Efficient syntheses of pyrofolic acid and pteroyl azide, reagents for the production of carboxyl-differentiated derivatives of folic acid

Reaction of folic acid (1) with excess trifluoroacetic anhydride provides access to both the previously unknown N-10-(trifluoroacetyl)pyrofolic acid (8) and pyrofolic acid (9). Reaction of either of these materials with hydrazine selectively affords pteroyl hydrazide (13), which may be oxidized to pteroyl azide (27) on a large scale (62% overall from 1 without the need for chromatography). Treatment of 27 with differentially protected glutamates provides a convenient and high-yielding synthesis of differentially protected, optically pure folates.