HETEROCYCLES

Short Paper | Regular issue | Vol. 87, No. 10, 2013, pp. 2047-2052
Received, 11th July, 2013, Accepted, 14th August, 2013, Published online, 30th August, 2013.
DOI: 10.3987/COM-13-12773

■ Alkaloids from Melodinus suaveolens

Ting-Ting Zhang, Zhi-Wen Liu, Wen-Jing Wang, Yong-Bin Tong, Fang-Fang Xu,* Jing-Quan Yuan, Bo Liu, Xiao-Qi Zhang,* and Wen-Cai Ye

Institute of Traditional Chinese Medicine and Natural Products, College of Pharmacy, Jinan University, 601 West Huangpu Avenue, Guangzhou, 510632, China

Abstract

Two new quinoline alkaloids, 14,15-β-epoxyscandine (3) and meloscine N-oxide (6), along with six known alkaloids scandine (1), scandine N-oxide (2), meloscandonine (4), meloscine (5), tubotaiwine (7), polyneuridine (8) were isolated from the twigs and leaves of Melodinus suaveolens. The structures of the new compounds were elucidated on the basis of spectroscopic methods and circular dichroism experiments.

The genus Melodinus (Apocynaceae) comprises 53 species mainly distributed in tropical or subtropical Asia and Australia.1 This genus has been regarded as a rich source of monoterpenoid indole alkaloids, which originate from the condensation of tryptophan with secologanin.2 Up to now, more than 100 monomeric and dimeric indole alkaloids as well as quinoline alkaloids have been isolated from Melodinus sp. The interesting chemical significance of the Melodinus plants prompted us to initiate a phytochemical study on the twigs and leaves of Melodinus suaveolens, which led to the isolation of two new quinoline alkaloids, 14,15-β-epoxyscandine (3) and meloscine N-oxide (6), along with six known alkaloids scandine (1),3 scandine N-oxide (2),3 meloscandonine (4),4 meloscine (5),5 tubotaiwine (7),6 and polyneuridine (8).7

14,15-β-Epoxyscandine (3) was obtained as yellow oil. The molecular formula of 3 was established to be C21H22N2O4 by its HR-ESI-MS (m/z 367.1648 [M+H]+, calcd for C21H23N2O4, 367.1652). The IR spectrum of 3 suggested the presence of NH (3456 cm-1), carbonyl (1744, 1663 cm-1) and aromatic ring (1596, 1498 cm-1). The UV spectrum showed absorption maxima at 341, 326, 287, 257 and 214 nm, suggesting that 3 was a quinoline alkaloid. The 1H NMR spectrum of 3 displayed signals for an ortho-disubstituted phenyl ring [δH 7.30 (1H, dd, J = 7.9, 1.3 Hz, H-9), 7.12 (1H, td, J = 7.9, 1.3 Hz, H-11), 7.03 (1H, td, J = 7.9, 1.3 Hz, H-10), and 6.74 (1H, dd, J = 7.9, 1.3 Hz, H-12)], a terminal double bond [δH 5.63 (1H, dd, J = 17.8, 11.1 Hz, H-19), 4.93 (1H, d, J = 17.8 Hz, H-18a) and 4.92 (1H, d, J = 11.1 Hz, H-18b)], and a methoxyl group [δH 3.63 (3H, s, CO2Me)]. The 13C NMR spectrum showed twenty-one carbon signals, including two carbonyls, eight olefinic carbons, three quaternary carbons, one methoxyl, three methines and four methylenes. With the aid of 1H-1H COSY, HSQC and HMBC experiments, the 1H and 13C NMR signals of 3 were assigned as shown in Table 1.
Comparison of the NMR data of 3 with those of the known compound scandine (1)3 revealed their structural similarity, except for an epoxy group [δC 58.2, 55.5; δH 3.48 (1H, m), 3.03 (1H, d, J = 3.7 Hz)] instead of a double bond [δC 132.7, 124.2; δH 5.72 (1H, m, H-14), 5.65 (1H, overlapped, H-15)]. The 1H-1H COSY spectrum of 3 revealed the presence of the spin system (C-3 to C-15) and the HMBC cross peaks between H-3α/H-3β/H-15/H-19/H-17α/H-17β and C-21 indicated that epoxy group was located at C-14 and C-15 (Figure 1). Based on the above evidence, the planar structure of 3 was elucidated as 14,15-epoxyscandine.4 However, the relative configuration of the epoxy group was not clearly defined in the previous work. In this paper, the relative stereochemistry of 3 was deduced by a ROESY experiment, in which correlations between H-19 and H-15/H-21 suggested that these protons had the same orientation. Furthermore, the CD spectrum of 3 showed the same Cotton effects as scandine (1) (Figure 2), indicating the presence of S, R, R, R configurations at C-7, C-16, C-20 and C-21, respectively. Thus, the structure of 3 was elucidated as 14,15-β-epoxyscandine.

Meloscine N-oxide (6) was isolated as yellow oil with a molecular formula of C19H20N2O2, as determined by the HR-ESI-MS at m/z 309.1597 [M+H]+ (calcd for C19H21N2O2, 309.1598). The IR and UV spectra of 6 were similar to those of 3, suggesting that 6 was also a quinoline alkaloid. The 1H NMR spectrum of 6 showed signals for an ortho-disubstituted phenyl ring [δH 7.90 (1H, dd, J = 7.7, 1.4 Hz, H-9), 7.24 (1H, td, J = 7.7, 1.4 Hz, H-11), 7.17 (1H, td, J = 7.7, 1.3 Hz, H-10), 6.91 (1H, dd, J = 7.7, 1.3 Hz, H-12)], a double bond [δH 6.21 (1H, m, H-14) and 6.08 (1H, dd, J = 9.8, 2.8 Hz, H-15)], and a terminal double bond [δH 5.79 (1H, dd, J = 17.2, 10.4 Hz, H-19), 5.16 (1H, d, J = 17.2 Hz, H-18a) and 5.04 (1H, d, J = 10.4 Hz, H-18b)]. The 13C NMR spectrum displayed nineteen carbon signals, including one carbonyl, ten olefinic carbons, two quaternary carbons, two methines and four methylenes. Detailed examination of 1D and 2D NMR spectra of 6 and comparison with those of the known compound meloscine (5)5 revealed that 6 was the N-oxide of compound 5, in particular the characteristic downfield shifts of the C-21 [δc 98.3; δH 4.48 (1H, s)] in 6. In the HMBC spectrum, the correlations between H-3α/H-3β/H-5α/H-5β/H-15/H-19/H-17α/H-17β and C-21 further confirmed the oxidation of N4 (Figure 1). In the ROESY spectrum, correlations between H-9 and H-6α/H-21 suggested that H-6α and H-21 were α-oriented, whereas the correlations H-16 and H-6β/H-17β suggested that H-16 was β-oriented. Furthermore, H-19, H-17α and H-21 were assigned on the same side on the basis of the ROESY correlations among them. The absolute configuration of 6 was identical to that of meloscine (5), since they showed similar Cotton effects in the CD measurement (Figure 2). Accordingly, the structure of 6 was determined as meloscine N-oxide.

The structures of the known compounds were identified by the comparison of their spectroscopic data (UV, IR, 1H NMR, 13C NMR, MS) with the data from the corresponding values in the literature as scandine (1),3 scandine N-oxide (2),3 meloscandonine (4),4 meloscine (5),5 tubotaiwine (7),6 and polyneuridine (8),7 respectively.

EXPERIMENTAL
General experimental procedures: Optical rotations were carried out using a Jasco P-1020 digital polarimeter. UV spectra were measured on a Jasco V-550 UV/VIS spectrophotometer with a 1 cm length cell. IR spectra were recorded on a Bruker Equinox 55 infrared spectrometer with KBr disc. CD spectra were measured on a Chirascan spectrometer (Applied Photophysics Ltd) at 25 ºC for 200-400 nm with a quartz cell of path length 1 cm. HR-ESI-MS data were measured on an Agilent 6210 ESI/TOF mass spectrometer. NMR experiments were performed on Bruker AV-400 and AV-600 spectrometers. Column chromatography (CC) were performed on silica gel (200-300 mesh; Qingdao Marine Chemical Inc., Qingdao, China) and ODS (YMC, Kyoto, Japan), respectively. Preparative high-performance liquid chromatography (HPLC) was carried on a Agilent 1260 system equipped with a G1310B Iso pump, a G1365D MWD VL detector and a CAPCELL PAK MGII C18 reversed-phase column (20×250 mm, 5 μm, Shiseido Fine Chemicals Ltd, Japan).
Plant material: The leaves and twigs of M. suaveolens were collected in Jinxiu, a Yao Autonomous County of Guangxi Province of China, in September 2010, and authenticated by Dr. Jing-Quan Yuan (Guangxi Medicinal Herb Garden). A voucher specimen (No. CP2010093001) is deposited in the herbarium of the College of Pharmacy, Jinan University, Guangzhou, China.
Extraction and isolation: The air-dry leaves and twigs (18 kg) of M. suaveolens was powdered and extracted with 95% EtOH (50 L) three times at room temperature, and the solution was concentrated under reduced pressure to afford a brownish residue (1700 g), which was suspended in water, and then successively partitioned with petroleum ether, EtOAc and n-BuOH, respectively. The EtOAc extract (326 g) was subjected to a silica gel column with gradient CHCl3-MeOH (100:0→0:100) to afford 9 fractions (Fr-E1~Fr-E9). Fraction E2 was then subjected to sephadex LH-20 (CHCl3-MeOH, 1:1) and ODS (MeOH-H2O, 30:90→90:10) column chromatography (CC) to yield 1 (17.5 mg) and 8 (2.3 mg). The n-BuOH extract (306 g) was subjected to a silica gel column with gradient CHCl3-MeOH (100:0→0:100) to afford 11 fractions (Fr-B1~Fr-B11). Fraction B3 was further separated on silica gel column eluting with CHCl3-MeOH (100:1→90:10), followed by HPLC (MeOH-H2O+0.1‰ Et2NH, 65:35) to afford 3 (2.4 mg), 4 (3.2 mg) and 5 (13.5 mg). The Fr-B5 was purified by Sephadex LH-20 column (CHCl3-MeOH, 1:1) and HPLC (MeOH-H2O+0.1‰ Et2NH, 60:40) to afford 2 (2.4 mg), 6 (2.5 mg) and 7 (2.7 mg).
14,15-β-Epoxyscandine (3). Yellow oil; [α]25 D +109.8° (c 0.9, MeOH); UV λ MeOH max (log ε): 341 (3.38), 326 (3.43), 287 (3.74), 257 (4.12), 214 (4.56) nm; CD (MeOH, Δε) λmax 257 (+8.5), 221 (−21.4) nm; IR ν KBr max: 3456, 2924, 1744, 1663, 1634, 1596, 1498, 1384, 1235, 1115, 756 cm-1; HR-ESI-MS m/z 367.1648 [M+H]+ (calcd for C21H23N2O4, 367.1652).
Meloscine N-oxide (6). Yellow oil; [α]25 D +128.6° (MeOH, c 0.6); UV λ MeOH max (log ε): 252 (4.40), 210 (3.91) nm; CD (MeOH, Δε) λmax 254 (+6.2), 224 (−5.3) nm; IR νKBr max: 3455, 2923, 1665, 1594, 1491, 1384, 763 cm-1; HR-ESI-MS m/z 309.1597 [M+H]+ (calcd for C19H21N2O2, 309.1598).

ACKNOWLEDGEMENTS
Financial support of this research was was provided by grants from National Natural Science Foundation of China (Nos. 81172947, 81202428), Ministry of Science and Technology of China (Nos. 2013DFM30080, 2013BAI11B05, 2012ZX09103201-056), China Postdoctoral Science Foundation (No. 2013M531910), Educational Commission of Guangdong Province (No. gjhz1003), and the Program for New Century Excellent Talents in University (No. NCET-12-0676).

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