PROCESS FOR THE RECYCLE OF A WASTE PRODUCT OF DILTIAZEM SYNTHESIS

Abstract: A process which allows the re-use of compounds of formula (III-(2R, 3R)) in diltiazem synthesis through a process of conversion to a mixture of enantiomers III-(2R,3R) and III-(2S,3S) is described.

Description: PROCESS FOR THE RECYCLE OF A WASTE PRODUCT OF DILTIAZEM SYNTHESIS ********************************* The present invention relates to a method for the of a waste product of diltiazem sMithesis and. more particularh . it relates to a method for the preparation of - 3-(2-amιnophen\lthιo)-3-(4-metho\yphen l)propιonιc acid esters, intermediates useful for the synthesis of diltiazem. starting from -3-(2-ammophenylthιo)-3-(4- metho\\phcn\ l)propιonιc acid or derivatives thereof Diltiazem. -5-f2-(dιmethvlammo)ethyl]-2.3-dιhydro-2-(4-methoxy- phen\ l)-1.5-benzothιazepιn-4(5H)-one (The Merck lndev XII ed . no 3247, page 541 ) is a known drug w ith calcium-antagonist activity described in the U K patent 1236467 (Tanabe Senaku Co Ltd ) Several methods for the preparation of diltiazem such as. for example, those described in the above cited U K patent 1236467 in the European patent application 0 059 335 and in the Japanese patent no 71/8982. all in the name of Tanabe Co Ltd . arc described in the literature Most of these methods substantially provide for the follo ing scheme Scheme 1 Diltiazem wherein R is a hydrogen atom or a lower alkyl and the asterisks mark the stereogenic carbon atoms. These methods use the compound of formula lll-threo in racemic form, as intermediate. However, diltiazem shows stereocenters with S configuration and then, following the above reported scheme, a step for the separation of the (2S.3S) enantiomer from the (2R.3R) one is necessary The separation of the two enantiomers can be carried out on the intermediate (III) in the form of an ester (R=alkyl) as well as in the form of acid (R=H) For example, the separation of the (2S.3S) and (2R.3R) enantiomers at the le\el of the ester III can be carried out b\ using an optically active acid as resolving agent (U.S patent 5, 144.025 - Zambon Group S p A ) or by spontaneous resolution (U S patent 5.097.059 - Zambon Group S p A ) In case of separation at the level of the acid III. the methods described in the literature provide for. inter alia, the resolution with opticalK' active bases such as α-phenylethylamine (European patent application 0 098 892 - Tanabe Seiyaku Co Ltd) or L-lysine (U.K. patent application 2130578 - Istituto Luso Farmaco d'lta a S p A ) or the acylation m the presence of a pase (European patent application 0 617 130 - Oπon-Yhtyma Oy Fermion) It is evident that the resolution methods have the disadvantage of giving the desired (2S.3S) isomer with a maximum theoretical yield of 50% on the raceme and of giving also the corresponding (2R.3R) isomer. at the same time The isomers with (2R.3R) configuration, unsuitable for diltiazem synthesis, are then a waste product in the industπal synthesis. As a consequence, a process for the recycle of said compounds m order to recover them for diltiazem synthesis should be useful. As far as we know, the only method described in the literature for the recycle of an intermediate with (2R,3R) configuration is the process claimed in the U.S patent 5.102,999 (Zambon Group S.p.A.) which provides for the racemization of the intermediate IV-cis (2R.3R). We have now found a process for the conversion of the intermediates III-(2R.3R) to a mixture of enantiomers III-(2R.3R) and III-(2S,3S) which allows to re-use the enantiomers with (2R.3R) configuration of the intermediates of formula III for diltiazem synthesis Therefore, object of the present invention is a process for the conversion of t 7reo-(2R.3R)-2- -3-(2-amιnophenylthιo)-3-(4-methoxyphenyl)propιonιc acid derivatives of formula wherein R' is a linear or branched d-C? alkyl or a hydrogen atom. into a mixture of enantiomers II1-(2R.3R) and III-(2S.3S). characterised by the following steps (a) the cvc zation of the compound III-(2R.3R) to afford the corresponding compound of formula (b) the conversion to the compound of formula wherein R " is a hydrogen atom or a C -C acy 1 group, (c) the reduction to afford the compound of formula (d) the opening reaction treating w ith a strong acid or w ith a strong base in an alcoholic or aqueous solvent The mixture of the two enantiomers III-(2R.3R) and III-(2S.3S) obtained with the process object of the present invention can be resolved according to the already reported methods in order to separate the (2S,3S) enantiomer useful for diltiazem synthesis In the present context, if not otherw lse specified, the term mixture of enantiomers means a substantially racemic mixture (2R.3R 2S.3S ratio about 1 1) or a mixture wherein the enantiomer 2S.3S prevails Analogously, if the absolute configuration of the compounds of formula III or IV is not indicated, it means that said compounds are a substantially racemic mixture (2R.3R 2S.3S ratio about 1 1) or a mixture wherein the enantiomer 2S.3S prevails The term linear or branched d-C~, alkyl means methyl, ethyl, propyl. isopropyl and the term C 2 -C 4 acyl group means acetyl. propionyl. butyryl. isobutyryl The intermediates of formula II1-(2R.3R) used as starting products m the process object of the present invention are known compounds and they are obtained as waste products in the optical separation processes for diltiazem synthesis Generally the compounds of formula III are in the form of acid (R 1= H) or in the form of methy l (R'=CH 3 ) or ethyl (R'=CH 2 -CH,) ester Preferabl . in the process object of the present invention the compounds of formula III are 1 esters The c\ clization of the compound III-(2R_3R) to afford the compound of formula rV-(2R,3R) can be carried out according to the known methods for the c\ clization of the corresponding (2S.3S) enantiomer For example, the c\chzatιon of the esters of formula III-(2R.3R) can be carried out b\ treating with fosfonic acids (U S patent 5.223.612 - Zambon Group S p A ) or treating with sulfonic acids (European patent application 0 447 135 - Tanabe Seiyaku Co Ltd ) Similarh . the cychzation of the acid of formula III-(2R.3R) can be earned out by treating with sulfonic acids (European patent application 0 395 323 - Tanabe Seiyaku Co Ltd ) or with bases (European patent application 0 450 705 - Stamicarbon B V ) In the process object of the present invention the c\ clization reaction is preferably carried out starting from the methvl ester of formula III-(2R.3R) bv treating with -propenvl-fosfonic The subsequent conversion reaction to the denvative V can be earned out according to known methods too For example, the methods described in J Org Chem 1996. 8586-8590 or m the already cited U S patent 5.102.999 can be used Preferably . in the process object of the present invention, the acetvl derivative of formula V (R " =COCH is prepared, then hydrolysed to afford the compound Va which will be in equilibrium with its tautomer (Vb) as herein below reported Va Vb The subsequent reduction reaction allows to obtain the compound IV- Optionally, the hydrolysis and reduction reactions can be earned out one-pot. that is in an unique reaction environment, w ithout isolating the compound Va (or Vb) The preparation of the acctyl derivative V is preferably carried out by treating with acetic anhydride in dimethy lsulfoxide. in the presence of catalytic amounts of pyndine The optional hy drolysis can be then earned out treating w ith bases such as sodium hvdroxide or sodium mesylate The reduction of the compound of foπnula V can be carried out with known methods, for example by treating with hydrides according to what reported in the alrcaαy cited U S patent 5.102.999 The resultant compound IV- n can be then converted into the corresponding compound of formula lϊl-threo by treating with a strong acid or with a strong base in an alcoholic or aqueous solvent The amount of acid or base is at least equimolar. preferably in excess, with respect to the compound IW-cis Generally the reaction is earned out by using an excess of acid or base equal to 10%-30% in moles with respect to the compound IV The strong acids used in the process of the invention are inorganic acids such as hydrochlonc, hydrobromic. sulfunc and fosfonc acid or organic acids such as sulfonic acids, preferably methanesulfonic, /Holuenesulfonic and camphorsulfonic acid Sodium hydroxide is preferably used as strong base In the process object of the present mvention methanesulfonic acid is preferably used The reaction is carried out in an alcoholic solvent such as. for example, methanol or ethanol. preferably in methanol. or in water optionally in admixture with a suitable co-solvent such as dimethylsulfoxide Dependently on the alcohol used as solvent the corresponding ester of formula lϊl-threo is obtained which can then be used again in the preparation of diltiazem according to the synthetic route illustrated in the previous scheme 1 When an aqueous solvent is used, the acid (R'=H) of formula lll-threo is obtained and used according to the synthetic route illustrated in scheme 1 too The opening reaction of the compound of formula IV-c/s represents the most charactensing feature of the process object of the present invention In fact, as far as we know, this reaction has never been described in the literature On the contrar . cyclizations of the compound lll-threo to afford the compound IV-cis by using acids or bases have widely been described in the literature Furthermore, it is evident that the possibility of racemizmg the waste products with (2R.3R) configuration of the process for diltiazem preparation at the level of one of the earliest synthetic intermediates represents a relevant advantage from a practical and economic viewpoint A particularh preferred embodiment of the process object of the present invention is the following The methyl ester of formula III-(2R.3R). obtained by resolution of the corresponding racemic mixture, is cychzed with cw-propenyl-fosfonic acid and then oxidised by treatment with acetic anhydnde/dimethylsulfoxide/pyπdine obtaining the acetyl denvative of formula V After basic hydrolysis and reduction with sodium borohydπde the racemic compound TV-cis is obtained and treated with an excess of methanesulfonic acid in methanol up to the obtainment of the racemic methyl ester lϊl-threo The resolution of the racemic methyl ester lll-threo allows to obtain the compound III-(2S,3S) which is used for diltiazem synthesis and the enantiomer III-(2R.3R) which can undergo a further recycle phase according to the process object of the present invention In order to better illustrate the present invention the following examples are now given. Example 1 A mixture of methyl (2R.3R)-2-hydroxy -3-(2-amιnophenyIthιo)-3-(4-methoxypheny )- propionate (5 g. 15 mmoles) and cπ -propeny l-fosfonic acid (0 183 g, 1 5 mmoles) in xylene (35 ml) was heated under reflux and stirring for 5 5 hours After distilling a xvlene/methanol mixture (about 3%), the reaction mixture was cooled to 15°C The resultant precipitate was filtered under vacuum, washed with xylene (2x5 ml) and then dπed in oven at 65°C obtaining (2R.3R)-2.3-dιhydro-3-hydroxy-2-(4-methoxyphenyl)-1.5- benzothιazepm-4(5H)-one (42 g. 89 6% yield) Example 2 A catalvtic amount of pyndine ( 19 7 g. 0 25 moles) was added to a solution of (2R.3R)-2.3- dιhydro-3-hydroxy-2-(4-methoxyphenyl)-1.5-benzothιazepιn-4(5H)-one (500 g. 1 66 moles) in dimethylsulfoxide (1100 g, 14 1 moles) and acetic anhydride (425 g. 4 17 moles) After keeping the solution under stirring at room temperature for 24 hours, water ( 1000 ml) was slowly added and the mixture was kept under stirring for 30 minutes The crystalline precipitate was filtered off. washed with methanol and dried obtaining 3- acetoxy-2-(4-methoxyphenyl)-1.5-benzothιazepm-4(5H)-one (481 6 g. 84 7% yield) Example 3 A suspension of 3-acetoxy-2-(4-methoxyphenyl)-1.5-benzothιazepm-4(5H)-one ( 17 1 g. 0 05 mmoles) m methanol (51 ml) was cooled in ice and then a solution of NaOH (5 g, 0 125 mmoles) in water (63 ml) was added The solution was then kept under stirnng for 2 hours at room temperature After neutralisation with HC1 2N (50 ml) and extraction with ethyl acetate, the organic phase was washed twice with water, dned and concentrated obtaining a viscous oil The oil was treated with ethyl ether obtaining 2-(4-methoxyphenyl)-1.5-benzothιazepm- 3,4(2H,5H)-dιone (12 98 g, 86 7% yield) as a crystalline solid Example 4 Sodium boroh\dπde (0 567 g. 15 mmoles) was added to a solution of 2-(4-methoxyphenyl)- 1.5-benzothιazepιn-3.4(2H.5H)-dιone (4 25 g. 14 2 mmoles) in methanol (65 ml), kept under stirring, at 15°C After 1 hour the reaction mixture was poured into a buffer solution (100 ml) at pH 7 and methanol was removed by distillation under vacuum The resultant mixture was extracted with methylene chlonde (2x30 ml) After evaporation of the solvent under reduced pressure c/5-2.3-dιhydro-3-hydroxy-2-(4- metho\yphenyl)-1.5-benzothιazepιn-4(5H)-one (4 15 g. 97% yield) as racemic mixture was obtained Example 5 In a 500 ml flask, equipped with thermometer and condenser. c«-2,3-dιhydro-3-hydroxy-2-(4- methoxyphenyl)-1.5-benzothιazepιn-4(5H)-one (50 g, 165 9 mmoles) was suspended in methanol (200 ml) and methanesulfonic acid (19 2 g, 200 mmoles) was added to the mixture The reaction mixture was brought to reflux and the progress of the reaction was followed by TLC (eluent ethylacetate hexane=6 4) After heating for 7 hours, the content of starting product was about 1% The reaction mixture was then cooled at room temperature before adding, with a dropping funnel, a 8% solution of sodium bicarbonate (208 g. final pH=7 0) The resultant precipitate was filtered off and washed three times with water (3x50 ml) The resultant solid was then dned at 0°C under vacuum up to constant weight obtaining methyl tΛreo-2-hydroxy-3-(2-amιnophenylthιo)-3-(4-methoxyphenyl)-propιonate (54 g, molar yield 94.2%. HPLC titre 96 5%) Example 6 30% Sodium methylate in methanol (0 1 ml, 0 5 mmoles) and, after 15 minutes, sodium borohydπde (54 mg, 1 42 mmoles) were added to a suspension of 3-acetoxy-2-(4- methoxyphenyl)-1.5-benzothιazepm-4(5H)-one (1 g, 2 9 mmoles) m methanol (5 ml) After keeping the mixture under stirnng at room temperature for 5 hours, methanesulfonic acid (0 6 ml. 9 3 mmoles) was added and the mixture was heated under reflux for 5 hours After cooling at room temperature. 8% sodium bicarbonate (9 ml) was added After adding toluene, the resultant solid was filtered off. washed with water (3x1 ml) and dned at 50°C under vacuum obtaining methyl tΛreø-2-hydroxy-3-(2-amιnophenylthιo)-3-(4- metho\yphenvl)propιonate (0 92 g. molar vield 90%. HPLC titre 95%) Example 7 30% Sodium methylate in methanol (0 5 ml. 2 5 mmoles) and, after 15 minutes, sodium borohydπde (0 28 g, 7 35 mmoles) were added to a suspension of 3-acetoxy-2-(4- methoxyphenyl)-l,5-benzothιazepιn-4(5H)-one (5 g. 14 7 mmoles) in methanol (25 ml) After keeping the mixture under stirnng at room temperature for 5 hours, a 6 5M solution of hydrochloric acid in methanol (2 7 ml 17 6 mmoles) was added and the mixture was heated under reflux for 5 hours After cooling at room temperature. 8% sodium bicarbonate (16 ml) was added The resultant precipitate was filtered off. washed with water (3x5 ml) and dried at 50°C under vacuum obtaining methyl //7reo-2-hydroxy-3-(2-ammophenylthio)-3-(4-methoxyphenyl)- propionate (5 g. molar yield 85% Η-NMR titre 75%)

Claims: Claims 1) A process for the conversion of t 7reo-(2R.3R)-2-hydroxy-3-(2-aminophenylthio)-3-(4- methoxypheny propionic acid derivatives of formula wherein R is a linear or branched Ci-Ci alkyl or a hydrogen atom; into a mixture of enantiomers III-(2R.3R) and III-(2S,3S), charactensed by the following steps: (a) the cyclization of the compound III-(2R,3R) to afford the corresponding compound of formula (b) the conversion to the compound of formula wherein R 2 is a hydrogen atom or a C2-C4 acyl group. (c) the reduction to afford the compound of formula (d) the opening reaction by treating with a strong acid or with a strong base in an alcoholic or aqueous solvent. 2) A process according to claim 1 for the conversion of the methyl ester of tλra>-(2R,3R)- 2-hydroxy-3-(2-aminophenylthio)-3-(4-methoxyphenyl)-propionic acid. 3) A process according to claim 1 wherein the compound of formula V wherein R 2 is acetyl is used. 4) A process according to claim 1 wherein the opening reaction is carried out by treating with a strong acid. 5) A process according to claim 4 wherein the strong acid is hydrochloric, hydrobromic, sulfuric. fosforic, methanesulfonic, /Holuenesulfonic or camphorsulfonic acid. 6) A process according to claim 5 wherein the strong acid is methanesulfonic acid. 7) A process according to claim 4 wherein an alcoholic solvent is used. 8) A process according to claim 7 wherein the solvent is methanol. 9) Diltiazem, (+)-(2S,3S)-3-acetoxy-5-[2-(dimethylamino)ethyl]-2,3-dihydro-2-(4-meth- oxyphenyl)-l,5-benzothiazepin-4(5H)-one, obtained by using an intermediate of formula III prepared with the process according to claim 1.

Inventors: ALLEGRINI PIETRO, BARRECA GIUSEPPE, MARCHIORO GAETANO, RUSSO LAURA, VILLA MARCO, ALLEGRINI, PIETRO, BARRECA, GIUSEPPE, MARCHIORO, GAETANO, RUSSO, LAURA, VILLA, MARCO
Applicants: ZAMBON SPA, ZAMBON GROUP S.P.A.
Application year: 1998
Main classification: C07C323
IPC codes: C07B55, C07B61, C07C319, C07C323, C07D281