Synthesis of Optically Active 2-Acetoxy-3-alkylidene-a-lycoranes for a Synthetic Approach toward (+)-Lycorine by Radical Reaction

Abstract

A radical-mediated synthesis of optically active 2α- and 2β-trimethylsilylmethyene-a-lycoranes (3,4), which are key intermediates for synthesis of (+)- lycorine (2), is described. Thus, both B and C rings in lycorine (2) were constructed by 6-exo mode radical cyclization. The former ring formation was performed in diastereoselective manner by radical cyclization via α-acylamino radical of (45,5R)-4-acetoxy-N-(2-methoxy-, benzyloxy-, and tert-butoxy-carbonylethenyl-4,5-methylenedioxybenzyl)- or (4S,5R)-N-(2-tert-butoxycarbonylethenyl-4,5-methylenedioxybenzyl)-4-triethylsilyloxy-5-phenyl-selenyl-2-pyrrolidinones (10-12 or 19). The latter ring formation was accomplished by the reaction of (1S,10R,10aR,2'S)- and (1S,10R,10aR,2'R)-10-(2'-acetoxy-4'-trimethylsilyl-3'-butynyl)- 1-imidazolylthiocarbonyloxy-7,8-methylenedioxy-1,2,3,5,10,10a-hexahydrobenz[f]indolizidines (28).