Thirteen New Xanthone Derivatives from Calophyllum caledonicum (Clusiaceae)

An EtOAc extract of the stem bark of Calophyllum caledonicum (Clusiaceae) yielded thirteen new hydroxylated and/or prenylated xanthone derivatives, namely 5-hydroxy-8-methoxyxanthone (1), 3,5-dihydroxy-1,2-dimethoxyxanthone (2), 1,8-dihydroxy-6,7-dimethoxyxanthone (3), 5,7-dihydroxy-2,6-dimethoxyxanthone (4), 6,8-dihydroxy-3,7-dimethoxy-xanthone (5), 2,5,6,7,8-pentahydroxyxanthone (6), 1,3,8-trihydroxy-5,7-dimethoxyxanthone (7) and according to a previously adopted nomenclature [3], caledonixanthone G-L. (8-13). The structural elucidation of 1-13 were mainly established on the basis of 1D and 2D NMR and HRMS spectroscopic analysis.


Introduction
The genus Calophyllum (Guttiferae) is already known as a rich source of oxygenated and prenylated polyphenol derivatives belonging to the coumarin or the xanthone types of secondary metabolites [1][2][3][4][5]. Among 187 known species, Calophyllum caledonicum Vieill. is an endemic one restricted to New-Caledonia where it is locally used by natives as a diuretic. We have already shown the presence of numerous xanthones in C. caledonicum [3][4][5], some of them exhibiting quite interesting antifungal activities [4]. In our continuing search for novel biologically active compounds, we thus have carried on our phytochemical study of this species and we report hereafter on the isolation and the structure determination of new xanthones 1-13 (Schemes1 and 2).

Results and discussion
The molecular formula of 1 (C 14 H 10 O 4 ) was established by HRMS(EI) analysis of its molecular ion at m/z 242.0579 (Calcd. 242.0576). The UV spectrum of 1 showed maxima at 230, 256, 304 and 363 nm similar to those of hydroxylated xanthones [6]. The 1 H-NMR spectrum (Table 1) showed the presence in the molecule of a 1,2-disubstituted benzene ring [δ H 8.34 (1H,dd,J = 8.0,1.5 Hz),7.39 (1H,dd,J = 8.0,8.0 Hz),7.71 (1H,ddd,J = 8.5,8.0,1.5 Hz) and 7.46 ppm (1H,d,J = 8.5 Hz)], a methoxyl group (δ H 3.99 ppm), a hydroxyl group (δ H 5.51 ppm) and two ortho-coupled aromatic protons [δ H 7.29 (1H, d, J = 9.0 Hz) and 6.74 ppm (1H, d, J = 9.0 Hz)]. The substitution pattern of the xanthone was finally deduced from the HMBC spectrum of 1, which was thus identified as 5-hydroxy-8-methoxyxanthone. It should be noticed that though 1 has already been synthesized [7], this is the first report on the isolation of this compound from a natural source. Compounds 2-7 were then firmly identified from the same kind of spectroscopic data sets (see Experimental and Tables 1 & 2) since these xanthones only differed from 1 in their number of hydroxyl or methoxyl groups and/or the substitution patterns of their aromatic rings.

Scheme 2.
The HRMS(EI) of 9 showed the molecular ion at m/z 312.1005 (Calcd. 312.0998) and the molecular formula to be C 18 H 16 O 5 . As for 8, the 1 H-NMR and HMBC spectra of 9 were indicative of a 5,6-disubstituted xanthone. The 1 H-NMR spectrum of 9 further showed the presence of a gem-dimethyl function [δ H 1.35 and 1.40 ppm (3H each, s), H-12 and H-13] whereas the characteristic deshielded protons of a dihydrogenated dihydroxypyran ring resonated as two cis coupled doublets (J = 5.0 Hz) at δ H 5.53 and 4.50 ppm. In the HMBC spectrum of 9, the quaternary oxygen-bearing carbon of this pyran ring (δ c 71.9 ppm) both correlated with H-12 and H-13 on one hand and the aforementioned gemdimethyl protons on the other hand. These elements thus characterized a 3,4-dihydroxy-2,2-dimethyl chroman moiety and the structure of this xanthone which we have named caledonixanthone H (9) was further confirmed by long-range proton-carbon correlations. Compound 9 was here isolated as a racemate since neither optical rotation nor CD effects could be measured for this compound.
Caledonixanthone I (10) was then readily identified since this compound showed UV, NMR and MS data very similar to those of caledonixanthone H, 9. The only significant differences in the 1 H-and 13 C-NMR spectra of 9 and 10 ( Table 3) were observed for the signals assignable to the chroman ring.
Indeed, a coupling constant of 8.0 Hz between H-12 and H-13 there indicated that caledonixanthone I (10) was the trans isomer of 9.
Compound 11 or caledonixanthone J had the molecular formula C 18 H 16 O 6  In the HMBC spectrum of 11, one of these ortho-coupled protons (δ H 7.78 ppm) was correlated to the carbonyl of the xanthone (C-9 at δ C 183.5 ppm). These results indicated that 11 had a 1-hydroxy-5,6-disubstituted xanthone structure by comparison with 9, The 1 H-NMR and HMBC spectrum of 11 showed the presence 3,4-cisdihydroxy-2,2-dimethylchroman in the molecule (Table 4). Further inspection of the HMBC data of 11 finally allowed us to firmly identify this compound as caledonixanthone J.
The HRMS(EI) revealed the molecular formula C 18 H 16 O 5 for 12 (M + at m/z 312.1013). The UV spectra of 12 (see Experimental) were those of a xanthone bearing hydroxyls at C-1 and either C-3 or C-6. The 1 H-NMR spectrum of 12 showed two AB spins systems at δ H 6.08 (1H, d, J = 2.0 Hz) and 6.17 ppm (1H, d, J = 2.0 Hz)] one the one hand and δ H 7.13 (1H, d, J = 9.0 Hz) and 7.29 ppm (1H, d, J = 9.0 Hz) on the other hand. From these elements, ring A was then identified as a 1,3-dihydroxy benzene system, while it was established that ring B was disubstituted by one hydroxyl and had one other substituent at C-7 and C-8. Furthermore, typical signals of a prenyl group [δ H 4.18 (2H, d, J = 6.5 Hz), 5.31 (1H, d, J = 6.5 Hz), 1.81 (3H, s) and 1.61 ppm (3H, s)] were also observed in the 1 H-NMR spectrum of 12. The relative location of this prenyl and the hydroxyl on ring B were then deduced by interlocking different HMBC correlations, i. e. the correlations observed between the aromatic proton at δ H 7.29 ppm (H-7) and C-10a (δ C 151.9 ppm) and C-8 (δ C 128.8 ppm), as well as the ones existing between the methylene protons of the isoprenyl (δ H 4.18 ppm) and C-7 (δ C 152.1 ppm), C-8 (δ C 128.8 ppm) and (C-8a δ C 119.6) ppm) and. Accordingly, the hydroxyl the prenyl substituents of ring B were located at C-7 and C-8, respectively. This way, the structure of the so-called caledonixanthone K was concluded to be 12.

General
Melting points were determined on a Electrothermal 8100 melting point apparatus and were uncorrected. Optical rotations were measured on a Schmidt-Haensch-polartronic-I polarimeter. UV spectra were taken on a Hitachi U-2000 spectrophotometer. HREIMS were recorded on Varian MAT 311 spectrometer at 70 eV for EI data and on a JMS-700 spectrometer with PEG matrix for APCI data. NMR spectra were recorded in CDCl 3 solutions on Jeol GSX 270 WB FT and Bruker AMX 500 (2-D experiments) instruments using TMS as internal standard. Si gel 60 (Macherey-Nagel, 230-400 mesh) was used for column chromatography, precoated Si gel plates (Macherey-Nagel, SIL G/UV254, 0.25 mm) were used for preparative TLC. The compounds were detected by UV at 254 and 366 nm.

Plant Material
The stem bark of Calophyllum caledonicum was collected from the "Rivière bleue" area, New Caledonia, during September 1997. A herbarium specimen is deposited at the Laboratoire des Plantes Medicinales, CNRS, Noumea, New Caledonia, under reference LIT 0315.