Enamide photocyclization has been shown to be a new useful tool for the synthesis of various nitrogen containing heterocyclic systems. Particularly, the stereoselective photocyclization of the enamide derived from cyclohexanone to trans-fused phenanthridone offered wide applications to the alkaloids of this type of structure, which are abundant in nature, e.g. crinine, lycorine, some benzo(c)phenan-thridine, protcberberine, and yohimbine-type alkaloids. Irradiation of the enamides derived from the 2-substituted cyclo-hexanones afforded the corresponding phenanthridones which were converted into crinan, the basic structure of crinine, and a key intermediate for the total synthesis of tazettine and haemanthidine. Photocyclization of the enamides derived from the 1-tetralones afforded the B/C-trans benzo(c)phenanthridones which were readily converted into the alkaloids, e.g. nitidine and avicine. N-Acylation of 1-alkyl-3,4-dihydroisoquinolines yielded the corresponding N-acylates which underwent similar photocyclization to the berbine derivatives, thus enabling total syntheses of proto-berberine alkaloids, e.g. xylopinine, sinactine and cavidine. Further, the N-acylates of harmalane were similarly photocyclized to the yohimbine-type derivatives, which furnished the first and one-step total synthesis of angustidine, a new type of hetero-yohimbine alkaloid.
