MARCHIORO GAETANO
BARRECA GIUSEPPE
VILLA MARCO
RUSSO LAURA
| US5102999A | 1992-04-07 |
| 1. | C3 alkyl or a hydrogen atom; into a mixture of enantiomers III. (2R3R) and III. (2S, 3S), characterised 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 R2 is a hydrogen atom or a Cl. . 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 I for the conversion of the methyl ester of threo. (2R, 3R). 2. hydroxy. 3. (2. aminophenvlthio). 3. (4. methoxyphenvl). propionic acid. |
| 3. | A process according to claim I wherein the compound of formula V wherein R2 is acetyl is used. |
| 4. | A process according to claim I wherein the opening reaction is carried out bv treating with a strong acid. |
| 5. | A process according to claim 4 wherein the strong acid is hydrochloric, hvdrobromic, sulfuric, fosforic, methanesulfonic, p. toluenesulfonic 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). 1, 5. benzothiazepin. 4 (5H). one, obtained by using an intermediate of formula III prepared with the process according to claim 1. |
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 Tanabc Sciyaku Co. Ltd., are described in the literature.
Most of these methods substantiallx provide for the following synthetic scheme.
Scheme I ° COOR SH \ w * 0 OCH3 OCH3 ocH, ocH, s s * * OH \ OH NH COOR/N IV-Cis H 0 O lII-t/lreo J Diltiazem
wherein R is a hvdrogen atom or a lower alkyl and the asterisks mark the stereogenic carbon atoms.
These methods use the compound of formula III-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 (2R3R) one is necessary The separation of the two enantiomers can be carried out on the intermediate (III) in the form of an ester (R=alkvl) as well as in the form of acid (R=H).
For example. the separation of the (2S. 3S) and (2R. 3R) enantiomers at the level of the ester III can be carried out by using an optically active acid as resolving agent (U. S. patent 5, 144, 025 - Zqambon Group S. p. A.) or bv spontaneous resolution (U. S. patent 5. 097.059- Zambon Group S. p A.) In case of separation at the levcl of the acid III. the methods described in the literature provide for, inter alia. the resolution with optically active bases such as α-phenylethylamine (European patent application 0 098 892-Tanabc Seivaku Co. Ltd) or L-lysine (U.K. patent application 2130578-Istituto Luso Farmaco d'Italia S. p. A.) or the acylation in the presence of a lipase (European patent application 0 617 130 - Orion-Yhtyma Oy Fermion).
It is evident that the resolution methods have the disadvantage of giving the desired (2S. 3S) isomer with a maximum theoretical vield of 50% on the raceme and of giving also the corresponding (2R3R) isomer. at the same time.
The isomers with (2R, 3R) configuration. unsuitable for diltiazem snnthesis. are then a waste product in the industrial synthesis.
As a consequence. a process for the recycle of said compounds in order to recover them for diltiazem svnthesis should be useful.
As far as we know. the onlv 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 threo- (2R,3R)-2- hydroxy-3-(2-aminophenylthio)-3-(4-methoxyphenyl)propionic acid derivatives of formula wherein R'is a linear or branched Ci-C alkvl or a hydrogen atom : into a mixture of cnantiomers III- (2R.3R) and III- (2S. 3S). thefollowingsteps:characterisedby (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 is a hvdrogen atom or a C2-C, 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.
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 svnthesis.
In the present context. if not otherwisespecified, the term mixture of enantiomers means a substantially racemic mixture (2R3R: 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 (2R3R : 2S. 3S ratio about 1: 1) or a mixture wherein the enantiomer 2S. 3S prevails.
The term linear or branched C1-C3 alkyl means methyl, ethyl, propyl, isopropyl and the term C2-C4 acyl group means acetyl, propionvl. butvryl. isobutvrvl
The intermediates of formula III- (2R. 3R) used as starting products in 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.
Generallv the compounds of formula III are in the form of acid (R'=H) or in the form of methyl (R'=CH3) or ethyl (R'=CH,-CH3) ester.
Preferably, in the process object of the present invention the compounds of formula III are methyl esters.
The eyclization of the compound III-(2R,3R) to afford the compound of formula IV-(2R, 3R) can be carried out according to the known methods for the cyclization of the corresponding (2S. 3S) enantiomer. For example. the cyclization of the esters of formula III- (2R. 3R) can be carried out by treating with fosfonic acids (U. S. patent 5.223.612-Zambon Group S. p. A.) or by treating with sulfonic acids (European patent application 0 447 135-Tanabe Seiyaku Co.
Ltd.). Similarly. the cvclization of the acid of formula III- (2R. 3R) can be carried 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 cyclization reaction is preferably carried out starting from the methvl ester of formula III- (2R. 3R) by treating with cis-propenvl-fosfonic acid.
The subsequent conversion reaction to the derivative V can be carried out according to known methods too. For example, the methods described in J. Org. Chem., 1996. 8586-8590 or in the already cited U. S. patent 5, 102,999 can be used.
Preferably. in the process object of the present invention, the acetyl derivative of formula V (R-=COCH3) is prepared, then hydrolyse to afford the compound Va which will be in equilibrium with its tautomer (Vb) as herein below reported.
The subsequent reduction reaction allows to obtain the compound IV-cis.
Optionally, the hydrolysis and reduction reactions can be carried out one-pot. that is in an unique reaction environment, without isolating the compound Va (or Vb).
The preparation of the accty) dcrivativc V is preferabl ! carried out by treating with acetic anhydride in dimethylsulfoxide. in the presence of catalytic amounts of pyridine. The optional hydrolysis can be then carried out by treating with bases such as sodium hydroxide or sodium mesvlate.
The reduction of the compound of formula V can be carried out with known methods, for example bv treating with hvdrides according to what reported in the alreaciv cited U. S. patent 5,102.999.
The resultant compound IV-cis can be then converted into the corresponding compound of formula III-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. preferabiy in excess. with respect to the compound IV-cis.
Generally the reaction is carried 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 hydrochloric, hydrobromic, sulfuric and fosforic acid or organic acids such as sulfonic acids, preferably methanesulfonic, p-toluenesulfonic and camphorsulfonic acid.
Sodium hydroxide is preferably used as strong base.
In the process object of the present invention 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 optionallv in mixture with a suitable co-solvent such as dimethvlsulfoxide.
Dependently on the alcohol used as solvent the corresponding ester of formula III-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 III-threo is obtained and used according to the synthetic route illustrated in scheme I too.
The opening reaction of the compound of formula IV-cis represents the most characterising 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 contrary. cyclizations of the compound III_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 possibilitv of racemizing the waste products with (2R3R) 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 particularly 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 cvclized with cis-propenvl-fosfonic acid and then oxidised by treatment with acetic anhvdride/dimethylsulfoxide/pyridine obtaining the acetyl derivative of formula V. After basic hydrolysis and reduction with sodium borohydride the racemic compound IV-cis is obtained and treated with an excess of methanesulfonic acid in methanol up to the obtainment of the racemic III-threo.ester The resolution of the racemic methvl ester III-threo allows to obtain the compound III- (2S, 3S) which is used for diltiazem svnthesis and the enantiomer III- (2R3R) 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 (2R3R)-2-hvdroxy-3-(2-aminophenvlthio)-3-(4-metlloxyphenvl)- propionate (5 g : 15 mmoles) and cis-propenvl-fosfonic l-fosfonic (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 xvlene (2x5 ml) and then dried in oven at 65°C obtaining (2R,3R)-2,3-dihydro-3-hydroxy-2-(4-methoxyphenyl)-1,5- benzothiazepin-4(5H)-one (42 yield).89.6% Example 2 A catalytic amount ofpyridine (19.7 g ; 0. 25 moles) was added to a solution of (2R. 3R)-2. 3- dihydro-3-hvdroxy-2- (4-methoxyhenvl)-1. 5-benzothiazepin-4 (5H)-one (500 g: 1.66 moles) in dimethylsulfoxide (1100 g ; 14. 1 moles) and acetic anhvdride (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- g;84.7%yield).acetoxy-2-(4-methoxyphenyl)-1,5-benzothiazepin -(4(5H)-one(48.16 Example 3 A supension (17.1g;0.053-acetoxy-2-(4-methoxyphenyl)-1,5-benzothiazepin- 4(5H)-one mmoles) in 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 stirring for 2 hours at room temperature.
After neutralisation with HCI 2N (50 ml) and extraction with ethyl acetate. the organic phase was washed twice with water, dried and concentrated obtaining a viscous oil.
The oil was treated with ethvl ether obtaining 2-(4-methoxyphenyl)-1,5-benzothiazepin- 3,4 (2H, 5H)-dione (12 98 g; 86.7% vield) as a crystalline solid.
Example 4
Sodium borohydride (0.567 g : 15 mmoles) was added to a solution of2- (4-methoxyphenyl)- 1.5-benzothiazepin-3.4 (2H. 5H)-dione (4.25 g : 14.2 mmoles) in methanol (65 ml), kept under stirring. at 15°C After I hour the reaction mixture was poured into a buffer solution (100 ml) at pH 7 and methanol was removed bv distillation under vacuum.
The resultant mixture was extracted with methvlene chloride (2x30 ml).
After evaporation of the solvent under reduced pressure cis-2.3-dihydro-3-hydroxy-2- (4- methoxyphenyl)-1.5-benzothiazepin-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, cis-2, 3-dihvdro-3-hydroxv-2- (4- methoxyphenvl)-l. 5-benzothiazepin-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 dried at 50°C under vacuum up to constant weight obtaining methyl threo-2-hydroxy-3- (2-aminophenylthio)-3- (4-methoxyphenyl)-propionate (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 borohydride (54 mg; 1.42 mmoles) were added to a suspension of 3-acetoxy-2- (4- methoxyphenyl)-1. 5-benzothiazepin-4 (5H)-one (1 g; 2.9 mmoles) in methanol (5 ml).
After keeping the mixture under stirring 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 (3xl ml) and dried at 50°C under vacuum obtaining methyl threo-2-hydroxy-3- (2-aminophenvlthio)-3- (4- methoxvphenvl) propionate (0.92 g: molar yield 90%: HPLC titre 95%).
Example 7 30% Sodium methylate in methanol (0.5 ml: 2.5 mmoles) and, after 15 minutes, sodium borohydride (0.28 g; 7.35 mmoles) were added to a suspension of 3-acetoxy-2- (4- methoxyphenyl)-1, 5-benzothiazepin4 (5H)-one (5 g ; 14 7 mmoles) in methanol (25 ml).
After keeping the mixture under stirring at room temperature for 5 hours, a 6.5M solution of hvdrochloric 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 mi) and dried at 50°C under vacuum obtaining methyl threo-2-hydroxv-3- (2-aminophenylthio)-3- (4-methoxyphenyl)- propionate (5 g: molar yield 85%:'H-NMR titre 75%).