Synthesis of 1,3,4,8-Tetraoxygenated Xanthones

Abstract

Six 1,3,4,8-tetraoxygenated xanthones are synthesized. The possibility of a 1,3,4,8-tetraoxygenated pattern for two xanthones from Centaurium linarifolium is disproved. New structures are proposed for these two xanthones as 1,3,5-trihydroxy-2-methoxyxanthone, already cited in the literature, and 1,3-dihydroxy-2,5-dimethoxyxanthone, not previously cited.During the past 30 years, a large number of xanthones have been isolated from higher plants and their natural occurrence and synthesis have gained considerable importance (1). In addition, in several cases the medical properties of plants have been attributed to their xanthone constituents ^and several pharmacological studies have been done on them (2).Centaurium. linarifolium (Lamark) G. Beck (Gentianaceae), a plant commonly found in eastern Spain, has been used in folk medicine as a digestive, an antipyretic, and a blood circulation stimulant (3). Several new xanthones were isolated from this plant, and two alternative structural proposals were advanced on the basis of spectral data for two of them (4). These were 1,3,8-trihydroxy-2-methoxy-or 1,3,8-trihydroxy-4methoxyxanthone and 3,8-dihydroxy-1,2-dimethoxy-or 3,8-dihydroxy-1,4-dimethoxyxanthone.No naturally occurring 1,3,4,8-tetraoxygenated xanthones have been described before, although the synthesis of 1,4,8-trihydroxy-3-methoxy (5) and 1,3,4,8-tetramethoxyxanthone (6) has been described. In a previous report we have discarded the 1,2,3,8-tetraoxygenated pattern for the xanthones from Centaurium linarifolium (7), and in the present paper six 1,3,4,8-tetraoxygenated xanthones have been synthesized in order to confirm the alternative structures. A systematic study of their spectral data, summarized in Tables 1,2, and 3, will be very valuable in the assignment of the correct structures for new 1,3,4,8-tetraoxygenated xanthones.For unambiguous assignment of the 13C-nmr signals from unsubstituted carbon atoms, 1H-13C correlation experiments were performed. This allowed the assignment

Synthesis of Xanthones: An Overview

“…Table 2 presents natural products whose syntheses have been reported in the last twenty years by the reasons stated above. Nevertheless, in the 80s and 90s, several studies still reported syntheses of xanthones as a methodology to assign unambiguously the structures of isolated compounds [174][175][176]. For previous studies, considering the synthesis of phytoxanthones, see references [8,9].…”

Tetraoxygenated naturally occurring xanthones

“…In a review of Hypericum genus, Kitanov and Blinova (1987) listed only ®ve simple tetraoxygenated xanthones; two years later, in a review of xanthones from 1-Hydroxy-2,3,7-trimethoxyxanthone (4) F. caroliniensis [313] Halenia elliptica [38,87] Veratrilla baillonii [348] 1-Hydroxy-3,4,5-trimethoxyxanthone (5) Frasera albicaulis [314] 1-Hydroxy-3,4,7-trimethoxyxanthone (6) Frasera albicaulis [314] F. albomarginata [90] 1-Hydroxy-3,5,6-trimethoxyxanthone (7) Canscora decussata [108] Centaurium erythraea [190] Hoppea dichotoma [110] (Haploxanthone) Calophyllum apetalum [165] Calophyllum bracteatum [312] C. inophyllum [171] C. tomentosum [28] C. trapezifolium [128] Centaurium linarifolium [114] Kayea stylosa [131] Monnina obtusifolia [283] Pentadesma butyracea [81] Tovomita pyrifolium [245] 1,3,5-Trihydroxy-6-methoxyxanthone (68)…”