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Pyranoxanthones 6-8 were obtained by dehydrogenation of the respective dihydropyranoxanthones 3-5 with DDQ in dry dioxane. Two prenylated xanthones 10,11 were obtained from the reaction of 1-hydroxyxanthone (9) with prenyl bromide in alkaline medium, or by condensation of xanthone 9 with isoprene in the presence of orthophosphoric acid. The structural elucidation of the two new compounds 6,11, as well as an update of data for the already described prenylated derivatives 7,8,10 were accomplished by IR, UV, HRMS and NMR (1H, 13C, HSQC and HMBC) techniques. The effect of the prenylated xanthone derivatives on the in vitro growth of human tumor cell lines MCF-7 (breast adenocarcinoma) and NCI-H460 (non-small cell lung cancer) is also reported. Compounds 10 and 11 have been found to exhibit a moderate growth inhibitory activity against the MCF-7 cell line.

Marine natural products provide an abundant source of biologically active compounds with high chemical diversity and a great potential to produce valuable therapeutic entities. For these reasons, marine natural products have fascinated many researchers over the years, and the research in this field has increased tremendously in the last few years. Inspired by the vastness of our oceans and an almost incomprehensible level of biodiversity in the marine environment, researchers have enthusiastically explored the pharmacological potential of secondary metabolites from marine organisms. Among them emerge xanthones, compounds with a broad spectrum of biological and pharmacological activities that have raised a growing interest in the pharmaceutical industry. Therefore, this chapter reports the isolation of 57 xanthone derivatives from marine‐derived microorganisms, namely fungi associated with algae, coral, sponges, and mangrove plants and from marine actinomycetes, with special emphasis on novel molecules. Relevant biological activities exhibited by these compounds, such as cytotoxic/antitumor, cancer chemopreventive, antioxidant, antibacterial, antifungal, and antiprotozoal, are also described. Finally, the sources and origins of the producer organisms are also briefly referred, and the methods of isolation and structure elucidation are also discussed.

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