Method for preparing an enantiomerically enriched form of 3-(2-chlorothiazol-5-yl)-8-methyl-7-oxo-6-phenyl-2,3-dihydrothiazolo[3,2-a]pyrimidin-4-ium-5-olate

A method for preparing an enantiomerically enriched form of 3-(2-chlorothiazol-5-yl)-8-methyl-7-oxo-6-phenyl-2,3-dihydrothiazolo[3,2-a]pyrimidin-4-ium-5-olate of formula (I):

• where the asterisk * shows the stereogenic center;
• which method comprises
• (a) reducing 2-[2-(2-chlorothiazol-5-yl)-2-oxo-ethyl]sulfanyl-6-hydroxy-3-methyl-5-phenyl-pyrimidin-4-one of formula 1
• or a tautomer thereof or a mixture of different tautomers thereof with a reduction agent selected from the group consisting of formic acid HC(═O)OH, formates of formula HC(═O)O−M+, and mixtures of formic acid HC(═O)OH and one or more formates of formula HC(═O)O−M+, where M+ is a cation equivalent;
• in the presence of a chiral transition metal catalyst and optionally a base, where in case that formic acid is used as the reduction agent, the reaction is carried out in the presence of a base;
• to obtain a reaction mixture comprising an enantiomerically enriched form of 2-[2-(2-chlorothiazol-5-yl)-2-hydroxy-ethyl]sulfanyl-6-hydroxy-3-methyl-5-phenyl-pyrimidin-4-one of formula 2
• or of a tautomer thereof or a mixture of different tautomers thereof;
• and
• (b) reacting the reaction mixture obtained in step (a) with an activating agent which enhances electrophilicity of the carbon atom marked with the asterisk in the compound of formula 1 without promoting racemization at said carbon atom;
• to obtain an enantiomerically enriched form of the compound of formula (I).