Bistellettazines A−C and Bistellettazole A: New Terpenyl−Pyrrolizidine and Terpenyl−Imidazole Alkaloids from a Southern Australian Marine Sponge, Stellettasp.

Four new alkaloids have been isolated during chemical investigations into a southern Australian marine sponge, Stellettasp. Detailed spectroscopic analysis, supported by chemical degradation and partial synthesis, permitted structure elucidation of bistellettazines A−C ( 1− 3), the first reported examples of terpenyl-pyrrolizidine conjugates, and bistellettazole A ( 4), a unique cyclic terpenyl-imidazole conjugate. The alkaloids 1− 4feature unprecedented carbon skeletons that are proposed to share a common convergent biosynthetic origin, arising via the biogenic equivalent of a Diels−Alder addition between two hypothetical polyenyl norsesquiterpene precursors. [ABSTRACT FROM AUTHOR]