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5,9,11-Trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl)pyrano[2,3-a]xanthen-12(2H)-one from the Stem Bark of Calophyllum tetrapterum Miq.

1
Natural Products Chemistry Research Group, Organic Chemistry Division, Department of Chemistry, Faculty of Science and Technology, Universitas Airlangga, Surabaya 60115, Indonesia
2
Airlangga Health Science Institute, Universitas Airlangga, Surabaya 60115, Indonesia
*
Author to whom correspondence should be addressed.
Academic Editor: Ping-Jyun Sung
Molbank 2017, 2017(1), M936; https://doi.org/10.3390/M936
Received: 15 February 2017 / Revised: 17 March 2017 / Accepted: 18 March 2017 / Published: 21 March 2017
(This article belongs to the Section Natural Products)

Abstract

A new pyranoxanthone namely 5,9,11-trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl)pyrano[2,3-a]xanthen-12(2H)-one (1) was isolated from the stem bark of Calophyllum tetrapterum Miq. The structure of compound 1 was determined by means of spectroscopic methods including UV, IR, HRESIMS, 1D and 2D NMR.
Keywords: Calophyllum tetrapterum Miq.; pyranoxanthone; natural product Calophyllum tetrapterum Miq.; pyranoxanthone; natural product

1. Introduction

The genus Calophyllum (Clusiaceae) comprises over 200 species of trees and shrubs native to tropical Asia, East Africa and Australia. This genus is well known to be a rich source of bioactive xanthones [1,2,3,4], coumarins [5,6,7], chromanone acids [8,9,10,11], and flavonoids [12]. Some these were reported to exhibit of biological activities including anti-HIV, anticancer, antimalarial and antimicrobial [13,14].
In this paper, we report the chemical constituents of the stem bark of Calophyllum tetrapterum Miq. with the isolation of a new pyranoxanthone, 5,9,11-trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl)pyrano[2,3-a]xanthen-12(2H)-one (Figure 1). The anti-HIV activity of isolated compound from this plant is also reported.

2. Result and Discussion

5,9,11-Trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl)pyrano[2,3-a]xanthen-12(2H)-one (1) was isolated as yellow solid, m.p. 156–158 °C. The HRESIMS displayed a negative molecular ion peak at m/z 393.1352 indicating a molecular formula of C23H21O6 and 13 degrees of unsaturation (See supporting information, Figure S8). The UV spectrum exhibited four absorption bands characteristic of a xanthone chromophore at λmax 249, 260, 320 and 335 nm [3]. The IR spectrum showed absorptions bands at νmax 3436, 1652, and 1510 cm−1 indicating the presence of a hydroxyl, conjugated carbonyl and aromatic groups, respectively. The 1H-NMR (Table 1) spectrum showed the presence of the proton signals of a pair of doublets (J = 2.2 Hz) in the aromatic region at δH 6.20 and 6.33 (each 1H) and a singlet at δH 6.80, suggest that compound 1 is a typical for a xanthone with five substituents. The presence of a chelated hydroxyl group at δH 13.48 assignable to the signal of 11-OH. The 1H-NMR revealed the presence of a 2,2-dimethylpyrano group [δH 1.50 (6H, s, H2a/H-2b), 8.17 (1H, s, H-4) and 1,1-dimethylallyl group [δH 1.41 (6H, s, H-1′, 2′-CH3), 5.08 (1H, dd, J = 1.1; 10.6 Hz, H-4′a), 5.16 (1H, dd, J = 1.1; 17.5 Hz, H-4′b), 6.02 (1H, dd, J = 10.6; 17.5 Hz, H-3′)]. See supporting information in Figures S1 and S2. The 13C-NMR spectrum (APT experiment, Table 1) of 1 showed 21 carbon signals representing for 23 carbon atoms were observed. See Figures S3 and S4, supporting material.
The presence of long-range correlations in the HMBC spectrum of 1 between the proton signal of a chelated hydroxyl group (δH 13.48, 11-OH) was correlated with two quaternary carbons (δC 164.7; 103.9) and a methine (δC 98.7) carbon signals, showing that C-10 is unsubstituted. One of them, proton signal of aromatic region at δH 6.33 (J = 2.2 Hz) that showed long-range correlations with two oxyaril carbon signals (δC 165.4 (C-9); 158.1 (C-7a)), a quarternary (δC 103.9) and a methine carbon signals (δC 98.7), showing that δH 6.33 at C-8. Furthermore, proton signal of isolated aromatic (δH 6.80, s, H-6) has correlation with two oxyaril carbons [δC, 155.6 (C-5), 153.6 (C-6a)] and a quaternary carbon signals (δC 108.3, C-4a), which showed that 2,2-dimethylpyrano group were fused at C-4a and C-12b. The methine signal of vinyl group at δH 8.17 (H-4) on the 2,2-dimethylpyrano group showed long-range correlations with four quarternary carbons [δC 137.8 (C-3), 108.3 (C-4a), 80.4 (C-2), 42.7 (C-2′)]. This showed that C-3 bonded with 1,1-dimethylallyl group. This HMBC correlation is similar to the previous reported of xanthone in Calphyllum pseudomole [3]. Long-range correlations in HMBC consistent with the structure 1 are shown in Figure 2 and supporting information in Figure S5–S7. Based on the above spectral evidence, 5,9,11-trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl) pyrano[2,3-a]xanthen-12(2H)-one was established to have structure 1 which is a novel compound and. had not been reported yet.
On inhibition of human immunodeficiency virus type-1 reverse transcriptase (HIV-1 RT) against human lymphocytes in vitro, compound 1 exhibited IC50 values of 84.60 μg/mL. Those anti-HIV-1 RT data suggested that compound 1 has weak activity.

3. Experiment Section

3.1. General

Melting points were obtained on a Thermo Scientific Fisher-Johns Melting Point Apparatus 220 VAC (Waltham, MA, USA). NMR spectra were recorded on a JEOL 400 ECA spectrophotometer (Tokyo, Japan) in acetone-d6 at 400 (1H), 100 (13C) MHz using APT experiment with TMS as the internal standard. The UV spectra was measured with Shimadzu series 1800 spectrophotometer (Kyoto, Japan). The IR spectra was recorded with Perkin-Elmer spectrum-100 FT-IR (Waltham, MA, USA). The mass spectra was recorded using a Waters LCT Premier XE (Santa Clara, CA, USA). Coloumn chromatography and radial chromatography were carried out using silica gel 60 G Cat. No. 1.07734.1000 and Si gel 60 PF254 Cat. No. 1.07749.1000 (Merck, Darmstadt, Germany).

3.2. Plant Material

The stem bark of C. tetrapterum Miq. was collected in October 2015 from Lungkut Layang Village, District Kapuas, West Kalimantan, Indonesia. The sample was identified by Mr. Ismail Rachman, Herbarium Bogoriense, Center of Biological Research and Development, National Institute of Science, Bogor, Indonesia.

3.3. Extraction and Isolation

The dried stem bark of C. tetrapterum Miq. (2.0 kg) were macerated in 10 L methanol twice for 2 days each. After evaporating of the solvent in a rotary evaporator, it was obtained 260 g of pale brown semi-solid. Further, the methanol extract were partitioned first with n-hexane (1:1 v/v). The methanol extract was added with water (10% v/v) to increase the polarity and then partitioned with ethyl acetate (1:1 v/v). The ethyl acetate extract (35 g) was subjected to coloumn chromatography over silica gel and eluted with n-hexane-ethyl acetate (from 9:1 to 1:1) to give fractions A–C. Fraction B was then subjected further to coloumn chromatography and eluted with n-hexane-ethyl acetate (from 9:1 to 1:1) to produce subfractions B1–B3. Subfraction B3 was purified by planar radial chromatography using n-hexane-chloroform (from 3:7 to 7:3), chloroform and chloroform-ethyl acetate 9:1 to yielded compound 1 (10 mg).

3.4. Anti-HIV Reverse Transcriptase Activity

The anti-HIV-1 RT inhibition of compound 1 was evaluated at Institute of Tropical Desease, Universitas Airlangga by a non-radioactive immunocolorimetric assay [13].

4. Conclusions

A new pyranoxanthone compound, 5,9,11-trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl) pyrano[2,3-a]xanthen-12(2H)-one (1) was isolated from the stem bark of Calophyllum tetrapterum Miq. This compound showed inactive toward anti-HIV-1 RT.

Supplementary Materials

1H-NMR, 13C-NMR, HMQC, HMBC and HRESIMS spectra are reported in the supplementary materials as Figures S1–S8 and structure refinement parameters as Table 1.
Supplementary File 1Supplementary File 2Supplementary File 3Supplementary File 4

Acknowledgments

This research was funded by Airlangga Health Science Institute, Universitas Airlangga, 2017.

Author Contributions

Tjitjik Srie Tjahjandarie designed the whole experiment of bioactivity and wrote the manuscript. Mulyadi Tanjung researched data, analyzed the NMR and HRESIMS spectra and contributed to the manuscript, Ratih Dewi Saputri designed the whole experiment. All authors read and approved the final manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Structures of 5,9,11-trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl)pyrano[2,3-a]xanthen-12(2H)-one (1).
Figure 1. Structures of 5,9,11-trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl)pyrano[2,3-a]xanthen-12(2H)-one (1).
Molbank 2017 m936 g001
Figure 2. Selected HMBC correlations for 1.
Figure 2. Selected HMBC correlations for 1.
Molbank 2017 m936 g002
Table 1. Data NMR of 5,9,11-trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl)pyrano[2,3-a]xanthen-12(2H)-one in acetone-d6.
Table 1. Data NMR of 5,9,11-trihydroxy-2,2-dimethyl-3-(2-methylbut-3-en-2-yl)pyrano[2,3-a]xanthen-12(2H)-one in acetone-d6.
No. CδH (Mult, J Hz)δCHMBC
2-80.4-
2a1.50 (s, 3H)27.3C-2; C-2b
2b1.50 (s, 3H)27.3C-2; C-2a
3-137.8 -
48.17 (s, 1H)118.7C-2; C-3; C-4a; C-2′
4a-108.3-
5-155.6-
66.80 (s, 1H)103.0C-4a; C-5; C-6a
6a-153.6-
7a-158.1-
86.33 (d, 2.2, 1H)93.9C-7a; C-9; C-10; C-11a
9-165.4-
106.20 (d, 2.2, 1H)98.7C-8, C-11, C-11a
11-164.7-
11a-103.9-
12-183.1-
12a-122.8-
12b-149.9-
1′1.41 (s)28.6C-2′; C-3′, 2′-CH3
2′-42.7-
2′-CH31.41 (s)28.6C-1′; C-2′, C-3′
3′6.02 (dd, 10.6; 17.6, 1H)147.9C-1′; C-2′, 2′-CH3
4′5.16 (dd, 1.1; 17.5, 1H)
5.08 (dd, 1.1; 10.6, 1H)
112.3C-2′, C-3′
11-OH13.48 (s, 1H)-C-10; C-11; C-11a
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