, for example, salts which can be obtained by neutralization of CDA with cyclohexylamine in a range of solvents, form another aspect of the invention. The cyclohexylammonium salt of CDA was examined under polarized light microscopy and analyzed by X-ray powder diffraction and was found to be crystalline. The X-ray powder diffraction pattern of the cyclohexylammonium salt of CDA exhibits major diffraction peaks (3 or more) at positions 5.6, 11.3, 14.6, 17.2, 19.7, 23.0 and 28.9 20 (± 0.1 20) [5.6, 11.3, 14.6, 17.2, 17.7, 18.2, 19.7, 23.0, 26.8 and 28.9 20 (± 0.1 20) ] . The cyclohexylammonium salt of CDA has single crystal parameters as tabulated in Table 5 below .

On acidification of the salt, e.g., with a strong mineral acid in water, the free acid is liberated as a distinct, easily separable oily phase showing an enhanced purity assay (e.g., >90%) compared to the crude CDA, with overall good yield. Thus, the method of the invention further comprises the steps of acidifying the ammonium salt of the cis-2-alkenoic acid to liberate the free acid and recovering the free acid.

The crude cis-2-alkenoic acids (R-CH=CH-COOH) purified by the present invention are preferably linear. That is, R is usually a straight alkyl chain CH3- (CH2)_n- (3<n, e.g., 3<n<10, 3<n<6) . Crude cis-2-alkenoic acid obtained by various synthetic pathways can be purified by the method of the invention, e.g,, grades with HPLC assay in the range from 50% to 75-85%, e.g., 55 to 75%. Crude cis-2-alkenoic acid is characterized by an impurity profile consisting of >3%, e.g. >5%, e.g., from 5 to 25%, of organic acids, and as explained above, such acidic impurities can be separated from the cis-2-alkenoic acid, owing to the selectivity of the amine base towards reaction with/precipitation of cis-2-alkenoic acid.

We now describe one synthetic pathway for preparing cis-2- alkenoic acids (see WO 2022/118309) , leading to the formation of crude cis-2-alkenoic acid with an impurity profile as set out above. The synthesis consists of a two-step process comprising the bromination of 2-alkanone to give 1 , 3-dibromo-2-alkanone ; and rearrangement of the 1 , 3-dibromo-2-alkanone (such as those depicted below) to the cis-2-alkenoic acid:

1, 3-dibromo-2-undecanone

The 1 , 3-dibromo-2-alkanone undergoing the rearrangement reaction is most conveniently prepared by brominating the corresponding 2-alkanone [R-CH2-C (0) -CH3] (e.g., 2-heptanone, 2-octanone, 2- nonanone, 2-decanone or 2-undecanone) in concentrated hydrobromic acid (e.g., from 30% to 48% by weight HBr solution) , by the slow addition of elemental bromine (stoichiometry dictates a ~2 : 1 molar ratio of Br2: 2-alkanone) . Alternatively, the bromination of the 2-alkanone may take place in an organic solvent such as halogenated hydrocarbon (CH2CI2 or CH2Br2) with the aid of acceptable bromination reagents.

When the bromination reaction occurs in aqueous HBr, then the weight ratio of the 2-alkanone starting material to the aqueous HBr is from 1:1 to 1:2. The reaction medium is chilled to a temperature in the range from 5 to 20°C, e.g., around 5 to 10°C. Under these conditions, elemental bromine adds smoothly to the 2-alkanone. After the addition of the elemental bromine has been completed, the reaction mixture is held at room temperature (15- 25°C) , optionally under stirring, for some time ("hold time") . Hold time may last between 6 and 24 hours, e.g., between 6 and 12 hours. The desired isomer, 1 , 3-dibromo-2-alkanone progressively becomes the predominant product with the passage of time, i.e., an extended hold time enables a significant interconversion of the 3 , 3-dibromo-2-alkanone isomer to the desired isomer 1 , 3-dibromo-2-alkanone .

To recover the crude 1 , 3-dibromo-2-alkanone, the reaction mixture is worked-up by the addition of water, followed by separation into an aqueous phase (consisting of ~ 48% w/w hydrobromic acid) and an organic phase, consisting of the crude product .

Accordingly, 1 , 3-dibromo-2-alkanone used in the rearrangement reaction is a crude 1 , 3-dibromo-2-alkanone obtained by the steps of : brominating the corresponding 2-alkanone in concentrated hydrobromic acid by the addition of elemental bromine, whereby 1 , 3-dibromo-2-alkanone is formed in the reaction mixture alongside 3, 3-dibromo-2-alkanone; maintaining the reaction mixture over a hold time adjusted to maximize the interconversion of 3 , 3-dibromo-2-alkanone to 1,3- dibromo-2-alkanone (e.g., to reach >65%, >67%, >69% (GC, area%) of 1 , 3-dibromo-2-alkanone ) ; and collecting the crude 1,3- dibromo-2-alkanone .

The crude 1 , 3-dibromo-2-alkanone, without further purification, can now proceed to the rearrangement reaction. A convenient way to carry out the rearrangement reaction comprises gradually adding 1 , 3-dibromo-2-alkanone to a reaction vessel that was previously charged with an alkaline aqueous solution (e.g., consisting of 10 to 30% w/w Na2COs, K2CO3 or a mixture thereof dissolved in water, or carbonate/bicarbonate mixtures) and preferably also a catalytically effective amount of an alkali metal salt of cis-2-alkenoic acid, at an elevated temperature, e.g., h35°C, for example, >40°C. For example, the gradual addition of the 1 , 3-dibromo-2-alkanone takes place when the reaction mixture is held at a temperature in the range of 40°C to 60°C. The molar ratio of 1 , 3-dibromo-2-alkanone to the carbonate salt is from 1:2 to 1:4, e.g., around 1:3-1: 3.5.

The catalytically effective amount of the alkali metal salt of the cis-2-alkenoic acid added to the alkaline solution before the rearrangement reaction starts is preferably from 1 to 5-10 molar percent relative to the 1 , 3-dibromo-2-alkanone . In the presence of the alkali metal salt of the cis-2-alkenoic acid, the reaction takes place during the addition of the 1,3-dibromo- 2-alkanone to the alkaline reaction mixture. The occurrence of the reaction is marked by pH drop (i.e., the initial, strongly alkaline pH of 12-14 drops by at least 2 pH units, e.g., 2-4 pH units, during the addition of the 1 , 3-dibromo-2-alkanone ) , and by temperature rise (i.e., AT reactor) of ~ 5 to 10°C. After the slow addition of the crude 1 , 3-dibromo-2-alkanone has been completed (on a laboratory scale, this may last from 30 to 120 min) , the reaction mixture is held under stirring for some time, i.e., a cooking period over a few (1-3) hours, at a temperature in the range from 50 to 55°C, for the reaction to reach completion. A pH drop of ~ 0.5-1.5 units is observed during the cooking period. The progress of the reaction can be monitored by pH measurement (a constant pH indicates the end of the reaction) and/or GC analysis of the organic phase (to determine the disappearance of the 1 , 3-dibromo-2-alkanone, i.e., down to dl%, areal) .

One exemplary rearrangement reaction is illustrated by the scheme depicted below, transforming 1 , 3-dibromo-2-decanone (1,3- DBD) using K2CO3 into the potassium salt of cis-2-decenoic acid (abbreviated CDA-K) :

1,3-DBD

CDA-K

AP-RM indicates the catalytically effective amount of the alkali metal salt of cis-2-alkenoic acid, added in advance to start up the rearrangement reaction.

On completion of the rearrangement reaction, the reaction mixture is cooled to room temperature and separated into aqueous (heavy) and organic (light) phases. The organic phase can be discarded. The aqueous phase, which contains the cis-2-alkenoic acid in the form of its alkali metal salt (namely, sodium or potassium salts, determined by the base selected) is worked-up to isolate the product. It should be noted that the catalytically effective amount of the alkali metal salt of the cis-2-alkenoic acid can be supplied to the reaction in an aqueous form by removing a minor portion of the aqueous phase, which was collected after the phase separation, and keeping this minor portion for addition in the next run of the process.

Next, the major portion of the aqueous phase is worked-up, by washing (repeated washing cycles may be needed) with a water- immiscible organic solvent such as a halogenated hydrocarbon, e.g., dichloromethane (DCM) , to extract and remove organic impurities from the product-containing aqueous solution.

To recover the product in the form of the free acid, the purified aqueous solution is acidified, e.g., with the aid of concentrated hydrochloric acid (for example, commercially available 32% HC1 solution) , which is slowly added to the aqueous solution to reach a strongly acidic pH (e.g., from 1 to 2) . The acidified reaction mixture is separated into aqueous (heavy) and organic (light) phases. The former contains bromide and chloride salts; the latter consists of the crude cis-2-decenoic acid, and possibly some residual organic solvent (e.g., DCM) which served in the washing stage, and water, which can be removed, e.g., by evaporation under vacuum, whereby the crude cis-2-alkenoic acid is obtained. The acidification reaction resulting in crude CDA is shown below:

CDA

Accordingly, the process of preparing crude cis-2-alkenoic acid further comprises acidification of the purified aqueous phase (i.e., recovered after the extraction with DCM) , to obtain a biphasic medium comprised of a heavy, salt-containing aqueous phase, and a light organic phase consisting essentially of a crude cis-2-alkenoic acid (as the free acid) , separating the crude cis-2-alkenoic acid and optionally removing residual organic solvents (e.g., DCM) from the crude cis-2-alkenoic acid by evaporation.

Turning now to describe the purification of crude cis-2-alkenoic acid, it should be first noted that the term "crude cis-2- alkenoic acid" as used herein includes an as-synthesized cis-2- alkenoic acid (i.e., the direct product of chemical synthesis, as shown above) and any cis-2-alkenoic acid with insufficient purity level (i.e., material that was already treated to remove impurities by other methods) .

The as-synthesized cis-2-alkenoic acid may be supplied to the purification stage in an isolated form free of residual organic solvents (e.g., after evaporation of residual extraction solvents, e.g., DCM, used in a work-up stage) . But the as- synthesized cis-2-alkenoic acid can enter the purification step while it still contains residual extraction solvents. In fact, because the purification is achieved by neutralizing the cis-2- alkenoic acid with an amine base in an organic solvent, a suitable extraction solvent can serve, at least in part, as the solvent for the neutralization reaction, from which the ammonium salt is crystallized.

As to low-purity cis-2-alkenoic acid, the purification method of the invention can be used as a final clean-up step, after the application of other purification methods to an as-synthesized impure product, to raise the purity of the cis-2-alkenoic acid to >85%, >90% (by HPLC assay against an external standard) . That is, any cis-2-alkenoic acid with insufficient purity level (<85%; e.g., <80%; <70%, e.g., 50-75% HPLC assay) could benefit from the method of the invention to remove impurities, especially acidic impurities, e.g., the trans isomer and brominated acids such BMNA.

In its most general form, the method comprises a reaction of crude cis-2-alkenoic acid with an amine base to form the corresponding ammonium salt in a solid (e.g., crystalline) form, separation of the ammonium salt from the mother liquor by filtration (or any other solid/liquid separation technique) , and acidification of the ammonium salt in an aqueous solution with the aid of a strong mineral acid, to recover the free acid.

For example, the method comprises dissolving crude cis-2-alkenoic acid in an organic solvent or a mixture of organic solvents, adding the amine base to precipitate/crystallize the ammonium salt of cis-2-alkenoic acid, and separating the solid ammonium salt from the liquid phase.

The amine bases used in the invention are usually liquid at room temperature, showing good miscibility in a range of organic solvents. In addition, their HC1 salts dissolve well in water. Suitable amine bases for use in the invention are selected from the group consisting of cyclic amines and medium-chain (straight or branched) acyclic aliphatic amines. C6-C10 primary amines with pKb <4 (25°C) are also useful.

The cyclic amines include cyclic aliphatic amines, e.g., cyclic aliphatic primary amines of the formula C_nH2n-i-NH2, with n ranging from 3 to 10, such as cyclopropylamine (C3H5-NH2) , cyclobutylamine (C4H7-NH2) , cyclopentylamine (C5H9-NH2) , cyclohexylamine (CeHn- NH2) and cycloheptylamine (C7H13-NH2) , and the corresponding secondary (Cnfbn-i-NHR¹) and tertiary (C_nH2n-i-NR¹R²) amines, where R¹ and R² are independently lower alkyl groups, e.g., methyl, such as N, N-dimethylcyclohexylamine (CeHn-N (CH3) 2) . Heterocyclic secondary amines, e.g., with molecular formula (CH2)_PNH, i.e., the nitrogen atom is a ring atom with p ranging from 4 to 9, such as pyrrolidine and piperidine can also be used. The term "cyclic aliphatic amine" is also meant to include hydrocarbon rings where the -NH2, -NHR¹ or NR²R² group is not attached directly to a carbon ring atom, but rather through a short chain, such as 1-cyclohexyl-ethylamine (CeHn-CH (CH3) NH2) and 2-cyclohexyl-ethylamine (C6H11-CH2-CHNH2) . The amine bases are usually monoamines. Aromatic amines (e.g., pyridine) can also be used; the rings may be optionally substituted.

Medium-chain (straight or branched) acyclic aliphatic amines have a carbon skeleton consisting of six to 10 carbon atoms. Preferred are normal primary aliphatic amines of the formula CH3 (CH2) _mNH2, where m is 6, 7 or 8, e.g., n-octylamine .

It is convenient to carry out the purification of the crude cis- 2-alkenoic acid by first dissolving (usually at room temperature) the acid in one or more organic solvents selected from the group consisting of saturated hydrocarbons (e.g., alkanes such as n- pentane, n-hexane, n-heptane) , halogenated hydrocarbons (e.g., dichloromethane) and ethers (e.g., tert-butyl methyl ether) . Other classes, e.g., esters, aromatic hydrocarbons and ketones can also be used. The concentration of the cis-2-alkenoic acid in the organic solution is from 5 to 50% by weight, e.g., from 10 to 30% by weight.

Next, an amine base is added to the organic solution. Amine bases that are mentioned above are liquid at room temperature and they usually show good miscibility with a range of organic solvents, namely, the saturated hydrocarbons, halogenated hydrocarbons and ethers mentioned above. Usually, an equimolar amount of the amine base is supplied to the neutralization reaction of the cis-2- alkenoic acid (the amount of the amine base is calculated as if the whole amount of crude material subjected to purification consists of the target cis-2-alkenoic acid) .

The addition of the amine base to a reaction vessel that was previously charged with the organic solvent (s) and the cis-2- alkenoic acid is preferably carried out in a gradual manner at ambient temperature under stirring. The neutralization reaction of the acid by the base takes place with the evolution of heat. Usually, a temperature rise of ~ 1 to 10°C is observed. Most of the reaction occurs during the addition of the base. The reaction mixture is allowed to cool down, either by leaving the solution to come to room temperature, or by placing the reaction vessel inside a cooling bath on a lab scale, or by using a conventional coolant system coupled to chemical reactors. Selective precipitation/crystallization occurs at a moderately low temperature, with crystals starting to separate out from the solution at ~20 °C. No significant cooling is needed to induce crystallization of the salt; cooling the reaction down to about 10-20°C, e.g., 15-20°C is sufficient to recover a major portion of the solute in the amine salt form.

Accordingly, another aspect of the invention is a method comprising dissolving the crude cis-2-alkenoic (e.g., cis-2- decenoic acid) in an organic solvent or in a mixture of organic solvents, gradually adding the amine base (e.g. cyclohexylamine) , e.g., at room temperature under stirring, allowing the reaction mixture to cool to crystallize the ammonium salt of cis-2-alkenoic acid, and separating the solid ammonium salt from the liquid phase, e.g., by filtration.

It should be noted that the order of steps may be reversed, i.e., first dissolution of the amine base in the organic solvents (s) , followed by a slow addition of the crude cis-2-alkenoic acid. Another approach involves a solventless salt formation reaction between the crude cis-2-alkenoic acid and the amine base, as both are liquids at room temperature. Addition of the amine base to a reaction vessel that was previously charged with the crude cis- 2-alkenoic acid (or vice versa) can afford the ammonium salt product, typically as a material with a paste consistency or as a thick slurry. Trituration/recrystallization of the solid in the solvent (s) listed above, affords the salt product as a filterable powder. But in general, a salt formation reaction conducted in one of the solvents listed above, namely, in a stirrable solution, is more readily scalable and is usually preferred .

The purification method of the invention offers an efficient utilization of process solvents, i.e., 1) solvents used during the production of the crude cis-2-alkenoic acid, and 2) solvents used for the salt formation reaction between the amine and the crude cis-2-alkenoic acid.

It was mentioned above in passing that the solvents used during production of the crude cis-2-alkenoic acid can act as solvents for the neutralization of the acid by an amine base and crystallization of the ammonium salt product. It is recalled that the synthesis of crude cis-2-alkenoic acid takes place in an alkaline aqueous solution. Upon completion of the synthesis, the reaction mixture is separated into aqueous and organic phases, and the aqueous phase, which contains the cis-2-alkenoic acid in the form of its alkali metal salt, is acidified to isolate the free acid. But before the acidification step, the aqueous phase is washed with a water-immiscible organic solvent - e.g., dichloromethane - to extract and remove organic impurities. The crude free acid that is ultimately liberated as an oil is accompanied by a residual amount of dichloromethane, e.g., the ratio DCM:CDA is from 1:10 to 1:3, e.g., 1 : 5 to 1 : 3. Experimental work reported below shows that the reaction between the crude cis-2-alkenoic acid and an amine base proceeds efficiently in DCM, or in a mixture of DCM and a second solvent, e.g., an ether solvent such as t-BME, e.g., with the mixture consisting of DCM and the second solvent being proportioned from 1:20 to 1:7 by weight (the second solvent, such as t-BME, is the major solvent in the solvent mixture) . The corresponding ammonium salt e.g., the cyclohexylamine salt of CDA, is separable from such DCM- containing solvent mixtures in a solid, e.g., crystalline form, with an increased content (HPLC assay) of the CDA relative to the crude material. That is, evaporation of the DCM, used at an earlier stage of the process, is not mandatory. The purification step of the invention can be coupled in a "telescopic-like" fashion to the liberation and separation of the crude cis-2- alkenoic acid.

Accordingly, the crude cis-2-alkenoic acid purified by the method of the invention is obtained by: rearranging 1 , 3-dibromo-2-alkanone in a reaction vessel which was previously charged with an alkaline aqueous solution, separating the reaction mixture into aqueous and organic phases, and working-up the aqueous phase, which contains the cis-2- alkenoic acid in the form of its alkali metal salt, to isolate the free acid, wherein the work-up comprises: washing the aqueous phase with a water-immiscible organic solvent (e.g., DCM) to extract and remove organic impurities ;

- acidifying the aqueous phase, to liberate the cis-2- alkenoic acid from its alkali salt;

- separating into aqueous and organic phases, wherein the organic phase consists of the crude cis-2-alkenoic acid and residual water-immiscible organic solvent that was used in the washing stage; and - either removing the residual water-immiscible organic solvent by evaporation from the crude cis-2-alkenoic acid, which then proceeds to the reaction with the amine base; or

- delivering the crude cis-2-alkenoic acid accompanied by the residual water-immiscible organic solvent directly to the reaction with the amine base, such that said water-immiscible organic solvent acts as a solvent in the reaction with the amine base .

Experimental work conducted in support of this invention also shows the recyclability of the filtrate streams produced by the separation of the solid ammonium salt from its mother liquor. The recycled mother liquor can be used as a solvent in successive reactions, i.e., on addition of crude cis-2-alkenoic acid and amine base to the recycled solvent, to precipitate/crystallize a salt product, without employing fresh solvent, or using smaller amounts of the fresh solvent.

Thus, another aspect of the invention is a method comprising separation of the ammonium salt from the liquid phase, recycling and using the liquid phase (i.e., filtrate, supernatant) as a solvent in a subsequent run, wherein a crude cis-2-alkenoic acid and an amine base is added to the recycled solvent to recover additional crop of an ammonium salt.

The amine salt that was separated from the organic solvent is optionally dried or proceeds without removing the solvent to the acidification step. The acidification reaction comprises adding the isolated ammonium salt into water, adding a strong mineral acid, e.g., hydrochloric acid (e.g., from 8 to 12% by weight HC1 solution) , thereby reducing the pH to below <3, e.g., l<pH<3, whereby the free acid is liberated from the amine salt, forming a light oily phase recoverable by phase separation. Owing to the miscibility of the amine HC1 salt in water, it remains in the aqueous phase. GC/¹H-NMR analysis of the purified free acid did not detect the amine base as an impurity.

In the drawings

Figure 1 relates to the crude CDA from preparation 1 (A) ¹H-NMR spectrum, (B) GC chromatogram and (C) HPLC chromatogram.

Figure 2 is a flowchart corresponding to the experiment described in Example 19, which involves the precipitation of CDA-CHA salt from t-BME, and filtrate recycling for 3 successive runs.

Figure 3 is a flowchart showing the recovery by acidification of free CDA from the CDA-CHA of Example 19.

Figure 4 is a HPLC chromatogram of the crude CDA (red, CDA assay 67%) used in Example 19, and the purified free CDA obtained (blue, CDA assay 88%) .

Figure 5 is a ¹H-NMR spectrum of the purified CDA (the free acid) obtained in Example 19.

Figures 6A-6C show GC chromatograms of cyclohexylamine (A, top) , CDA-cyclohexylamine salt (B, middle) and purified free CDA (C, bottom) .

Figure 7 shows X-ray powder diffraction patterns of cyclohexylammonium salt of cis-2-decenoic acid prepared with different starting materials.

Figure 8 shows configuration of the crystalline cyclohexylammonium salt of cis-2-decenoic acid. Examples

Methods

GC : Gas-chromatograph HP 7890A

Method (CDA) : Initial temp. 50°C, held 2 min, then raised to 280°C at 10°C/min and held for 5 min, then raised to 300°C at 10°C/min and held for 2 min.

Injector: 250°C

Detector: 300°C

Split ratio: 1:40

Concentration of the product sample: ~20 mg/ml DCM

Injection amounts: 1 pl sample

Column: Agilent J&W Columns, HP-5, 30 m x 0.32 mm x 0.25p

¹H-NMR spectroscopy

Spectra were taken on an Avance III, 500 MHz instrument.

HPLC : Agilent 1220 LC system

HPLC analysis was performed by an Agilent 1200 LC system equipped with a quaternary pump, autosampler and diode array detector. CDA assay was determined against an external standard. Column: Kromasil C18 5p 250x4.6 mm i.d.

Mobile phase (a) 0.1% H3PO4/water, (b) acetonitrile

Gradient table:

Time (min) % (b)

0 58

19 58

24 95

29 95

34 58

Flow: 1 ml/min.

Injection volume: 10 pL Wavelength: X=214 nm Run time: 35 min. Post run time: 5 min.

Column temperature: 25°C

Sample temperature: ambient

Retention time of CDA: 13.4 min.

The assay of CDA was determined using an external standard calibration curve.

Preparation 1 (Example 1 of WO 2022/118309 and WO 2023/238135)

Preparation of crude cis -2 -decenoic acid

Step 1 :

Into a mixture of 2-decanone (200 g, 1.28 mol) and aq. 48% HBr (300 g) , stirred and cooled to ~10°C, was added bromine (410 g, 2.56 mol) , dropwise over 2 h. The reaction started immediately with the start of the addition of the bromine and no accumulation of bromine was observed.

The reaction was exothermic and accompanied by the emission of HBr gas, just before the end of the addition of the bromine, which was absorbed in a scrubber.

Most of the reaction took place during the addition of the bromine and cooking at room temperature (~20°C) for 6 hours. After standing overnight (~15 h) at room temperature, without stirring, the composition of the reaction mixture stabilized. Partial conversion of the 3 , 3-dibromo-2-decanone (3,3-DBD) to the desired product, 1 , 3-dibromo-2-decanone (1,3-DBD) , took place. To the reaction mixture was added water (160 g) at RT, with stirring for 30 min, and the phases were separated.

An aqueous phase (627 g) was obtained containing ~50% HBr (d = 1.51 g/ml) and crude DBD (404 g, d = 1.43 g/ml) . The concentration of 1,3-DBD in the crude product was 69.6% (GC, area%) .

Step 2 :

An aqueous solution of K2CO3, in a concentration of 25% w/w, was prepared in a IL stirred reactor by the batchwise addition of K2CO3 (200 g) to water (600 g) . The reaction was exothermic. To this solution was added a part of the aqueous phase (which contained CDA-K) of the reaction mixture (50 g) remaining from a previous run (named AP-RM) . The clear solution obtained was heated to 40°C and crude DBD of step 1 (200 g) was added to it dropwise over 60 min. The progress of the reaction was monitored by GC and by the change of the pH. The reaction was completed by cooking at 50°C for 3.0 h, with mechanical stirring.

It should be pointed out that without the addition of AP-RM, the reaction only starts spontaneously two hours after the addition of the crude DBD.

The end of the reaction was determined by the pH (drop in the pH from 13.3 to 9.3) and by GC analysis of the reaction mixture (disappearance of 1,3-DBD to <1% , area%) . After completion of the reaction, cooling to RT and stopping the stirring, an organic phase appeared above the aqueous phase which contained unreacted 3-bromo-2-decanone (3-BD) and 3,3-DBD, and byproducts formed by a condensation reaction of crude DBD. The phases were separated. The organic phase (39 g) was organic waste. 50 g of the aqueous phase was taken for use in the next run .

In order to reduce the amount of impurities to a minimum, the remainder of the aqueous phase (950 g) was washed three times with dichloromethane (DCM, 3 x 250 g) . After the washing stage, an aqueous phase was obtained containing cis-2-decenoic acid potassium salt (CDA-K) and organic by-products, KBr and KHCO3. In order to obtain the crude cis-2-decenoic acid (CDA) , the aqueous phase was acidified by the dropwise addition of aq. 32% HC1 (193 g) over 1 h. During the acidification (final pH=l.l) , CO2 (calculated at 63 g) was emitted .

After stopping the stirring, an aqueous phase (955 g) was obtained containing salts: KC1 and KBr (heavy phase, d = .19 g/ml) and wet crude CDA (light phase, 71 g, d = 1.07 g/ml)

Evaporation of the DCM and lights from the wet CDA under vacuum (at TB = 30-40°C, 15-25 mbar) gave crude CDA (50.5 g) , which was analyzed by GC, HPLC and ¹H-NMR (see Figures 1A, IB and 1C for ¹H-NMR spectrum and the GC and HPLC chromatograms) . The calculated yield of crude CDA was ~68%, based on 1,3-DBD, or 46.8%, based on 2-decanone.

The purity of the crude CDA obtained was 88.2% (by GC area%) .

The main impurity in the crude product was 2-bromomethylidene nonanoic acid (BMNA) : 8.8% (by GC, area%) . HPLC assay determined against an external standard was 69% assay.

Preparation 2

CDA (assay 92%) was obtained from CV-Chem and was used as is for assessment of precipitation by various amine bases.

Preparation 3

Crude CDA (assay 62%) was obtained from CV-Chem and filtered before use. Example 1 (comparative)

Reaction of CDA with potassium hydroxide

The potassium salt was prepared by the addition of 10g CDA to an equimolar amount of KOH aqueous solution (0.06M) in a 250 mL vessel. The resultant solution was evaporated, to give a slurry/paste like product. Samples of the product (0.1-0.6 g) were added to, and triturated in, different organic solvents (1-3 ml acetonitrile, THF, water or acetone) , but none of the treatments resulted in the formation of a workable, filtrable, powdery precipitate.

CDA potassium salt in water was prepared (5g CDA / 1.8g KOH in 55 mL, pH = 12-13) . To 5 ml aliquots of the potassium salt solution, an antisolvent was added up to 2.5 ml (the organic solvents tested were acetonitrile, THF, acetone and IDA) . No precipitation occurred upon cooling the aqueous/organic solution to 2-8°C. The solution was left at 2-8°C for 48 hours, but no precipitate was observed.

Example 2 to 4 (comparative) and 5 (of the invention) Reaction of CDA with organic bases

A series of experiments was conducted to test solventless reactions between crude CDA and a few amine bases, to determine if the resultant ammonium salts are solids and whether these salts show selectivity towards precipitation of CDA over acidic impurities present in the crude CDA.

A typical procedure consists of two steps. In the first step, a solventless salt formation was carried out, by adding an equimolar amount of the amine base to the neat crude CDA (1-3 g CDA of Preparation 1; as if all the crude material consists of CDA) . In the second step, the resultant material (~ 5g) , was triturated in an organic solvent (15-40 mL) in an attempt to obtain a workable slurry that can undergo filtration to collect a solid. The conditions of each experiment and the observations are tabulated below.

Table 1

The results tabulated above indicate that with cyclohexylamine, a solventless neutralization reaction between the crude CDA and the base affords the cyclohexylammonium salt of CDA in a solid form. Trituration of the cyclohexylammonium salt of CDA (CDA- CHA) in different solvents enabled the removal of soluble impurities (apparently the corresponding salts of acidic impurities) , as shown by the high HPLC assay (~90%) of CDA-CHA compared to the crude CDA (~70%) , indicating that a significant portion of the acidic impurities remain in solution. The results suggest that with a proper choice of a solvent, a reaction between CDA and cyclohexylamine could lead to selective crystallization of CDA-CHA with enhanced purity, leaving impurities in solution. Examples 6 to 9 (of the invention)

Reaction of CDA with cyclohexyl amine in organic solvents and selective crystallization of cyclohexylammonium salt of CDA

A series of experiments was conducted to neutrali ze crude CDA dissolved in di f ferent organic solvents by addition of the cyclohexylamine base , to determine whether the resultant cyclohexylammonium salt of CDA is crystalli zed in a selective manner, leaving acidic ( and other) impurities in solution .

The salt formation reaction took place in a round bottom flask equipped with a magnetic bar and thermometer . A typical procedure consisted of dissolution of the crude CDA ( Preparation 1 ) in the organic solvent (usually in 1 : 8 w/v ratio ) , followed by gradual addition of an equivalent amount of the cyclohexylamine ( calculated as i f all crude CDA consisted of 100% CDA) . Usually, the addition of base to the solution was exothermic . The solution was cooled to 15-20 ° C, at which temperature precipitation started . The suspension was stirred at 15-20 ° C for about an hour or two and then the solid was collected by vacuum filtration and washed with cold fresh solvent ( re-suspended on a filter ) . The salt was dried under reduced pressure at 35 °C and analyzed by HPLC . The experimental conditions and results are tabulated below .

Table 2 yield was calculated for CDA content not product weight The results indicate that crude CDA could be purified owing to the selective crystallization of CDA-CHA from different solvents at moderate temperature, with increased content of CDA in the salt, as compared to the crude CDA.

In addition, CDA-CHA of Example 6A was acidified by suspending the salt (2 g) in water (lOmL) and the addition of 10% HC1 solution (2.2 g) to a pH~l-2. The free acid (oil, 1.27 g) was quantitatively recovered and collected by phase separation. HPLC analysis indicated an assay of 83% and a total purification yield of 62%.

Examples 10-11 (comparative) and 12-18 (of the invention) Reaction of CDA with amine bases in DCM or DCM/t-BME mixture and selective crystallization of ammonium salts

A series of experiments was conducted to examine the reaction of CDA with amine bases in dichloromethane or in a mixture of dichloromethane and t-butyl methyl ether, to determine if the corresponding ammonium salt is obtained in a selective manner, in an easily separable, crystalline form, from which the free acid can be liberated with enhanced purity assay.

Salt formation reaction: the reaction took place in a round bottom flask equipped with a magnetic bar and thermometer. A typical procedure consisted of dissolving the crude CDA from preparations 1, 2 or 3 in DCM, with optional addition of MTBE to the stirred solution. Then the amine base (triethylamine (TEA) , tetradecylamine (TDA) , n-octylamine (NOA) and cyclohexylamine (CHA) ) was added over about 10-15 min into the stirred solution. The salt formation reaction was exothermic, marked by a temperature rise up to 30 °C. The flask was placed in a water bath and the reaction mixture was allowed to cool down. The temperature at which precipitation/crystallization started is indicated in the table below. The reaction mixture was further cooled down to 13-17 °C, as appropriate, and was held at this temperature for about 2h.

When precipitation occurred, the solid was filtered (A41 filter paper) under reduced pressure, and the filter cake was washed twice with 50 ml (10-15% of total solvent volume) of cold MTBE (vacuum was stopped before each solvent addition and resumed immediately after addition) . The cake was dried under reduced pressure (at 36 °C) to yield the corresponding ammonium salt.

Recovery of free CDA: 30g of the salt was suspended in 80 ml of DI water and acidified by addition of 40g of 10% HC1. The pH of the aqueous phase was pH=l . The phases were separated, and the upper organic phase was collected and evaporated to a constant weight, yielding the free CDA as a pale-yellow oil. GC analysis showed no traces of the base in the product.

The conditions and HPLC purity assay are tabulated below.

Table 3

(*) The total purification yield is calculated as for the assay of CDA in the relevant crude CDA and CDA assay in the purified free CDA.

(**)no phase separation Example 19

Reaction of CDA with cyclohexyl amine in t-BME using recycled filtrate streams as solvent

Filtrate recyclability was tested, as shown in the flowchart appended in Figure 2.

Crude CDA prepared according to Preparation 1 (8.01g; ~65% HPLC assay) was dissolved in t-BME (48g, 65 mL) , followed by addition of an equimolar amount of CHA (4.74g, 5.38 mL) . The temperature of the reaction mixture increased from 21°C to 33°C. The reaction mixture was cooled down in a water bath to 17 °C, and the precipitate formed was separated by filtration, and washed with t-BME (15 ml at 15-17°C) . 5.5 g of CDA-CHA were collected.

The filtrate was recycled as a solvent in three consecutive salt formation reactions. In each cycle, the amount of crude CDA dissolved in the recycled solvent was equal to the equivalent amount of CDA isolated as the CHA salt in the previous run. Then an equimolar amount of cyclohexylamine salt was gradually added, with evolution of heat. The reaction mixture was cooled down to 14-17°C, to induce crystallization. A crop of CDA cyclohexylamine salt was collected by filtration, washed as above, with the filtrate moving to the next run. The conditions of the four successive precipitation reactions and the results are shown in Figure 2 and Table 4.

Table 4 The assay and yield of the CDA in the precipitated CDA-CHA salt was carried out by HPLC : a sample was acidified in the sample medium (ACN/HsPCh/water) , so the actual analyte was the free CDA. The expected CDA assay was predicted assuming that all CDA in the precipitate is CDA-CHA, this value allows the evaluation of the selectivity of the precipitation (i.e., no selectivity will result in a predicted assay of ~65% as in the starting crude CDA, maximum selectivity - only CDA-CHA salt is precipitated - will give a predicted assay of 100%) . The theoretical yield of the cycle is the ratio between the content of CDA in the crude CDA added in the cycle and the content of CDA in the precipitation.

The total amount of CDA cyclohexylamine salt collected after the four successive runs was 13.4g. The combined salt was suspended in water (65 g) , acidified by HC1 to pH=3 (10% HC1, 16 g) whereby the free CDA was separated and collected as oil (8.4g, 98% recovery) . The assay of the free CDA was 88%. The acidification is shown in the flowchart appended as Figure 3. The total yield of CDA from the crude product was 63% of the CDA content in the crude.

Figure 4 is an HPLC chromatogram of the crude CDA (red, CDA assay 67%) and purified free CDA (blue, CDA assay 88%) . The characteristic CDA peaks at 13.4' have the same height in both chromatograms, but minor peaks assigned to the impurities are significantly smaller in the chromatogram of the purified CDA (blue) . Figure 5 is a ¹H-NMR spectrum of the purified free CDA and is devoid of peaks assigned to the cyclohexylamine base. Examples 20A-20B Reaction of CDA with cyclohexylamine in DCM/t-BME and characterization of the CDA-CHA salt

20A (starting from crude CDA of a commercial source) :

18g Dichloromethane were added to 50g of filtered crude CV-Chem CDA (Preparation 3; CDA assay ~63%) to afford CDA- DCM-solution with a composition matching the organic phase that is separated from the acidic aqueous phase in the last stage of crude CDA production .

To this solution, 330mL of t-butyl methyl ether were added to give a clear CDA solution. Cyclohexylamine (29g) was added slowly to the stirred solution, the CHA addition was exothermic, and the temperature of the reaction mixture was controlled by the rate of CHA addition and kept below 31°C. Clear solution was obtained. The solution cooled while stirring, and at about 22-20°C precipitation started. The slurry was further cooled to 16-15°C and filtered on a paper filter (A41) under vacuum. The precipitate was washed twice with cold t-butyl methyl ether (2x50mL) and dried under reduced pressure to yield 30g of CDA- CHA salt with an assay of 96% (crystallization yield by CDA: 58%) .

20B (starting from crude CDA of Preparation 1) :

7g Dichloromethane were added to 20g of crude CDA (according to Preparation 1, with CDA assay ~65%) to afford CDA- DCM-solution with a composition matching the organic phase that is separated from the acidic aqueous phase in the last stage of crude CDA production .

To this solution, 132mL of t-butyl methyl ether were added to give a clear CDA solution. Cyclohexylamine (12g) was added slowly to the stirred solution, the CHA addition was exothermic, and the temperature of the reaction mixture was controlled by the rate of CHA addition and kept below 27°C. Clear solution was obtained. The solution cooled while stirring, and at about 23°C precipitation started. The slurry was further cooled to 16-15°C and filtered on a paper filter (A41) under vacuum. The precipitate was washed twice with cold t-butyl methyl ether (2x20mL) and dried under reduced pressure to yield 14g of CDA- CHA salt with an assay of 94% (crystallization yield by CDA: 63%) .

Figure 6 shows GC chromatograms of commercial cyclohexylamine (top) , a CDA-cyclohexylamine salt corresponding to the procedure of Example 20A (middle) and the purified free CDA resulting from Example 20A (bottom) .

Samples of cyclohexylammonium salt of cis-2-decenoic acid prepared by procedures 20A and 20B were examined under polarized light microscopy and were found to be crystalline. In addition, the samples were analyzed by X-ray powder diffraction. X-ray powder diffraction patterns were recorded from 3 to 80° 20 using CuKa (1.54178 A) radiation with the following measurement conditions: tube voltage of V=40 kV, tube current of 1= 30 mA. The two measured X-ray diffraction patterns corresponding to procedures 20A and 20B, presented as nearly overlaid as possible in Figure 7, are in fact the same, exhibiting major diffraction peaks at positions 5.6, 11.3, 14.6, 17.2, 17.7, 18.2, 19.7, 23.0, 26.8 and 28.9 20 (± 0.1 20) .

Example 21

Single crystal structure of CDA-CHA salt

A suitable crystal obtained using the procedures of the previous Example was selected and the data was collected on a diffractometer (wavelength: 1.54184 A) . The crystal was kept at 100.15K during data collection. Using 01ex2, the structure was solved with the olex2. solve structure solution program using Charge Flipping and refined with the SHELXL refinement package using Least Squares minimization. Details of the crystal are tabulated in Table 5.

Table 5

Additional details: 7155 reflections measured (5.882° < 20 < 142.368°) , 3076 unique (A±_nt = 0.0334, R_sigma = 0.0461) which were used in all calculations. The final Ri was 0.0457 (I > 2o(I) ) and WR2 was 0.1333 (all data) . Bond lengths and bond angles determined from the single crystal data are tabulated in Tables 6 and 7, respectively (see also Figure 8) .

Table 6

AtomAtom Length/A AtomAtom Length/A

01 CIO 1.2723 (17) C8 C9 1.332 (2)

Cl C2 1.522 (2) C9 CIO 1.4958 (19)

02 CIO 1.2459 (17) N1 Cll 1.4957 (19)

C2 C3 1.520 (2) Cll C12 1.5231 (19)

C3 C4 1.521 (2) Cll C16 1.5199 (19)

C4 C5 1.526(2) C12 C13 1.528 (2)

C5 C6 1.521 (2) C13 C14 1.523 (2)

C6 C7 1.524 (2) C14 C15 1.521 (2)

C7 C8 1.497 (2) C15 C16 1.528 (2) Table 7

Atom Atom Atom Angle/ ° AtomAtomAtom Angle/ °

C3 C2 ci 113.72 (13) 02 CIO C9 119.39 (13)

C2 C3 C4 113.04 (12) N1 Cll C12 109.53 (11)

C3 C4 C5 113.81 (12) N1 Cll C16 110.23 (11)

C6 C5 C4 113.66(12) C16 Cll C12 111.31 (12)

C5 C6 C7 112.91 (12) Cll C12 C13 110.59 (12)

C8 C7 C6 113.20 (12) C14 C13 C12 111.84 (13)

C9 C8 C7 128.75 (14) C15 C14 C13 111.14 (13)

C8 C9 CIO 126.07 (14) C14 C15 C16 111.05 (12)

01 CIO C9 116.06(13) Cll C16 C15 110.36(12)

02 CIO 01 124.54 (13)

Claims

1) A method comprising reacting crude cis-2-alkenoic acid with an amine base and isolating the resultant ammonium salt of cis- 2-alkenoic acid in a solid form, from an organic solvent or a mixture of organic solvents.

2) A method according to claim 1, comprising dissolving the crude cis-2-alkenoic acid in an organic solvent or in a mixture of organic solvents, adding the amine base to precipitate the ammonium salt of cis-2-alkenoic acid, and separating the solid ammonium salt from the liquid phase.

3) A method according to claim 2, comprising dissolving the crude cis-2-alkenoic acid in an organic solvent or in a mixture of organic solvents, adding the amine base at room temperature under stirring, allowing the reaction mixture to cool, if needed, whereby the ammonium salt of cis-2-alkenoic acid is precipitated, and separating the solid ammonium salt from the liquid phase.

4) A method according to any one of claims 1 to 3, wherein the amine base is selected from the group consisting of:

A) cyclic amines; and

B) medium-chain, straight or branched, acyclic aliphatic amines.

5) A method according to claim 4, wherein the cyclic amine is cyclic aliphatic primary amine of the formula C_nH2n-i-NH2, with n ranging from 3 to 10.

6) A method according to claim 5, wherein the cyclic aliphatic primary amine is cyclohexylamine.

7) A method according to any one of the preceding claims, wherein the organic solvent (s) is (are) selected from the group consisting of saturated hydrocarbons, halogenated hydrocarbons, and ethers . 8 ) A method according to claim 7 , wherein the saturated hydrocarbon comprises n-pentane, n-hexane or n-heptane , the halogenated hydrocarbon comprises dichloromethane , and the ether comprises tert-butyl methyl ether .

9 ) A method according to any one of the preceding claims , wherein the crude cis-2-alkenoic acid is crude cis-2-decenoic acid and the amine base is cyclohexylamine .

10 ) A method according to claim 9 , comprising di ssolving crude cis-2-decenoic acid in an organic solvent or in a mixture of organic solvents , adding cyclohexylamine at room temperature under stirring, optionally cooling the reaction mixture , whereby the cyclohexylammonium salt of cis-2-decenoic acid is crystalli zed, and separating the crystalline salt from the liquid phase .

11 ) A method according to claim 10 , wherein the mixture of organic solvents comprises dichloromethane and tert-butyl methyl ether .

12 ) A method according to any one of claims 9 to 11 , wherein the crude cis-2-decenoic acid has an HPLC purity assay of less than 70% .

13 ) A method according to any one of the preceding claims for puri fying crude cis-2-alkenoic acid, the method further comprises the steps of acidi fying the ammonium salt of the cis-2-alkenoic acid in water and separating a puri fied free acid as an oil .

14 ) A method according to claim 13 for puri fying crude cis-2- decenoic acid, the method comprises the steps of acidi fying the crystalline cyclohexylammonium salt of cis-2-decenoic in water and separating a puri fied free cis-2 -decenoic acid as an oil . 15 ) A method according to claim 14 , wherein the puri fied free cis-2-decenoic acid shows an increased HPLC purity assay compared to the crude cis-2- decenoic acid by at least 10% .

16 ) A method according to any one of the preceding claims , wherein the crude cis-2-alkenoic acid is obtained by rearranging 1 , 3- dibromo-2-alkanone in a reaction vessel which was previously charged with an alkaline aqueous solution, separating the reaction mixture into aqueous and organic phases , and working-up the aqueous phase , which contains the cis-2-alkenoic acid in the form of its alkali metal salt , to isolate the free acid .

17 ) A method according to claim 16, wherein the work-up comprises : washing the aqueous phase with a water-immiscible organic solvent to extract and remove organic impurities ; acidi fying the aqueous phase , to liberate the cis-2-alkenoic acid from its alkali salt ; separating into aqueous and organic phases , wherein the organic phase consists of the crude cis-2-alkenoic acid and residual water-immiscible organic solvent used in the washing stage ; wherein either the residual water-immiscible organic solvent is removed by evaporation from the crude cis-2-alkenoic acid, which proceeds to the reaction with the amine base ; or the crude cis- 2-alkenoic acid accompanied by the residual water-immiscible organic solvent proceeds directly to the reaction with the amine base , such that said water-immiscible organic solvent acts as a solvent in the reaction with the amine base .

18 ) A method according to claim 17 , wherein the water-immiscible organic solvent is dichloromethane .

19 ) A method according to any one of the preceding claims , wherein the crude cis-2-alkenoic acid is crude cis-2-decenoic acid that contains, as impurities, trans-2-decenoic acid and/or 2- bromomethylidene nonanoic acid.

20) A method according to any one of the preceding claims, wherein, following separation of the ammonium salt from the liquid phase, the liquid phase is recycled and used as a solvent in a subsequent run, wherein a crude cis-2-alkenoic acid and an amine base is added to the recycled solvent to recover additional crop of an ammonium salt.

21) Crystalline cyclohexylammonium salt of cis-2-decenoic acid.

22) Cyclohexylammonium salt of cis-2-decenoic acid of claim 19, which has an X-ray powder diffraction pattern that exhibits three or more diffraction peaks at positions 5.6, 11.3, 14.6, 17.2, 19.7, 23.0 and 28.9 20 (± 0.1 20) .

23) Cyclohexylammonium salt of cis-2-decenoic acid of claim 19, characterized by the following single crystal parameters: the crystal system is monoclinic, the space group is P2i/n, the unit cell has dimensions a=8.4339 ( 4 ) A, b=6.5174 ( 3 ) A, c=30.1470 ( 16 ) A, a=90°, p=94.556° (5) , y=90°, the volume is 1651.86 ( 14 ) A³, Z is 4, and the calculated density 1.083 g/cm³.