On the regioselectivity of imidoyl radical cyclisations

The previously reported tandem cyclisation of N-aryl alpha-(2-cyanophenyl)s ulfanyl imidoyl radicals affords one quinoxaline derivative arising from ex clusive 1,6-cyclisation of the final iminyl radical onto the N-aryl ring. W hen the imidoyl radicals are generated by addition of photolytically genera ted (2-cyanophenyl)sulfanyl radicals to isocyanides, the reaction also give s small amounts of a by-product that is formed by an analogous route and wh ose X-ray crystallo-graphic data are reported here. The formation of this p roduct entails a rare ortho-selective photo-Fries rearrangement of the star ting disulfide, followed by addition to the isocyanide and regioselective 1 ,5-cyclisation of the resulting imidoyl onto only one of the two available radical accepters, i.e. the cyano group and the sulfide moiety. Semiempiric al MNDO-d calculations were performed in order to throw some Light on the f actors affecting these competitive cyclisations.