Amaranzoles B-F, Imidazole-2-carboxy Steroids from the Marine Sponge Phorbas amaranthus. C24-N- and C24-O-Analogues from a Divergent Oxidative Biosynthesis

Five new steroidal imidazoles, amaranzoles B-F, were isolated from extracts of the marine sponge Phorbas amaranthus along with the known amaranzole A. The C24-N-(4-p-hydroxyphenyl)imidazol-5-yl constitution found in amaranzoles A, C, and D is replaced by a C24-O-(4-p-hydroxyphenyl)imidazole-2-carboxylate motif in amaranzoles B, E, and F. The structures were elucidated by interpretation of spectroscopic data. The C24 side chain configuration was assigned by synthesis of a model ester followed by chiroptical comparisons of its CD spectrum with that of an amaranzole B derivative.