A SYNTHETIC APPROACH TOWARD OPTICALLY-ACTIVE LYCORINE - SYNTHESIS OF OPTICALLY-ACTIVE 2-METHYLLYCORANES BY RADICAL REACTION

A synthesis of optically active 2-methyl-alpha-lycoranes (2a) and thei r stereoisomers (2b, b', c) using radical-mediated cyclization is desc ribed. The B ring in lycorane was constructed by 5-exo cyclization of (4S, 1''E)-4-hydroxy (11)-, 4-acetoxy (12)-, 4-benzyloxy (13)-, 4-T-bu tyldimethylsilyloxy (14)- and 4-methoxymethoxy ylenedioxybenzyl]-5-phe nylselenylpyrrolidin-2-ones by using Bu3SnH-AIBN in boiling benzene. A mong these radical cyclizations, C-acetoxy phenylselenide (12) gave th e best product ratio (1.70) of a desired stereoisomer. On the other ha nd, formation of C ring underwent 6-endo cyclization of oxy-10-(2'-met hyl-2'-propenyl)benz[f]indolizidines (28. 29) in a manner similar to t hat noted for 11 to produce optically active 2-methyllycoranes (2).