The ocean is a rich source of natural products with novel structural features and biological activities. We have investigated bioactive compounds from marine organisms such as sponges, ascidians, and marine-derived microorganisms. In the course of our search program for novel and useful compounds, three new sesquiterpenes (1-3), designated euryspongin A-C, were isolated from a marine sponge Euryspongia sp. collected at Iriomote Island. In this presentation, we described the isolation and structure elucidation including the stereochemistry of euryspongins A-C. From the EtOH extract of the sponge, compounds 1-3 were purified by ODS column chromatography and HPLC. The structure of 1 was assigned on the basis of spectroscopic data including various NMR experiments. Compound 1 had a unique bicyclic furanosesquiterpene structure with six- and eight-membered rings. The stereochemistry of 1 was assigned by ^1H-^1H coupling constants, NOESY spectrum, and a comparison of the specific rotation of 1 with that of the related compound. ^1H-NMR spectra of 2 and 3 were very similar to that of 1. Comparison of NMR data among compounds 1-3 indicated that a furan ring in 1 was replaced by an α,β-unsaturated-γ-lactone ring in 2 and 3. The stereochemistry of compounds 2 and 3 were deduced to be the same as that of 1 by the analysis of ^1H-^1H coupling constants and NOESY data. Sesquiterpenes having a six- and eight-membered bicyclic structure are quite rare in the nature. To our knowledge, only four compounds have been reported as natural products in this class. Investigation on the biological activity of 1-3 is now in progress.
