Structure and synthesis of a progesterone homologue from the skin of the dorid nudibranch Aldisa smaragdina.

The short side-chain steroid 24-norchol-4-ene-3,22-dione has been found to be the main metabolite of both skin and mucus of the mollusc Aldisa smaragdina. This compound, previously reported as a microbial degradation product of cholesterol, has been never isolated from natural sources. The absolute stereochemistry has been determined by synthesis from commercial stigmasterol. A full NMR characterisation has been also reported.