Xanthones are widely distributed in the genus Garcinia (Guttiferae), which provides compounds with a wide range of pharmacological effects such as cytotoxic, anti-bacterial, antioxidant and anti-human immunodeficiency virus (HIV) activities.1-3) The diverse substitution patterns of xanthones provide a good basis for different biological activity and for structure-activity relationship studies.
4)a-Mangostin (1), the dominating constituent of the fruit hulls of Garcinia mangostana, together with b-mangostin (2) and g-mangostin (3) (Fig. 1), have exhibited various biological activities, such as cytotoxicity in different cell lines together with anti-oxidative, anti-inflammatory, and anti-bacterial activities.5) As examples, inhibition of the cell proliferation of DLD-1 cells by 1 was observed at concentrations above 2 mM with an IC 50 value at 7.5 mM.6) This value corresponds to the activity observed in HL-60 cells.7) 3 seems to have a stronger effect compared with 1 and 2 8) but this is not confirmed by all reports. [7][8][9] Besides their slightly different potencies, three mangostins (1-3) differ in their mechanism of action. 3 caused accumulation of the cells in S-phase, whereas the cells treated with 1 and 2 showed G 0 /G 1 accumulation. Furthermore, the three mangostins (1-3) showed different expression profiles for proteins involved in the cell cycle regulation.8) The cytotoxic activity of 1 is only partly explained by apoptosis, as higher concentrations are needed for apoptosis than necessary for cytotoxicity.6) Apoptosis seems to be caused by 1-induced dysfunction of mitochondria.
10)Beside 1-3, several other xanthones have been tested for their cytotoxic activity and a structure-activity relationship can partly be elucidated. In all such experiments, the mangostins showed the strongest effect.7-9) Analysis of the cytotoxic effect of 6-methoxy-b-mangostin 8) indicated that the free hydroxyl group at C-6 was necessary for the cytotoxic activity. Furthermore, the apoptosis activity also depends on the number of free hydroxyl groups. 7,9,10) Pyranoxanthones, produced naturally or synthetically from 1 by acid-catalysed cyclisation of the prenyl group, have been found to have reduced cytotoxic potency compared with 1. 9) For 3,6-di-O-alkylated a-mangostin, the inhibitory effects on antifungal activity were reduced dramatically as compared with a-mangostin.11) However, the cytotoxic effects of these derivatives on MCF-7 cells (human breast cancer cells) or DLD-1 (human colon cancer cells) have not been investigated yet.In this paper, we report the isolation of two new geranylated xanthones (4, 5) and five known compounds (2, 6-9) from the bark of Garcinia oliveri PIERRE, a Vietnamese tall tree whose young buds and sour fruit are used for soup cooking. However, no use in folk medicine nor identification of specific compounds has been previously reported.12) Furthermore, conversion of a-mangostin into O-alkylated a-mangostin derivatives (10-17) is described. Cytotoxic activity for compounds 1-3 and 6-17 was analy...