Synthesis of the indolo[2′,3′:3,4]pyrido[1,2-b][2,7]naphthyridinone alkaloid nauclefine and its ring- E isomers

Nauclefine has been reported to be the only product from the photocyclisation of the enamide derived from harmalan and nicotinoyl chloride. Re-investigation has shown, however, that two compounds are formed: the alkaloid and an isomer in which cyclisation to the 2-position of the pyridyl unit has occurred. These components have been separated and the remaining ring-E isomers have been synthesized. 1H and 13C n.m.r. data for these structures are analysed.