Soulamarin, a New Coumarin from Stem Bark of Calophyllum soulattri

Abstract

The extracts of the stem bark of Calophyllum soulattri gave a new pyranocoumarin, soulamarin (1), together with five other xanthones caloxanthone B (2), caloxanthone C (3), macluraxanthone (4), trapezifolixanthone (5) and brasixanthone B (6) one common triterpene, friedelin (7), and the steroidal triterpene stigmasterol (8). The structures of these compounds were established based on spectral evidence (1D and 2D NMR).

1. Introduction

The genus Calophyllum comprises over 200 species of trees and shrubs native to tropical Asia, East Africa, India and Australia. Calophyllum species have been used in traditional Chinese folk medicine for the treatment of wounds, inflammation and rheumatism. Phytochemical studies on Calophyllum species have revealed the presence of xanthones [1,2,3], coumarins [4], triterpenoids [5] and flavonoids [6]. The coumarins have been reported to have displayed anti-HIV biological effects [7] and are used as cancer chemo-preventive agents [8]. Various xanthone derivatives show antifungal [9], antimicrobial [10] and molluscicidal [11] effects. We report here the isolation and characterization of a new pyranocoumarin, soulamarin (1), from the stem bark of Calophyllum soulattri.

2. Results and Discussion

Soulamarin (1) was isolated as yellowish oil from the hexane extract of the stem bark of Calophyllum soulattri. The HRESIMS displayed a negative molecular ion peak at m/z 387.1818 [M−H]⁻ indicating a molecular formula C₂₂H₂₈O₆. Compound 1 gave typical coumarin IR absorptions at 3,296 (OH), 2,930 (sp² and sp³ CH), 1,706 (C=O) and 1,621 (C=C) cm⁻¹. Maximum absorptions were observed at 314, 300, 274 and 268 nm in the UV spectrum [12].

The ¹H-NMR spectrum revealed the presence of one chelated hydroxyl proton signal at δ 12.46 (s, 1H), two vinylic proton signals at δ 6.60 (d, 1H, J = 10.1 Hz) and 5.46 (d, 1H, J = 10.1 Hz,) for H-9 and H-10 respectively, three methine signals at δ 2.53 (m, H-2'), 4.09 (m, H-3') and 3.80 (m, H-4) and three methylene signals at δ 1.26 (m, H-14), 1.38 (m, H-15) and 2.71 and 2.81 (both dd, J = 7.8 Hz) for H-3a and H-3b. Five methyl signals at δ 1.19 (d, J = 6.4 Hz), 1.27 (t, J = 3.4 Hz), 1.42 (s), 1.44 (s) and 1.49 (d, J = 6.4 Hz) were also observed. The ¹³C-NMR spectrum and DEPT experiment showed the presence of seven quaternary carbons at δ 78.2 (C-11), 102.1 (C-5), 102.9 (C-7), 110.9 (C-4a), 157.0 (C-6), 159.6 (C-8) and 159.7 (C-8a), five methines at δ 25.4 (C-4), 45.8 (C-2'), 79.0 (C-3'), 115.8 (C-9), 125.8 (C-10), three methylenes (δ 39.5, 28.4, 29.8 for C-3, C-14 and C-15, respectively) and five methyls (δ 10.4, 19.3, 19.7, 28.3 × 2 for C-5', C-16, C-4' and C-12 and C-13, respectively). The ¹³C-NMR spectrum also indicated a carbonyl (δ 179.1) and an aliphatic ketone (δ 199.4).

The structure was further elucidated by HMBC spectral analysis after the assignment of the protons to their direct bonding carbons by the HMQC spectrum. The low field chelated proton at δ 12.46 was due to hydrogen-bonding with the carbonyl group (δ 199.4, C=O) which resulted in the deshielding effect. The ³J and ²J connectivity of the chelated hydroxyl proton at δ 12.46 with δ 102.1 (C-5), 102.9 (C-7) and 157.0 (C-6) confirmed its location at C-6 (see Figure 1).

Figure 1. HMBC ²J and ³J correlations between ¹H and ¹³C in 1.

Figure 1. HMBC ²J and ³J correlations between ¹H and ¹³C in 1.

The HMBC experiment also demonstrated long-range ²J and ³J correlations between the two doublet protons at δ 5.46 (H-10, J = 10.1 Hz) and 6.60 (H-9, J = 10.1 Hz) with the carbon signal at δ 78.2 (C-11) respectively. The linkages between the two aliphatic methyls at δ 1.42 (H-12) and δ 1.44 (H-13) to the two carbon signals at δ 78.2 (C-11) and 125.8 (C-10) were also seen. These data together with the COSY spectrum analysis suggested the existence of a pyran ring. The pyran ring was clearly fused onto the non-oxygenated carbon C-7 and an oxygenated carbon C-8, as confirmed by the long range (³J) correlations of H-9 (δ 6.60) with C-8 (δ 159.6) and H-10 (δ 5.46) with C-7 (δ 102.9).

Meanwhile, the HMBC correlations of δ 1.19 (H-5') with δ 45.8 (C-2'), 79.0 (C-3') and 199.4 (C-1') proved that these methyl protons were located at position C-2'. The coupling of H-2' and H-3' in the COSY spectrum established the connectivity between δ 45.8 (C-2') and 79.0 (C-3'). The linkage between δ 1.49 (H-4') with δ 45.8 (C-2') and 79.0 (C-3') confirmed its position at C-4'.

An n-propyl substituent was found to be attached to C-4 via long range correlations of δ 1.26 (H-14) with δ 25.4 (C-4), 39.5 (C-3) and 110.9 (C-4a). The existence of the n-propyl group was also observed in the COSY spectrum through cross peaks between δ 1.26 (H-14) and 1.38 (H-15) and between δ 1.38 (H-15) and 1.27 (H-16). NOESY experiments predicted the relative configurations of soulamarin. Cross peaks were observed between H-3' and H-5' and between H-2' and H-4' suggesting the two proton pairs were oriented on opposite sides. Cross peaks between H-3b and H-4 and between H-3b and H-14 indicate these protons have similar orientations. The NOESY correlations for compound 1 are shown in Figure 2. Taken together, the evidence suggested that compound 1 is 6-hydroxy-4-propyl-5-(3-hydroxy-2-methyl-1-oxobutyl)-6",6"-dimethylpyrano-[2",3":8,7]-benzopyran-2-one (Figure 3).

Figure 2. NOESY correlations between ¹H and ¹H in 1.

Figure 2. NOESY correlations between ¹H and ¹H in 1.

3. Experimental

3.1. Plant Material

The stem bark of Calophyllum soulattri was collected from the Sri Aman district in Sarawak, Malaysia. This plant was identified by Dr. Rusea Go from the Department of Biology, Faculty of Science, Universiti Putra Malaysia.

3.2. General

EIMS were recorded on a Shimadzu GC-MS model QP2010 Plus spectrometer. NMR spectra were obtained using a JEOL 500 MHz FT-NMR spectrometer using tetramethylsilane (TMS) as an internal standard. Ultraviolet spectra were recorded in EtOH on a Shimadzu UV-160A, UV-Visible Recording Spectrophotometer. Infrared spectra were measured using the universal attenuated total reflection (UATR) technique on a Perkin-Elmer 100 Series FT-IR spectrometer.

3.3. Extraction and Isolation

Approximately 1 kg of air-dried stem bark of Calophyllum soulattri was ground to a fine powder and extracted successively in a Soxhlet apparatus with n-hexane (68-72 °C, 3 × 2 L) and dichloromethane (40 °C, 3 × 2 L) for 24 hours. The extracts were evaporated to dryness under vacuum to give 101.2 g of n-hexane extract and 15.3 g of dichloromethane extract. Part of each extract was subjected to column chromatography over silica gel and eluted with a stepwise gradient system of n-hexane, dichloromethane, ethyl acetate and methanol. Further purification of the n-hexane extract afforded the new coumarin soulamarin (1, 8 mg) and the triterpene friedelin (7, 450 mg). Compound 1 was isolated from the n-hexane:chloroform (1:4) eluate mixture, followed by several further purifications using a Chromatotron® (Harrison Research) eluting with an n-hexane:chloroform (3:2) mixture. Soulamarin (1): Yellowish Oil; UV (EtOH) λ_max nm: 314, 300, 274, 268; IR ν_max cm⁻¹: 3296, 2930, 1706, 1621; EIMS m/z (rel. int.): 388 [M⁺] (9), 373 (53), 360 (42), 345 (100), 301 (14), 285 (13), 245 (9), 229 (24), 55 (11); HRESIMS: 387.1818 [M-H]⁻ (Calc’d. for C₂₂H₂₈O₆: 388.1886); ¹H-NMR (CDCl₃): δ 12.46 (OH-6, s), 6.60 (1H, d, J = 10.1 Hz, H-9), 5.46 (1H, d, J = 10.1 Hz, H-10), 4.09 (1H, m, H-3'), 3.80 (1H, m, H-4), 2.81 (1H, dd, J = 7.8 Hz, H-3a), 2.71 (1H, dd, J = 7.8 Hz, H-3b), 2.53 (1H, m, H-2'), 1.49 (3H, d, J = 6.4 Hz, H-4'), 1.44 (3H, s, H-13), 1.42 (3H, s, H-12), 1.38 (2H, m, H-15), 1.27 (3H, t, J = 3.4 Hz, H-16), 1.26 (2H, m, H-14), 1.19 (3H, d, J = 6.4 Hz, H-5'); ¹³C-NMR (CDCl₃): δ 199.4 (C-1'), 179.1 (C-2), 159.7 (C-8a), 159.6 (C-8), 157.0 (C-6), 125.8 (C-10), 115.8 (C-9), 110.9 (C-4a), 102.9 (C-7), 102.1 (C-5), 79.0 (C-3'), 78.2 (C-11), 45.8 (C-2'), 39.5 (C-3), 29.8 (C-15), 28.3 × 2 (C-12 & C-13), 25.4 (C-4), 19.7 (C-4'), 19.3 (C-14 & C-16), 10.4 (C-5'). Meanwhile, purification of the dichloromethane extract afforded the five known xanthones caloxanthone B (2, 12 mg), caloxanthone C (3, 14 mg), macluraxanthone (4, 6 mg), trapezifolixanthone (5, 10 mg), brasixanthone B (6, 21 mg) and stigmasterol (8, 22 mg) (Figure 3).

• Caloxanthone B (2). Yellow needles; M.P. 157–158 °C (lit. 160.5 °C [13]); Spectral data were consistent with published data [13].
• Caloxanthone C (3). Yellow needles; M.P. 210–212 °C (lit. 217 °C [14]); Spectral data were consistent with published data [14].
• Macluraxanthone (4). Yellow crystal; M.P. 174–175 °C (lit. 170–172 °C [13]); Spectral data were consistent with published data [13].
• Trapezifolixanthone (5). Yellow crystal; M.P. 171–172 °C (lit. 171–172 °C [15]); Spectral data were consistent with published data [15].
• Brasixanthone B (6). Yellow crystal; M.P. 227–229 °C (lit. 227–229 °C [16]); Spectral data were consistent with published data [16].
• Friedelin (7). White needles; M.P. 245–246 °C (lit. 246–248 °C [2]); Spectral data were consistent with published data [2].
• Stigmasterol (8). White needles; M.P. 155–157 °C (lit. 168–169 °C [17]); Spectral data were consistent with literature data [17].

Figure 3. Structures of soulamarin (1), caloxanthone B (2), caloxanthone C (3), macluraxanthone (4), trapezifolixanthone (5), brasixanthone B (6), friedelin (7) and stigmasterol (8).

Figure 3. Structures of soulamarin (1), caloxanthone B (2), caloxanthone C (3), macluraxanthone (4), trapezifolixanthone (5), brasixanthone B (6), friedelin (7) and stigmasterol (8).

4. Conclusions

The stem bark of Calophyllum soulattri furnished one new pyranocoumarin, soulamarin (1), together with five xanthones caloxanthone B (2), caloxanthone C (3), macluraxanthone (4), trapezifolixanthone (5) and brasixanthone (6), a common triterpene, friedelin (7), and the steroidal triterpene stigmasterol (8).