RADICAL ANNULATIONS AND CYCLIZATIONS WITH ISONITRILES - THE FATE OF THE INTERMEDIATE IMIDOYL AND CYCLOHEXADIENYL RADICALS

The reaction of 4-methoxyphenylisonitrile with phenylacetylene and AIB N produces a novel cyclopenta-fused quinoxaline through addition of 2- cyanoprop-2-yl radical to the alkyne; the resulting vinyl radical atta cks isonitrile to afford an imidoyl radical, which gives rise to a tan dem 5-exo, 6-endo cyclisation. The whole process entails a new example of a rare 4 + 1 radical annulation. The cyanopropyl radical can also attack isonitrile to yield small amounts of quinolines deriving from 4 + 2 and 3 + 2 annulation between the resulting imidoyl radicals and p henylacetylene. The oxidation step leading to the final aromatic produ cts involves the starting isonitrile, which is converted to an a-unsub stituted imidoyl radical and affords 2-unsubstituted quinolines. This behaviour was also found in cyclisations of biphenyl-2-ylisonitrile un der various radical conditions. Finally, the title reaction gives smal l amounts of an alpha,beta-unsaturated nitrile, which can arise from a spirocyclohexadienyl radical through fragmentation and subsequent bet a-scission of the resulting iminyl. This could be the first, direct ev idence of the intermediacy of iminyl radicals in the rearrangements of the spirocyclohexadienyls obtained by 3 + 2 annulation between imidoy l radicals and alkynes.