Pteridines CV: Selective n(3)-and 0⁴-Alkylation of L-Biopterin: A Convenient Synthesis of 3-and 0⁴-Methyl-L-Biopterin and the Versatile N²-(N,N-dimethylami-nomethylene)-N(3)-p-nitrophenethyl-Protected L-Biopterin

Treatment of L-biopterin (1) with N, N-dimethyformamide dimethyl- (or diethyl)acetal and then with acetic anhydride in pyridine gave 1,2-di-0-acetyl-N^:2-(N, N-dimethylaminomethylene)-L-biopterin (4), which was converted by the Mitsunobu reaction into 3-methyl (5) and 3-p-nitrophenetyl derivatives (7). The protective groups on the side chain diols and N of these compounds were selectively cleaved to furnish products 6, 8-10 among which 9 is naturally occurring 3-methyl-L-biopterin and 8 is N^2:,N(3)-protected biopterin, a versatile intermediate for various reactions on the side-chain diol. In contrast, the same Mitsunobu reactions of tri-N^2::1,2'-0-acetyl-L-biopterin (11) afforded 0⁴-methyl (12) and 0⁴-NPE derivatives (13), both of which yielded 0⁴-methyl-L-biopterin (14) and subsequently led to 4-amino-L-biopterin (18).