Caloxanthones O and P: Two New Prenylated Xanthones from Calophyllum inophyllum

Chemical investigation of the EtOH extract of the twigs of Calophyllum inophyllum collected in Hainan Province of China resulted in the isolation of two new prenylated xanthones, caloxanthone O (1) and caloxanthone P (2). Their structures were elucidated by a study of their physical and spectral data. Compound 1 exhibited cytotoxicity against human gastric cancer cell line (SGC-7901), with an IC50 value of 22.4 μg mL−1.


Results and Discussion
Compound 1, was obtained as a yellow powder that reacted positively to the Gibbs and FeCl 3 reagent, indicating the presence of phenolic groups. Its HR-ESI-MS spectrum showed the quasimolecular [M+Na] + ion peak at m/z 467.1681 (calc. 467.1676), corresponding to the molecular formula C 24 H 28 O 8 . This formula could also be validated through its 1 H-NMR, 13 C-NMR and DEPT data. The IR spectrum displayed free hydroxyl (3,560 cm −1 ), chelated hydroxyl (3,160 cm −1 ), conjugated carbonyl (1684 cm −1 ), and aromatic ring (1,589, 1,540 cm −1 ) absorptions. These data, together with those obtained from the UV spectrum [λ max (MeOH) 217, 222, 252, 282, 321 nm] were consistent with the presence of a xanthone skeleton [4,5]. In the 1 H-NMR spectrum, a chelated hydroxyl group δ H 13.73 (s, 1H), two isolated aromatic protons δ H 6.19 (s, 1H), 6.78 (s, 1H) and one methoxyl (δ H 3.84) were observed. The 1 H-NMR spectrum of 1 also showed two methyl singlets (δ H 1.25 and δ H 1.55), a methyl doublet (δ H 1.36, J = 6.5 Hz) and one-proton quartet (δ H 4.56, J = 6.2 Hz), which suggested the presence of an α,α,β-trimethyldihydrofuran ring [4]. Furthermore, two methyl singlets (δ H 1.15, δ H 1.17), an oxymethine proton (δ H 3.41, 1H, dd, J = 6.3, 9.6 Hz ， ) and two methylene protons (δ H 2.62 and 3.78, each 1H) were observed in the 1 H-NMR spectrum, which indicated the presence of a C 5 unit characterized as a 2,3-dihydroxy-3-methylbutyl chain in 1 [13]. Assignment of the 13 C-NMR spectral data are shown in Table 1. A combination of the 1 H-1 H COSY and HMQC experiments permitted the assignment of all the protonated carbons. It remained to establish the positions of the substituents on the xanthone skeleton. In the HMBC spectrum (Figure 2), the proton of chelated hydroxyl group 1-OH (δ H 13.73) was correlated to three carbons C-9a (δ C 102.7), C-2 (δ C 92.9), and C-1 (δ C 163.5), which suggested that the chelated hydroxyl group was located at C-1. Aromatic carbons with an oxygen function were observed at C-1 (δ C 163.5), C-3 (δ C 164.9) and C-4a (δ C 151.3) in the 13 C-NMR spectrum, which indicated that this aromatic ring was a phloroglucinol ring [14]. Therefore, the α,α,βtrimethyldihydrofuran ring was clearly fused at C-4 through an oxygen at C-3 position. This result was further confirmed by the long-range correlations of H-12 (δ H 1.25) and H-13 (δ H 1.55) with C-4 (δ C 112.0) and H-14 (δ H 4.56) with C-3 (δ C 164.9) in the HMBC spectrum. The positions of the 2,3-dihydroxy-3-methylbutyl chain and the remaining phenolic hydroxyl group were established as follows. In the HMBC spectrum (Figure 2), the methylene protons H 2 -16 (δ H 3.78 and δ H 2.62) were correlated to three aromatic carbons C-8a (δ C 110.8), C-7 (δ C 117.7), and C-8 (δ C 139.4). The latter resonance at C-8 also gave cross-peak with the oxymethine proton H-17 (δ H 3.41). These results demonstrated clearly that the 2,3-dihydroxy-3-methylbutyl moiety was located at C-8. The cross-peaks from the aromatic proton (δ H 6.78) to C-16 (δ C 36.7), C-8a (δ C 110.8), and C-5 (δ C 132.8) in the HMBC spectrum indicated that the aromatic proton (δ H 6.78) was assigned at C-7 position. The methoxyl group was deduced to be located at C-5 position by ROESY experiment revealing the crosspeak from the 5-OMe (δ H 3.84) to H-13 (δ H 1.55) (Figure 2). The downfield shifts of C-6 (δ C 155.3) and C-18 (δ C 71.9) revealed that each carbon should be connected to a hydroxyl group. The configuration at C-17 has not been determined in this study because the amount of compound 1 is too low. On the basis of the above results, the structure of compound 1 was thus elucidated and named caloxanthone O.    Figure  2). The configuration at C-17 has not been determined in this study because the amount of compound 2 is too low. From the above evidence, the structure of the compound 2 was thus confirmed, and given the trivial name caloxanthone P. Compounds 1 and 2 were evaluated for their cytotoxic activity against human gastric cancer cell line (SGC-7901) using the MTT method. Compound 1 showed cytotoxic activity against the SGC-7901 cell line with the IC 50 value of 22.4 μg mL -1 , while compound 2 was inactive (IC 50 > 100 μg mL -1 ).

General
Melting points were obtained on a Beijing Taike X-5 stage apparatus and are uncorrected. Optical rotation was recorded using a Rudolph Autopol III polarimeter (Rudolph Research Analytical, NJ, USA). The UV spectra were measured on a Shimadzu UV-2550 spectrometer. The IR spectra were obtained on a Nicolet 380 FT-IR instrument, as KBr pellets. The NMR spectra were recorded on a Bruker AV-400 spectrometer, using TMS as an internal standard. The HRESIMS spectra were measured with an API QSTAR Pulsar mass spectrometer. Column chromatography was performed with silica gel (Marine Chemical Industry Factory, Qingdao, China) and Sephadex LH-20 (Merck). TLC was preformed with silica gel GF254 (Marine Chemical Industry Factory, Qingdao, China).

Plant Material
The twigs of Calophyllum inophyllum L. were collected in Wenchang county, Hainan Province, China in May 2006, the plant was identified by Associate Professor Zheng-Fu Dai of the Institute of Tropical Bioscience and Biotechnology, Chinese Academy of Tropical Agricultural Sciences, where a voucher specimen (No. 20060508) of C. inophyllum was deposited.

Extraction and Isolation
The dried and crushed twigs of C. inophyllum (19.9 kg) were extracted with 95% EtOH (70 L) three times at room temperature. After removal of EtOH by evaporation, the EtOH extract was suspended in water (6.0 L) and successively partitioned with petroleum ether (4 L) three times to give a Petrosoluble extract (290.0 g) and an aqueous residue. The Petro-soluble extract (290.0 g) was applied to a silica gel (200-300 mesh) column packed in CHCl 3 . The column was then eluted in gradient elution with CHCl 3 -acetone (1:0, 20:1, 10:1, 5:1, 2:1, 0:1) to afford 16 fractions. The active fraction (fraction 16, 44.0 g) was then subjected to repeated column chromatography over silica gel using Pet-EtOAc as eluent and further separated by column chromatography over Sephadex LH-20 using 95% EtOH as eluent to afford 1 (5.6 mg). The 95% MeOH fraction (245.0 g) was subjected to vacuum liquid chromatography (VLC) over silica gel, eluting with gradient elution CHCl 3 -MeOH to afford 10 fractions. The active fraction (fraction 3, 5.7 g) was then subjected to repeated column chromatography over silica gel using Pet-acetone as eluent and further separated by column chromatography over Sephadex LH-20 using 95% EtOH as eluent to afford 2 (6.2 mg).  Table 1.

Bioassay
The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was performed according to the previously reported method [15]. The inhibition rates (IR%) were calculated using OD mean values from IR% = (OD control − OD sample )/OD control . The IC 50 value, which was defined as the concentration of sample needed to reduce 50% of absorbance relative to the vehicle-treated control, was determined using the Bliss method. The same experiment was repeated independently three times to obtain a mean IC 50 and its standard deviation.

Conclusions
It was reported that a wide variety of bioactive natural products exist in the plant Calophyllum inophyllum, such as pyranocoumarins, triterpenes, and flavonoids, together with xanthones, especially prenylated xanthones. In our continuous search for bioactive constituents, two new prenylated xanthones, named caloxanthones O (1) and P (2) were obtained. Compound 1 showed cytotoxic activity against the SGC-7901 cell line with the IC 50 value of 22.4 μg mL -1 , while compound 2 was inactive (IC 50 > 100 μg mL -1 ).