Carbene-Catalyzed [4 + 2] Annulation of 2 H-Azirine-2-carboxaldehydes with Ketones via Azolium Aza-Dienolate Intermediate.

A new carbene-catalyzed [4 + 2] annulation of 2 H-azirine-2-carbaldehydes with ketones was developed, thus providing the 2,3-dihydro-6 H-1,3-oxazin-6-one core structures with broad scope and good to excellent yields. Notably, the azolium aza-dienolates generated from the addition of NHCs to 2 H-azirines are first uncovered.