Five-membered Monoheterocyclic Compounds Alkaloids (continued): Amaryllidaceae Alkaloids

The Amaryllidaceae alkaloids comprise some one hundred structures isolated from the bulbs of plants from almost all genera of the subfamily Amaryllidoidae. A number of different types of alkaloids are recognized, all of which contain a basic skeleton of fifteen carbon atoms subdivided into a phenylmethyl (C₆–C1) unit and a cyclohexylethyl or cyclohexenylethyl (C₆-C₂) unit, and the majority contain a simple or fused pyrrolidine ring. The best known alkaloid of the Amaryllidaceae is lycorine, the carbon skeleton of which forms the basic framework for a series of natural products. The biosynthetic derivation of the various Amaryllidaceae alkaloids from belladine type precursors suggests that lycorine, crinane and alanthamine alkaloids are derived by the oxidative cyclization of norbelladine derivatives and this hypothesis has been substantiated subsequently by radio-tracer experiments utilizing doubly and triply labeled norbelladines. In this chapter individual alkaloids have been cited as examples. These include pyrrolo[1,2,3-d,e]phenanthridine alkaloids, 2-benzopyrano[3,4-g]indole alkaloids, galanthamine-type alkaloids, [5,10b]-ethanophenanthridine alkaloids, pretazettine-type alkaloids, montanine, coccinine and manthine, and many other miscellaneous alkaloids. Narciprimine and narciclasine, both present in the extracts of the plant Narcissus incomparabilis Mill., are unusual in that their basic skeleton contains only thirteen carbon atoms instead of the usual fifteen.