Abstract: The intramolecular photocycloaddition reactions of a number of o-allyloxyphenyl substituted 2H-azirines have been examined in mechanistic detail. Upon irradiation with ultraviolet light, these systems undergo intramolecular I, I-and/or 1, 3-cycloaddition depending on the substituent groups attached to the 2 position of the azirine ring. The internal cycloaddition reactions have been shown to proceed through transient nitrile ylide ...