合成路线1

Alternatively, folic acid (IV) was chemically reduced to the (6R,S)-epimeric mixture of tetrahydrofolates (VII) with sodium borohydride in aqueous NaOH. After derivatization of (VII) with (-)-menthyl chloroformate (VIII), the desired (6S) carbamate (IX) was isolated in high diastereoisomeric purity, taking advantage of the different solubility of the diastereoisomers in n-butanol. The menthyloxycarbonyl derivative (IX) was converted to the 5,10-methenyltetrahydrofolate (III) using a mixture of formic acid and acetic acid, saturated with HBr. The cyclic formamidinium salt (III) was then hydrolyzed to the 5-formyl compound, which was finally converted to the corresponding calcium salt. In a closely related method, folic acid (IV) was hydrogenated in the presence of rhodium (I) catalysts supported on an optically active phosphane to provide a diastereomeric mixture of tetrahydrofolate epimers (VII) with moderate diastereomeric excess of the desired (6S) isomer. After derivatization with menthyl chloroformate, the resultant mixture of epimeric mono- and bis-menthyl carbamates was separated using preparative HPLC. The target carbamate (IX) was then converted into the title compound as outlined above.