Radical annulations with nitriles: Novel cascade reactions of cyano-substituted alkyl and sulfanyl radicals with isonitriles

New radical annulation reactions are described involving addition of cyano-substituted alkyl and sulfanyl radicals to aromatic isonitriles. The tandem cyclisation of the resulting imidoyl radicals onto the cyano group affords cyclopenta- and thienoquinoxalines, respectively. The intervention of the isonitriles in the aromatisation process of the cyctohexadienyl radicals is discussed as well as the regiochemistry of the cyclisation of the iminyl radicals obtained by addition of the imidoyls to the nitrile moiety. The hypothesis of an exclusive 6-membered ring closure onto the aromatic ring is also supported by the results of semiempirical AM1 calculations. (C) 1998 Elsevier Science Ltd. All rights reserved.