Stereoselective radical cascade approach to benzo[a]quinolizidines

Article abstract of DOI:10.1016/S0040-4039(98)02550-7

The treatment of easmide 15, having an (E)-4-ethoxycarbonyl-3-butenyl group on the nitrogen atrom, with Bu₃SnH-AIBN in boiling benzene, afforded a 1.2:1 mixture of two benzo[a]quinolizidine stereoisomers 16 and 17 as a result of cascade radical cyclization. A similar treatment of the (Z)-4- ethoxycarbonyl-3-butenyl congener 19 gave 16 and 17 in a ratio of 3.4:1. The high stereoselectivity (16:17=37:1) from 19 was obtained using Et₃B as the initiatior at -78 °C in toluene.