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The defensive secretion of the nudibranch Phyllidia varicosa consists of two isocyanosesquiterpenes: the previously described 9‐isocyanopupukeanane (1), and its 2‐isomer (4), for which we report the structure and properties. The mixture originates with a sponge, Hymeniacidon sp., where it is produced in varying proportions. CD. measurements and X‐ray diffraction data establish the absolute configuration of the two metabolites.

SummaryThe defensive secretion of the nudibranch Phyllidia varicosa consists of two isocyanosesquiterpenes: the previously described 9-isocyanopupukeanane (l), and its 2-isomer (4), for which we report the structure and properties. The mixture originates with a sponge, Hymeniacidon sp., where it is produced in varying proportions. CD. measurements and X-ray diffraction data establish the absolute configuration of the two metabolites.Nudibranchs (sea slugs) are marine mollusks which lack an external shell and which are frequently associated with sessile marine invertebrates, e.g. sponges, bryozoans, or coelenterates, in a grazer-prey or in a symbiotic relationship [I]. We recently [2] elucidated the molecular basis of such a relationship between the nudibranch Phyllidia varicosa and its prey, a sponge, Hymeniacidon S P .~) , and we reported the structure of 9-isocyanopupukeanane (l), the defense allomone of the mollusk, which the mollusk in turn obtains from its preferred diet, a sponge. The mollusk accumulates the allomone, which is the active constituent of its mucous skin secretion that is lethal to fish and crustaceans [3].We recognized early in our investigation that the isonitrile fractions of the mollusk and of the sponge were identical binary mixtures of isonitriles. However, the two compounds proved inseparable by chromatography or complexation. On the other hand, degradation of the mixture to 9-pupukeanone (2) [2], or derivatization of 9-methylaminopupukeanane (3) with phenylisothiocyanate [2] led in each instance to isolation of a single compound, having a sesquiterpene skeleton corresponding to 1. These observations indicated that either isomerization of one of the constituents had occurred, or that one constituent had been eliminated during reaction or work-up. We now report the identity and properties of the second iso- I ) 3,In part from the M.S. Dissertation of M.R.H., University of Hawaii, 1978. Present address: