Allos-hemicalyculin A, a photochemically converted calyculin from the marine sponge Discodermia calyx

Allos-hemicalyculin A (1), a new derivative of calyculin A, was isolated from the marine sponge Discodermia calyx collected off Shikine-jima Island, Japan. The structure of 1, including the absolute configurations, was elucidated by spectroscopic analyses and photochemical degradation experiments. Consequently, its structure was identical to the distal end of the peptide side chain of calyculin A (2), previously isolated from D. calyx and generated by photochemical oxidative cleavage of the oxazole moiety. In stark contrast to the potent cytotoxicity of 2, 1 is no longer cytotoxic, in agreement with the previously reported structure–activity relationship data. Here we describe the isolation and structural elucidation of 1.