intramolecular conjugate addition reaction leading to the formation of dihydroxypyrrolidine-ester 6a and monohydroxypyrrolidine-γ-lactone 6b. Intermediates 6a and 6b were efficiently converted to (−)-lentiginosine 3a, its 8a-epimer 3b, and pyrrolizidine azasugar 4 in good overall yield.
d-
葡萄糖衍生的α,β-不饱和酯5在1,2-
丙酮化物脱保护,氧化二醇裂解后,在NaBH 3 CN存在下用N-
苄胺处理,经过还原胺化和伴随的分子内共轭加成反应,导致形成二羟基
吡咯烷-酯6a和单羟基
吡咯烷-γ-内酯6b。中间体6a和6b以良好的总产率有效地转化为(-)-
龙胆苷3a,其8a-顶基3b和
吡咯烷核苷氮杂糖4。