New mycalolides from the marine sponge Mycale magellanica and their interconversion.

Three new macrolides, 30-hydroxymycalolide A (4), 32-hydroxymycalolide A (5), and 38-hydroxymycalolide B (6), were isolated from the marine sponge Mycale magellanica. Their structures were assigned on the basis of spectroscopic data. They were cytotoxic against L1210 cells with IC50 values of 0.019, 0.013, and 0.015 microg/mL, respectively. Chemical interconversion of the known mycalolides A-C (1-3) with 4-6 established their stereochemical relationships.