Silicon Directed N-Acyliminium Ion Cyclizations. Highly Selective Syntheses of (+/-)-Isoretronecanol and (+/-)-Epilupinine

Article abstract of DOI:10.1021/jo00221a011

Intramolecular reactions of N-acyliminium ions with allyl- and propargylsilanes are described.The cyclization precursors are hydroxy lactams 4b-10b, derived from succinimide and glutarimide, with the ?-nucleophile connected to the nitrogen via an ethylene or propylene chain.Cyclizations are induced by treatment with trifluoroacetic or formic acid and lead to products containing the pyrrolizidine (4c, 7c), indolizidine (5c, 8c), or quinolizidine (6c, 9c) ring system with a vinylidene substituent (propargylsilane cyclization) or a vinyl substituent (allylsilane cyclization).Reactions proceed in high yield with complete regiocontrol (govered by the β-effect of silicon) and with complete stereocontrol (chair-like transition-state conformations).Two allylsilane cyclization products 7c and 9c are further transformed into racemic isoretronecanol (7e) and epilupinine (9e), respectively.