Abstract: A method for the resolution of (6R,S)-folinates into their diastereomers, comprising adding a water-soluble alkali metal, ammonium or alkaline earth metal salt of an inorganic or organic acid to an aqueous solution of a salt of (6R,S)-folinic acid, removing by filtration the salt which separates out, which consists predominantly of the corresponding (6R)-folinates, and isolating the desired (6S)-folinate from the filtrate.

Abstract: For the preparation of 5,10-methenyl-(6R)-, 5-formyl-(6S)- or 5-methyl-(6S)-tetrahydrofolic acid or salts thereof, 5,10-methenyl-(6R,S)-tetrahydrofolic acid, the inner salt thereof or one of the salts thereof with strong acids is subjected to diastereomer separation by fractional crystallization and, if desired, the resulting 5,10-methenyl-(6R)-tetrahydrofolic acid is converted by treatment with a hydrolyzing agent into 5-formyl-(6S)-tetrahydrofolic acid or by treatment with a reducing agent into 5-methyl-(6S)-tetrahydrofolic acid, and the resulting free acid may be converted into one of its salts by treatment with a base.

Abstract: The invention relates to a selective process for the preparation of N-L-.alpha.-aspartyl-L-phenylalanine-1-lower alkyl ester which comprises:A.