Synthesis of (-)-lentiginosine, its 8a-epimer and dihydroxylated pyrrolizidine alkaloid from D-glucose

Abstract

The D-glucose derived α,β-unsaturated ester 5 on 1,2-acetonide deprotection; oxidative diol cleavage followed by treatment with N-benzylamine in the presence of NaBH₃CN undergoes reductive amination; a concomitant intramolecular conjugate addition reaction leading to the formation of dihydroxypyrrolidine-ester 6a; monohydroxypyrrolidine-γ-lactone 6b. Intermediates 6a and 6b were efficiently converted to (−)-lentiginosine 3a, its 8a-epimer 3b, pyrrolizidine azasugar 4 in good overall yield.