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 a re generated in situ by deprotonation of the corresponding 2-alkylbenz azolium tetrafluoroborates, i.e. 14a --> 15a, 14b --> 15b, and 18 --> 19. Ring-expanded products are formed by extrusion of molecular nitrog en 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 210, the N-sulphonylimine 24 and t he N-sulphonylamine 27 are elucidated by means of X-ray diffraction an alyses. The ratio of the (useful) ring-expansion reactions vs. the unw anted formation of 5 + 6 is hardly influenced by the solvent employed and temperature of the experiment but strongly by the nature of the po tential migrating atom and the substituents at the alpha-carbon atom.