Under the period covered by this review, several studies were carried out to identify biologically active marine compounds, of which the xanthones stand out. Around 100 xanthones were isolated, from which 51 are considered new natural compounds.

Nowadays, the isolation of xanthone derivatives from marine sources usually involves fermentation to increase the compound’s final amount. Fermentation involves putting the fungi in a culture broth, which comprises all the nutrients, such as glucose, iron phosphate, calcium carbonate, malt extract, and controlled pH [21,26], necessary for its growth. The duration of fermentation can be variable but usually involve several days, at least 14 days and the highest 45 days [14,17]. Then, the cultured broth is filtered and extracted, most often, with EtOAc. This extract is subjected to a vacuum liquid chromatography over a silica gel column [15,16,21,23], flash chromatography [19] or, most commonly, to normal column chromatography (CC) [14,17,18,20,22], originating different fractions, which can be subjected to analysis by HPLC or TLC, obtaining sub-fractions, or be directly purified to obtain pure compounds. Most of the xanthones herein referred to were obtained after semi-preparative HPLC purifications of these sub-fractions. Naturally, the eluents and gradients used differ depending on the extracts/fractions being analyzed. The most common eluents used are organic solvents mixtures, such as n-hexane/CH₂Cl₂/MeOH [16], n-hexane/EtOAc [15], CH₂Cl₂/MeOH [15,18,20,21,23], gradient of petroleum ether/EtOAc [14,17,18], but mixtures of H₂O/MeOH can also be used successfully [19].

The new xanthone derivatives isolated include simple ones (Figure 2), mainly isolated from marine-derived fungi found in marine sediments [15,18,20,31,32,33,34,35,36,46,47,48,49]. These xanthones are usually highly substituted, with different substituent patterns being the substituents found to be essential for four types; hydroxy, methoxy, methyl, and methoxycarbonyl groups. It seems that the methoxy group is not common in bioactive marine xanthones [50], but yicathins B (3) and C (4) (Figure 2), which were recently prepared by total synthesis [51], present such a group at C-8. Other examples include xanthones bearing a 6-methoxy group (2) and (6) and 7-methoxy group (13) and (15) (Figure 2). On the other hand, marine xanthones (14) and (16) present two methoxy groups, respectively, at C-3 and C-7 and at C-2 and C-7 (Figure 2).

The more complex xanthones isolated during the covered period are depicted in Figure 3 and present the xanthone skeleton fused with other rings. The most interesting examples include (2S,3aS,12cS)-8-hydroxy-2,6-dimethoxy-1,2,3a,12c-tetrahydro-7H-furo [3’,2’:4,5] furo [2,3-c] xanthen-7-one (18), isolated from the marine fungus Aspergillus versicolor and named oxisterigmatocystin D due to its similarity with the known oxisterigmatocystin C [37]. Although the similarities with oxisterigmatocystin C and other isomers were previously isolated, the authors established the stereogenic centers’ configurations and demonstrated that oxisterigmatocystin D is a new natural compound [37]. Compound (19), named monacyclione G, is another example isolated from Streptomyces sp. HDN15129 [21]. Its stereochemistry was confirmed by the NMR data mainly through the correlations found in HMBC [21] and was established as depicted in Figure 3.

The most prevalent nucleus in these new marine xanthones is the 11-hydroxy-5-methyl-2-(prop-1-en-2-yl)-2,3-dihydropyrano [3,2-a]xanthen-12(1H)-one, found in fourteen of the twenty-one new derivatives (Figure 3). Twelve marine xanthones with this core were isolated from the fungus Aspergillus sp. ZA-01, respectively, aspergixanthones A-K (20–24 and 26–31) and the 15-acetyltajixanthone hydrate (25) (Figure 3) [14,17]. Varioxiranols F (37) and G (38) (Figure 3) were isolated from the Emericella variecolor, a fungus associated with a Cinachyrella sp. Sponge [28].

The aspergixanthones (20–24, 26 and 27) and the 15-acetyltajixanthone hydrate (25) have a 3-hydroxy-3-methylbutyl moiety at C-8, whereas the remaining aspergixanthones (28–31) have at the same position a 3-methylbut-3-en-1-yl substituent (Figure 3), in both cases, hydroxy, methoxy or acetyloxy groups at C-1′ and C-2′ are also present (Figure 3).

The last examples are xanthone derivatives bearing a 1,7,8,10-tetrahydroxy-11-methoxy-5,6-dihydro-9H-naphtho [2,1-c]xanthen-9-one (32, 33) and a 1,9,14-trihydroxy-5,6-dihydro-8H-naphtho [2,1-b]xanthen-8-one nucleus (34–36) (Figure 3). Buanmycin (34) was extracted from a marine Streptomyces cyaneus [52], and the remaining examples, citreamicin θ A (32), citreamicin θ B (33), citreaglycon A (36), and dehydrocitreaglycon A (35), were isolated from Streptomyces caelestis [19].

In the last years, complex tetrahydroxanthone derivatives were also isolated (Figure 4) [23,26,27,40]. Compounds (39–44), named versixanthones A-F, were isolated from the fungus Aspergillus versicolor HDN1009 [27]. Compounds (45, 46) were isolated from the fungus Engyodontium album strain LF069 [26] and the Tritirachium sp. SpB081112MEf2 a marine sponge fungus [40]. Finally, compounds (47–51) were also isolated from the fungus Aspergillus versicolor HDN1009 [23].

Versixanthones A–F have a methyl (3R,4S,4aS)-1,4,8-trihydroxy-3-methyl-9-oxo-2,3,4,9-tetrahydro-4aH-xanthene-4a-carboxylate moiety linked to a methyl (2′R,2″S,3″S)-5-hydroxy-2-(3-methyl-5-oxotetrahydrofuran-2-yl)-4-chromanone-2-carboxylate (39, 41–43) and methyl (2′R,1″R,2″S)-5-hydroxy-2-(1-hydroxy-4-methoxy-2-methyl-4-oxobutyl)-4-chromanone-2-carboxylate (40, 44). The main differences are the linkage between the two moieties since the authors establish the stereocenter configurations as equal in all isomers (Figure 4) [27].

Compounds (45, 46) represent two diastereomers fully characterized by Wu et al. in 2016 [26], although the isomers JBIR-99 (45a) and JBIR-97/98 (46a) were previously isolated by Ueda et al. in 2010 [40]. The absolute configuration of these isomers and the engyodontochone A (46b) and B (45b) was established using NMR experiments and is depicted in Figure 4 [26].

Our final examples are the dimeric tetrahydroxanthones (47–51) isolated from the fungus Aspergillus versicolor HDN1009 and named versixanthones G-K (Figure 4) [23]. The structure depicted in Figure 4, including the absolute configuration, was established using NMR experiments.