Nonconcerted Cycloaddition of 2<i>H</i>-Azirines to Acylketenes: A Route to N-Bridgehead Heterocycles

Reactions of acylketenes, generated from diazo diketones, with 2-unsubstituted and 2-monosubstituted 3-aryl-2<i>H</i>-azirines lead to 1:1 or 2:1 adducts, which are derivatives of 5-oxa-1-azabicyclo[4.1.0]­hept-3-ene or 5,7-dioxa-1-azabicyclo[4.4.1]­undeca-3,8-diene. According to DFT B3LYP/6-31G­(d) computations, the formation of (4+2)-monoadducts proceeds via a stepwise non-pericyclic mechanism. Reaction with methanol transforms quantitatively both 1:1 and 2:1 adducts into 1,4-oxazepine derivatives.