Preparation and reactions of 3-phenyl-4a,5,6,8a-tetrahydro-4H-1,2-benz-[e]oxazines

Abstract

α-Nitrosostyrene (1) undergoes [4 + 2] cycloaddition reactions with cyclohexa-1,3-dienes (2) to give, in good yield, tetrahydro-4H-1,2-benzoxazines (3). In the case of 1,4-dimethoxycyclohexa-1,3-diene the regioisomeric adduct (4) is isolated. Hydrolysis of the adducts (3b–f) gives the ketone (5; R = H). This undergoes Beckmann rearrangement when heated in trifluoroacetic acid to give the dione (9). Pyrolysis of the oxazines (3b, c) and and (5; R = H) affords 3-(6-phenyl-3-pyridiyl)propionaldehyde (10) as the only product.