Intramolekulare 1,3‐dipolare Cycloadditionen mit alkenyl‐substituierten 3,4‐Diarylsydnonen

Intramolecular 1,3‐Dipolar Cycloaddition Reactions of Alkenyl‐substituted 3,4‐Diarylsydnones The 3,4‐diarylsydnones 1 and 12a–e with an allyl or alkenyloxy substituent on the N(3)‐phenyl group have been synthesized by classical methods starting from 2‐allylaniline and the corresponding alkenyloxyanilines, respectively ( Schemes 2 and 3 ). Whereas the allyl‐substituted sydnone 1 undergoes an intramolecular 1,3‐dipolar cycloaddition at room temperature in solution to yield 13 ( Scheme 4 ), the alkenyloxy‐substituted sydnones 12a–e are thermally stable. On irradiation of 1 and 12a–d , formation of the fused dihydropyrazoles 2 and 14a–d ( Schemes 1 and 5 ) is observed. In the case of 12d , the yield of 14d is very low, and the symmetric 1,2,3‐triazole 15a has been isolated as the main product. A very likely reaction mechanism for the formation of the photoproducts involves decarboxylation of the sydnones to give a nitrile‐imine which undergoes an intramolecular 1,3‐dipolar cycloaddition onto the CC bond.