C-Branched pyrrolidines from 2-C-acetylmethyl-glycosylazides. Reduction of imines formed by monosaccharide ring opening

Reduction of the protected 2-C-acetylmethyl-β-glucopyranosyl azide derivative 1 produced the corresponding β-glucosylamine 3. Rather than forming a conformationally strained 1,2-trans-fused bicyclic imine, we propose that the β-glycosylamine underwent anomerization to an acyclic imine (4) followed by an intramolecular ring closure by the 5-hydroxy group. The resultant 2-C-acetylmethyl-α-glucopyranosylamine 5, which possesses the 1,2-cis-configuration was immediately converted to a bicyclic imine (2) in excellent yield. Attempts to selectively reduce the CN double bond of 2 using sodium borohydride and cyanoborohydride failed to produce bicyclic amine 6. Instead, compound 6 underwent another ring-opening elimination and further reduction to produce a C-branched pyrrolidine (8) in good yield. Catalytic hydrogenation of 1 and 2 also provided the C-branched pyrrolidine (10).