SYNTHESIS OF TETRAHYDROPTERIDINE C6-STEREOISOMERS, INCLUDING N(5)-FORMYL-(6S)-TETRAHYDROFOLIC ACID

Chiral N1-protected vicinal diamines derived from amino acids were condensed with 2-amino-6-chloro-5-nitro-4(3H)-pyrimidinone, the nitro group reduced, and the amine deprotected. oxidative cyclization of the resulting triaminopyrimidinone via quinoid pyrimidine intermediates gave a quinoid dihydropteridine, which was then reduced to a tetrahydropteridine C6-stereoisomer. Thus, 6(R)- and 6(S)-propyltetrahydropterin were stereospecifically synthesized (99% enantiomeric purity) in good yield from D- and L-norvaline, respectively. Reductive alkylation of (p-aminobenzoyl)-L-glutamate with a nitropyrimidine aldehyde derived from D- or L-serine similarly afforded, after cyclization and reduction, (6R)- or (6S)-tetrahydrofolic acid. The latter was then converted to the natural isomer of leucovorin by regioselective N5-formylation with carbonyl diimidazole/formic acid without loss of enantiomeric purity.