Chumao Wen‡
a,
Su Chen‡a,
Fang Yuana,
Xiangming Liua,
Fajun Songc,
Zhinan Meib,
Xiaofei Yang*a and
Guangzhong Yang*b
aCollege of Biomedical Engineering, South-Central University for Nationalities, Wuhan 430074, P. R. China. E-mail: sunlittlefly@hotmail.com; Fax: +86 27 6784 1196; Tel: +86 27 6784 1196
bSchool of Pharmaceutical Sciences, South-Central University for Nationalities, Wuhan 430074, P. R. China. E-mail: yanggz888@126.com
cCollege of Life Science, South Central University for Nationalities, Wuhan 430074, P. R. China
First published on 9th December 2019
Six new ent-kaurane diterpenoids, isodonrubescins A–F (1–6), together with twenty-five known ent-kaurane diterpenoids (7–31), a known ent-atisane diterpenoid (32), and two known ent-abietane diterpenoids (33–34), were isolated from Isodon rubescens. Their structures were established by means of extensive MS and NMR data analysis. Among the all isolates, compound 7 was found in a natural product for the first time, and ent-atisane diterpenoid was discovered from I. rubescens in Hubei Province, P. R. China for the first time. Furthermore, all the isolated compounds were tested for their NO production inhibitory activity in LPS stimulated RAW264.7 cells. Compounds 7–9, 12, 13, 16, and 17 displayed NO production inhibitory activities with IC50 values ranging from 1.36 to 18.25 μM, respectively.
Isodon rubescens is a perennial herb distributed widely in Henan, Guizhou, Hebei, Jiangxi, Hubei, and some other provinces of P. R. China.4 It has attracted great attention due to the traditional uses in folk medicine for the treatment of respiratory and gastrointestinal bacterial infections, inflammation, and cancer.5–8 Oridonin, an important ent-kaurane from I. rubescens showed the anti-tumor and anti-inflammatory activities. Previous studies have demonstrated that it exhibits anti-tumor effects on human cancer cells, such as HepG2, SGC-7901, MCF-7, mainly by blocking the cell cycle, inducing apoptosis and autophagy of tumor cells, and shows anti-inflammatory effects by inhibiting the expression of inflammatory factors through nuclear factor-kappa B (NF-κB) signal pathway.9,10 In addition, previous investigations on the chemical constituents of I. rubescens collected from different provinces, P. R. China revealed that they contained different structure types of diterpenoids. For example, the chemical constituents of I. rubescens collected from Guizhou Province were mainly 6,7-seco-ent-kaurane diterpenoids, however, 7,20-epoxy-ent-kaurane diterpenoids were main chemical constituents of I. rubescens collected from Henan Province.11,12 Furthermore, the chemical constituents of I. rubescens collected from Hubei Province have not been extensively investigated, only 16 new diterpenoids have been reported, including diterpene alkaloids with an aza-ent-kaurane skeleton.13–17 Therefore, in order to fully understand the active constituents of I. rubescens from different regions, a reinvestigation of I. rubescens collected from Badong county, Hubei Province was undertaken in the hope of discovering diterpenoids with interesting structures and biological activities. As a result, six new diterpenoids (1–6), together with twenty-five known ent-kaurane diterpenoids (7–31), a known ent-atisane diterpenoid (32) and two known ent-abietane diterpenoids (33–34) were isolated from this plant, and it should be noted that compounds 10, and 13–17 have been reported in our previous work.18 Herein we reported the isolation, structural elucidation of six new diterpenoids and biological activities of all isolated compounds (Fig. 1).
No. | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
1 | 4.89 (m) | 4.92 (dd, 6.9, 10.5) | 4.35 (m) | 3.74 (m) | 1.74–1.86 (m) | 6.38 (dd, 2.4, 10.8) |
2 | 1.87 (m) | 1.91 (m) | 1.96 (m) | 1.86 (m) | 2.07 (m); 1.09 (d, 12.6) | 5.73 (ddd, 1.8, 6.0, 10.2) |
3 | 1.31 (m) | 1.37 (m); 1.32 (m) | 1.51 (m); 1.42 (m) | 1.34 (m) | 3.75 (s) | 1.94 (overlap); 1.77 (dd, 6.0, 16.8) |
4 | ||||||
5 | 3.48 (s) | 3.27 (s) | 1.74 (d, 5.4) | 1.77 (d, 6.6) | 2.44 (d, 6.6) | 2.16 (d, 6.6) |
6 | 5.03 (s) | 5.76 (s) | 4.24 (dd, 5.4, 3.6) | 5.86 (d, 6.6) | 4.33 (dd, 2.4, 6.6) | 4.31 (dd, 4.8, 6.6) |
7 | ||||||
8 | ||||||
9 | 2.65 (m) | 2.93 (d, 10.8) | 2.82 (d, 9.6) | 2.92 (dd, 6.0, 12.9) | 2.84 (overlap) | 2.88 (dd, 1.2, 9.6) |
10 | ||||||
11 | 4.46 (m) | 4.55 (dd, 8.7, 18.9) | 4.73 (m) | 2.31 (m); 1.91 (m) | 1.54 (m); 1.23 (m) | 4.58 (m) |
12 | 2.91 (m); 1.60 (dd, 9.0, 13.8) | 2.95 (m); 1.58 (dd, 9.0, 14.4) | 2.86 (m); 1.91 (m) | 2.42 (m); 1.73 (m) | 2.32 (m); 1.61 (m) | 2.96 (m); 1.91 (overlap) |
13 | 2.61 (dd, 4.2, 9.6) | 2.68 (dd, 3.6, 9.6) | 2.72 (dd, 9.6, 3.6) | 2.87 (d, 9.6) | 2.84 (overlap) | 2.76 (dd, 5.4, 10.4) |
14 | 2.68 (m); 2.33 (dd, 4.2, 12.0) | 2.72 (d, 12.0); 2.39 (dd, 3.9, 12.3) | 2.22 (d, 12.6); 2.17 (dd, 12.6, 4.2) | 5.15 (overlap) | 5.08 (s) | 2.19 (dd, 4.8, 12.6); 2.01 (d, 12.6) |
15 | 6.55 (s) | 5.57 (d, 2.4) | 6.93 (s) | 5.20 (overlap) | ||
16 | 2.67 (m) | 2.63 (br t, 5.4) | ||||
17 | 3.60 (m); 3.52 (dd, 4.2, 9.0) | 3.61 (m) | 5.29 (s); 5.12 (s) | 5.69 (s); 5.40 (s) | 5.41 (s); 5.28 (s) | 5.51 (s); 5.22 (overlap) |
18 | 0.96 (s) | 0.98 (s) | 1.24 (s) | 0.92 (s) | 1.57 (s) | 1.20 (s) |
19 | 0.93 (s) | 0.98 (s) | 1.21 (s) | 1.22 (s) | 1.21 (s) | 1.12 (s) |
20 | 4.32 (d, 9.6); 4.28 (d, 9.6) | 4.42 (d, 9.0); 4.30 (d, 9.0) | 4.81 (d, 9.6); 4.50 (d, 9.6) | 4.86 (d, 9.6); 4.46 (d, 9.6) | 4.36 (d, 9.6); 4.05 (d, 9.6) | 4.36 (d, 9.6); 4.16 (dd, 1.2, 9.6) |
OAc | 2.20 (s) | 2.21 (s) | 2.28 (s) | |||
OMe | 3.19 (s) | |||||
OMe | 3.15 (s) | |||||
OCH2CH3 | 3.32 (m); 1.07 (t, 7.2) | |||||
HO-1 | 6.75 (d, 4.2) | 5.97 (d, 4.2) | ||||
HO-3 | 6.00 (s) | |||||
HO-6 | 9.11 (s) | 6.25 (d, 3.0) | 5.91 (s) | 8.17 (d, 4.2) | ||
HO-7 | 7.91 (s) | 8.31 (s) | 8.01 (s) | 8.07 (s) | ||
HO-11 | 5.75 (s) | 7.24 (overlap) | 7.10 (d, 6.0) | 5.77 (br s) | ||
HO-14 | 8.01 (s) | 8.06 (s) | ||||
HO-15 | 4.40 (d, 3.0) | 6.84 (d, 2.4) |
No. | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
1 | 76.9(d) | 77.1(d) | 74.2(d) | 73.5(d) | 26.4(t) | 130.2(d) |
2 | 24.2(t) | 24.4(t) | 28.8(t) | 30.7(t) | 24.6(t) | 125.3(d) |
3 | 37.1(t) | 37.3(t) | 40.5(t) | 39.1(t) | 74.8(d) | 41.4(t) |
4 | 31.7(s) | 31.9(s) | 34.5(s) | 34.2(s) | 38.7(s) | 32.7(s) |
5 | 53.1(d) | 54.2(d) | 57.3(d) | 55.5(d) | 50.3(d) | 57.5(d) |
6 | 109.7(d) | 102.5(d) | 74.5(d) | 75.3(d) | 73.5(d) | 74.3(d) |
7 | 171.3(s) | 171.4(s) | 97.2(s) | 98.8(s) | 100.1(s) | 97.6(s) |
8 | 57.7(s) | 57.4(s) | 53.1(s) | 53.8(s) | 53.0(s) | 53.8(s) |
9 | 52.6(d) | 52.7(d) | 52.1(d) | 45.7(d) | 46.5(d) | 50.8(d) |
10 | 51.2(s) | 51.3(s) | 42.9(s) | 41.7(s) | 35.9(s) | 39.4(s) |
11 | 64.8(d) | 63.9(d) | 63.6(d) | 18.8(t) | 15.4(t) | 62.8(d) |
12 | 42.1(t) | 41.9(t) | 41.4(t) | 33.0(t) | 32.4(t) | 45.7(t) |
13 | 32.2(d) | 32.1(d) | 37.7(d) | 46.2(d) | 46.1(d) | 37.2(d) |
14 | 34.3(t) | 34.1(t) | 28.9(t) | 76.4(d) | 76.4(d) | 28.0(t) |
15 | 212.9(s) | 213.1(s) | 75.3(d) | 73.4(d) | 74.2(d) | 75.9(d) |
16 | 58.5(d) | 58.8(d) | 160.7(s) | 161.5(s) | 160.0(s) | 161.7(s) |
17 | 71.8(t) | 69.5(t) | 107.7(t) | 110.2(t) | 110.6(t) | 107.9(t) |
18 | 33.1(q) | 33.3(q) | 33.8(q) | 32.0(q) | 29.8(q) | 31.4(q) |
19 | 23.6(q) | 23.4(q) | 23.3(q) | 22.2(q) | 23.8(q) | 22.7(q) |
20 | 74.5(t) | 74.0(t) | 64.7(t) | 64.1(t) | 67.0(t) | 66.8(t) |
OAc | 171.4(s) | 169.6(s) | 171.6(s) | |||
22.3(q) | 21.7(q) | 22.5(q) | ||||
OMe | 54.8(q) | |||||
OMe | 59.0(q) | |||||
OCH2CH3 | 66.9(t) | |||||
15.5(q) |
Compound 2 was isolated as colorless crystals (MeOH), and its molecular formula was the same as 1, as established to be C22H32O7 by HR-ESI-MS at m/z 431.20407 [M + Na]+ (calcd for C22H32O7Na, 431.20402) and 13C NMR data. A comparison of the NMR data of 2 (Tables 1 and 2) with those of 1 suggested that compound 2 had a 6,7-seco-ent-kaurane skeleton as 1, except for the disappearance of two methoxyl signal and the presence of an additional ethoxyl signal. The location of the ethoxyl group at C-17 was revealed by the HMBC correlations of H2-17 (δH 3.61) with the carbon (δC 66.9) of the ethoxyl group. In addition, the chemical shift of C-6 was shifted upfield from δC 109.7 in 1 to δC 102.5 in 2 due to the change of the substituent at C-6 from a methoxyl group in 1 to a hydroxyl group in 2. The relative stereochemistry of 2 was consistent with those of 1 and was ensured by the ROESY correlations (Fig. 3). Accordingly, the structure of compound 2 was established as 6β,11α-dihydroxy-16β-ethoxymethyl-6,7-seco-6,20-exoxy-1α,7-olide-ent-kaur-15-one and given the name isodonrubescin B.
Compound 3 was exhibited to have the molecular formula C22H32O7 by HR-ESI-MS (m/z 431.20288 [M + Na]+, calcd 431.20402). The 1H-NMR spectra (Table 1) of 3 established the existence of three single methyls [δH 1.24 (s), 1.21 (s), 2.20 (s)], one olefinic methylene [δH 5.29 (s), 5.12 (s)], one oxygenated methylene [δH 4.81 (d, J = 9.6 Hz), 4.50 (d, J = 9.6 Hz)], four oxygenated methines [δH 4.35 (m), 4.24 (dd, J = 5.4, 3.6 Hz), 4.73 (m), 6.55 (s)]. The methyl at δH 2.20 (3H, s) and the carbonyl group at δC 171.4 in the NMR spectrum suggested the presence of an acetoxyl group in 3. Apart from the acetoxyl group, there were 20 carbon resonances, consisting of two methyls, six methylenes (one oxygenated carbon at δC 64.7 and one olefinic carbon at δC 107.7), seven methines (four oxygenated carbons at δC 74.2, δC 74.5, δC 63.6 and δC 75.3, respectively), and five quaternary carbons (one hemiacetal group at δC 97.2 and one olefinic carbon at δC 160.7). The above-mentioned data suggested compound 3 to be a 7,20-epoxy-ent-kaurane diterpenoid. Comparison of the NMR date of 3 with those of hebeirubescensin K20 indicated that their structures were closely related. The only structural difference between them was that the hydroxyl group at C-15 in the latter was replaced by an acetoxyl group in 3, which can be deduced by the change of the chemical shift of H-15 from δH 5.06 in the latter to δH 6.55 in 3 and was further confirmed by the HMBC correlations (Fig. 2) from H-15 to C-16 (δC 160.7) and OAc (δC 171.4). The remaining structure was corroborated by the HMBC experiment.
The relative configuration of 3 was revealed by analysis of the ROESY spectrum (Fig. 3), in which the correlations of H-6/H3-19α (δH 1.21), H-11/H-12α (δH 2.86)/H-13α (δH 2.72), H-15/H-14β (δH 2.17)/H-13α were clearly observed, indicating that HO-6, HO-11, and AcO-15 were β-orientation. Correlations of H-1/H-5β assigned HO-1 to be α-oriented. Thus. Compound 3 was determined as 1α,6β,11β-trihydroxy-15β-acetoxy-7,20-exoxy-ent-kaur-16-ene, and named as isodonrubescin C.
Compound 4 had the same molecular formula C22H32O7 as that of 3, which was established by HR-ESI-MS at m/z 431.20404 [M + Na]+ (calcd for C22H32O7Na, 431.20402). Its 1H and 13C NMR spectra (Tables 1 and 2) showed that compound 4 possessed the same 7,20-epoxy-ent-kaurane skeleton as that of 3. A comparision of the NMR data of 4 (Tables 1 and 2) with those of enmenol21 disclosed that 4 was a 6-acetyl derivative of enmenol. The key HMBC correlation (Fig. 2) from H-6 (δH 5.86) to OAc (δC 169.6) in 4 confirmed this conclusion. Furthermore, the 1H–1H COSY correlations (Fig. 2) of H-1 (δH 3.74) with H2-2 (δH 1.86), of H-14 (δH 5.15) with H-13 (δH 2.87) and the HMBC correlations of H-15 (δH 5.57) with C-16 (δC 161.5) and C-17 (δC 110.2) indicated that three hydroxyl groups were located at C-1, C-14 and C-15 respectively. The relative configuration of 4 was assigned by the ROESY correlations (Fig. 3) of H-1/H-9β (δH 2.92), H-6/H3-19α (δH 1.22), HO-15 (4.40)/H-9β (2.92), which revealed the α-orientation of HO-1 and the β-orientation of AcO-6, HO-14, HO-15. Therefore, the structure of 4 was elucidated as 1α,14β,15β-trihydroxy-6β-acetoxy-7,20-exoxy-ent-kaur-16-ene, and given the name isodonrubescin D.
Compound 5 was obtained as a white amorphous powder with a molecular formula of C22H32O7 as assigned by HR-ESI-MS (m/z 431.20380 [M + Na]+, calcd 431.20402). Its 1H and 13C NMR data (Tables 1 and 2) resembled those of hikiokoshins G,22 suggesting that 5 had the same carbon skeleton as that of hikiokoshins G. The difference between them was that hikiokoshins G had two acetoxyl groups while compound 5 only possessed one acetoxyl group, and in the HMBC spectrum of 5 (Fig. 2), the cross-peak of H-15 with OAc (δC 171.6) indicated that the acetoxyl group was located at C-15. Thus, 5 was a 6-deacetyl derivative of hikiokoshins G, this conclusion was further supported by the change of the chemical shift of H-6 from δH 5.98 in hikiokoshins G to δH 4.33 in 5. The relative stereochemistry of 5 was consistent with those of hikiokoshins G, and was confirmed by the ROESY analysis (Fig. 3). Accordingly, compound 5 was established as 3β,6β,14β-trihydroxy-15β-acetoxy-7,20-exoxy-ent-kaur-16-ene, and named as isodonrubescin E.
Compound 6 had the molecular formula of C20H28O5 as determined by its HR-ESI-MS (m/z 349.20029 [M + H]+, calcd 349.20095) and 13C NMR data, indicating seven degrees of unsaturation. The 1H NMR and 13C NMR spectra (Tables 1 and Table 2) of 6 implied that compound 6 was a 7,20-epoxy-ent-kaurane diterpenoid. However, unlike the normal type of 7,20-epoxy-ent-kaurane diterpenoids, such as compound 3–5, a cis double bond signal [δH 6.38 (dd, J = 2.4, 10.8 Hz), 5.73 (ddd, J = 1.8, 6.0, 10.2 Hz); δC 130.2, 125.3] was presented in the NMR spectra of 6, and the double bond was assigned to C-1 and C-2 by the key 1H–1H COSY correlations of 6 (Fig. 2) from H-1 (δH 6.38) to H-2 (δH 5.73), from H-2 to H-3 (δH 1.94) and the key HMBC correlations (Fig. 2) from H3-18 (δH 1.20) to C-3 (δC 41.4), from H-2 to C-10 (δC 39.4). The remaining three hydroxyl groups were respectively assigned to C-6, C-11 and C-15 by interpretation of the 1H–1H COSY and HMBC correlations. The relative configuration of 6 was determined by the ROESY correlations (Fig. 3) of H-6 (δH 4.31)/H3-19α (δH 1.12), H-11 (δH 4.58)/H-20 (δH 4.36) and H-15 (δH 5.20)/H-13α (δH 2.76), which suggested the β-orientation of HO-6, HO-11 and HO-15. Consequently, the structure of 6 was assigned as 6β,11β,15β-trihydroxy-7,20-exoxy-ent-kaur-1,16-diene, and given the name isodonrubescin F.
The other twenty-eight known diterpenoids (7–34) were identified by comparison of their NMR data with those reported in the literature. As a result, they were identified to be 3β-hydroxy-6β-methoxy-6,7-seco-6,20-epoxy-1α,7-olide-ent-kaur-16-en-15-one (7),23 enmein (8),20 rabdosin A (9),19 epinodosinol (10),18 isojaponin A (11),24 epinodosin (12),25 oridonin (13),18 hubeirubesin K (14),18 neolaxiflorin U (15),18 hubeirubesin I (16),18 lasiokaurin (17),18 hebeirubescensin K (18),20 maoyecrystal F (19),24 rabdoternin D (20),26 lasiodonin (21),27 enmelol (22),28 rabdonervosin G (23),29 rabdonervosin D (24),29 hikiokoshins G (25),22 isodonhenrin E (26),30 maoyecrystal L (27),20 dayecrystal B (28),31 lushanrubescensin F (29),32 ponicidin (30),33 rubescensin D (31),34 isorosthornin D (32),35 isoadenolin M (33),36 rubescensin J (34).37
In addition, all the isolated compounds were assessed for their inhibitory activity against NO production in LPS stimulated RAW264.7 cells with dexamethasone as a positive control (IC50 = 9.58 μM). The cell viability of the tested compounds was firstly measured using CCK-8 assay to determine whether the NO production inhibitory activities were induced by the cytotoxicity. As a result, compounds 7, 9, 13, 16, and 17 exhibited obvious NO production inhibitory effects with IC50 values of 3.97, 2.25, 6.51, 1.48 and 1.36 μM, respectively. Compounds 8 and 12 displayed mild NO production inhibitory effects with IC50 values of 17.43 and 18.25 μM, respectively, while the rest of the tested compounds had no obvious NO production inhibitory activity (IC50 > 20 μM). In the present study, the 6,7-seco-ent-kaurane diterpenoids, such as 7–9 and 12 which possessed an α,β-unsaturated ketone moiety, exhibited NO production inhibitory effects, the result indicated that α,β-unsaturated ketone moiety was an essential pharmacophore. However, this conclusion did not fully be applied to 7,20-epoxy-ent-kaurane diterpenoids. For compounds 13, 17, 21 and 30, they shared an α,β-unsaturated ketone moiety, but compound 21 and 30 did not show the activity. This could be caused by the lack of HO-14β in 21 and 30. Additionally, compound 16 without an α,β-unsaturated ketone moiety also exhibited obvious NO production inhibitory effects. This result further demonstrated that the α,β-unsaturated ketone moiety was not absolutely essential active center for the activity. Besides, it was interesting that compound 25 was a 3-deacetyl derivative of 16, but it did not show NO production inhibitory effect, the result suggested that 3β-OAc might played an important role in the NO production inhibitory activity.
Fr. E (56.3 g) was separated on RP-18 CC into six fractions (Fr. E1–Fr. E6) by eluting with MeOH–H2O (3:
7, 5
:
5, 7
:
3, 0
:
10). Fr. E2 and Fr. E4 was purified by recrystallizing in MeOH to afford compounds 8 (735 mg). Fr. E3 was firstly purified by a silica gel column (eluted with CH2Cl2–MeOH, 100
:
1, 50
:
1, 25
:
1, 15
:
1, 12
:
1 gradient) to yield nine fractions Fr. E3A–Fr. E3I. Fr. E3C was purified by recrystallizing in MeOH to afford compound 12 (8 mg), then Fr. E3A was subjected to silica gel CC (petroleum ether–EtOAc, 9
:
1, 8
:
2, 7
:
3 gradient) to obtain fractions E3A1–E3A8. Fr. E3A8 was finally purified by semi-preparative HPLC (MeOH–H2O 43
:
57) to afford compounds 1 (10 mg, tR 16.3 min) and 9 (18 mg, tR 17.6 min). Similarly, compound 7 (5 mg, tR 15.8 min) was obtained from Fr. E3B by semi-preparative HPLC (MeOH–H2O, 40
:
60). Fr. E3F was successively chromatographed over silica gel CC (CH2Cl2–MeOH, 50
:
1, 25
:
1, 12
:
1) and semi-preparative HPLC to yield compounds 6 (1 mg, MeOH–H2O, 36
:
64, tR 41.8 min), 26 (9 mg, MeOH–H2O, 36
:
64, tR 43.3 min), 18 (25 mg, MeOH–H2O, 45
:
55, tR 22.4 min), 19 (19 mg, MeOH–H2O, 47
:
53, tR 22.3 min) and 32 (5 mg, MeOH–H2O, 36
:
64, tR 46.3 min). Fr. E6 was similarly purified with semi-preparative HPLC to yield compounds 33 (1.5 mg, CH3CN–H2O, 77
:
23, tR 14.2 min) and 34 (7 mg, MeOH–H2O, 79
:
21, tR 15.5 min).
Fr. F (51.3 g) was separated over RP-18 CC (MeOH–H2O, 3:
7, 5
:
5, 7
:
3, 0
:
10 gradient) into five fractions (Fr. F1–Fr. F5), Fr. F2 and Fr. F3 was separated over repeatedly chromatographed by silica gel column, and then further purified by semi-preparative HPLC to afford compounds 3 (46 mg, MeOH–H2O, 49
:
51, tR 23.9 min), 4 (5.0 mg, MeOH–H2O, 35
:
65, tR 12.2 min), 5 (8.0 mg, MeOH–H2O, 25
:
75, tR 19.5 min), 11 (3.0 mg, MeOH–H2O, 60
:
40, tR 12.9 min), 20 (13 mg, CH3CN–H2O, 35
:
65, tR 11.7 min), 21 (23 mg, CH3CN–H2O, tR 7.7 min), 22 (2.5 mg, CH3CN–H2O, 25
:
75, tR 11.7 min), 23 (9 mg, MeOH–H2O, 45
:
55, tR 11.4 min), 24 (12 mg, MeOH–H2O, 35
:
65, tR 34.7 min), 27 (6.0 mg, MeOH–H2O, 40
:
60, tR 17.0 min), 28 (12 mg, CH3CN–H2O, 35
:
65, tR 8.8 min), 29 (23 mg, CH3CN–H2O, 35
:
65, tR 6.8 min), 30 (1.7 mg, MeOH–H2O, 35
:
65, tR 26.9 min), 31 (3 mg, CH3CN–H2O, 35
:
65, tR 10.8 min).
Isodonrubescin B (2): colorless crystals (MeOH); [α]D = −47.6° (c 0.01, MeOH); UV (MeOH) λmax (logε): 205 (2.92) nm; 1H and 13C NMR data see Tables 1 and 2; HR-ESI-MS m/z 431.20407 [M + Na]+ (calcd for C22H32O7Na, 431.20402).
Isodonrubescin C (3): colorless crystals (MeOH); [α]D = −101.0° (c 0.02, MeOH); UV (MeOH) λmax (logε): 205 (3.21), 250 (2.43) nm; 1H and 13C NMR data see Tables 1 and 2; HR-ESI-MS m/z 431.20288 [M + Na]+ (calcd for C22H32O7Na, 431.20402).
Isodonrubescin D (4): white amorphous powder; [α]D = +6.2° (c 0.03, MeOH); UV (MeOH) λmax (logε): 210 (3.24), 250 (2.29) nm; 1H and 13C NMR data see Tables 1 and 2; HR-ESI-MS m/z 431.20404 [M + Na]+ (calcd for C22H32O7Na, 431.20402).
Isodonrubescin E (5): white amorphous powder; [α]D = +9.1° (c 0.02, MeOH); UV (MeOH) λmax (logε): 205 (3.29), 250 (2.86) nm; 1H and 13C NMR data see Tables 1 and 2; HR-ESI-MS m/z 431.20380 [M + Na]+ (calcd for C22H32O7Na, 431.20402).
Isodonrubescin F (6): white amorphous powder; [α]D = +12.2° (c 0.02, MeOH); UV (MeOH) λmax (logε): 210 (3.38) nm; 1H and 13C NMR data see Tables 1 and 2; HR-ESI-MS m/z 349.20029 [M + H]+ (calcd for C20H29O5, 349.20095).
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra08831h |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2019 |