Several in-depth studies from these laboratories have demonstrated that arylazirines undergo photocycloaddition with electron deficient olefins. 1-3 It has been suggested that these reactions proceed by way of irreversible opening of the three-membered ring to form a nitrile ylide intermediate which is subsequently trapped by a suitable dipolarophile. 4 The purpose of the present study was to investigate the generality of the photoaddition when the double bond and the azirine ring were constrained to be within the same molecule. In this communication we describe some novel reactions that 3-phenyl-2-formyl-2H-azirine (L) and some of its derivatives undergo when subjected to both ultra-violet and thermal excitation. Our results are also of some interest in connection with recent accounts concerning the role of azirines as reactive intermediates in the oxazole-isoxazole transformation. 4-10 Addition of iodine azide to the dimethylacetal of cinnamaldehyde followed by dehydrohalogenation, thermolysis, and aqueous hydrolysis afforded 3-phenyl-2-formyl-2H-azirine (&), mp 45-470, in good yield:(ir (KBr) 5.60 and 5.85: nmr (100 mHz) r 7.10 (lH, a, J = 7.OHz), 2.3-1.8 (5H,m), and 1.03 (lH, 4, J = 7.OHz))." When a solution of & in benzene was irradiated through a Vycor filter, the only product isolated was 2-phenyloxazole (2). Heating azirine & for 148 hr in benzene at 200' afforded 3-phenylisoxazole (2) in high yield (80%). Azirine L could be smoothly converted to the corresponding N-phenylimine 2, which gave 1,3_diphenylpyrazole (5) upon heating in xylene.