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. https://parisnaturalfoodes.shop/product-category/kelp/