<jats:p>The unstable isomeric compounds 5-hydroxy-7-prop-2-en-(E)-ylidene-7,7adihydro-2H-cyclopenta[b]pyran-6-one (1) and 5-hydroxy-7-prop-2-en-(Z)-ylidene-7,7adihydro-2H-cyclopenta[b]pyran-6-one (2), previously described as antimicrobialmetabolites from the sponge Ulosa sp., were isolated and identified as major componentsof the ascidian Diplosoma virens. In this paper, full spectral data for 2 and complete 13CNMRdata for 1, based on 2D NMR measurements, are provided for the first time.Compounds 1 and 2 showed cytotoxity against HCT116 cells (human colorectal cancercells) by triggering apoptotic cell death.