form) might result from initial nucleophilic attack by the lone pair of the azirine nitrogen on the reactive electrophilic carbon of carbon disulfide followed by 1,3-bond scission, cyclization, and 1,5 sigmatropic rearrangement. Spectroscopic data did not provide an unambiguous assignment. Structural differentiation came from treatment of the adduct with nitric acid,15 which gave the known 5phenylthiazole (5), mp 45°.16 Compound 5 must arise from Thermally induced [2 + 2] cycloadditions are rarely encountered. Orbital symmetry analysis reveals that additions of this type that involve relatively low activation energies require special inherent geometric and/or electronic properties of the component(s).1 The dienophilic and dipolarophilic character of 1-azirines in their thermal cycloadditions has already been established.2-7 Most reac-