1-Oxa-2,3-cyclohexadiene ("2H-isopyran"): A strained heterocyclic allene undergoing cycloaddition reactions with characteristic typo-, regio- and stereoselectivities

Article abstract of DOI:10.1016/S0040-4020(01)86466-8

1-Oxa-2,3-cyclohexadiene can be readily generated by treatment of 5-bromo-3,4-dihydro-2H-pyran with potassium tert-butoxide in the presence of "18-crown-6" and can be trapped with dienes or olefins. The remarkable regio- and stereoselectivities with which [4+2] and [2+2] adducts are formed suggests a concerted, non-radical cycloaddition mechanism.