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

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