The alkaloids of the madangamine group

This chapter is focused on madangamines, a small group of complex diamine alkaloids isolated from marine sponges of the order Haplosclerida, and covers their isolation, characterization, biogenesis, biological activity, and synthesis. Structurally, madangamines are pentacyclic alkaloids with an unprecedented skeletal type, characterized by a common diazatricyclic core and two peripheral macrocyclic rings. The isolation of these alkaloids from Xestospongia ingens (madangamines A-E) and Pachychalina alcaloidifera (madangamine F) is described in detail. Physical and complete spectroscopic 1H and 13C NMR data are included. The proposed biogenesis of madangamines from ammonia, a functionalized three-carbon unit, and saturated or unsaturated linear long-chain dialdehydes, via partially reduced bis-alkylpyridine macrocycles, is discussed. The synthesis of alkaloids of the madangamine group has been little explored, with only one total synthesis reported so far, that of (+)-madangamine D. This review also describes several model synthetic approaches to the diazatricyclic ABC core of these alkaloids, as well as model studies on the construction of the (Z,Z)-unsaturated 11-membered E macrocycle common to madangamines A-E, the 13- and 14-membered D rings of madangamines C-E, and the all-cis-triunsaturated 15-membered D ring of madangamine A. Some members of this group have shown significant in vitro cytotoxicity against a number of cancer cell lines.