Xanthone Glucosides: Isolation, Bioactivity and Synthesis

Xanthones are secondary metabolites found in plants, fungi, lichens, and bacteria from a variety of families and genera, with the majority found in the Gentianaceae, Polygalaceae, and Clusiaceae. They have a diverse range of bioactivities, including anti-oxidant, anti-bacterial, anti-malarial, anti-tuberculosis, and cytotoxic properties. Xanthone glucosides are a significant branch of xanthones. After glycosylation, xanthones may have improved characteristics (such as solubility and pharmacological activity). Currently, no critical review of xanthone glucosides has been published. A literature survey including reports of naturally occurring xanthone glucosides is included in this review. The isolation, structure, bioactivity, and synthesis of these compounds were all explored in depth.

More recently, xanthone glucosides have been explored, and the mutation of these glycosyl groups can change the biological activity of xanthone, which has a wide range of clinical applications [21,22]. However, xanthones usually have poor solubility; herein, many studies are being devoted to the synthesis of glycosylated xanthones to improve their solubility and activity and minimize their toxicity [23,24]. Xanthone glucosides are an important class of xanthones that are extensively dispersed in the plant families Gentianaceae and Polygalaceae. For natural xanthone glucosides, each xanthone site can be connected to a sugar group, which can be either monosaccharide or disaccharide. Recent research has revealed that xanthone glucosides have anti-oxidant [25], anti-inflammatory [26], anticancer [21,27], and other pharmacological properties. We separated xanthone glucosides into xanthone C-glucoside and xanthone O-glucoside and classified the substances accordingly. C-C bonds connect the sugar moiety to the xanthone nucleus in C-glucosides, which are usually resistant to acidic and enzymatic hydrolysis, whereas O-glucosides have normal glycosidic linkages. In glucosides whose glycosyl group is disaccharide, the second sugar residue is often glucose, xylose, or rhamnose and is usually associated with C-6 of the first glucose unit. However, when the second residue is rhamnose, it is linked to the C-2 of the first residue. The structures and connection site of sugars to the xanthone core that may be used in their full names are shown below.
In general, xanthone glucosides have received much interest due to their unique structures and significant bioactivities. As a result, we examined the separation, bioactivity, and synthesis of naturally occurring xanthone glucosides, with the goal of providing a reference for future relevant studies.

Xanthone C-Glucoside
This class of xanthone glucosides is composed of xanthone and sugar groups that are linked together by carbon atoms in the structure. D-glucose is a sugar group that is commonly found in these compounds. The majority of the sugar binding sites are located at position 2, and glycosylation can often boost the activity to a certain amount [28]. All of the xanthones have hydroxyl substitutions on their skeletons, and some of them have methoxy groups. The scavenging of free radicals and the anti-oxidant activity of these compounds are their most notable impacts. We will classify these compounds by distinct genera in the order in which they were discovered, followed by a description of their biological activity.
In 1970, Aritomi and Kawasaki isolated homomangiferin (2) and isomangiferin (3) from Anemarrhena asphodeloides Bunge [56]. These two compounds were similar to mangiferin in structure. Homomangiferin has a methoxy group at position 3 compared to mangiferin, while the sugar group of isomangiferin is attached at position 4. Isomangiferin can also be isolated from Cyclopia genistoides (L.) Vent. (honeybush) and has a strong effect in the treatment of rheumatoid arthritis [57].

Others
Mangiferoxanthone A (34) is a xanthone dimer isolated from M. indica by bioassay in 2014, and is a symmetric homodimer of mangiferin. The compound showed moderate influenza neuraminidase inhibition activity. According to the research, dimerization increased the activity of the compound compared with mangiferin [80].

Xanthone O-Glucoside
In contrast to xanthone C-glucosides, xanthone O-glucosides are glucosides that are linked to the tricyclic body of xanthones by an oxygen atom. Xanthone glucoside is generally found at the C-1 position of the xanthone nucleus. Glucosides are typically monosaccharides or disaccharides that contain glucose, xylose, rhamnose, and other glycosyl groups. At present, most xanthone O-glucosides isolated from natural resources contain hydroxyl, and methoxy groups, and a few have methyl groups, aliphatic side chains, or aromatic rings. Xanthone O-glucosides, in general, are a well-studied class of compounds. The glycosylation of xanthones improves not only their physical properties (such as solubility) but also their biological activity.
In 1978, Ghosal extracted and isolated five compounds (43-47) that had not been reported before from Swertia angustifoh Buch.-Ham. Their study showed that xanthone Oglucosides in the plant could be identified after the onset of maturity (i.e., 4-to 6-week-old plants) and were not present at the beginning of growth [86].
After extraction and analysis, Sun's group obtained three compounds from Swertia mussotii Franch. in 1991, (52). They all have hydroxyl groups at positions 1 and 8. These three compounds were isolated from watersoluble components, demonstrating that the glycosyl group in the structure was the main factor influencing their solubility [88].
In 1995, Hostettmann's group isolated and identified eight xanthone O-glucosides (56-63, shown in Table 2) from Halenia corniculata. These eight compounds share the following characteristics: (1) they all have three or four methoxy groups, and (2) they are disaccharides with gentiobiose or primeverose at the C-1 position. Their structures are similar to those discovered by Hosteyitman (53-55) [90].   (73) from Gentiana campestris by Kaldas and co-workers in 1974 [94], was isolated for the first time from Swertia davidii Franch. by the same group [93].
In  (89) from Swertia mussotii. These three compounds were found to have moderate anti-oxidant activity. Their oxygen radical absorbance capacity (ORAC) values at a concentration of 3.1 µM were 30.2 ± 0.2, 33.1 ± 0.2 and 33.2 ± 0.7, respectively. The experiment in this study also showed that the bio-activity of glycosylated xanthones was higher than that of xanthones without glycosylation [78].
Ishiguro and colleagues isolated two new compounds, patuloside A (120) and patuloside B (121) in 1999 from cell suspension cultures of Hypericum patulum [116]. This is the first report on the isolation of 1,3,5,6-tetrahydroxyxanthone glucosides from cell suspension cultures of H. patulum.
A phytochemical study on the aerial parts of Hypericum elatoides led to the isolation of five previously undescribed phenolic metabolites, hyperelatones E-H (123)(124)(125)(126), along with tenuiside A (127) in 2019 by Gao's group. Compound 123 has a hydroxyethyl group at the C-1 position and 126 is a compound with only a glucoside side chain. It was experimentally verified that 125, 126 and 127 had neuroprotective activity and could improve the survival rate of PC-12 cells in a dose-dependent manner, among which 126 and 127 had the strongest activity. Compounds 125, 126 and 127 also inhibited neuroinflammation induced by lipopolysaccharide (LPS) in BV-2 microglial cells without cytotoxicity to cells with IC 50 values of 3.84 ± 0.15, 0.75 ± 0.02, and 1.39 ± 0.03 µM, respectively. In addition, 125, 126, and 127 showed stronger activity than 123 and 124 [118].

Xanthone O-Glucoside from Iridaceae
An and coworkers separated 1-hydroxy-3,5-dimethoxy-xanthone-6-O-β-D-glucoside (128) from Iris minutiaurea Makino in 2016. To assess the anti-inflammatory activity of this compound, they measured its inhibitory rate of it on nitric oxide (NO) production, and tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and IL-6 release by LPS-induced RAW 264.7 macrophage cells. The results showed that the compound could exert an anti-inflammatory effect by inhibiting the production of the pro-inflammatory cytokine NO [119].

Xanthone O-Glucoside from Polygalaceae
Li's group isolated polycaudoside A (129) from the roots of Polygala caudata Reld et Wils in 1999. As seen from the structure, the glucoside side chains of 129 and 121 are the same, but the difference is that 121 has two more hydroxyl groups than 129 [120]. In 2008, two xanthone glucosides, polyhongkongenosides A (138) and B (139) and a known compound called polygalaxanthone V (140) [123], were isolated from Polygala hongkongensis [70].  [125,126].

Xanthone
Microluside A (158) is a unique O-glycosylated disubstituted xanthone isolated from the broth culture of Micrococcus sp. EG45 cultivated from the Red Sea sponge Spheciospongia vagabunda. Anti-microbial activity evaluations showed that 158 exhibited anti-bacterial potential against Enterococcus faecalis JH212 and Staphylococcus aureus NCTC 8325 with MIC values of 10 and 13 µM, respectively [127].
Recently, Xiong's group isolated sporormielloside (159) from an EtOAc extract of Sporormiella irregularis in 2016. The presence of a methyl group in the structure of compound 159 is unusual [128].

NMR Difference of Xanthone Glucosides
After investigation on the NMR data of xanthone C-glucosides and xanthone Oglucosides reported in the literature, it was discovered that there was no significant difference in the chemical shift of protons in 1 H NMR spectrum. However, the 13 C NMR data showed regular difference in the chemical shifts of C-1 of sugars which connected to the xanthone structures.
Generally, the chemical shifts of the sugar group appear among the range of δ 60-110 ( 13 C NMR). It was found that the chemical shift of C-1 on the sugar group in xanthone C-glucosides is obviously smaller than that of xanthone O-glucosides. The chemical shift value of the former is basically distributed around δ 74, while that of the latter is mainly distributed between δ 100-110. Conversely, for the chemical shifts of C-3 and C-5 of sugar group, xanthone C-glucosides is slightly greater than xanthone O-glucosides. For example, neomangiferin is a compound bearing both Cand O-glycosides. The chemical shifts of C-1, C-3, and C-5 of the sugar group via O-linker are 103.1, 76.5, and 77.2, respectively, while the chemical shifts of C-1, C-3, and C-5 via C-linker are 73.2, 79.1, and 81.4, respectively [31].
For more examples, please see the chemical shifts listed in the Table 5 below.
In addition to chemical methods, enzyme catalysis can also be used to synthesize xanthone glucosides. For example, Zarena et al. used enzyme catalysis to achieve glycosylation of α−mangostin (193) in a supercritical carbon dioxide system [134], and Sohng completed the diversified glycosylation of 193 by a one-pot enzymatic catalysis [135]. In addition, Kim and coworkers modified 1 with glucansucrase to obtain the disglycation product mangiferin-(1→6)-α-D-glucopyranoside (194), thus improving the activity and solubility of mangiferin [24].

Conclusions and Outlook
In this review, we summarized 160 xanthone glucosides, of which xanthone O-glucoside was the most abundant (136 included). These compounds are derived from a variety of sources, with mangiferin being the most widely distributed and having the most investigated pharmacological activities. There was no significant difference in bioactivity between glucosylxanones and xanthones, but glycosylation can usually improve bioactivity.
We reviewed 93 monosaccharide xanthone glucosides and 66 disaccharide xanthone glucosides. Disaccharide xanthone glucosides are composed primarily of two glucose or glucose and xylose sugars, with a small amount of glucose combined with rhamnose, apiose, or arabinose. In terms of sugar binding sites, xanthone C-glucosides have glucosyl groups primarily at C-2, whereas xanthone O-glucosides have glucosyl groups primarily at C-1. Hydroxyl and methoxy groups are the most common substituents on the xanthone skeleton. Only two compounds out of 160 contain a methyl group (159 and 160). Prenylated xanthone glucosides are also extremely rare and have only been discovered in lichens (142-157). With the exception of a few examples containing tetrahydroxanthones, xanthone glucosides all have a xanthone skeleton (80)(81)(109)(110)(111)(112)(113)(114).
Despite the fact that a number of xanthone glucosides have been discovered, the medicinal study and health benefits of this type of compound have largely been limited to mangiferin. Synthesis and structural modification based on xanthones and glucosyl groups are also underdeveloped. Future research could concentrate on the synthesis of xanthone glucoside derivatives and the investigation of their pharmacological activities.