Ring Expansion of Heterocyclic Ketene N,X‐Acetals and 2‐Alkylidenedihydroindoles with Methanesulphonyl Azide by [3 + 2] Cycloaddition and Subsequent Extrusion of Molecular Nitrogen

Methanesulphonyl azide (2) reacts readily with isolated cyclic ketene N,X‐acetals of type 1, viz. 10a, d, 15b, d, and 22a–c, or those that are generated in situ by deprotonation of the corresponding 2‐alkylbenzazolium tetrafluoroborates, i.e. 14a → 15a, 14b → 15b, and 18 → 19. Ring‐expanded products are formed by extrusion of molecular nitrogen from intermediate labile [3 + 2] cycloadducts 3 with concomitant 1,2‐shift of N (route A1 → 12, 24) or X (route A2 → 16, 20, 21). In addition, 3 may undergo [3 + 2] cycloreversion into N‐sulphonylimine 5 and diazoalkane 6 (route B → 13, 17, 25). The configurations of the cyclic N‐sulphonylamidines 16b and 21b, the N‐sulphonylimine 24 and the N‐sulphonylamine 27 are elucidated by means of X‐ray diffraction analyses. The ratio of the (useful) ring‐expansion reactions vs. the unwanted formation of 5 + 6 is hardly influenced by the solvent employed and temperature of the experiment but strongly by the nature of the potential migrating atom and the substituents at the α‐carbon atom.