Onchidin B: A New Cyclodepsipeptide from the Mollusc Onchidium sp.

Abstract

Onchidin B (4) is a cyclic depsipeptide isolated from the pulmonate mollusc Onchidium sp. Its structure was determined by extensive 2D-NMR, FABMS, tandem FAB MS/MS, selective hydrolysis, and synthesis. It contains four α-amino acids [two units of N-methyl valine (MeVal), two units of proline (Pro)], four α-hydroxy acids [two 2-hydroxyisovaleric acids (Hiv), two 2-hydroxy-3-methylpentanoic acid moieties (Hmp)] and two units of the new β-hydroxy acid: 3-hydroxy-2-methyloct-7-ynoic acid (Hymo)]. Selective hydrolysis and direct comparison by chiral GC-MS with authentic samples of the α-amino and α-hydroxy acids allowed us the assignment of the entire absolute stereochemistry of onchidin B. In this way, the α-hydroxy acids were found to be (S)-Hiv and (S,S)-Hmp, and the α-amino acids (R)-proline, (S)-proline, and (R)-MeVal. In order to establish the absolute configuration of the new β-hydroxy acid, Hymo, its four possible stereoisomers were stereoselectively synthesized using chiral N-propionyl oxazolidinones...