The Rh2 (OAc) 4-catalyzed domino reaction of 2H-azirines with 2-acyl-2-diazoacetates provides a unique synthetic approach to non-fused 2H-1, 4-oxazines. The reaction proceeds via sequential formation of a rhodium carbenoid, an azirinium ylide, and a 1-acyl-1- (alkoxycarbonyl)-2-azabuta-1, 3-diene to give, after 1, 6-π-electrocyclization, a 2H-1, 4- oxazine derivative in good yield. The observed stereoselectivity of 2-azadiene formation is ...