Chemical and photochemical cyclisations of 1-alkylidene-1,2,3,4-tetrahydro-2-nicotinoyl- and -isonicotinoyl-β-carbolines: a regiospecific synthesis of naucléfine

Abstract

1,2,3,4-Tetrahydro-1-methylene-2-nicotinoyl-β-carboline undergoes oxidative photocyclisation to give the indol-[2′,3′ : 3,4]pyrido[1,2-b][2,7]naphthyridinone alkaloid naucléfine and a ring-E isomer. A similar reaction is observed when the β-carboline is treated with acids, but on treatment with nicotinoyl or benzyl halides cyclisation takes place to give one isomer only. Analogous reactions occur with other 2-substituted 1-alkylidene-1,2,3,4-tetrahydro-β-carbolines. The mechanisms of these processes have been investigated and the ¹H n.m.r. spectra of both substrates and products have been analysed.