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 Delta(2)-thiazolines 4 were previously reported to react with acyl Meldrum's acid derivatives 5 under acidic conditions (HCI (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 beta-lactams, the optically active isomers 3R,9R-1,3-oxazinones 3a-g are obtained stereoselectively in 38-93% yields.