New cyclic peptides from the ascidian Lissoclinum patella.

Four new cyclic peptides, patellamide G (2) and ulithiacyclamides E-G (3-5), along with the known patellamides A-C (6-8) and ulithiacyclamide B (9), were isolated from the ascidian Lissoclinum patella collected in Pohnpei, Federated States of Micronesia. The planar structures of these peptides were determined from 1D and 2D 1H and 13C NMR spectra. The absolute stereochemistries of the amino acid units, except for cysteine, were assigned by chiral GC analysis of N(O)-trifluoroacetyl isopropyl ester derivatives of amino acids obtained by acid hydrolysis of the intact and ozonized peptides. The structures of ulithiacyclamides E-G (3-5) were confirmed by chemical conversion. Patellamides B (7) and C (8) exhibited in vitro modulation of multidrug resistance in CEM/VBL100 cells.