Keronopsamides, a New Class of Pigments from Marine Ciliates

Abstract

AbstractNew pigments with an unprecedented skeleton, named keronopsamides A–C, were isolated from the marine ciliate Pseudokeronopsis riccii. The structure of the most abundant secondary metabolite, keronopsamide A, was established through extended Nuclear Magnetic Resonance (NMR) analysis and mass spectrometric (MS) data obtained by using electrospray (ESI) and matrix assisted laser desorption (MALDI) ionizations. Structures of the minor analogues (keronopsamide B and C) were inferred from 1H NMR, chemical correlation, and LC‐MS measurements. The analysis of NOE dipolar couplings and quantum chemical calculations, carried out by density functional theory (DFT) on the preferred rotamers of keronopsamide A, suggested a fully planar structure with amide bond in anti stereochemistry capable to break down the extended conjugation of the whole π electronic system. Although the presence of a 2‐substituted 3,4‐dibromo‐pyrrole moiety is reminiscent of previously isolated metabolites isolated from a cell extract of Pseudokeronopsis rubra (keronopsins), keronopsamides show a new molecular skeleton formally derived from a peptide‐like condensation of a 3‐pyrrolepropenoic acid with an unsaturated bromotyramine.