Marine natural products. XXIII. Three new cytotoxic dimeric macrolides, swinholides B and C and isoswinholide A, congeners of swinholide A, from the Okinawan marine sponge Theonella swinhoei.

Following the characterization of swinholide A (1), the major cytotoxic dimeric macrolide, three new congeneric dimeric macrolides, named swinholide B (2), swinholide C (3) and isoswinholide A (10), have been isolated from the Okinawan marine sponge Theonella swinhoei. The structures of these dimeric macrolides have been elucidated on the basis of chemical and physicochemical evidence. These dimeric macrolides were shown to exhibit potent cytotoxicities toward KB cell lines.