Novel Meroditerpenoid-Related Metabolites from the Formosan Soft Coral
Nephthea chabrolii
Jyh-Horng Sheu,*,†Jui-Hsin Su,†Ping-Jyun Sung,‡Guey-Horng Wang,§and Chang-Feng Dai⊥ Department of Marine Resources, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, Republic of China, National Museum of Marine Biology and Aquarium, 2 Houwan Road, Checheng, Pingtung 944, Taiwan,
Republic of China, Center for General Education, Hsing-Kuo University, Tainan 709, Taiwan, Republic of China, and Institute of Oceanography, National Taiwan University, Taipei 106, Taiwan, Republic of China
Received June 9, 2004
Nine new metabolites, including one novel naphthoquinone derivative, chabrolonaphthoquinone A (1), four tetraprenyltoluquinol-related metabolites, chabrolohydroxybenzoquinones A-D (2-5), and four tetraprenyltoluquinone-related compounds, chabrolobenzoquinones A-D (6-9), were isolated from the organic extract of a Taiwanese soft coral, Nephthea chabrolii. The structures of 1-9 were elucidated on the basis of spectral data.
In previous studies a series of novel secondary metabo-lites, including cembranes and norditerpenes,1 polyhydroxy-steroids,2and sesquiterpenes,3-5have been isolated from the soft coral Nephthea chabrolii (Audouin). During the course of our investigation on new natural substances from Taiwanese marine invertebrates, we initiated a study on the chemical constituents of N. chabrolii, which has af-forded nine meroditerpene-derived metabolites. These include one novel naphthoquinone derivative, chabrolonaph-thoquinone A (1), four tetraprenyltoluquinol-related me-tabolites, chabrolohydroxybenzoquinones A-D (2-5), and four tetraprenyltoluquinones, chabrolobenzoquinones A-D (6-9). The structures of metabolites 1-9 were character-ized by spectral analysis.
Results and Discussion
A collection of N. chabrolii was homogenized with EtOH, filtered, and further extracted with EtOH. The combined extracts were concentrated and subsequently subjected to further purification to yield the new compounds, 1-9 (see Experimental Section).
Chabrolonaphthoquinone A (1) was obtained as an optically inactive yellow oil. The HREIMS of 1 established the molecular formula C27H32O4, implying 12 degrees of unsaturation. The EIMS showed peaks at m/z 420 (M)+and 374 (M - HCOOH)+, suggesting the presence of a carboxyl group. The UV spectrum showed absorptions at 343, 266, and 257 nm, indicative of a 1,4-naphthoquinone moiety.6 From the1H and13C NMR spectral data (Table 1), together with the HMQC data, 27 signals were assigned to four methyl, six sp3methylene, seven sp2methine, and seven sp2 quaternary olefinic carbons, two carbonyls, and a carboxyl group. The1H NMR spectrum of 1 also showed signals of seven olefinic protons (δ 7.96, d, J ) 8.0 Hz; 7.91, d, J ) 1.5 Hz; 7.53, dd, J ) 8.0, 1.5 Hz; 6.84, t, J ) 7.5 Hz; 6.81, d, J ) 1.5 Hz; 5.18, t, J ) 7.0 Hz; 5.12, t, J ) 7.5 Hz) and four methyls (δ 2.19, 3H, d, J ) 1.5 Hz; 1.68, 3H, s; 1.59, 3H, s; 1.56, 3H, s).
The planar structure and all of the1H and13C chemical shifts of 1 were elucidated by 2D NMR experiments,
especially the 1H-1H COSY and HMBC experiments (Figure 1). The proton sequence from H-1 to H-2 and the HMBC correlations from H-7′to C-4′, C-5′, C-6′; H-1 to C-1′, C-3′, C-3; H-2 to C-2′; H-20 to C-2′, C-4′, C-2; and H2-4 to C-2, C-3, C-20 suggested a naphthoquinone moiety in 1. This together with an EIMS peak at m/z 185 (C12H9O2)+ further revealed this moiety (10) to be an important structural unit of 1 (Scheme 1). The structure of the side chain was elucidated by the1H-1H COSY correlations from H2-4 to H-6; H2-8 to H-10; and H2-12 to H-14 and the HMBC correlations from H2-5 to C-3, C-7; H2-8 to C-7; H2-9 to C-7, C-11; H-10 to C-8, C-11; H2-12 to C-10, C-11, C-14; H3-16 to C-14, C-15; and H3-17 to C-14, C-15, and thus the * To whom correspondence should be addressed. Tel: +886-7-5252000,
ext. 5030. Fax: +886-7-5255020. E-mail: [email protected]. †National Sun Yat-Sen University.
‡National Museum of Marine Biology and Aquarium. §Hsing-Kuo University.
⊥National Taiwan University.
10.1021/np0401314 CCC: $27.50 © 2004 American Chemical Society and American Society of Pharmacognosy Published on Web 12/03/2004
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connectivity from C-4 to C-17 was fully established. The methyl group attached at C-7 was confirmed by HMBC correlations between H3-19/C-6, C-7, and C-8. The HMBC correlations between H-10/C-18 and H2-12/C-18 revealed the attachment of a carboxyl group at C-11. On the basis of these findings, the skeleton of 1 was unambiguously established. The geometries of the double bonds between C-6/C-7 and C-10/C-11 were shown to be E and Z, respec-tively, by comparison with data for related compounds.7,8 The abnormal downfield shift of H-10 (δ 6.84) probably arises from the strong anisotropic effect of the neighboring carboxyl group.
Chabrolohydroxybenzoquinone A (2) was obtained as a pale oil. The molecular formula C27H38O5for 2 was sug-gested by HREIMS data, which exhibited a peak at m/z 424.2614 [C27H36O4, M - H2O]+, and13C and 1H NMR spectral data (Tables 2 and 3). The structure elucidation and full assignment of1H and 13C NMR data of 2 were achieved by 2D NMR, including1H-1H COSY, HMQC, and HMBC experiments. The HMBC of 2 (Figure 1) possessed correlations between H-3′and C-1′, C-4′, C-5′; H-6′and C-1′, C-2′, C-4′; and H-7′and C-3′, C-4′, C-5′, C-6′, thus charac-terizing the 1,4-dihydroxy-5-methylbenzene subunit of 2. In addition, a1H-1H COSY correlation between H-1 and H-2 and the HMBC correlations from H-1 to C-1′, C-2′, C-3′, C-3; H-2 to C-2′, C-3, C-4, C-20; and H3-20 to C-2, C-3, C-4 were consistent with an isoprene unit attached to the aromatic moiety. The configuration of the double bonds in 2 was assigned according to the 1H NMR data. The Z geometry of the C-1/C-2 double bond was indicated by a 10.0 Hz coupling constant between H-1 and H-2. Compari-son of 1D and 2D NMR data, particularly the1H-1H COSY and HMBC correlations (Figure 1), showed that the partial structure of the side chain from C-5 to C-19 in 2 is very close to that of 1. The structure of compound 2 could be then further established.
Chabrolohydroxybenzoquinone B (3) was isolated as a pale oil with the molecular formula C28H40O6, which possesses nine units of unsaturation, as indicated by HREIMS (438.2768 m/z, [M - H2O]+) and NMR spectral data (Tables 2 and 3). The1H and13C NMR spectral data of 3 revealed that the structure of metabolite 3 should be very similar to that of 2. Also, comparison of the spectral data of both compounds showed that the carboxylic acid attached at the C-11 position of compound 2 was replaced by a carbomethoxyl group in 3.
HREIMS and NMR spectral data indicated that chabrolo-hydroxybenzoquinone C (4) has the same molecular for-mula, C27H38O5, as that of 2 (Tables 2 and 3). The1H NMR spectral data of 4 were found to be similar to those of 2, except that the signal of H-10 of 4 (δ 5.99) was shifted significantly to upper field in comparison with that of 2 (δ 6.87), and the methylene protons H2-9 (δ 2.59) were found to show downfield shifted resonance in comparison with that of 2 (δ 2.28). Thus, 4 was found to be the 10Z isomer of 2. Also, we isolated a metabolite, chabrolohydroxyben-zoquinone D (5), possessing structure similar to that of 4. The NMR spectral data of 5 (Tables 2 and 3) are almost identical with those of 4 except for the presence of an additional oxymethyl group (δH3.73, 3H, s) in 5. Also, the 13C NMR spectrum of 5 showed the same number of methylene, methine, and quaternary carbons as that of 4, except for the presence of one more oxymethyl carbon, which showed a signal at δC 51.1 (q). Furthermore, the oxymethyl protons gave an HMBC cross-peak with a carbonyl carbon (δ 168.5, s), indicating the presence of the carbomethoxyl group in 5. All of the data indicated that 5 is the methyl ester of 4.
The new metabolite chabrolobenzoquinone A (6) was obtained as a pale oil. Its molecular formula, C27H36O4, was established by HREIMS (m/z 424.2607, [M]+). Thus, 10 degrees of unsaturation were determined for 6. It displayed UV (λmax 251 nm) and IR (νmax 1657 and 1614 cm-1) absorptions characteristic of benzoquinones.7,9The1H NMR spectrum of 6 (Table 4) showed signals for two quinone protons (δ 6.59, d, J ) 1.5 Hz; 6.49, s) and four olefinic protons (δ 6.87, t, J ) 7.5 Hz; 5.16, t, J ) 7.0 Hz; 5.14, m; 5.13, m). The13C NMR spectra (Table 5) of 6 contained a total of 27 resonances for five methyl, seven methylene, Table 1. 1H and13C NMR Data for Compound 1
C/H δHa δCb 1′ 185.0 (C)d 2′ 130.3 (C) 3′ 132.1 (C) 4′ 185.9 (C) 5′ 148.0 (C) 6′ 6.81 d (1.5)c 135.7 (CH) 7′ 2.19 d (1.5) 16.5 (CH3) 1 7.96 d (8.0) 126.3 (CH) 2 7.53 dd (8.0, 1.5) 133.9 (CH) 3 148.8 (C) 4 2.78 t (7.5) 36.1 (CH2) 5 2.38 m 29.2 (CH2) 6 5.18 t (7.0) 123.6 (CH) 7 135.5 (C) 8 2.10 m 38.4 (CH2) 9 2.28 m 27.3 (CH2) 10 6.84 t (7.5) 144.9 (CH) 11 131.2 (C) 12 2.31 m 26.8 (CH2) 13 2.10 m 27.6 (CH2) 14 5.12 t (7.5) 123.6 (CH) 15 132.4 (C) 16 1.68 s 25.7 (CH3) 17 1.59 s 17.6 (CH3) 18 171.8 (C) 19 1.56 s 16.0 (CH3) 20 7.91 d (1.5) 126.4 (CH) aSpectra recorded at 500 MHz in CDCl 3.b125 MHz in CDCl3.
cJ values (in Hz) parentheses.dDeduced from DEPT.
Figure 1. Key1H-1H COSY and HMBC correlations for 1, 2, and 6.
Scheme 1. Mode of Fragmentation of 1 in the EMS
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and six methine groups and nine quaternary carbons, including a carbonyl group (δ 173.1, s) and two quinone carbonyls (δ 187.9, s; 188.3, s). The1H-1H COSY (Figure 1) showed correlations of H3-7′/H-6′and H-3′/H2-1, and the HMBC spectrum (Figure 1) showed long-range correlations from H2-1 to C-1′, C-2′, C-3′; H-3′to C-5′; H-6′to C-7′; and H3-7′ to C-4′, C-5′, C-6′ and established the 5′ -meth-ylquinone structural unit of 6 and an isoprene unit at-tached to the quinone moiety. The13C NMR chemical shift of the allylic methyl H3-20 (δ 16.0) established the E configuration of the C-2/C-3 double bond. Careful analyses of the NMR data (1H and 13C) showed that the partial structure of the side chain from C-5 to C-19 in 6 should be very similar to that of 2, and the structure of compound 6 was further established. Furthermore, a metabolite 7 (optically inactive pale oil) with a structure closely related to 6 was found. The molecular formula of chabrolobenzo-quinone B (7) was assigned as C28H38O4from the HREIMS.
The NMR data (Tables 4 and 5) of 7 were similar to those of 6. However, an additional methoxyl group (δH3.72, 3H, s; δC51.5, q) was observed in 7. In addition, the methoxyl group positioned at C-18 was confirmed by the connectivity between the oxymethyl (δH3.72) and the carbonyl carbon (δC168.4, s, C-18). All of the above information suggested that chabrolobenzoquinone B (7) is the methyl ester of 6. Compound 8 (chabrolobenzoquinone C), with a molecular formula of C27H36O4(HREIMS), was obtained as an opti-cally inactive oil. Careful comparison of its1H and13C NMR data (Tables 4 and 5) with those of 6 suggested that metabolite 8 is the 10Z isomer of 6. Similarly, HREIMS and 1H and 13C NMR data of compound 9, an optically inactive pale oil, revealed that this metabolite has the molecular formula C28H38O4. Comparison of its1H and13C NMR (Tables 4 and 5) with those of 7 also clearly indicated that 9 is the 10Z isomer of 7.
The biosynthetic pathways of the concerned meroditer-penoidal carboxylic acids were proposed as shown in Scheme 2. The oxidation of the 1,4-dihydroxybenzene unit of a proposed intermediate 11 would lead to the formation of 1,4-benzoquinone 6 and the following isomer 8 (pathway a). Isomerization of the 2,3-double bond of 11 to the 1,2-double bond and the subsequent hydroxylation at C-3 Table 2. 1H NMR Chemical Shifts for Compounds 2-5a
2 3 4 5 3′ 6.42 s 6.43 s 6.42 s 6.42 s 6′ 6.56 s 6.56 s 6.56 s 6.56 s 7′ 2.18 s 2.18 s 2.18 s 2.18 s 1 6.25 d (10.0)b 6.26 d (10.0) 6.25 d (9.5) 6.25 d (9.5) 2 5.53 d (10.0) 5.53 d (10.0) 5.53 d (9.5) 5.53 d (9.5) 4 1.64 m; 1.71 m 1.64 m; 1.71 m 1.64 m; 1.71 m 1.64 m; 1.71 m 5 2.12 m 2.12 m 2.13 m 2.12 m 6 5.15 t (7.5) 5.14 t (7.0) 5.14 t (7.0) 5.13 t (7.2) 8 2.08 m 2.06 m 2.06 m 2.04 m 9 2.28 m 2.25 q (7.5) 2.59 q (7.5) 2.50 q (7.5) 10 6.87 t (7.5) 6.72 t (7.5) 5.99 t (7.5) 5.84 t (7.5) 12 2.31 m 2.30 m 2.26 m 2.24 m 13 2.11 m 2.08 m 2.12 m 2.08 m 14 5.14 t (7.5) 5.13 t (7.0) 5.09 t (7.0) 5.08 t (7.2) 16 1.68 s 1.68 s 1.68 s 1.68 s 17 1.59 s 1.59 s 1.58 s 1.57 s 19 1.59 s 1.59 s 1.59 s 1.57 s 20 1.36 s 1.36 s 1.36 s 1.36 s COOMe 3.73 s 3.73 s aSpectra recorded at 500 MHz in CDCl
3.bJ values (in Hz) in parentheses.
Table 3. 13C NMR Chemical Shifts for Compounds 2-5a
2 3 4 5 1′ 146.6 (C)b 146.7 (C) 146.7 (C) 146.7 (C) 2′ 119.5 (C) 119.6 (C) 119.6 (C) 119.6 (C) 3′ 112.5 (CH) 112.4 (CH) 112.5 (CH) 112.4 (CH) 4′ 147.4 (C) 147.4 (C) 147.4 (C) 147.4 (C) 5′ 124.6 (C) 124.5 (C) 124.5 (C) 124.4 (C) 6′ 118.1 (CH) 118.1 (CH) 118.1 (CH) 118.1 (CH) 7′ 15.9 (CH3) 15.9 (CH3) 15.9 (CH3) 15.9 ((CH3) 1 122.5 (CH) 122.5 (CH) 122.4 (CH) 122.4 (CH) 2 129.7 (CH) 129.7 (CH) 129.7 (CH) 129.8 (CH) 3 77.9 (C) 77.9 (C) 77.9 (C) 77.9 (C) 4 40.8 (CH2) 40.8 (CH2) 40.9 (CH2) 40.9 (CH2) 5 22.6 (CH2) 22.7 (CH2) 22.6 (CH2) 22.6 (CH2) 6 125.1 (CH) 125.0 (CH) 124.9 (CH) 124.7 (CH) 7 133.9 (C) 134.1 (C) 134.3 (C) 134.5 (C) 8 38.3 (CH2) 38.5 (CH2) 39.0 (CH2) 39.1 (CH2) 9 27.4 (CH2) 27.2 (CH2) 28.1 (CH2) 28.0 (CH2) 10 145.3 (CH) 142.7 (CH) 145.7 (CH) 142.1 (CH) 11 131.2 (C) 131.8 (C) 130.4 (C) 131.4 (C) 12 26.7 (CH2) 27.0 (CH2) 34.5 (CH2) 34.7 (CH2) 13 27.6 (CH2) 27.7 (CH2) 27.9 (CH2) 27.8 (CH2) 14 123.6 (CH) 123.7 (CH) 123.4 (CH) 123.5 (CH) 15 132.3 (C) 132.2 (C) 132.3 (C) 132.1 (C) 16 25.7 (CH3) 25.7 (CH3) 25.7 (CH3) 25.7 (CH3) 17 17.6 (CH3) 17.6 (CH3) 17.7 (CH3) 17.7 (CH3) 18 173.2 (C) 168.4 (C) 172.8 (C) 168.5 (C) 19 15.9 (CH3) 15.9 (CH3) 15.8 (CH3) 15.8 (CH3) 20 26.0 (CH3) 26.1 (CH3) 26.1 (CH3) 26.1 (CH3) COOMe 51.6 (CH3) 51.1 (CH3) aSpectra recorded at 125 MHz in CDCl 3.bDeduced by DEPT.
Table 4. 1H NMR Chemical Shifts for Compounds 6-9
6a 7a 8b 9a 3′ 6.49 s 6.49 s 6.50 s 6.50 s 6′ 6.59 d (1.5)c 6.59 d (1.5) 6.59 d (1.5) 6.59 s 7′ 2.03 d (1.5) 2.03 d (1.5) 2.04 d (1.5) 2.04 s 1 3.11 d (7.0) 3.11 d (7.5) 3.11 d (7.2) 3.11 d (7.0) 2 5.16 t (7.0) 5.15 md 5.15 t (7.2) 5.15 t (7.0) 4 2.08 m 2.07 m 2.08 m 2.08 m 5 2.13 m 2.12 m 2.12 m 2.13 m 6 5.14 md 5.14 md 5.13 t (7.2) 5.12 t (7.0) 8 2.12 m 2.11 m 2.13 m 2.10 m 9 2.30 m 2.28 m 2.60 m 2.52 m 10 6.87 t (7.5) 6.73 t (7.5) 6.00 t (7.5) 5.86 t (7.5) 12 2.31 m 2.32 m 2.26 m 2.25 m 13 2.10 m 2.08 m 2.10 m 2.08 m 14 5.13 md 5.13 md 5.10 t (7.2) 5.09 t (7.5) 16 1.67 s 1.67 s 1.67 s 1.68 s 17 1.59 s 1.59 s 1.59 s 1.58 s 19 1.62 s 1.62 s 1.60 s 1.60 s 20 1.62 s 1.62 s 1.62 s 1.62 s COOMe 3.72 s 3.74 s aSpectra recorded at 500 MHz in CDCl 3.bSpectra recorded at
300 MHz in CDCl3.cJ values (in Hz) in parentheses.d
Overlap-ping of signals was observed.
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would afford 2 and the following isomer 4 (pathway b). Furthermore, oxidation at C-20 of 11 formed intermediate 12, which could be ring-closed and subsequently oxidized to afford a naphthoquinone 1 (pathway c). To the best of our knowledge, the incorporation of a methyl group of the related meroditerpene to form a naphthoquinone was here found for the first time.
Experimental Section
General Experimental Procedures. IR spectra were recorded on a Jasco FT-5300 infrared spectrophotometer. Ultraviolet spectra were recorded on a Hitachi U-3210 UV spectrophotometer. NMR spectra were recorded on a Bruker
AVANCE DPX300 FT-NMR at 300 MHz for1H and 75 MHz
for13C or on a Varian Unity INOVA 500 FT-NMR at 500 MHz for1H and 125 MHz for13C, respectively, in CDCl
3. EIMS was obtained with a VG Quattro GC/MS spectrometer. HREIMS spectra were recorded on a Finnigan MAT-95XL mass spec-trometer. Silica gel (Merck, 230-400 mesh) was used for column chromatography. Precoated silica gel plates (Merck, Kieselgel 60 F-254, 0.2 mm) were used for analytical TLC.
Animal Material. The soft coral N. chabrolii was collected by hand using scuba off the coast of Pingtung County, southern Taiwan, in July 2001, at depths of 15-20 m, and stored in a
freezer until extraction. A voucher sample was deposited at the Department of Marine Resources, Sun Yat-Sen University. Extraction and Separation. The sliced bodies of N. chabrolii (1.8 kg, wet wt) were homogenized with EtOH and
filtered. The ground organism was repeatedly extracted with EtOH. The combined extract was concentrated under vacuum to afford a dark brown viscous residue (20.8 g). The residue was triturated with n-hexane first to afford an n-hexane-soluble layer and then with EtOAc. The combined EtOAc extract was evaporated under vacuum to yield an oily residue (15.8 g), which was subjected to column chromatography on silica gel, using n-hexane, n-hexane, and EtOAc mixtures of increasing polarity, and finally pure EtOAc, to yield 28 fractions. Fractions 7 and 9 eluted with n-hexane-EtOAc (15: 1) and were further purified on silica gel using n-hexane-acetone (gradient, 30:1 to 20:1) to yield 7 (8.9 mg) and 9 (3.1 mg) from fraction 7 and 3 (2.5 mg) and 5 (3.7 mg) from fraction 9. Fraction 12 eluted with n-hexane-EtOAc (8:1) and was purified by normal-phase HPLC using n-hexane-acetone (12: 1) to afford 6 (50.2 mg), 8 (8.2 mg), and 1 (8.0 mg). Fraction 15 eluted with n-hexane-EtOAc (4:1) and was purified by normal-phase HPLC using n-hexane-acetone (1:8) to afford 2 (150.2 mg) and 4 (15.2 mg).
Chabrolonaphthoquinone A (1): yellow oil; UV (MeOH) λmax(log ) 343 (2.79), 266 (3.63), 257 (3.69) nm; IR (neat) νmax 3292, 2922, 1680, 1662, 1631, 1601 cm-1;1H (CDCl
3, 500 MHz) and13C (CDCl
3, 125 MHz) NMR, see Table 1; EIMS (30 eV)
m/z 420 (4, [M]+), 402 (3), 374 (0.5), 185 (11); HREIMS m/z
420.2288 (calcd for C27H32O4, 420.2302).
Chabrolohydroxybenzoquinone A (2): pale oil; UV (MeOH) λmax(log ) 332 (3.69), 266 (3.65) nm; IR (neat) νmax 3398, 2926, 1684, 1637 cm-1;1H NMR (CDCl
3, 500 MHz) and 13C NMR (CDCl
3, 125 MHz), see Tables 2 and 3; EIMS (30 eV) m/z 424 (0.9, [M - Η2Ο]+), 409 (0.5), 378 (0.2), 175 (100), 137 (4), 69 (14); HREIMS m/z 424.2614 (calcd for C27H36O4, M+- Η
2Ο, 424.2615).
Chabrolohydroxybenzoquinone B (3): pale oil; UV (MeOH) λmax(log ) 331 (3.72), 266 (3.70) nm; IR (neat) νmax 3433, 2924, 1712, 1680, 1641 cm-1;1H NMR (CDCl
3, 500 MHz) and13C NMR (CDCl
3, 125 MHz), see Tables 2 and 3; EIMS (30 eV) m/z 438 (0.4, [M - Η2Ο]+), 423 (0.2), 392 (0.5), 175 (100), 137 (4), 69 (13); HREIMS m/z 438.2768 (calcd for C28H38O4, M+- Η2Ο, 438.2771).
Chabrolohydroxybenzoquinone C (4): pale oil; UV (MeOH) λmax(log ) 330 (3.77), 267 (3.80) nm; IR (neat) νmax 3396, 2926, 1684, 1637 cm-1;1H NMR (CDCl
3, 500 MHz) and 13C NMR (CDCl
3, 125 MHz), see Tables 2 and 3; EIMS (30 eV) m/z 424 (0.9, [M - Η2Ο]+), 409 (0.4), 378 (0.3), 175 (100), 137 (8), 69 (13); HREIMS m/z 424.2611 (calcd for C27H36O4, M+- Η
2Ο, 424.2615).
Chabrolohydroxybenzoquinone D (5): pale oil; UV (MeOH) λmax(log ) 331 (3.67), 267 (3.70) nm; IR (neat) νmax 3422, 2924, 1714, 1684, 1645 cm-1;1H NMR (CDCl
3, 500 MHz) and13C NMR (CDCl
3, 125 MHz), see Tables 2 and 3; EIMS (70 eV) m/z 438 (1, [M - Η2Ο]+), 423 (0.4), 392 (0.2), 175 (100), 137 (5), 69 (64); HREIMS m/z 438.2769 (calcd for C28H38O4, M+- Η2Ο, 438.2771).
Scheme 2. Proposed Biosynthetic Pathways of the Related Meroditerpenoids
Table 5. 13C NMR Chemical Shifts for Compounds 6-9
6a 7a 8b 9a 1′ 187.9 (C)c 187.9 (C) 187.9 (C) 187.9 (C) 2′ 148.4 (C) 148.4 (C) 148.6 (C) 148.4 (C) 3′ 132.3 (CH) 132.3 (CH) 132.4 (CH) 132.3 (CH) 4′ 188.3 (C) 188.3 (C) 188.5 (C) 188.4 (C) 5′ 145.6 (C) 145.6 (C) 145.7 (C) 145.6 (C) 6′ 133.5 (CH) 133.5 (CH) 133.6 (CH) 133.5 (CH) 7′ 15.4 (CH3) 15.5 (CH3) 15.5 (CH3) 15.5 ((CH3) 1 27.4 (CH2) 27.2 (CH2) 27.2 (CH2) 27.1 (CH2) 2 118.0 (CH) 118.0 (CH) 118.0 (CH) 117.9 (CH) 3 139.7 (C) 139.8 (C) 139.9 (C) 139.9 (C) 4 39.5 (CH2) 39.5 (CH2) 39.6 (CH2) 39.6 (CH2) 5 26.3 (CH2) 26.4 (CH2) 26.4 (CH2) 26.4 (CH2) 6 124.8 (CH) 124.7 (CH) 124.4 (CH) 124.4 (CH) 7 134.2 (C) 134.3 (C) 134.7 (C) 134.7 (C) 8 38.3 (CH2) 38.5 (CH2) 39.1 (CH2) 39.1 (CH2) 9 27.1 (CH2) 27.1 (CH2) 28.3 (CH2) 27.9 (CH2) 10 145.3 (CH) 142.6 (CH) 145.4 (CH) 142.2 (CH) 11 131.2 (C) 131.8 (C) 130.6 (C) 131.4 (C) 12 26.7 (CH2) 27.1 (CH2) 34.6 (CH2) 34.7 (CH2) 13 27.6 (CH2) 27.7 (CH2) 27.9 (CH2) 28.0 (CH2) 14 123.6 (CH) 123.7 (CH) 123.5 (CH) 123.5 (CH) 15 132.3 (C) 132.2 (C) 132.3 (C) 132.1 (C) 16 25.7 (CH3) 25.7 (CH3) 25.7 (CH3) 25.7 (CH3) 17 17.6 (CH3) 17.6 (CH3) 17.8 (CH3) 17.7 (CH3) 18 173.1 (C) 168.4 (C) 172.6 (C) 168.5 (C) 19 16.1 (CH3) 16.1 (CH3) 16.2 (CH3) 16.1 (CH3) 20 16.0 (CH3) 16.0 (CH3) 16.0 (CH3) 15.9 (CH3) COOMe 51.5 (CH3) 51.1 (CH3) aSpectra recorded at 125 MHz in CDCl 3.bSpectra recorded at 75 MHz in CDCl3.cDeduced by DEPT.
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Chabrolobenzoquinone A (6): pale oil; UV (MeOH) λmax (log ) 251 (4.00) nm; IR (neat) νmax3273, 2924, 1684, 1657, 1614 cm-1;1H NMR (CDCl
3, 500 MHz) and13C NMR (CDCl3, 125 MHz), see Tables 4 and 5; EIMS (30 eV) m/z 424 (0.7, [M]+), 409 (0.4), 378 (0.2), 175 (92), 137 (54), 69 (100); HREIMS
m/z 424.2607 (calcd for C27H36O4, 424.2615).
Chabrolobenzoquinone B (7): pale oil; UV (MeOH) λmax (log ) 252 (4.02) nm; IR (neat) νmax2924, 1712, 1657, 1614 cm-1;1H NMR (CDCl
3, 500 MHz) and13C NMR (CDCl3, 125 MHz), see Tables 4 and 5; EIMS (30 eV) m/z 438 (0.6, [M]+), 423 (0.3), 392 (0.2), 175 (93), 137 (93), 69 (100); HREIMS m/z 438.2741 (calcd for C28H38O4, 438.2771).
Chabrolobenzoquinone C (8): pale oil; UV (MeOH) λmax (log ) 251 (4.06) nm; IR (neat) νmax3271, 2924, 1684, 1657, 1614 cm-1;1H NMR (CDCl
3, 300 MHz) and13C NMR (CDCl3, 75 MHz), see Tables 4 and 5; EIMS (30 eV) m/z 424 (0.8, [M]+), 409 (0.4), 378 (0.2), 175 (94), 137 (76), 69 (100); HREIMS m/z 424.2605 (calcd for C27H36O4, 424.2615).
Chabrolobenzoquinone D (9): pale oil; UV (MeOH) λmax (log ) 251 (4.03) nm; IR (neat) νmax2924, 1712, 1657, 1614 cm-1;1H NMR (CDCl
3, 500 MHz) and13C NMR (CDCl3, 125 MHz), see Tables 4 and 5; EIMS (30 eV) m/z 438 (1, [M]+), 423 (0.4), 392 (0.7), 175 (93), 137 (92), 69 (100); HREIMS m/z 438.2768 (calcd for C28H38O4, 438.2771).
Acknowledgment. This work was supported by a grant from the National Science Council (Contract No. NSC 92-2323-B-110-003), Republic of China, awarded to J.-H.S.
References and Notes
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