Three novel prenylated xanthones, 1-3, have been isolated from the root bark of Carcinragerrardii HARVEY ex T. R. SIM (G. natalensis SCHLECHTEK; G. transuaalensis BUKTT DAVY; Guttiferae). Structure elucidation was achieved by a combination of one-and two-dimensional NMR spectroscopic techniques, including long-range HETCOR, mass spectroscopy, and chemical methods. Garcigerrin A (1) and B (2) are pyrano derivatives of 3. They all show a rare 1,4,5 oxygenation pattern of the xanthone nucleus. Xanthone 3 is fungicidal against Cladosporium cucumerinum.
Introduction.
~ Garcinia gerrardii HARVEY (Guttiferae) is a large shrub or small tree 4 to 5 m in height, but sometimes reaching 10 to 13 m. It occurs in evergreen forest and on mountain outcrops and is endemic to South Africa and Swaziland [I] [2]. The fruit is edible. G. gerrardii, which is investigated phytochemically for the first time here, belongs to the largest genus of the tropical family Guttiferae (excluding the Hypericaceae). The genus contains ca. 400 species and is a significant source of xanthones [3-51. Dioxygenated, tetraoxygenated, and various prenylated xanthones have been isolated [4]. Benzophenones, biflavonoids, and condensed tannins are also a major feature of some of the species [6]. In addition, gum and seed oil of some plants yield fatty acids and their esters.
The most studied species of the genus are C. mangostana and G . kola. Investigations on G.mangostana have resulted in the isolation of anthocyanin glycosides, a benzophenone, maclurin, and several xanthones [7]. One of these, mangostin and four of its derivatives have antimicrobial activity [8]. The seeds of G. kola contain flavonoids, antihepatotoxic biflavonoids, xanthones [9], and kolanone, a polyisoprenylated benzophenone with antimicrobial properties [ 101.
During our study of different Guttiferae species, it was found that the root bark extracts of G. gerrardii exhibited fungicidal activity. Here we report the isolation of the major xanthonoid constituents.
Results. -The root bark of G.gerrardii, collected in South Africa, was extracted successively with CH,Cl, and MeOH. In a TLC bioassay [ 111, the CH,Cl, extract was fungicidal to Cladosporium cucumerinum, a plant pathogenic fungus. Flash chromatography of the orange-brown gummy C H Q , extract on silica gel gave 6 fractions.
Subsequent Sephadex-LH-20 and low-pressure liquid chromatography on RP-8 and RP-18 adsorbents yielded three yellow compounds, garcigerrin A (l), garcigerrin β¬3 (2), and 12b-hydroxy-des-D -garcigerrin A (3).