Allelochemistry of some Hawaiian dorid nudibranchs

Abstract

Several novel metabolites were isolated from Hawaiian dorid nudibranchs, an order of brightly colored marine mollusks. The metabolites are believed to serve as defense allomones. This belief is based on bioassay results and on the extremely high concentration of metabolites per animal. Investigation of the sponge prey of two of the nudibranchs led to a variety of biogenetically related metabolites. Structure elucidation of the compounds was accomplished by chemical degradation and spectroscopic analysis. Nakafuran-8(26) and -9(35) were isolated from the dorid nudibranchs Hypselodoris godeffroyana and Chromodoris maridadilus and their prey, the sponge Dysidea fragilis. The structures of 26 and 35, including stereochemical features, were secured by chemical degradation, interpretation of spectral data, and a qualitative lanthanide induced shift(LIS) study. Both 26 and 35 demonstrated repellent activity against common reef fish at low concentrations. Further investigation of D. fragilis led to the structure elucidation of preupial(46) , upial(42), and the known microcionin-2(47) and -4(48), all biogenetically related to nakafuran-8(26) and -9(35). The structure of preupial(46) was determined from spectroscopic data. The structure of upial(42) was rigorously proven by a series of chemical degradations, interpretation of spectral data, and by a qualitative LIS study on a degradation product. Crude extraction of the nudibranch Hypselodoris daniellae provided nearly pure spiniferin-2(55), a known metabolite previously isolated from a Mediterranean sponge. Chemical investigation of another dorid nudibranch, Chromodoris albonotata, provided crystalline doridal(49) as the sole constituent of the crude extract. Conversion of doridal(49) to polygodial(50), proved its gross structure. Stereochemical features were demonstrated by a qualitative LIS study and by comparison of IH NMR data with the data for two isomeric synthetic intermediates. A survey of the nudibranch genus Phyllidia demonstrated that all six species described from the Hawaiian Islands are characterized by foul smelling secretions reminiscent of isonitriles. Spectral analysis confirmed the isonitrile function, which is attached to different terpenoid skeletons.