The preparation of the (6R)- and (6S)-diastereoisomers of 5-formyltetrahydrofolate (leucovorin)

Abstract

The separation of the (6R)- and (6S)-diastereoisomers of tetrahydrofolic acid by derivatisation on N-5 with chiral auxiliary reagents followed by fractional crystallisation or extraction is described. Chloroformates of chiral alcohols were used as chiral auxiliaries and those derived from cyclic terpene alcohols were found to be most effective for the separation. The cleavage of the derivative and conversion in situ into 5,10-methenyltetrahydrofolic acid which was subsequently hydrolysed to afford 5-formyltetrahydrofolate (leucovorin) was investigated for the derivatised tetrahydrofolates; that from (–)-menthol was the only one that combined satisfactory separation with sufficient lability for efficient conversion into 5-formyltetrahydrofolate. The characterisation and optical purity of the products is described.