Complementary Use of Iminium Ion and N-Acyliminium Ion Cyclization Initiators for Asymmetric Synthesis of Both Enantiomers of Hydroxylated Indolizidines

Article abstract of DOI:10.1021/jo00272a025

The indolizidinediol 4 and its antipode were prepared by short efficient sequences which diverge from amide 9; the latter intermediate was prepared in three steps from commercially available D-isoascorbic acid.Key steps are (a) BF3*OEt2-promoted N-acyliminium ion cyclization of 2-acetoxy lactam 11 to form the (1R,2R,8aR)-tetrahydroindolizinone 13, (b) the formation of 2-(ethylthio)pyrrolidine 26, from amide 17, and (c) copper(II) triflate promoted iminium ion cyclization of 26 to afford the (1S,2R,8aS)-tetrahydroindolizine 27.