The Dipolar Route to Azepin‐3‐one Derivatives by Heterocyclization of Linear and Monocyclic Enallenyl Nitrones as the Key Step

The recently discovered transformation of benzopentenynyl nitrones into 1,2‐dihydrobenz[ c ]azepin‐3‐ones has been extended to the synthesis of structurally different systems including the monocyclic derivatives 17 and 18 , the bicyclic azepinones 10a – d , and the tricyclic heterocycles 37 – 40 , 42 , 43 , 45 , 47 , and 48 . In addition, the applicability of the reaction principle was demonstrated through the preparation of the benzo‐piperidino compound 52 which acts as a model compound for the alkaloid astrocasine ( 53 ). A multistep reaction mechanism is proposed that involves enallenyl nitrones as precursors of a 1,7‐dipolar electrocyclization process followed by further bond reorganizations. The occurrence of cyclopropanones (type E and H ) as intermediates is supported by the formation of isoindoles ( 41 , 44 , and 46 ) as minor products in some cases. Photochemical studies with selected dihydroazepinones gave different results: Whereas the monocyclic azepinone 17 reacted under both direct and sensitized excitation to form the bicyclic diastereomers 20 and 21 , the reactions of the bicyclic compounds 10 were more complicated. Only 10a , b were observed to undergo the 4π‐cyclization reaction upon direct irradiation, resulting, after CO extrusion and hydrolysis, in the formation of the aldehydes 22a , b . Under triplet‐sensitized conditions 10b – d undergo a 1,5‐phenyl shift to afford the isomers 25b – d . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)