Efficient synthesis of (-)--lycorane alkaloid by two Bu3SnH-mediated radical cyclizations

Serry A.A.El Bialy

Abstract

An asymmetric induction using (S)-1-arylethylamine-based chiral auxiliary and two Bu3SnH-mediated radical cyclizations have been developed for a total synthesis of (-)--Lycorane (1). The first cyclization proceeded in 5-endo-trigmannerwithmoderate diastereoselectivtiy to give (3aR,7aR)- octahydroindol-2-one (6b) as the major product using -Iodo-N-(6- oxocyclohexen-1-yl)-N-[(S)-1-phenylethyl] acetamide (5b). In the second cyclization, the radical precursor (8) was used as substrate to construct the optically active lycorane skeleton (15)whichwas reduced using LiAlH4 into (-)--lycorane (1).