A Novel Cytotoxic Alkaloid of Lamellarin Class from a Marine Ascidian Didemnum sp.

Many structurally and pharmacologically novel natural products have been isolated to date from a variety of species of marine ascidians, invertebrate chordates (phylum Chordata, subphylum Urochordata, class Ascidiacea).Major secondary metabolites of ascidians are amino acid-derived compounds.1In the chemistry of the genus Didemnum, tyrosine and tryp tophan are the major amino acid components.2Lamellarins, isolated originally from prosobranch mollusc Lamellaria sp.3 and later from the ascidian D. chartaceum,4 the sponge Dendrilla cactos?。unidentifiedDidemnum sp.8,9 and a species of unidentified ascidian,10,11 are presumably conden sation products of 3-(3,4-dihydroxyphenyl) alanines (or 3hydroxytyrosines, viz DOPA's)12 and showed various bio activities, such as cytotoxicity,3,13,14 immunomodulating 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 of the 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 of the 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 Experiment지 SectionInstruments and Data C이lection.Low and high resolu tion mass measurements of DEI and FAB were supplied by the Mass Spec.Facility at the Scripps Research Institute, La Jolla, California, USA.Optical rotations were measured on a Roudolf polarimeter with a 10-cm cell.Infrared spectra were recorded on a Perkin Elmer FT-IR spectrophotometer Model 1600 and ultraviolet spectra on a Perkin Elmer Model Lambda 3B.