The addition of 2-tert-butyldimethylsilyloxyfuran to cyclic N-acyliminium ions containing cyclohexyl-based chiral auxiliaries

The vinylogous Mannich addition of silyloxyfuran 5 to chiral five- and six-membered N-acyliminium ions derived from 3/4a,b occurred exclusively through the addition to the N-acyliminium Si face to provide threo-6/7a,b as the major isomer (73–84% yield, 2:1–7:1 diastereoisomeric ratio) which were converted to the corresponding bicyclic lactams 10 and 11 with efficient recovery of the chiral auxiliary.