Vinyl azides in heterocyclic synthesis. Part 5. Thermal and photochemical decomposition of azidocinnamates containing ortho-cycloalkylidene substituents

Abstract

Heating the ortho-cycloalkylidene azidocinnamates (2b–e) in boiling toluene gives the indeno[1,2-b]tetrahydroazirines (8), derived by intramolecular ene reaction involving the CN bond of the intermediate 2H-azirines (11), as the major product. The intramolecular ene reaction is a highly favourable process, and also occurs at ca. 40 °C when the azirines (11) are formed by irradiation of the azides (2), although photochemical ring opening of the azirines does compete. Only when the possibility for such an intramolecular ene reaction is removed, by epoxidation of the cycloalkylidene double bond, are vinylnitrene derived products formed to any extent, the epoxy azides (7) giving the 4-substituted indoles (14) in high yield.