Cycloaddition of Δ²-Thiazolines and Acyl Ketenes under Acidic Conditions Results in Bicyclic 1,3-Oxazinones and Not 6-Acylpenams as Earlier Reported

Optically active Δ2-thiazolines 4 were previously reported to react with acyl Meldrum's acid derivatives 5 under acidic conditions (HCl (g) in benzene) to stereoselectively give 6-acylpenams 1. Recently we have discovered that the structure elucidation of these compounds was incorrect. Thus, we report new data showing that instead of acyl β-lactams, the optically active isomers 3R,9R-1,3-oxazinones 3a−g are obtained stereoselectively in 38−93% yields.