Trapping highly reactive dipolarophiles. Exploiting the mechanism associated with the azomethine ylide strategy for beta-lactam synthesis

Highly reactive thioaldehydes 7, which are generated transiently by thermol ysis of thiosulfinates 6, are trapped using azomethine ylide (derived from the beta -lactam based oxazolidinone 1) to give 2-substituted penams 8. Die thyl thioxomalonate 10 and the selenoxo analogue 13, both of which are gene rated transiently via a retro Diels-Alder reaction, undergo 1,3-dipolar cyc loaddition reactions with 1 to give the isomeric penam 11a and isopenam 11b , and the 2,2-disubstituted selenapenam 14 respectively.