α-Glucosidase Inhibitory Flavonoids and Oxepinones from the Leaf

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α‑Glucosidase Inhibitory Flavonoids and Oxepinones from the Leaf and Twig Extracts of Desmos cochinchinensis Pornphimol Meesakul,†,‡,§ Christopher Richardson,§ Stephen G. Pyne,§ and Surat Laphookhieo*,†,‡ †

Center of Chemical Innovation for Sustainability (CIS) and ‡School of Science, Mae Fah Luang University, Chiang Rai 57100, Thailand § School of Chemistry and Molecular Biosciences, University of Wollongong, Wollongong, New South Wales 2522, Australia

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ABSTRACT: Four new flavonoids (1−4), a new benzyl benzoate derivative (5), five new oxepinones (6−10), and 14 known compounds (11−24) were isolated from the leaf and twig extracts of Desmos cochinchinensis. Their structures were established by spectroscopic methods. The structure of 1 was also confirmed by X-ray diffraction data. The absolute configurations of 3, 4, and 6−10 were determined from comparisons of their ECD spectra with those of relevant reported compounds. Compounds 1, 2, 6, 8, 10, 12−15, and 17 showed α-glucosidase inhibitory activities with IC50 values ranging from 0.2 to 4.9 μM. Desmos cochinchinensis Lour. is a shrub belonging to the Annonaceae family, which is distributed throughout tropical and subtropical Asia.1−5 This plant is used for the treatment of malaria in the South of China.4 The genus produces various types of compounds including terpenoids,1,4 alkaloids,6−8 chalcones,9,10 and flavonoids.2,3,7,11−13 Some of these compounds showed cytotoxicity,5 anti-HIV,14 and aromatase inhibitory activities.2,3 In an ongoing search for compounds showing α-glucosidase inhibitory activities from Annonacea plants, we have studied the twig and leaf extracts of D. cochinchinensis collected from Mae Fah Luang University Health Park, Chiang Rai Province, Thailand. Herein, the isolation and identification of compounds from these extracts are reported. In addition, the α-glucosidase inhibitory activities of some of the compounds are also reported.

were isolated from the twig extract. The known compounds were identified as tectochrysin (11),15 chrysin (12),16 pinocembrin (13),16 isochamanetin (14),17 dichamanetin (15),17 benzyl benzoate (16),18 2-hydroxybenzyl benzoate (17),19 2-hydroxybenzyl alcohol (18), flexuvarin B (19),20 flexuvarin D (20),20 zeylenone (21),21 benzoic acid (22), lysicamine (23),22 and liriodenine (24)22 by analysis of their spectroscopic data and by comparisons of these data with those reported. The structure of desmoscochinflavone A (1) was established by X-ray diffraction data analysis (Figure 1), and its structure was supported by HRESIMS data, which showed an ion at m/z [M − H]− 375.0882 (calcd for C22H15O6, 375.0869). The 1H and 13C NMR data of 1 (Tables 1 and 2) displayed resonances for a monosubstituted aromatic ring at δH 7.83 (m, H-2′, H6′)/δC 127.5, 7.50 (m, H-3′, H-5′)/δC 130.1, and 7.47 (m, H4′)/δC 132.8, two methine protons at δH 6.68 (s, H-3)/δC 105.5 and 6.34 (s, H-6)/δC 99.9, and a 3″,4″-dihydroxybenzyl unit at δH 4.16 (s, H2-1″)/δC 22.9, 6.36 (dd, J = 7.8, 1.2 Hz, H5″)/δC 113.8, 6.50 (t, J = 7.8 Hz, H-6″)/δC 120.3, and 6.63

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RESULTS AND DISCUSSION The EtOAc extracts of the twigs and leaves of D. cochinchinensis were individually separated and purified by various column chromatographic techniques to yield 24 compounds, including 10 new (1−10) and 14 known compounds (11−24). Compounds 1−9, 11, 12, 14−20, and 22 were isolated from the leaf extract, while compounds 6, 10, 12, 13, 19, and 21−24 © XXXX American Chemical Society and American Society of Pharmacognosy

Received: July 20, 2018

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DOI: 10.1021/acs.jnatprod.8b00581 J. Nat. Prod. XXXX, XXX, XXX−XXX

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Chart 1

Table 1. 1H NMR (500 MHz) Spectroscopic Data of Compounds 1−4 (δ in ppm, J in Hz) δH (methanol-d4) position

1

2

2 3a

6.68, s

6.73, s

6.34, s

6.36, s

7.83, 7.50, 7.47, 7.50, 7.83, 4.16,

7.86, 7.47, 7.52, 7.47, 7.86, 4.13,

3b 6 8 2′ 3′ 4′ 5′ 6′ 1″ 3″ 4″ 5″

Figure 1. X-ray ORTEP drawing of compound 1.

(brd, J = 7.8 Hz, H-7″)/δC 120.5. The HMBC correlations confirmed the structure and are shown in Figure 2. The molecular formula of desmoscochinflavone B (2), C22H16O6, was deduced from HRESIMS data, which displayed an ion at m/z 375.0873 [M − H]− (calcd for C22H15O6, 375.0869). The 1H and 13C NMR spectroscopic data of 2 (Tables 1 and 2) were similar to those of 1 except that the resonances for the 2,3-disubstituted benzyl unit of 1 were replaced by resonances for a 2,5-disubstituted benzyl unit in the spectra of 2. The 1H NMR spectroscopic data of 2 showed resonances for an aromatic ring ABX spin system at δH 6.69 (d, J = 8.6 Hz, H-4″)/δC 116.3, 6.46 (dd, J = 8.6, 3.0 Hz, H-5″)/ δC 114.1, and 6.36 (d, J = 3.0 Hz, H-7″)/δC 116.3. The structure of 2 was vindicated by the HMBC correlations shown in Figure 2. (−)-(2S)-Desmoscochinflavanone A (3) was isolated as a yellow solid, mp 218−221 °C. Its molecular formula of C22H18O6 was deduced from the HRESIMS data, which showed an ion at m/z 377.1024 [M − H]− (calcd for C22H17O6, 377.1025). Analysis of the NMR spectroscopic data of 3 (Tables 1 and 2) indicated that it was related in structure to 2. The core structure of 3 was a flavanone, which displayed 1 H NMR resonances for an oxymethine proton at δH 5.38 (dd, J = 12.8, 2.4 Hz, H-2) and two methylene protons at 2.83 (dd, J = 17.2, 2.4 Hz, H-3a) and 3.06 (dd, J = 17.2, 12.8 Hz, H-3b).

6″ 7″ OH-5

m m m m m s

6.63, dd (7.8, 1.2) 6.50, t (7.8) 6.36, brd (7.8)

δH (CDCl3)

m m m m m s

6.69, d (8.6) 6.46, dd (8.6, 3.0)

3

4

5.38, dd (12.8, 2.4) 2.83, dd (17.2, 2.4) 3.06, dd (17.2, 12.8)

5.38, dd (13.0, 3.0) 2.86, dd (17.2, 3.0) 3.06, dd (17.2, 13.0)

6.03, 7.42, 7.37, 7.37, 7.37, 7.42, 3.81, 7.05,

6.04, 7.41, 7.40, 7.40, 7.40, 7.41, 3.88, 7.04,

s m m m m m s d (2.2)

6.57, dd (8.2, 2.2) 6.70, d (8.2)

6.36, d (3.0)

s m m m m m s t (4.6)

6.74, t (4.6)

6.73, t (4.6) 12.68, s

12.89, s

An additional difference was the position of the 2,5disubstituted benzyl unit, which was located at C-6 in 3 but at C-8 in 2. This difference was supported by the H-8 singlet of 3 at δH 6.03. The structure of 3 was confirmed by the HMBC correlations shown in Figure 2. The electronic circular dichroism (ECD) spectrum of 3 displayed a positive Cotton effect at 223 nm and a negative Cotton effect at 286 nm, similar to that of (2S)-5,6,7-trimethoxyflavanone from Oxymitra velutina23 as well as the known and related isolated compounds 13−15 (Figure 3). Thus, compound 3 was assigned the (2S) absolute configuration. The HRESIMS data of (−)-(2S)-desmoscochinflavanone B (4) displayed an ion at m/z [M − H]− 377.1024 (calcd for C22H17O6, 377.1025) corresponding to the molecular formula B

DOI: 10.1021/acs.jnatprod.8b00581 J. Nat. Prod. XXXX, XXX, XXX−XXX

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oxymethylene protons at δH 5.24 (s, H2-7)/δC 61.9, benzoyl group protons at δH 8.04 (d, J = 7.6 Hz, H-2′, H-6′)/δC 129.7, 7.43 (t, J = 7.6 Hz, H-3′, H-5′)/δC 128.4, and 7.55 (t, J = 7.6 Hz, H-4′)/δC 133.2, and an acetoxy group at δH 2.26 (s, AcO2)/δC 20.8. The acetoxy group was placed at C-2 based on the HMBC cross-peaks between the methyl protons of the acetoxy group (δH 2.26) and the oxymethylene protons (δH 5.24) with C-2 (δC 142.3). From this data and the HMBC correlations shown in Figure 2, the structure of desmoscochin benzoate (5) was assigned as shown. (−)-(5R)-Desmoscochinoxepinone A (6) showed an ion at m/z 325.0687 ([M + Na]+, calcd for C16H14O6Na, 325.0688) in the HRESIMS data corresponding to the molecular formula C16H14O6. Analysis of the 1D and 2D NMR spectroscopic data indicated that compound 6 was an oxepinone derivative24,25 that displayed resonances for three olefinic protons at δH 6.70 (dd, J = 2.0, 6.6 Hz, H-7), 5.24 (dd, J = 4.2, 6.6 Hz, H-6), and 7.51 (s, H-2), an oxymethine proton at δH 5.78 (dd, J = 2.0, 4.2 Hz, H-5), acetoxy protons at δH 2.14 (s, H-2″), oxymethylene protons at δH 5.04 and 5.14 (d, J = 12.0 Hz, H-8), and benzoyl group protons at δH 8.02 (m, H-2′, H-6′), 7.41 (m, H-3′, H5′), and 7.54 (m, H-4′). The (S)-enantiomer of 6, isolated from the extracts of D. chinensis and other related species, is claimed in the patent literature;26 however, proof of the configuration of this compound was not provided. The NMR spectroscopic data of 6 (Tables 4 and 5) match closely with the NMR spectroscopic data reported in this patent, which also includes a copy of the ECD spectrum, which is the mirror image of the spectrum of 6. Thus, compound 6 reported here is the enantiomer of that isolated from D. chinensis. Finally, the (5R) absolute configuration defined by comparison of its ECD spectrum to that of (+)-(5S)-grandiuvarone, previously isolated from Uvaria valderramensis,24 which had a mirror image ECD spectrum to 6 (Figure 4) isolated here. Thus, the structure of (−)-(5R)-desmoscochinoxepinone A was assigned as shown. (−)-(5R)-Desmoscochinoxepinones B (7), C (8), and D (9) were isolated as viscous oils. The NMR spectroscopic data of these compounds (Tables 4 and 5) were similar to those of (−)-(5R)-desmoscochinoxepinone A (6) except for the lack of resonances for the C-8 benzoyl group. Compound 7 (HRESIMS m/z 263.0535 [M + Na] + , calcd for C11H12O6Na, 263.0532) displayed NMR resonances for an acetoxy group at δH 2.04/δC 20.5 and δC 171.0 (C-1′) instead of the benzoyl unit present in 6. This was confirmed by HMBC correlations (Figure 2) between H2-8 (δH 4.77 and 4.87) and

Table 2. 13C NMR (125 MHz) Spectroscopic Data of Compounds 1−4 (δ in ppm) δC (methanol-d4)

δC (CDCl3)

position

1

2

3

4

2 3 4 5 6 7 8 9 10 1′ 2′ 3′ 4′ 5′ 6′ 1″ 2″ 3″ 4″ 5″ 6″ 7″

165.6 105.5 184.2 161.2 99.9 164.6 107.4 157.0 105.6 132.6 127.5 130.1 132.8 130.1 127.5 22.9 128.3 144.2 146.0 113.8 120.3 120.5

165.6 105.3 184.2 161.3 100.0 164.6 107.2 157.0 105.5 132.6 127.5 130.1 132.8 130.1 127.5 23.0 128.9 149.0 116.3 114.1 151.1 116.3

79.1 43.2 195.9 161.1 107.8 163.3 96.1 161.2 102.8 138.7 126.1 128.8 128.8 128.8 126.1 21.9 127.6 118.1 149.7 114.3 116.4 146.1

79.1 43.1 196.1 160.6 108.0 163.1 95.9 161.1 102.7 138.1 126.1 128.8 128.9 128.8 126.1 22.0 126.8 122.8 144.3 113.1 140.9 120.8

of C22H18O6. The 1H and 13C NMR spectroscopic data of 4 (Tables 1 and 2) were similar to those of 3 except that compound 4 showed resonances for a 3,5-disubstituted benzyl unit comprising an AMX aromatic 1H NMR spin system, at δH 7.04 (t, J = 4.6 Hz, H-3″), 6.74 (t, J = 4.6 Hz, H-5″), and 6.73 (t, J = 4.6 Hz, H-7″), instead of that for a 2,5-disubstituted benzyl unit as in 3. The (2S) absolute configuration of 4 was determined from a comparison of its specific rotation, as well as its ECD spectrum, to those of 3 and 13−15 (Figure 3). The molecular formula of desmoscochin benzoate (5), C16H14O5, was deduced from HRESIMS data, which showed an ion at m/z 309.0745 ([M + Na]+, calcd for C16H14O5Na, 309.0739). Analysis of the 1H and 13C NMR spectroscopic data of 5 (Table 3) suggested that this compound was a benzyl benzoate derivative. The 1H and 13C NMR data (Table 3) displayed resonances for an ABX aromatic ring spin system at δH 6.97 (dd, J = 8.8 Hz, H-3)/δC 123.6, 6.81 (dd, J = 3.2, 8.8 Hz, H-4)/δC 116.2, and 6.95 (d, J = 3.2 Hz, H-6)/δC 116.5,

Figure 2. Selected HMBC correlations of compounds 1−10. C

DOI: 10.1021/acs.jnatprod.8b00581 J. Nat. Prod. XXXX, XXX, XXX−XXX

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Figure 3. ECD spectra of compounds 3, 4, and 13−15 (in MeOH).

the same (5R) absolute configuration. The 1H and 13C NMR data of 7−9 are shown in Tables 4 and 5, and the HMBC correlations in Figure 2. Compound 10, (5R)-acetoxy-6-benzoyloxymethyl-5H-oxepin-4-one [(+)-grandiuvarone], was first reported from Uvaria grandif lora by Ankisetty et al. in 2006,25 and the (5R) absolute configuration assigned by a comparison of the ECD data of 10 with those of (7S,8R)-leptosphaerone.27 This configurational assignment was shown to be incorrect in 2014 by Macabeo and co-workers, who isolated the same compound from U. valderramensis24 and revised the absolute configuration from (5R) to (5S) by comparison of experimental and calculated ECD data.24 In this study, the enantiomer of (+)-(5S)grandiuvarone was isolated. The 1 H and 13 C NMR spectroscopic data of 10 (Tables 4 and 5) closely matched those of (+)-(5S)-grandiuvarone. Compound 10 and (+)-(5S)-grandiuvarone had specific rotations of opposite 24 sign {[α]23 and D +34 (c 0.1, CHCl3) for (5S)-grandiuvarone 25 [α] D −22 (c 0.4, CHCl3) for 10} and mirror image ECD spectra [252 nm (+2.69), 316 nm (−0.34), lit.24 and 253 nm (−6.28), 316 nm (+1.14) for 10] (Figure 4). Therefore, compound 10 was identified as (−)-(5R)-grandiuvarone. Compounds 1, 2, 6, 8, 10−15, 17, and 19−21 were evaluated for their α-glucosidase inhibitory activities. Compounds 1, 2, 6, 8, 10, 12−15, and 17 showed potent αglucosidase inhibitory activities with IC50 values ranging from 0.2 to 4.9 μM (Table 6) compared to 170.7 μM for the positive control (acarbose). Compounds 11, 19, 20, and 21 showed weaker α-glucosidase inhibitory activities with IC50 values of 158.2, 40.7, 139.0, and 71.1 μM, respectively, which were better than that of the positive control, acarbose (IC50 170.7 μM). These screening results suggested that flavonoids 1, 2, and 15 with IC50 values of 0.9, 0.9, and 0.2 μM, respectively, may have potential for further development as antidiabetic agents.

Table 3. 1H (400 MHz) and 13C NMR (100 MHz) Spectroscopic Data of Compound 5 (δ in ppm) position

δC

1 2 3 4 5 6 7 1′ 2′ 3′ 4′ 5′ 6′ 7′ OCOMe-2 OCOMe-2

129.0 142.3 123.6 116.2 153.6 116.5 61.9 129.0 129.7 128.4 133.2 128.4 129.7 166.3 169.9 20.8

δH (J in Hz, CDCl3)

6.97, d (8.8) 6.81, dd (3.2, 8.8) 6.95 d (3.2) 5.24, s 8.04, 7.43, 7.55, 7.43, 8.04,

d (7.6) t (7.6) t (7.6) t (7.6) d (7.6)

2.26, s

C-1′ (δC 171.0). The structure of 8 (HRESIMS m/z 221.0418 [M + Na]+, calcd for C9H10O5Na, 221.0426) was closely related to that of 6 except that the benzoate group of 6 was replaced by a hydroxy group in 8. Its 1H and 13C NMR data (Tables 4 and 5) displayed resonances for oxymethylene protons at δH 4.28 (d, J = 12.4 Hz, H-8a) and 4.37 (d, J = 12.4 Hz, H-8b)/δC 61.7. The HMBC correlations (Figure 2) of H8a (δH 4.28) and H-8b (δH 4.37) with C-5 (δC 183.6) and C-6 (δC 118.8) supported this assignment. The structure of 9 (HRESIMS m/z 235.0579 [M + Na]+, calcd for C10H12O5Na, 235.0582) was closely related to that of 8. The only difference was that the hydroxy group at C-8 in 8 was replaced by a methoxy group (δH 3.35/δC 58.2) in 9. The HMBC correlations (Figure 2) confirmed the position of the methoxy group at C-8 by the 2J correlation of the methyl proton resonance with C-8 (δC 69.3). The specific rotation and ECD spectra (Figure 4) of 7−9 were similar to those of 6, suggesting D

DOI: 10.1021/acs.jnatprod.8b00581 J. Nat. Prod. XXXX, XXX, XXX−XXX

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Table 4. 1H NMR (400 MHz) Spectroscopic Data of Compounds 6−10 in CDCl3 (δ in ppm, J in Hz) position 2 3 5 6 7 8a 8b 2′ 3′ 4′ 5′ 6′ 2″ OMe-8

6a

6

7

8

9

7.51, s

7.70, s

7.41, s

7.27, s

7.25, s

5.78, 5.24, 6.70, 5.04, 5.14, 8.02, 7.41, 7.54, 7.41, 8.02, 2.14,

5.78, 5.40, 6.91, 5.00, 5.14, 8.04, 7.48, 7.60, 7.48, 8.04, 2.11,

5.75, 5.24, 6.69, 4.77, 4.87, 2.04,

5.74, 5.20, 6.70, 4.28, 4.37,

5.75, 5.18, 6.68, 4.15, 4.20,

dd (2.0, 4.2) dd (4.2, 6.6) dd (2.0, 6.6) d (12.0) d (12.0) m m m m m s

dd (2.0, 4.6) dd (4.6, 6.6) dd (2.0, 6.6) d (11.8) d (11.8) m m m m m s

dd (2.1, 4.2) dd (4.2, 6.6) dd (2.1, 6.6) d (12.0) d (12.0) s

2.19, s

dd (2.1, 4.2) dd (4.2, 6.6) dd (2.1, 6.6) d (12.4) d (12.4)

2.19, s

10

dd (2.2, 4.0) dd (4.0, 6.6) dd (2.2, 6.6) d (11.8) d (11.8)

2.19, s 3.35, s

7.11, d (7.8) 5.71, d (7.8) 5.85, s 7.01, 4.96, 4.97, 8.00, 7.44, 7.57, 7.44, 8.00, 2.10,

s d (12.0) d (12.0) dd (8.0, 0.8) t (8.0) t (8.0) t (8.0) dd (8.0, 0.8) s

a

Recorded in methanol-d4.

Table 6. α-Glucosidase Inhibitory Activities of Compounds 1, 2, 6, 8, 10−15, 17, and 19−21

Table 5. 13C NMR (100 MHz) Spectroscopic Data of Compounds 6−10 in CDCl3 (δ in ppm) position

6

6a

7

8

9

10

2 3 4 5 6 7 8 1′ 2′ 3′ 4′ 5′ 6′ 7′ 1″ 2″ OMe-8

153.5 115.2 182.4 75.9 108.8 144.8 62.0 133.2 129.6 129.5 128.5 129.5 129.6 166.0 169.3 20.5

157.0 116.2 183.8 76.1 107.5 144.5 63.5 130.7 130.6 134.2 129.5 134.2 129.7 167.8 171.0 20.3

155.3 115.3 181.7 74.4 106.9 142.6 61.8 171.0 20.5

153.1 118.8 183.6 74.4 106.3 142.8 61.7

153.3 116.4 181.7 74.4 106.3 142.5 69.3

169.5 21.0

169.7 20.5

169.4 20.9 58.2

142.7 106.7 181.8 74.5 115.3 155.1 62.1 129.9 129.7 133.0 128.3 133.0 129.7 166.5 169.4 20.5

α-glucosidase inhibitory activity compound

% inhibition at 200 μg/mL

IC50 (μM)

1 2 6 8 10 11 12 13 14 15 17 19 20 21 acarbose

96.1 95.8 95.5 92.1 98.2 90.9 95.4 97.7 93.2 99.1 97.5 99.1 94.0 98.7

0.9 0.9 3.6 4.9 3.1 158.2 4.9 4.3 2.9 0.2 4.3 40.7 139.0 71.7 170.7

a

Recorded in methanol-d4.

Figure 4. ECD spectra of compounds 6−10 (in MeOH). E

DOI: 10.1021/acs.jnatprod.8b00581 J. Nat. Prod. XXXX, XXX, XXX−XXX

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3342, 2924, 2851, 1698, 1636, 1479, 1453, 1342, 1250, 1152, 1081, 769, 702 cm−1; see Table 1 for 1H NMR (CDCl3, 500 MHz) and Table 2 for 13C NMR (CDCl3, 125 MHz); HRESIMS m/z 377.1024 [M − H]− (calcd for C22H17O6, 377.1025). (−)-(2S)-Desmoscochinflavanone B (4): pale yellow crystalline solid; mp 225−228 °C; [α]26D −50 (c 0.1, MeOH); ECD (c 3.4 × 10−3 M, MeOH) λmax (Δε) 223 (+0.61) and 286 (−0.53) nm; UV (MeOH) λmax (log ε) 236 (1.68), 288 (1.75) nm; IR (neat) νmax 3284, 2924, 2852, 1700, 1637, 1497, 1453, 1343, 1297, 1155, 1063, 743 cm−1; see Table 1 for 1H NMR (CDCl3, 500 MHz) and Table 2 for 13C NMR (CDCl3, 125 MHz); HRESIMS m/z 377.1029 [M − H]− (calcd for C22H17O6, 377.1025). Desmoscochin benzoate (5): white crystalline solid; mp 103−105 °C; UV (MeOH) λmax (log ε) 229 (2.02), 272 (1.23) nm; IR (neat) νmax 3389, 2922, 2852, 1759, 1718, 1600, 1496, 1451, 1370, 1215, 1115, 712 cm−1; see Table 3 for 1H NMR (CDCl3, 400 MHz) and 13 C NMR (CDCl3, 100 MHz); HRESIMS m/z 309.0745 [M + Na]+ (calcd for C16H14O5Na, 309.0739). (−)-(5R)-Desmoscochinoxepinone A (6): viscous oil; [α]25D −30 (c 0.4, CHCl3); ECD (c 3.6 × 10−4 M, MeOH) λmax (Δε) 253 (−4.59) and 316 (+5.58) nm; UV (MeOH) λmax (log ε) 228 (2.66), 258 (1.73) nm; IR (neat) νmax 2925, 2854, 1720, 1603, 1271, 1228 cm−1; see Table 4 for 1H NMR (CDCl3, 500 MHz) and Table 5 for 13 C NMR (CDCl3, 125 MHz); HRESIMS m/z 325.0687 [M + Na]+ (calcd for C16H14O6Na, 325.0688). (−)-(5R)-Desmoscochinoxepinone B (7): viscous oil; [α]25D −22 (c 0.3, CHCl3); ECD (c 7.4 × 10−4 M, MeOH) λmax (Δε) 253 (−2.11) and 316 (+1.82) nm; UV (MeOH) λmax (log ε 228 (1.32), 258 (1.11) nm; IR (neat) νmax 2955, 2923, 2852, 1737, 1684, 1606, 1231 cm−1; see Table 4 for 1H NMR (CDCl3, 500 MHz) and Table 5 for 13C NMR (CDCl3, 125 MHz); HRESIMS m/z 263.0535 [M + Na]+ (calcd for C11H12O6Na, 263.0532). (−)-(5R)-Desmoscochinoxepinone C (8): viscous oil; [α]25D −29 (c 1.0, CHCl3); ECD (c 4.0 × 10−3 M, MeOH) λmax (Δε) 253 (−1.78) and 316 (+0.64) nm; UV (MeOH) λmax (log ε) 228 (2.48), 258 (2.14) nm; IR (neat) νmax 3469, 2928, 1746, 1681, 1610, 1229 cm−1; see Table 4 for 1H NMR (CDCl3, 500 MHz) and Table 5 for 13 C NMR (CDCl3, 125 MHz); HRESIMS m/z 221.0418 [M + Na]+ (calcd for C9H10O5Na, 221.0426). (−)-(5R)-Desmoscochinoxepinone D (9): viscous oil; [α]25D −51 (c 0.8, CHCl3); ECD (c 3.0 × 10−3 M, MeOH) λmax (Δε) 253 (−1.86) and 316 (+0.60) nm; UV (MeOH) λmax (log ε) 228 (1.63), 258 (1.29) nm; IR (neat) νmax 2925, 1747, 1684, 1609, 1227 cm−1; see Table 4 for 1H NMR (CDCl3, 500 MHz) and Table 5 for 13C NMR (CDCl3, 125 MHz); HRESIMS m/z 235.0579 [M + Na]+ (calcd for C10H12O5Na, 235.0582). (−)-(5R)-Grandiuvarone (10): viscous oil; [α]25D −22 (c 0.4, CHCl3); ECD (c 1.0 × 10−3 M, MeOH) λmax (Δε) 253 (−6.28) and 316 (+1.14) nm; UV (MeOH) λmax (log ε) 238 (1.23), 274 (1.20) nm; IR (neat) νmax 2924, 1748, 1718, 1691, 1603, 1267, 1224 cm−1; see Table 4 for 1H NMR (CDCl3, 500 MHz) and Table 5 for 13C NMR (CDCl3, 125 MHz); HRESIMS m/z 325.0689 [M + Na]+ (calcd for C16H14O6Na, 325.0688). Single-Crystal X-ray Data for 1. Single crystals of C22H16O6, 0.61 × 0.11 × 0.08 mm3, M = 376.35, P21/n with a = 4.5167(2) Å, b = 16.8440(6) Å, c = 22.4387(8) Å, β = 92.700(3)°, V = 1705.23(11) Å3, Z = 4, T = 150.00(10) K, μ(mm−1) = 0.107 mm−1, Dcalc = 1.466 g/cm3, 18 642 reflections measured (3.634° ≤ 2θ ≤ 61.602°), 5122 unique (Rint = 0.0397, Rsigma = 0.0379), which were used in all calculations. The final R1 was 0.0497 (I > 2σ(I)), and wR2 was 0.1332 (all data). Crystallographic data for 1 (CCDC 1851984) can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. α-Glucosidase Assay. A colorimetric α-glucosidase (Sigma, St. Louis, MO, USA, cat. no. G5003) assay was performed by the previous method.28 Acarbose was used as a positive control with an IC50 value of 170.7 μM.

EXPERIMENTAL SECTION

General Experimental Procedures. The chromatographic materials and information on instruments were the same as previously reported28−31 except for single-crystal X-ray diffraction measurements that were done on an XtaLAB Mini II diffractometer at 150 K with Mo Kα radiation. Plant Material. The twigs and leaves of D. cochinchinensis were collected in August 2015 from an authentically identified plant growing at Mae Fah Luang University Health Park, Chiang Rai Province, Thailand. The plant was identified by Assoc. Prof. Surat Laphookhieo, and a voucher specimen (No. MFU-NPR0157) was deposited at the Natural Products Research Laboratory, School of Science, Mae Fah Luang University. Extraction and Isolation. Air-dried twigs (2.93 kg) and air-dried leaves of D. cochinchinensis (2.01 kg) were individually extracted with EtOAc to yield 151.8 and 210.0 g of extracts, respectively. The twig extract was subjected to quick column chromatography (QCC) over silica gel (100% hexanes to 100% EtOAc) to give 12 fractions (Fr.1A−1L). Fr.G (1.18 g) was subjected to CC using reversed-phase silica gel (4:1 MeOH−H2O) to obtain 12 (6.6 mg), 19 (7.6 mg), and three fractions (Fr.1GA−1GC). Compound 22 (13.2 mg) was obtained from Fr.1GA (223.1 mg) by CC (100% CH2Cl2). Fr.I (345.6 mg) was further separated over Sephadex LH-20 CC to give Fr.1IA and Fr.1IB. Fr.1IB (28.3 mg) was further purified by CC (5:95 acetone−CH2Cl2) to afford 13 (2.5 mg). Fr.1J (428.2 mg) was subjected to Sephadex LH-20 CC to afford Fr.1JA and Fr.1JB. Further purification of Fr.1JA (247.6 mg) by repeated CC using (5:95 EtOAc−CH2Cl2) gave 10 (6.3 mg) and 6 (5.4 mg). Fr.1K (916.8 mg) was further separated by CC using reversed-phase silica gel (4:1 MeOH−H2O) to afford three fractions (Fr.1KA−1KC). Fr.KA (227.7 mg) was chromatographed by CC (1:1 EtOAc−n-hexane), yielding 21 (5.4 mg). Fr.1L (9.68 g) was subjected to Sephadex LH-20 CC to give Fr.1LA and Fr.1LB. Compounds 23 (3.1 mg) and 24 (3.6 mg) were obtained from Fr.1LB (416.5 mg) by repeated CC (5:95 MeOH−CH2Cl2). Similarly, the leaf extract of D. cochinchinensis was subjected to QCC over silica gel (100% hexanes to 100% acetone) to give eight fractions (Fr.2A−2H). Fr.2C (2.01 g) was subjected to CC using reversed-phase silica gel (4:1 MeOH−H2O) to afford six fractions (Fr.2CA−2CF). Fr.2CA (341.4 mg) was further purified by CC (1:4 acetone−n-hexane), yielding 6 (2.0 mg), 19 (39.0 mg), and 22 (15.2 mg). Fr.2CB (4.48 g) was further purified by CC (1:4 acetone−nhexane) to afford 11 (2.3 mg), 16 (20.0 mg), 17 (9.9 mg), and 20 (17.1 mg). Fr.2F (14.3 g) was washed with MeOH to yield 12 (4.1 mg), and the remaining part of Fr.2F was further separated by CC using reversed-phase silica gel (4:1 MeOH−H2O) to obtain Fr.2FA and Fr.2FB. Purification of Fr.2FA (12.1 g) by CC (2:3 EtOAc−nhexane) gave 7 (5.3 mg), 8 (14.5 mg), 9 (12.9 mg), 5 (2.9 mg), and 18 (4.2 mg). Fr.2G (4.67 g) was subjected to CC using reversedphase silica gel (4:1 MeOH−H2O) to afford three fractions (Fr.2GA−2GC). Compounds 1 (1.8 mg), 2 (2.0 mg), 3 (3.8 mg), 4 (2.1 mg), 14 (9.1 mg), and 15 (2.2 mg) were obtained from Fr.2GC (3.01 g) by repeated CC (5:95 MeOH−CH2Cl2). Desmoscochinflavone A (1): yellow crystalline solid; mp 232−235 °C; UV (MeOH) λmax (log ε) 258 (3.02), 296 (3.14) 330 (3.22) nm; IR (neat) νmax 3323, 2916, 1654, 1453, 1355, 1280, 1000, 774 cm−1; see Table 1 for 1H NMR (methanol-d4, 500 MHz) and Table 2 for 13 C NMR (methanol-d4, 125 MHz); HRESIMS m/z 375.0882 [M − H]− (calcd for C22H15O6, 375.0869). Desmoscochinflavone B (2): yellow crystalline solid; mp 228−230 °C; UV (MeOH) λmax (log ε) 258 (1.23), 296 (1.56) 330 (2.04) nm; IR (neat) νmax 3295, 2918, 1654, 1453, 1355, 1279, 1000, 774 cm−1; see Table 1 for 1H NMR (methanol-d4, 500 MHz) and Table 2 for 13 C NMR (methanol-d4, 125 MHz); HRESIMS m/z 375.0873 [M − H]− (calcd for C22H15O6, 375.0869). (−)-(2S)-Desmoscochinflavanone A (3): pale yellow crystalline solid; mp 218−221 °C; [α]26D −53 (c 0.9, MeOH); ECD (c 1.0 × 10−2 M, MeOH) λmax (Δε) 223 (+0.53) and 286 (−0.38) nm; UV (MeOH) λmax (log ε) 236 (2.03), 292 (1.69) nm; IR (neat) νmax F

DOI: 10.1021/acs.jnatprod.8b00581 J. Nat. Prod. XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.8b00581.

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HRESIMS and 1D and 2D NMR spectra of new compounds 1−10 (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +66-5391-6238. Fax: +66-5391-6776. E-mail: surat.lap@ mfu.ac.th (S. Laphookhieo). ORCID

Stephen G. Pyne: 0000-0003-0462-0277 Surat Laphookhieo: 0000-0002-4757-2781 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Thailand Research Fund (BRG5980012 and PHD/0010/2558) and Mae Fah Luang University. The University of Wollongong is also acknowledged for laboratory facilities.

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DOI: 10.1021/acs.jnatprod.8b00581 J. Nat. Prod. XXXX, XXX, XXX−XXX