Ascidiolides, nitrogenous phthalides from the solitary ascidian Ascidia virginea from the Northeastern Atlantic, and structure revision of the closely related ascidines

Abstract

As part of our investigation into the chemical diversity of Irish marine invertebrates, we identified the solitary ascidian Ascidia virginea as a promising organism. A fractionated extract from this organism displayed toxicity in a brine shrimp assay and its chemical profiling indicated the presence of an unknown family of highly unsaturated nitrogenous metabolites. The targeted isolation and NMR based structure elucidation of members of this family led to the identification of four new phthalide derivatives, named ascidiolides A-D (1-4). The presence of a furanone ring was indicative of their similarity with both previously described polyaromatic butenolides from other ascidian species and isoquinolinediones isolated from the same species. Sixteen additional derivatives were characterised using only LC-MS/MS, HSQC NMR analysis, and analogy with major metabolites due to the minute amounts present in the extract. The unique structures of the ascidiolides allowed us to propose a biosynthetic pathway from anthranilic acid. Their structures also led to the revision of the structures of the ascidines A-C, related metabolites previously isolated from A. virginea collected in Norway. The isolated compounds 1-4 exhibited no cytotoxicity against a panel of tumour cell lines nor antimicrobial activity against several human pathogens, while the antioxidant activity measured for ascidiolide D (4) was comparable to that of the ascorbic acid positive control.2