Many structurally and pharmacologically novel natural products have been isolated to date from a variety of species ofmarine ascidians, invertebrate chordates (phylum Chordata, subphylum Urochordata, class Ascidiacea). Major secondary metabolites ofascidians are amino acid-derived compounds. 1 In the chemistry ofthe genus Didemnum, tyrosine and tryp tophan are the major amino acid components. 2 Lamellarins, isolated originally from prosobranch mollusc Lamellaria sp. 3 and later from the ascidian D. chartaceum, 4 the sponge Dendrilla cactos?. unidentified Didemnum sp. 8, 9 and a species of unidentified ascidian, 10, 11 are presumably conden sation products of 3-(3, 4-dihydroxyphenyl) alanines (or 3-hydroxytyrosines, viz DOPA’s) 12 and showed various bio activities, such as cytotoxicity, 3, 13, 14immunomodulating acti vity, 8 HIV integrase-inhibitory activity. 11 We studied a purple unidentified Didemnum sp.(other than D. chartaceum) to search for novel cytotoxic compounds. In this paper, we describe the structure elucidation of lamellarin § (1) using spectroscopic methods including the exten sive use of 2-dimensional NMR correlation experiments. Molecular modeling study ofthe alkaloid was also conduct ed. The issue of chirality and the shielding effect of phenyl rings are discussed.

The specimens were lyophilized and extracted twice each with 70% methanolic chloroform and methanol. The combin ed extract was concentrated under vacuum and fractionated between hexane and methanol. The methanol-soluble material was further partitioned between ethyl acetate and water. The ethyl acetate fraction showed cytotoxicity against human acute promyelocytic leukemia cells (HL-60). Bioactivityguided separation ofthe ethyl acetate-soluble material through Sephadex LH-20 (MeOH), followed by chromatography on reversed-phase HPLC (ODS-silica), gave a cytototoxic com pound, lamellarin § (1), along with known compounds, lamellarins G (2) 4 and L (3). 8