The hetero Diels–Alder reaction featuring 2-phenyl-4-dimethylamino-1-thia-3-azabuta-1,3-diene as diene component: a clear example of a thermodynamically-controlled process

Cycloaddition reactions of diene 1a with various dienophiles lead to trans 4,5-disubstituted dihydro-1,3-thiazine adducts. We demonstrate that a thermodynamically-controlled process, in which the adduct N-3 nitrogen plays a prominent role, accounts for the observed trans relative configuration.