Method of Synthesis of 5-Methoxy-2-[(4-Methoxy-3,5-Dimethyl-2-Pyridinyl)-Methyl]sulfinyl-1h-Ben Zimidazole (Omeprazole)

The present invention relates to an improved method for the synthesis of 5-methoxy-2[-(4-methoxy-3,5-dimethyl-2-pyridinyl)-methyl]sulfinyl-1H-benzi midazole, referred to under its generic name omeprazole throughout the following specification and claims.

U.S. Pat. No. 4,255,431 discloses a process for the synthesis of omeprazole comprising the steps of reacting 5-methoxy-2-[(4-methoxy-3,5-dimethyl-2-pyridinyl)-methylthio]-1H-benzimida zole in a methylene chloride solution with m-chloroperoxybenzoic acid resulting in the formation of omeprazole and m-chlorobenzoic acid. omeprazole is highly sensitive to acids, and the reaction mixture has to be maintained at a low temperature to prevent excessive decomposition in the reaction mixture.

The product is worked-up by filtering-off of m-chlorobenzoic acid formed during the reaction. The filtrate is diluted with methylene chloride, is extracted with Na.sub.2 CO.sub.3 solution, dried and evaporated. The resulting omeprazole product is contaminated with starting materials and byproducts.

The object of the present invention is to provide an improved method for the synthesis of omeprazole, which eliminates the drawbacks of previously known methods.

This object is achieved according to the present invention, which is characterized by the steps of reacting 5-methoxy-2-[(4-methoxy-3,5-dimethyl-2-pyridinyl)-methyl-thio]-1H-benzimid azole (below denoted Compound I) with m-chloroperoxybenzoic acid in a methylene chloride solution at a substantially constant pH of about 8.0 to 8.6; extracting the reaction with aqueous NaOH; separating the aqueous phase from the organic phase; and adding an alkyl formate to the aqueous phase, resulting in crystallization of omeprazole.

The m-chloroperoxybenzoic acid is suitably used in an amount of 0.7-1.4 molar equivalents of Compound I, and preferably in an amount of 0.9-1.2 molar equivalents.

According to one embodiment of the invention, the alkyl formate is methylformate or ethylformate, methylformate being preferred.

The alkyl formate is suitably used in an amount of 1.2-2.0 molar equivalents of Compound I, and preferably in an amount of 1.5-1.8 molar equivalents.

One important feature of the method according to the invention is that unreacted sulfide is not transferred into the aqueous phase upon the extraction with aqueous NaOH. Another important feature is that m-chlorobenzoic acid does not crystallize upon the addition of methylformate formate to the aqueous phase, thereby eliminating the need of filtering-off of m-chlorobenzoic acid in a previous step.

The pH of the reaction mixture may be maintained within the pH range of 8.0-8.6 with the aid of pH static titration with NaOH or with the use of a buffer. Preferred buffers are sodium bicarbonate and potassium bicarbonate. A great advantage of the method according to the invention is that the reaction takes place in the organic methylene chloride phase while the m-chlorobenzoic acid formed during the reaction goes into the aqueous phase containing the buffer, in the case a buffer is used. Because of this, omeprazole formed does not stay in contact with the acid and the reaction may be performed at a temperature above 0.degree. C.

According to one embodiment of the invention the pH of the aqueous NaOH phase is kept at above about 12.

According to another embodiment of the invention the crystallization of omeprazole is performed at a pH of above 9.

The invention will be further illustrated below with a non-limiting example .

5-methoxy-2-[(4-methoxy-3,5-dimethyl-2-pyridinyl)methylthio]-1H-benzimidazo le (16.2 g; 0.0492 mol) is reacted with m-chloroperoxybenzoic acid (13.6 g; 0.0537 mol) in CH.sub.2 Cl.sub.2 acting as a solvent at a pH of 8.6, which is maintained by the presence of KHCO.sub.3 (5.6 g; 0.056 mol) acting as a buffer. The temperature is maintained at about 0.degree. C. during the addition.

Diluted NaOH is added to a pH above 12 and the CH.sub.2 Cl.sub.2 phase is separated off.

Methylformate (4.7 g) is charged to the water phase and the pH is kept above 9, whereupon omeprazole crystallizes. The crystals are filtered off and are washed with water and methanol at a temperature of about 0.degree. C. The washed crystals are dried under vacuum. Yield: 15.6 g (92%).