F0RM2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. Title of the invention. - IMPROVED PROCESS FOR PREPARATION OF
HIGHLY PURE TRANDOLAPRIL

2. Applicant(s)
(a) NAME :
(b) NATIONALITY
(c) ADDRESS :

LUPIN LIMITED
An Indian Company
159, CST Road, Kalina, Santacruz (East), Mumbai -400 098, Maharashtra, India

3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed



FIELD OF THE INVENTION
The present invention relates to process for manufacturing trandolapril of formula I of high enantiomeric purity.


COOH

Formula

Trandolapril

BACKGROUND OF THE INVENTION
Trandolapril [CAS Reg. No. [87679-37-6]], chemically known as N- (1(S)-carboethoxy-3~ phenylpropyl)-S-alanyl- (2S, 3aR, 7aS)-octahydroindole-2-carboxylic acid, was first disclosed in US Patent No. 4,933,361. Trandolapril is a well-known antihypertensive agent due to its Angiotensin Converting Enzyme (ACE) inhibitory activity.
US patent No. 4,933,361 describes the synthesis of trandolapril that employs racemic (2S, 3aR, 7aS)-trans-octahydro-1H-indole-2-carboxylic acid (la) and (2R, 3aS, 7aR)-trans-octahydro-1H-indole-2-carboxylic acid (lb) as intermediate.

US Patent No. 4,933,361 discloses several methods for the preparation of the above mentioned octahydro-1H-indole-2-carboxylic acids (la-h). Such methods for preparation of trans octahydro-1H-indole-2-carboxylic acids (la-d) employ the reduction of the mixture of enamine of the formula (A) and imine of formula (B) by catalytic hydrogenation using Raney Nickel, or Pt/C in glacial acetic acid or reduction with complex borohydrides or borane-amine complexes. However these methods are commercially non-viable since the undesired cis isomers (le-h) are produced in major amount (i.e. more than 60%).
In copending application No. 1033/MUM/2003 there is disclosed and claimed an improved method for the production of desired racemic trans octahydroindole-1H-2-


carboxylic acids (la and lb) by the reduction of mixture of enamine compound formula (A) and imine compound of formula (B) using Rh/C under alkaline condition in presence of water and water miscible organic solvent.
This method provided diastereomeric mixture of octahydroindole-1H-2-carboxylic acids (la-h) in which the ratio of trans acids (la-lh) to cis acids (le-lh) was greater than or equal to 1:1. In the subsequent process the mixture of acids (la-h) was enriched to >94% racemate of trans octahydroindole-1H-2-carboxylic acids (la and lb) by selective fractional crystallization initially from isopropanol and then from methanol. The resulting racemate of trans exo amino acids (la and lb) was > 94% containing <1% of the trans endo isomers (Ic and Id); and <5% of the cis isomers (le-h). The composition of cis and trans acids in the mixture was determined by converting the mixture to benzyl esters (lla-h) and then checking the purity of benzyl ester by HPLC method.


The synthesis described in US Patent No. 4,933,361 is shown in scheme 1 which involves conversion of racemic trans acids la and lb to corresponding mixture of hydrochloride salts lla.HCI and llb.HCI with benzyl alcohol and thionyl chloride. The mixture of hydrochloride salts lla.HCI and llb.HCI was neutralised with N-methyl morpholine in dimethyl formamide to give racemic mixture of free benzyl esters lla and lib which was condensed with N- [1-(S)-ethoxycarbonyl-3-phenylpropyl]-(S)-alanine (NEPA, lllb) by using 1-hydroxybenzotriazole and dicyclohexylcarbodiimide to obtain a diastereomeric mixture of trandolapril benzyl esters IVa and IVb.



The diastereomers IVa and IVb were separated by column chromatographic method to obtain pure isomer IVa which was then subjected to hydrogenolysis with 10%Pd/C in ethanol to afford trandolapril as a foamy material.
The method described in US Patent No. 4,933,361 suffers from the several drawbacks such as:
i) it gives very low yield of required trans acids la and lb,
ii) it requires separation of trandolapril benzyl ester (IVa) from its diastereomer IVb by column chromatography which is not suitable for large-scale production, and iii) it provides trandolapril as foamy solid that is difficult to isolate.
US 6,335,453 assigned to Kaneka Corporation discloses a general method for preparation of N- [1-(S)-ethoxycarbonyl-3-phenylpropyl]-(S)-alanyl-amino acids (IlIC) having low content of diketopiperazine (IIId) which involve reaction of corresponding amino acid with NEPA-NCA (Ilia) under basic condition at pH 9-12 in aqueous medium or in biphasic medium consisting mixture of organic solvent and water in the ratio 96:4 to 0:100. In this method at least 2 molar equivalent of amino acid is used. Moreover, we found that trandolapril prepared by following this method was contaminated with NEPA (lllb) which was formed presumably by hydrolysis of NEPA-NCA (Ilia). Thus, the method disclosed in US 6,335,453 B1 suffers from the following disadvantages:
4

i) it requires at least 2 molar equivalent of amino acid which increases the cost,
and ii) it provides trandolapril contaminated with NEPA (lllb)

The resolution of the racemic benzyl esters Ha and lib is disclosed in Drug Design and Discovery, 1992, vol 9, pp 11-28 by using DBTA. The DBTA precipitates the salt of benzyl (2S, 3aR, 7aS)-trans-octahydro-1H-indole-2-carboxylate (lla.DBTA) which is the required one for synthesis of trandolapril. As described in this publication, the resolution is achieved by treating the racemic benzyl esters IIa and IIb with DBTA in absolute ethanol followed by crystallization of crude solid from ethanol. It was found that by following this method of preparation of pure enantiomer IIa, transesterification of the benzyl ester takes place leading to the formation of undesired ethyl ester (llJ). The formation of salt llj.DBTA was revealed from the mass spectrum which showed a peak at m/z 197 amu (M+1) arising from ethyl ester llj. It was also found that when resolution and crystallization was carried out in methanol as solvent then the transesterification of the benzyl ester leading to the formation of undesired methyl ester (Hi) occurs.

This was evident from the fact that trandolapril manufactured from enantiomer Ha obtained by following the method of resolution as described in above publication had the contamination of the trandolapril ethyl ester (IVj) as indicated by peak at m/z 459.3 amu (M+1) (when ethanol was used for resolution and recrystallization). Similarly, trandolapril methyl ester (IVi) as indicated by peak at m/z 445 amu (M+1) was formed when methanol was used as solvent for resolution and recrystallization. These impurities were detected by their mass spectra were formed in the range of 5-12% as per HPLC
5

analysis. The removal of these trandolapril methyl ester (IVi) or trandolapril ethyl ester (IVj) impurities from trandolapril resulted in significant loss in yield.


IVj
IVi

Thus, the resolution method described in Drug Design and Discovery, 1992, vol 9, pp 11-28 suffers from the disadvantage of undergoing side reaction i.e. transesterification of benzyl ester which complicates the subsequent steps and finally leads to contamination of impurities in the trandolapril that are arising from the transesterification products.
It is an object of the present invention to solve the problem of transesterification and provide a process for the preparation of highly pure trandolapril of Formula I which is simple and industrially suitable process and which can provide trandolapril in very high purity (i.e. >99%).
It is a further object of the present invention to provide a process for preparation of highly pure trandolapril of Formula I which is cost effective and also easy to operate on plant scale.
The applicants have found that the problem of transesterification may be solved by carrying out the resolution of racemic benzyl esters lla and lib in aprotic solvent selected from dimethyl formamide, dimethyl sulphoxide, acetonitrile or a mixture thereof.
SUMMARY OF THE INVENTION
A process for the preparation of highly pure trandolapril of Formula I


comprising the steps of:
a) enriching a racemic mixture of benzyl trans (2S, 3aR, 7aS)-octahydro-1 H-indole-
2-carboxylate p-toluene sulphonic acid salt (lla.p.TsOH) and benzyl trans (2R, 3aS,
7aR)-octahydro-1H-indole-2-carboxylate p-toluene sulphonic acid salt (llb.p-TsOH)
to more than 99% from a mixture containing the other diastereomers (IIc-h.p-TsOH)
up to 6%,
b) converting the mixture of the said salts lla.p-TsOH and llb.p-TsOH to
corresponding mixture of free bases benzyl trans (2S, 3aR, 7aS)-octahydro-1H-
indole-2-carboxylate (lla) and benzyl trans (2R, 3aS, 7aR)-octahydro-1H-indole-2-
carboxylate (lib),

c) optically resolving the racemate benzyl trans (2S, 3aR, 7aS)-octahydro-1H-indole-2-carboxylate (lla) and benzyl trans (2R, 3aS, 7aR)-octahydro-1 H-indole-2-carboxylate (lib) with (-)-dibenzoyl-L-tartaric acid monohydrate (DBTA hereafter) to obtain pure enantiomer lia,
d) reacting benzyl ester lla with N- [1-(S)-ethoxycarbonyl-3-phenylpropyl]-(S)-alanine N-carboxy anhydride (Ilia) to prepare trandolapril benzyl ester,

e) converting trandolapril benzyl ester (IVa) to crude trandolapril by hydrogenolysis, and
f) crystallizing crude trandolapril from the mixture of ethanol-diisopropyl ether to yield pure trandolapril (>99%).
According to a preferred aspect of the invention there is provided a process for the preparation of highly pure trandolapril of Formula I comprising the following steps:


a. converting octahydroindole-1 H-2-carboxylic acids (la-h) to corresponding benzyl
ester p-toluene sulphonic acid salts (Ila-h. p-TsOH) by the reaction of benzyl
alcohol and p-toluene sulphonic acid monohydrate in refluxing cyclohexane and
simultaneously removing the water formed during reaction by azeotropic
distillation,
b. distilling out cyclohexane under reduced pressure and stirring the residue in
diisopropyl ether,
c. filtering the solid and drying under reduced pressure,
d. heating mixture of salts lla-h.p-TsOH in mixture of dichloromethane and
cyclohexane to reflux temperature,
e. addition of extra quantity of cyclohexane at reflux temperature,
f. continuing reflux for some period of time, and
g. crystallizing of the mixture of Ha.p-TsOH and llb.p-TsOH at 25-30°C and
followed by filtration of the same.
According to a further preferred aspect there is provided step of resolution comprises the following steps:
a. conversion of p-toluene sulphonate salts Ha.p-TsOH and llb.p-TsOH to
mixture of racemic esters lla and lIb,
b. preparing solution of racemic mixture of lla and lib in acetonitrile,
c. cooling the solution to 15-20°C,
d. dilution with dimethyl formamide,
e. addition of solution of DBTA at 15-20°C,
f. optionally seeding with salt lla.DBTA,
g. stirring at 15-20°C for 4-5 hours for crystallization of DBTA salt of pure
enantiomer lla (lla.DBTA), and
h. filtration and washing of salt lla.DBTA with acetonitrile.
According to still further aspect of the invention the step of recrystallization of crude trandolapril comprises of the following steps:
a. dissolving crude trandolapril in mixture of ethanol-diisopropyl ether (2:5)
by heating to reflux temperature;
b. continuing reflux for 10-15 minutes;
c. cooling the solution to 25-30°C; and
d. crystallizing at 25-30°C followed by filtration and washing with
diisopropyl ether.
8

DETAILED DESCRIPTION OF THE INVENTION
The present invention has four parts as shown below in scheme 2.


PartB

Racemic trans octahydroindole-1 H-2-carboxylic acids (la and lb) were prepared as per process described in the copending application No. 1033/MUM/2003 by the reduction of mixture of enamine compound formula (A) and imine compound of formula (B) using Rh/C under alkaline condition in presence of water and water miscible organic solvent. The purity of racemate of trans exo amino acids (la and lb) was > 94% and it contain <1% of the trans endo isomers (Ic and Id); and <5% of the cis isomers (le-h). The purification of trans exo acids (la and lb) upto 99% was achieved after repeatedly crystallization from methanol but the yield was poor and hence this method of purification was not commercially feasible.
The process for enriching the p-toluene sulphonic acid salts of Ha and lib to >99% purity is achieved by the present invention. The octahydroindole-1 H-2-carboxylic acid (la-h) containing >94% of the trans racemate la and lb; <1% of the trans isomers (Ic) and (Id); and <5% of the c/s-diasteromers (le-h) was converted to its corresponding benzyl ester p-toluene sulphonate salts (lla-h.p-TsOH) by treatment with benzyl alcohol and p-toluene sulphonic acid monohydrate by refluxing in cyclohexane and simultaneously removing the water formed during reaction by azeotropic distillation. The mixture of p-toluene sulphonic acid salts of benzyl esters (lla-llj). p-TsOH was then purified by crystallization from various solvents selected from cyclohexane, dichloromethane, ethyl acetate and diisopropyl ether or mixtures thereof, preferably from a mixture of dichloromethane-cyclohexane dichloromethane-diisopropyl ether or ethyl acetate-diisopropyl ether. A comparison of purity and yield obtained by using various solvents for crystallization is indicated in Table 1.
Table 1. Enrichment of purity by crystallization of mixture of benzyl ester p-toluene sulphonate salts (lla-h.p-TsOH) from various solvents.

Sr. No. Solvent Ratio of solvent (v/v) Purity obtained (%) Yield (%)
1 Cyclohexane — 98.2 95
2 Dichloromethane -Cyclohexane 2:6 99.89 62
3 Dichloromethane -Cyclohexane 1:5 99.27 94
4 Dichloromethane -Cyclohexane 1.5:5 99.72 95
5 Dichloromethane -Cyclohexane 1.5:5 99.44 89
6 Dichloromethane -Diisopropyl ether 2:5 98.47 90
7 Ethyl acetate -Cyclohexane 1:5 99.25 96


The invention involves the appropriate selection of solvent for purification and to provide a process for obtaining the mixture of p-toluene sulphonic acid salts of benzyl esters lla and lib in a purity >99%.
The conversion of salts Ha.p-TsOH and llb.p-TsOH to free esters (lla and lib) has been achieved by treatment with inorganic bases such as sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, sodium hydroxide, potassium hydroxide etc in biphasic mixture containing water immiscible organic solvent such as ethyl acetate, dichloromethane and water at lower temperature such as 0-10°C, preferably 0-5°C.
The resolution of the racemic mixture of benzyl esters lla and lib with DBTA was accomplished in various solvents such as ethanol, methanol, acetonitrile, ethyl acetate, acetone mixture of dimethyl sulphoxide and acetonitrile, mixture of dimethyl formamide and acetonitrile. The chiral purity and yield obtained in different solvents is indicated in Table 2. The preferred solvent for resolution is mixture of dimethyl formamide-acetonitrile or dimethyl sulphoxide- acetonitrile. The most preferred solvent is mixture of dimethyl formamide-acetonitrile.
Table 2 Resolution of racemic benzyl esters lla and lib in various solvents.

Sr. No. Solvent Volume of solvent Ratio of lla:llb (% by HPLC on chiral column) Yield of lla.DBTA (%)

lla lib

1 Ethanol 7 87.5 12.5 *
2 Methanol 15 97.1 2.9 50.6
3 Acetonitrile 35 52.3 47.7 No resolution
4 Ethyl acetate 10 51.9 48.1 No resolution
5 Acetone 8 59.5 40.5 No resolution
6 Dimethyl sulphoxide-acetonitrile (20:80) 20 99.4 0.6 386
7 Dimethyl formamide-acetonitrile (30:70) 25 98.5 1.5 64
*This crude product on further recrystallisati on from etha nol afford ed pure la.DBTA salt
in 99.4% chiral purity and 64% yield.


In a preferred aspect the resolution of the racemic mixture of benzyl esters lla and lIb was carried out with DBTA in a mixture of dimethyl formamide and acetonitrile at temperature between 15°C to 35°C. When resolution was carried out at 25-35°C impurity formation was upto 2-3.6%. In a further preferred aspect the resolution carried out at 15-20°C in which the unknown impurity formation was controlled below 2%. The effect of variation in ratio of dimethyl formamide to acetonitrile is shown in table 3.
Table 3 Effect of variation in ratio of dimethyl formamide and acetonitrile in resolution of lla and lib

Sr. No. Ratio of dimethyl formamide-acetonitrile in the solvent Temperature (°C) Ratio of lla:llb (% by HPLC on chiral column) Yield of lla.DBTA (%)

Dimethyl formamide Acetonitrile
lla lIb

1 20 80 25-30 75 25 Poor resolution
2 25 75 25-30 75.8 24.2 Poor resolution
3 30 70 25-30 98.5 1.5 64
4 35 65 25-30 98 2 64
5 40 60 25-30 97.7 2.2 41
6 45 55 25-30 98 1.9 33
7 50 50 25-30 95.8 4.1 11
8 30 70 15-18 97.4 2.5 72
The conversion of salt lla.DBTA to free benzyl ester (lla) has been achieved by treatment with inorganic bases such as sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, sodium hydroxide, potassium hydroxide etc in biphasic mixture containing water immiscible organic solvent such as ethyl acetate, dichloromethane and water at lower temperature such as 0-10°C, preferably 0-5°C.
The optically pure enantiomer benzyl ester lla is converted to trandolapril benzyl ester (IVa) by treating with NEPA-NCA (Mb) in dichloromethane which on deprotection of the benzyl group by catalytic hydrogenation over Pd/C in ethanol furnished crude trandolapril.


^ The crude trandolapril is purified by recrystallization from solvents such as ethanol,
mixture of ethanol-diisopropyl ether, ethyl acetate, acetone, methyl ethyl ketone, acetonitrile, tetrahydrofuran, nitromethane and dimethoxy propane. Among these preferred is a mixture of ethanol and diisopropyl ether. In a preferred embodiment the ratio 3:5 and 2:5 of ethanol and diisopropyl ether was studied. The preferred ratio is 2:5 in which purity >99.5% and yield >70% (from crude trandolapril) was obtained. The crystallization from ethanol-diisopropyl ether minimizes the formation of diketopiperazine impurity. Also it resulted in reduction of trandolapril analogues below 0.1% which were arising from cis endo ester (II) and unknown impurity formed by epimerisation in resolution. The results of crystallization of crude trandolapril are shown in table 4.
Table 4. Crystallization of crude trandolapril in various solvents.

Sr. No. Solvent Assay by HPLC* (%) Yield (%)
1 Ethyl acetate 98.2 92
2 Acetone 98.7 76
3 Methyl ethyl ketone 98.8 84
4 Acetonitrile 98.6 81
5 Tetrahydrofuran 97.9 31
6 Nitromethane 96.3 65.5
7 Dimethoxy propane 97.7 74
8 Ethanol 98.6 85
9 Ethanol-Diisopropyl ether (3:5) 98 83
10 Ethanol-Diisopropyl ether (2:5) 99.3 89.2
* Isocratic system. Column: RP18 (150x4.6 mm), 4u; Flow: 1.5 ml/minute; Detector: UV 210 nm; Buffer: 0.05 molar Na2HPO4+triethylamine+acetonitrile (1500:3:555), pH adjusted to 1.5-2.5.
The infrared spectrum of crystallized trandolapril obtained by the process of the present invention is given in Fig.1 and the characteristic X-ray powder diffraction pattern is given in Fig.3.
Though in the example 42 (c) of the product patent US 4,933,361 the nature of trandolapril is mentioned as foam, it was found that while repeating the same procedure

f and evaporating the solvent under reduced pressure (2-4 mm Hg) for longer time (20
hours) trandolapril as solid was obtained.
The infra red spectrum and X-ray powder diffraction pattern of trandolapril solid obtained by practicing the process disclosed in product patent US 4,933,361 is given in Fig.2 and Fig. 4 respectively.
The infrared spectrum crystallized trandolapril obtained by the process of the present invention (Fig. 1) and that of the product obtained by the process disclosed in the product patent US'361 shown in (Fig. 2) are identical.
The powder XRD of crystallized trandolapril obtained by the process of the present invention (Fig. 3) that of the product obtained by the process disclosed in the product patent US'361 shown in (Fig. 4) are also identical.
The invention is further illustrated by the following non-limiting examples.
Example 1
Step 1. Preparation of benzyl ester p-toluenesulphonate salt (lla-h.p-Ts-OH) A mixture of racemic amino acid la-h (83 gm, 0.491 mole), p-toluenesulphonic acid monohydrate (186.6 gm, 0.982moles), and benzyl alcohol (265.2 gm, 2.455 moles) in cyclohexane (830 ml), was slowly heated to reflux temperature (79-80X) for about 10-12 hours. The cyclohexane was distilled under reduced pressure till thick mobile residue was left. The residue was cooled to 25-30°C and diisopropyl ether (2490 ml) was added. The white solid separated out was filtered, washed with diisopropyl ether (274 ml). Yield: 323.7 g (wet solid) and HPLC purity 94.9%.
Step 2. Purification of benzyl ester p-TsOH salts (lla-h.p-TsOH) A flask was charged with dichloromethane (448.2 ml), wet solid benzyl ester p-TsOH salt (323.7 gm) obtained above in step 1 was added with stirring at 25-30°C. Cyclohexane (747 ml) was added to the slurry at 25-30°C. The reaction mixture was heated further to 50-55°C. Cyclohexane (747 ml) was added to the slurry and heating continued further at for 1hour. The reaction mixture was then cooled to 25-30°C, filtered and the solid was washed with a mixture of dichloromethane (80 ml) and cyclohexane (280 ml). Solid dried under reduced pressure at 50-55°C for 4-5 hours Yield: 257.3 gm and HPLC purity 99.1%.


f Step 3. Preparation of racemic benzyl ester (lla+llb) free base
Racemic benzyl ester p-TsOH salt (211.6 gm, 0.491 moles) obtained in step 2 above was added to flask containing dichloromethane (622.5 ml). Cooled to O'C. A cooled aqueous solution of cold 5% sodium bicarbonate (2905 ml) was added maintaining the temperature below 5°C. Stirred at 2-5°C for 15-20 minutes to get a clear biphasic mixture. The organic layer was separated and washed twice with 5% sodium bicarbonate solution (581 ml) followed by saturated sodium chloride solution (83 ml). The organic layer was concentrated under reduced pressure to give thick light brownish liquid. Yield 95.68 gm and HPLC purity 98.96%.
Step 4. Resolution of the racemic benzyl esters Ha and lib
The racemic benzyl ester lla+llb (41 gm, 0.158 mole) obtained in step 3 above was charged to flask containing acetonitrile (574 ml). Cooled to 15-20°C and then dimethyl formamide (246 ml). A solution of (-)-dibenzoyl-L-tartaric acid monohydrate (61.29 gm, 0.163 mole) in mixture of acetonitrile (143.5 ml) and dimethyl formamide (61.5 ml) was slowly added at 15-20° C. Seed of salt lla.DBTA (0.041 gm) was added. The resulting solution was stirred for 5 hrs at 15-20°C. The dibenzoyi tartarate salt of the benzyl ester lla (lla.DBTA) separated as solid was filtered and washed with acetonitrile (20.5). The solid was dried at 50-55° C under reduced pressure for 10 hrs. Yield of lla.DBTA was 28.7 gm and chiral purity by HPLC 98.18%.
Step 5. Preparation of benzyl ester lla
The dibenzoyi tartarate salt lla.DBTA (26 gm, 0.042 mole) obtained in step 4 above was charged into dichloromethane (130 ml), cooled to 0-2°C. An aqueous solution of cold 5% NaHC03 (260 ml) was added with maintaining the temperature 2-4° C. The organic layer was separated and washed twice with 5% NaHCO3 (78 ml) followed by saturated sodium chloride solution (13 ml). The organic layer was concentrated under reduced pressure at 35-40° C to give benzyl ester lla as a thick gummy mass. Yield 10.87 gm and HPLC purity 98.06%. The ester lla was converted to its hydrochloride salt and its specific optical rotation [a]D of ester hydrochloride (lla.HCI) checked which was - 41.8° (c=0.5, acetone) [Lit.-43°]
Step 6. Preparation of trandolapril benzyl ester (IVa)
Benzyl ester lla (10.87 gm, 0.042 moles) obtained in step 5 above was dissolved in dichloromethane (40 ml) and cooled to 0-2° C. N- [1-(S)-ethoxycarbonyl-3-phenylpropyl]-(S)-alanine N-carboxy anhydride (NEPA-NCA, IlIa) (13.49 gm, 0.044 mole) was added and stirred at 2-3°C for 2 hours. Solution of 5% sodium bicarbonate


< (130 ml) and triethyl amine (0.85 gm) was added and stirred for 19 hours. The layers
were separated. The organic layer washed twice with 5% sodium bicarbonate (52 ml) followed by water (13 ml). The organic layer was concentrated under reduced pressure at 40-45° C to get a gummy solid. Yield was 21.84 gm and HPLC purity 97.8%.
Step 7. Preparation of crude trandolapril
The gummy mass of trandolapril benzyl ester IVa (21.84 gm, 0.042 mole) obtained above in step 6 was dissolved in ethanol (410 ml) at 25-30°C and charged to autoclave. 10% Pd/C (2.184 g) was added under nitrogen at 25-30° C. The reaction mixture was stirred at 25-30°C for 2 hours maintaining the hydrogen pressure at 50 psi. The contents were filtered off, and catalyst washed with ethanol (60 ml). The combined filtrate was charged into another flask and ethanol was distilled off under reduced pressure at 35-40° C till solid was left. Yield of crude trandolapril was 16.5 gm.
Step 8. Crystallization of crude trandolapril
Mixture of crude trandolapril (16.5 gm) obtained in step 7 above, ethanol (36.4 ml), and
diisopropyl ether (91 ml) was refluxed for 10 minutes. Slowly cooled to 25° C. The solid
obtained was filtered off, washed with diisopropyl ether (7.8 ml). Yield of pure
trandolapril was 11.848 gm and HPLC purity 99.94% on gradient system and assay
99.2% (on gradient system).
M.P.: 122-124°C,
IR (KBr): 3278.7, 2942.2, 1735.2, 1654.3, 1456.7, 1433.7, 1366.5, 1192.8, 1101.5,
1063.8 and 1023.8 cm'1 (Fig. 1).
1H NMR (CD3OD, 6 ppm): 7.33 (s, 5H), 4.34 (m, 3H), 3.86 (q, 2H), 3.28-1.46 (m, 17H)
and 1.39 (d + t,6H),
Mass (m/z, amu): 453.5 (M+Na) and 431.7 (M+H)+ molecular ion.
Powder XRD: The (d) spacings and relative intensities (l/lo) are listed below.

d Relative intensity (%)
7.30 100
8.88 20
11.66 8
12.4 15
12.9 6
14.6 34
15.7 9


16.42 8
17.02 49
17.8 19
18.14 11
18.68 21
19.72 19
21.08 7
21.32 11
21.50 20
22.12 16
22.92 10
23.15 6
24.38 11
25.16 19
25.98 6
26.66 8
27.78 11
29.5 12
38.22 8
The crystalline trandolapril obtained by the above process of the present invention has the characteristic X-ray powder diffraction pattern as given in Fig. 3
Example 2 Preparation of trandolapril as per example 42 (c) described in product patent US 4,933,361
The gummy mass of trandolapril benzyl ester IVa (42 gm, 0.0807 mole) was dissolved in ethanol (1482.3 ml) at 23°C and solution was charged into autoclave. 10% Pd/C (4.94 gm) was added reaction mixture was hydrogenated under normal pressure at 23°C for 2 hours. The contents were filtered and filtrate was evaporated to give foamy solid.
The resulting foamy solid was further concentrated under reduced pressure (2-4 mm Hg) for 5 hours to remove the traces of solvent. The trandolapril was obtained was further dried under reduced pressure (2-4 mm Hg) for 20 hours. Yield was 17.2 gm and HPLC purity 98.8%. M.P.: 117.5-118.5°C,


IR (KBr): 3278.5, 2942.4, 1735.1, 1654.3, 1457.8, 1433.8, 1366.7, 1192.5, 1101.4.
1063.7 and 1023.7 cm"1 (Fig.2)
Powder XRD: The (d) spacing and relative intensities (l/lo) are listed below.

d Relative intensity (%)
7.46 100
9.02 15
11.8 9
12.52 10
12.64 14
14.72 24
15.82 5
16.56 6
17.16 27
17.94 16
18.26 14
18.80 14
1986 21
21.18 10
21.64 17
22.26 13
23.04 10
23.30 8
2364 7
24.48 9
25.32 18
26.08 5
26.78 9
27.90 10
29.62 10
38.34 8
The characteristic X-ray powder diffraction pattern of trandolapril obtained by the above example 2 is given in Fig. 4


WE CLAIM :
1. A process for preparation of highly pure trandolapril comprising the steps of:
a. enriching a racemic mixture of benzyl trans- (2S, 3aR, 7aS)- octahydro-
1H-indole carboxylate p-toluene sulphonic acid salt (lla.p-TsOH) and
benzyl trans- (2R, 3aS, 7aR)- octahydro-1H-indole carboxylate p-toluene
sulphonic acid salt (llb.p-TsOH) from a mixture containing the other
diastereomers (Hc-h.p-TsOH) up to 6%;
b. effecting optical resolution of racemic mixture of benzyl trans- (2S, 3aR,
7aS)- octahydro-1 H-indole carboxylate (lla) and benzyl trans- (2R, 3aS,
7aR)- octahydro-1 H-indole carboxylate (Mb) with (-)-dibenzoyl-L-tartaric
acid monohydrate;
c. reacting benzyl ester lla with N- [1-(S)-ethoxycarbonyl-3-phenylpropyl]-
(S)-alanine N-carboxy anhydride (Ilia, NEPA-NCA) to get trandolapril
benzyl ester (IVa);
d. effecting hydrogenolysis of the trandolapril benzyl ester (IVa) to obtain
crude trandolapril; and
e. effecting crystallization of crude trandolapril.
2. Benzyl octahydro-1 H-indole-2-carboxylate p-toluene sulphonic acid salts (lla-h.p-TsOH) of the formula

3. A process for preparation of benzyl octahydro-1 H-indole-2-carboxylate p-toluene sulphonic acid salt (lla.-h.p-TsOH) comprising :
a. heating a mixture of octahydroindole-1H-2-carboxylic acids (la-h) with benzyl
alcohol and p-toluene sulphonic acid monohydrate in water immiscible organic
solvent under reflux and distilling the water azeotropically,
b. distilling out solvent,
c. adding a non-polar solvent to the syrupy mass, and
d. stirring the mass at room temperature to obtain free flowing solid followed by
isolation of solid by filtration.


4. A process according to claim 3, wherein 1.0-4.0, preferably 2.0-2.5 mole equivalent of p-toluenesulphonic acid monohydrate is used.
5. A process according to claim 3, wherein 1.0-10.0, preferably 2.0-5.0 mole equivalent of benzyl alcohol is used.
6. A process according to claim 3, wherein the water immiscible solvent is aliphatic cyclic or acyclic hydrocarbon such as hexane, cyclohexane or aromatic hydrocarbon such as benzene, toluene.
7. A process according to claim 3, wherein the reaction is carried out at 50-100°C, and preferably at 70-90°C.
8. A process according to claim 3, wherein the reaction is carried out for 1-30 hours, and preferably for 8-12 hours.

9. A process according to claim 3, wherein the non-polar solvent is diisopropyl ether.
10. A process according to claim 3, wherein non-polar solvent is added to reaction mixture at room temperature.

11. A process according to claim 3, wherein reaction mixture is stirred at room temperature to get free flowing solid.
12. A process for enriching a racemic mixture of benzyl trans- (2S, 3aR, 7aS)-octahydro-1H-indote carboxylate p-toluene sulphonic acid salt (lla.p-TsOH) and benzyl trans- (2R, 3aS, 7aR)- octahydro-1H-indole carboxylate p-toluene sulphonic acid salt (llb.p-TsOH) comprising :
a. heating lla-h.p-TsOH salts in a mixture of organic solvent of first type and
organic solvent of second type, or heating in organic solvent of the first type and
adding the organic solvent of the second type during heating,
b. refluxing the mixture, and
c. cooling and isolating the solid by filtration.
13. A process according to claim 12, wherein the organic solvent of first type is selected
from dichloromethane, ethyl acetate, cyclohexane and diisopropyl ether or mixtures
thereof.


14. A process according to claim 12, wherein the organic solvent of second type is selected from cyclohexane and diisopropyl ether.
15. A process according to claim 12, wherein the temperature during recrystallisation is between 60-80°C.
16. A process according to claim 12, wherein the reaction mixture is cooled to 25-30°C.
17. A process for the conversion of racemic mixture of benzyl trans- (2S, 3aR, 7aS)-octahydro-1H-indole-2-carboxylate p-toluene sulphonic acid salt (lla.p.TsOH) and benzyl trans- (2R, 3aS, 7aR)-octahydro-1H-indole-2-carboxylate p-toluene sulphonic acid salt (llb.p-TsOH) to the corresponding mixture of their free benzyl esters lla and Mb by treatment with aqueous sodium bicarbonate in dichloromethane wherein the temperature of reaction is in between 0-40°C, preferably 0-10°C.

18. A process for optical resolution of racemic mixture of benzyl trans- (2S, 3aR, 7aS)-octahydro-1H-indole carboxylate (lla) and benzyl trans- (2R, 3aS, 7aR)- octahydro-1H-indole carboxylate (IIb) with (-)-dibenzoyl-L-tartaric acid monohydrate (DBTA) comprising:
a. providing a solution of racemic mixture of lla and lib in a mixture of aprotic
solvents,
b. cooling the solution,
c. adding a solution of DBTA in mixture of aprotic solvents,
d. mixing of DBTA solution with cold solution of esters lla and lib at lower
temperature,
e. optionally seeding with salt lla.DBTA,
f. stirring at lower temperature to crystallize DBTA salt of pure enantiomer lla
(lla.DBTA), and
g. isolating solid by filtration and washing of salt (lla.DBTA) with aprotic solvent
of first type.


19. A process according to claim 18 wherein (-)-dibenzoyl-L-tartaric acid monohydrate (DBTA) is 0.9 to 1.2 mole equivalent preferably 1.0 to 1.1 equivalent.
20. A process according to claim 18, wherein the aprotic solvent is selected from acetonitrile, dimethyl sulfoxide, and dimethyl formamide.
21. A process according to claim 18, wherein the solvent mixture is selected from acetonitrile and dimethyl formamide.
22. A process according to claim 18, wherein the mixing of DBTA solution to solution of esters lla and lib is carried out 0-50°C, preferably 10-20°C.
23. A process according to claim 18, wherein the optical resolution is carried out in mixture of dimethyl formamide-acetonitrile at 0-50°C, preferably 10-20°C.
24. A process according to claim 18, wherein the ratio of dimethyl formamide-
acetonitrile is in the range between 10:90 to 90:10 preferably 30:70.
25. A process for the conversion of benzyl trans- (2S, 3aR, 7aS)-octahydro-1H-indole-2-carboxylate (-)-dibenzoyl -L-tartarate salt (lla.DBTA) to free benzyl ester lla by treatment with aqueous sodium bicarbonate in organic solvent such as dichloromethane at temperature between 0-40°C, preferably 0-10°C.
26. A process for preparing trandolapril comprising the steps of
a. reacting benzyl ester lla with NEPA-NCA of formula Ilia in an organic
solvent in presence of organic base to get trandolapril benzyl ester of
formula IVa,

b. effecting hydrogenolysis of trandolapril benzyl ester (IVa).


27. A process according to claim 26, wherein the reaction is carried out at temperature between 0-40°C, preferably 0-10°C.
28. A process according to claim 26, wherein the reaction is carried out in organic solvent such as dichloromethane.
29. A process according to claim 26, wherein the reaction is carried out in organic base such as triethyl amine.
30. A process according to claim 26, wherein hydrogenolysis of trandolapril benzyl ester (IVa) is carried out in alcoholic solvent with transition metal catalyst such as 10%Pd/C in under hydrogen pressure.
31. A process according to claim 26, wherein hydrogenolysis of trandolapril benzyl ester (IVa) is carried out at temperature between 10-50°C, preferably 20-30°C under hydrogen pressure.
32. A process according to claim 26 wherein hydrogenolysis of trandolapril benzyl ester (IVa) is carried out at hydrogen pressure from 0-100 psi, preferably 40-50 psi.
33. A process for crystallization of crude trandolapril comprises the following steps:
a. dissolving an appropriate amount of crude trandolapril in a mixture of
ethanol-diisopropyl ether,
b. heating the mixture to reflux,
c. cooling the solution to below 40°C, and
d. stirring at temperature below 40°C followed by filtration and washing of
solid with solvent.
34. A process according to claim 33, wherein the organic solvent is selected from ethanol, diisopropyl ether, acetone, methyl ethyl ketone ethyl acetate, tetrahydrofuran, acetonitrile, nitro methane or mixtures there of.
35. A process according to any of claims 33 or 34, wherein crystallization of crude trandolapril is carried out from mixture of ethanol-diisopropyl ether in the ratio 1:9 to 9:1, preferably 1:1 to 1: 3.
23

36. A process according to any of claims 33 or 34 wherein crystallization of crude trandolapril is carried out at temperature of between 30-100°C, preferably 60-80°C.
37. A process according to any of claims 33 or 34, wherein crystallization of crude trandolapril is carried out from mixture of ethanol-diisopropyl ether containing substrate to solvent ratio from 1:10, preferably 1:5.
38. A process according to any of claims 33 or 34, wherein mixture is cooled below 40°C, preferably between 20-30°C.
39. A process according to any of claims 33 or 34, wherein stirring is done at 20-30°C for 20 hours, preferably for 2-4 hours.
40. A process according to any of claims 33 or 34, wherein crystalline solid was washed with solvent, preferably diisopropyl ether.
Dated this 10th day of February 2005
S. MAJUMDAR
Of S. MAJUMDAR & CO.
Applicants' Agent