FORM 2
THE PATENTS ACT 1970
(Act 39 of 1970)
& THE PATENTS RULE, 2003
PROVISIONAL SPECIFICATION
(SECTION 10 and Rule 13)


"A METHOD OF PREPARING TRANS-4-AMINO-l-CYCLOHEXANE CARBOXYLIC ACID
DERIVATIVES"
Emcure Pharmaceuticals Limited.,
an Indian company, registered under the Indian Company's Act 1957
and having its registered office at
Emcure House, T-184, M.I.D.C., Bhosari, Pune-411026, India.
THE FOLLOWING SPECIFICATION DESCRIBES THE NATURE OF THE INVENTION.


FIELD OF INVENTION
The present invention relates to a method for preparation of isomerically pure trans-4-amino-1-cyclohexanecarboxylic acid derivatives, which are useful as intermediates for synthesis of several pharmacologically active compounds.
BACKGROUND OF THE INVENTION
Trans-4-amino-l-cyclohexanecarboxyIic acid derivatives are useful as antihypertensive agents and for the prevention and treatment of coronary, cerebral and renal circulatory diseases because of their long lasting coronary cerebral and renal blood increasing actions. These agents are also useful as intermediates for the preparation of medicines such as neuropeptide YY antagonists (WO 0137826), thrombin inhibitor (Tetrahedron Letters 1996, 37(29), 5045-5048). These derivatives are also used as intermediates for the preparation of various pesticides (US 5,831,118).
Trans form of 4-amino-l-cyclohexanecarboxylic acid derivatives are more stable than cis form because of equatorial configuration of both amine and carboxylic acid functional groups. Therefore, it is always desirable to synthesize 4-amino-l-cyclohexanecarboxylic acid derivative in its stable trans form. Various methods are known for conversion of cis 4-amino-l-cyclohexanecarboxyIic acid to the corresponding trans form. These methods are as follows:
Journal of Medicinal Chemistry, 1993, 36(8), 1100-1103 discloses a method for preparation of trans form involving fractional crystallization of a mixture of cis and trans isomers obtained by hydrogenation of 4-aminobenzoic acid derivatives. However, the disclosed method of fractional crystallization involves repeated crystallization and purification steps. Further, the obtained yields are very low, the maximum being 18%; even the obtained isomeric purity is only 65-90%.
Toyoshima et. al. in US 4,816,484 discloses a method for epimerization of cis isomer to trans isomer by using strong base (i.e. sodium hydride) in an anhydrous atmosphere with heating of a neat reaction mass at very high temperature i.e. 150°C. The resulting isomer


ratio of trans to cis is ratio of 85:15 with yield of about 78%. Thus, the use of stringent conditions like pyrophoric reagent (sodium hydride), high temperature are limiting factor for mass scale production.
Epimerization is also disclosed in JP 60258141 of Fujimoto Masaki et. al. The document discloses alkali and alkaline earth metal were used to give an equilibrium mixture in a ratio of trans : cis of 75-85:25-15. Further, JP 56120636 of Hoshino Shigetaka et. al. also disclose epimerization by direct heating to mixtures of cis and trans isomers.
US 5,831,118 of Inagaki et.al discloses a method for obtaining an isomeric purity of about 93% to 100 % of the trans isomer of 2 or 4-substituted cyclohexanecarboxylic acid or derivatives thereof. The disclosed method uses potassium hydroxide instead of sodium hydride and temperature in range 130°C to 220°C for the epimerization of cis to trans isomer. The said process involves drastic conditions of strong base and high temperature, and these types of isomerisation, as disclosed in the specification, works only with lower alkyl substituted cyclohexanecarboxylic acid.
Recently granted US 7,314,950 of Kawanish et. al. also gives a process for the preparation of trans 4-aminocyclohexanecarbixylic acid derivatives. Epimerization process utilizes base sodium methoxide in a non-polar aprotic solvent. Epimerization was obtained only when the amino group of the 4-aminocyclohexanecarboxylic acid was protected by using substituted sulfonyl and benzylidene groups.
The reference further recommends crystallization to achieve desire isomeric purity in the range of 85-97:15-3. However, the reported yield after crystallization is in the range of 56% to 70%. Even the disclosed process for protection of amino group is tedious as it involves conversion of sulfurous acid gas to sulfinic acid and then to sulfinyl chloride. Sulfinyl chloride is then treated with amino cyclohexylcarboxylic acid to get sulfinylaminocyclohexane carboxylate, which is then oxidized to sulfoxide by using molybdenum complex in presence of hydrogen peroxide.
Though, the obtained isomeric purity is up to 97%, this method involves a number of steps raising concerns in the mind of manufactures about feasibility and yield; making


such process commercially not particularly viable. Further, presence of undesired isomer in amount as high as 3 % has regulatory implications associated with it. Generally, presence of unwanted isomer is considered as impurity by the regulatory health authorities all over the world. This impurity may have significant untoward clinical and/or toxicological effects.
Thus, in view of the above, there is a need for novel epimerization method of 4-amino-l-cyclohexanecarboxylic acid from cis and/or mixture of cis/trans isomer such that the manufacturing processes itself would control the level of the undesired isomer formed during the reaction to the desired level.
OBJECTS OF THE INVENTION
An object of the present invention is to provide a novel process for the preparation of highly pure trans-4-amino-l-cycIohexanecarboxylic acid derivatives.
Another object of the present invention is to provide a process which would epimerize cis form and/or mixture of cis and trans form of 4-amino-l-cyclohexanecarboxylic acid derivatives to highly pure trans- 4-amino-l-cyclohexanecarboxylic acid derivatives.
Still another object of the present invention is to provide an epimerization process which is simple, economical, environmentally safe and commercially viable.
A further object of the present invention is to provide a process which produces trans form of 4-amino-l-cyclohexanecarboxylic acid derivatives having isomeric purity greater than 99%.
SUMMARY OF THE INVENTION
In their endeavor to prepare highly pure trans-4-amino-l-cyclohexanecarboxylic acid derivatives, the present inventors have found that the epimerization rate and yield of 4-amino-1-cyclohexanecarboxylic acid is remarkably increased by using a specific protection at amino group.


By taking recourse to such N-protecting groups, it is possible to synthesize trans form of 4-amino-1-cyclohexanecarboxylic acid derivative in very high purity. According to the present invention, mixture of cis/trans 4-amino-l-cyclohexanecarboxylic acid derivatives having cis form almost 100% is efficiently converted to the trans form having isomeric purity greater than 99%.
Apart from N-substitution, the present epimerization process of 4-amino-1 -cyclohexanecarboxylic acid derivatives is preferably carried out in an alcoholic solvent in the presence of a very mild base and at ambient to reflux temperature utilizing protecting group, which include a phthalimiodo group.
Thus, in one aspect the present invention provides a novel method for synthesis of highly pure trans-4-amino-l -cyclohexanecarboxylic acid derivatives.
In another aspect the present invention provides a method to synthesize trans-4-amino-l-cyclohexanecarboxylic acid derivatives with isomeric purity greater than 99%.
In still another aspect the present invention provides a method to synthesize trans-4-amino-1-cyclohexanecarboxylic acid derivatives which is simple, economical and commercially viable.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a process for the preparation of highly pure trans-4-amino-1-cyclohexanecarboxylic acid derivatives. The high purity of the trans form is obtained by the protection of amino group of 4-amino-l-cyclohexanecarboxylic acid derivatives with N-protecting groups.
As used herein the term, 'Highly pure trans isomer of 4-amino-1-cyclohexanecarboxylic acid derivatives' means a isomer with trans configuration and having isomeric purity in the range of 95% to 100%, preferably from about 97.5% to 100%, and more preferably from 99% to 100%. The words 'trans form' and 'trans isomer' as used herein are interchangeable.


As used herein the term 'N-protecting groups' or 'PG' refers to a substituent that blocks or protects amino group of 4-amino-l-cyclohexanecarboxylic acid while favoring epimerization. The rate of epimerization of 4-amino-l-cyclohexanecarboxylic acid and/or derivatives is dependent on substituent at amino group. Therefore, the present N-protecting groups cover all substituents, which are capable of providing sterically hindered atmosphere for promoting efficient epimerization. For example, suitable amino-protecting groups include, but are not limited to, aliphatic acids, aliphatic acid anhydrides, aromatic acids, aromatic acid anhydride, substituted aromatic acid anhydride such as phthalic anhydride, substituted phthalic anhydride, succinic anhydride, maleic anhydride, 1,2,4-benzenetricarboxylic anhydride, trimellitic anhydride chloride, 3-hydroxyphthalic anhydride etc. Preferably the N-protecting group is substituted or unsbstituted phthalic anhydride.
As used herein the term '4-amino-l-cyclohexanecarboxyIic acid derivatives' refers to derivatives, salts of 4-amino-l-cyclohexanecarboxylic acid. The preferred derivatives are represented by the compound of formula I,

wherein R1 is hydrogen, 1-6 carbon alkyl and R2 is any substitution at 2, 3, 5 and/or 6 position of cyclohexane ring and preferably includes hydrogen, halogen, or lower alkyl.
According to the present invention, a method for obtaining trans-4-amino-l-cyclohexanecarboxylic acid derivatives, in isomeric purity of above 99% with a yield of at least 70% is disclosed.
Scheme-I:



Scheme-I: Preparation of highly pure trans-4-aminocyclohexane carboxylic derivatives Wherein R1, R2 and PG are the same as defined above.
The process for epimerization of compound-II to compound-III is carried out by reacting mixture of cis : trans form (Compound-II) with a base in presence of alcoholic solvents. In this process cis form epimerizes to trans form. Alcoholic solvents are selected from straight or branched C1 to C6 alcohols. Examples of such alcoholic solvents include methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, t-butyl, n-pentyl, neopentyl, tert-pentyl and n-hexyl. Preferably the alcohol is isopropyl alcohol.
The base helps in efficiently carrying out epimerization of cis form to trans form. The choice of the base is not limited as long as it efficiently proceeds towards the desired epimerization reaction. The preferred bases are weak bases and are selected from alkali metal alkoxides, such as sodium methoxide, sodium ethoxide, potassium methoxide, potassium ethoxide. The preferred base is potassium tert-butoxide.
Typically, the amount of base used for epimerization reaction is preferably in the range of 0.05 to 1.0 mole equivalent per mole of compound-II. Preferably the amount of the base is in the range of 0.1 to 0.5 mole equivalent.
Because of use of N-protecting groups as disclosed herein before, the reaction temperature required to carry out epimerization reaction is not a critical factor for the present reaction. Typically, reactions are carried out between ambient to reflux temperature of the alcoholic solvent used.
Further, the present inventors have found that with the use of disclosed N-protecting groups, the reaction time is also not a critical factor. With various protecting groups, the epimerization reaction proceeds rapidly. Typically the required reaction time is 1.5 hours to 2.5 hours.
The disclosed N-protecting groups also offer another advantage as it is very easy to remove. For deprotection of amino group protected by phthalic anhydride, a simple one step reaction with base (i.e. alkaline earth metal hydroxide or organic bases) at ambient


temperature in suitable solvent is required. Similarly, other N-protecting groups could also be removed by suitable one step reaction.
Reaction of epimerization is not limited to mixtures of cis; trans forms. The epimerization reaction of the present invention also works with pure cis form. Normally, epimerization rate of the reaction is 95% or more, preferably above 97% and more preferably it is 99% to 100%.
A typical process for the preparation of an isopropyl-4-amino-cyclohexanecarboxylate starting from methyl-4-amino-cycIohexanecarboxylate hydrochloride using Phthalic anhydride and isopropyl alcohol is as depicted in Scheme-H
Scheme-II:

The invention is further explained with the help of following illustrative examples, however, in no way these examples should be construed as limiting the scope of the invention.
EXAMPLES:
The process of preparation of isopropyl-trans-4-aminocyclohexanecarboxylate. Example -1: Preparation of Methyl-4-aminobenzoate hydrochloride 4-Amino benzoic acid (100 g, 0.729 mol) was charged into methanol (1000 ml) and stirred to get clear solution at room temperature. Thionyl chloride (112.8 g, 0.948 mol) was charged in the reaction mixture at 0°C to -5°C under stirring. Reaction mass was refluxed for 2 to 4 hours. After completion of the reaction, methanol was evaporated


under vacuum at 45°C to 50°C. Product was isolated from diisopropyl ether after stirring for 1 hour at room temperature (Yield-133g).
Example - 2: Preparation of Methyl-4-amino-cyclohexanecarboxylate hydrochloride Methyl-4-aminobenzoate hydrochloride (100 g, 0.533 mol) obtained in Example-1 and methanol (1100 ml) were charged into a 2 L autoclave under nitrogen atmosphere at room temperature. Platinum-Carbon (10 g, 10% w/w) was added under nitrogen atmosphere. The temperature of autoclave was raised to 60°C at 7-8 kg/cm2 hydrogen pressure for 6 to 8 hours. Methanol was distilled out after filtration of Pt-C through hyflo bed. Chloroform was added to the residue and methyl-4-amino-cyclohexanecarboxylate hydrochloride was obtained, which was then purified from ethyl acetate (Yield-74 g).
Example - 3: Preparation of Methyl-4-phthalimidocyclohexanecarboxylate Methyl-4-amino-cyclohexanecarboxylate hydrochloride (100 g, 0.517 mol) obtained in Example-2, toluene (1000 ml) and phthalic anhydride (99.5 g, 0.67 Irnol) were charged into a flask at room temperature. Triethyl amine (130.54 g, 1.292 mol) was added drop wise over a period of 30 min at room temperature. The reaction mass was refluxed for 5 to 6 hours. After completion of the reaction toluene was distilled at 50°C to 55°C under reduced pressure. DM water was added and stirred for 30 min at ambient temperature, after which methyl-4-phthalimidocyclohexa-carboxylate was isolated by filtration, (Yield-100 g). MP: BOX; MS(CI) calcd for C16H17NO4 (M+) 288.32, found 288.3.
Example - 4: Preparation of Isopropyl-trans-4-pthalimidocycIohexanecarboxylate Isopropyl alcohol (1000ml) and Methyl-4-phthalimidocyclohexa-carboxylate (100 g, 0.348 mol) in Example-3 were mixed together and potassium tert-butoxide (19.54 g, 0.174 mol) was added to the mixture at ambient temperature. Reaction mixture was stirred for 2 to 3 hours at 60°C to 65°C. After completion of reaction, pH was adjusted to 7.0 by using acetic acid at room temperature. The reaction, was stirred for 30 min at 10°C to 12°C, Product was filtered and washed with isopropyl alcohol and dried at 40°C to 45°C, (Yield-50 g; Enantiomeric purity >99% based on ]H NMR).


MP: 120°C; 1H NMR (400 MHz, CDCl3): δ 1.22 (d,6H), 1.65 (m,2H ), 1.75 (m,2H), 2.11 (m,2H), 2.24 (m,3H), 4.11 (m,lH), 4.98 (sep.lH), 7.69 (d,2H), 7.81 (d,2H). MS(CI) calcd for C18H21NO4 (M+) 316.37, found 316.3.
Example - 5: Preparation of Isopropyl-trans-4-amino-l-cyclohexanecarboxylate Methanol (600 ml), Dichloromethane (250 ml) and Isopropyl-trans-4-phthalimidocyclohexa-carboxylate (100 g, 0.317 mol) were charged in the flask at ambient temperature. Hydrazine hydrate (110.64 g, 1.904 mol) in DM water (50 ml) was charged drop wise at 25°C to 30°C under stirring. Reaction mass was stirred for 12 to 14 hours and solid was filtered off. DM water (200 ml) was charged to the filtrate; the dichloromethane layer was separated and concentrated under reduced pressure, (Yield-54
g).
MP: 89°C; 1H NMR (400 MHz, CDCl3): δ 1.11 (m,2H), 1.22 (d,6H), 1.37 (m,2H), 1.48 (m,2H), 1.98 (m,4H), 2.16 (m,lH), 2.65 (m,lH), 4.98 (SepJH). MS(CI) calcd for C10H19NO2 (M+) 186.27, found 186.2.
Although, the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as described above.
All patent and non-patent publications cited in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated herein by reference.
Dated this Nineteenth (19th) day of August, 2008
(Signed)
Dr. Mukund. K. Gurjar Chief Scientific Officer Emcure Pharmaceuticals Ltd.