Yan-duo Wanga,
Yuan-yuan Lia,
Xiang-mei Tana,
Lin Chenb,
Zhong-qi Weic,
Li Shen*d and
Gang Ding*a
aKey Laboratory of Bioactive Substances and Resources Utilization of Chinese Herbal Medicine, Ministry of Education, Institute of Medicinal Plant Development, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100193, People's Republic of China. E-mail: gding@implad.ac.cn
bZhengzhou Key Laboratory of Synthetic Biology of Natural Products, Henan Joint International Research Laboratory of Drug Discovery of Small Molecules, Huanghe Science and Technology College, Zhengzhou, Henan 450063, People's Republic of China
cNanjing Vocational Health College, Nanjing, Jiangsu 210038, People's Republic of China
dInstitute of Translational Medicine, Medical College, Yangzhou University, Yangzhou, Jiangsu 225001, People's Republic of China. E-mail: shenli@yzu.edu.cn
First published on 23rd June 2020
Isochaetoglobosin Db is a new chaetoglobosin possessing a unique 3,4-substituted pyrrole ring isolated and named by Qiu et al., and it is different from any one of the 14 sub-types in the macrocyclic ring of chaetoglobosins classified in our previous work. Its chemical shift values, coupling constants and biosynthetic consideration implied that the proposed structure of isochaetoglobosin Db was incorrect. In this report, based on detailed NMR data analysis together with biosynthetic consideration, the structure of isochaetoglobosin Db is suggested to be revised to that of penochalasin C. The NMR spectra of penochalasin C measured in the same solvent (DMSO-d6) as that of isochaetoglobosin Db supported the above conclusion. The results imply that reasonable biosynthetic consideration could complement spectroscopic structural determination, and also support that the 1H-NMR rule of chaetoglobosin summarized in our previous work can provide help for dereplication and rectification.
If the fragment A is right, the chemical shift values of β-H/C (H/C-21) on the pyrrole ring are not reasonable compared with compounds possessing similar pyrrole units. There is an α,β-unsaturated ketone group in fragment A, and the β-position is connected with a nitrogen atom, which will lead the chemical shift values of β-H/C to be deshielded. The chemical shift values of β-H/C are 7.40/127.3, and 7.67/123.7 in verrucarin E9,10 and azamonosporascone11 with the similar α,β-unsaturated ketone group, whereas chemical shift values of β-H/C in fragment A of isochaetoglobosin Db were 6.60/113.7.6 The differences of β-H/C in similar fragment of three compounds are significant, implying that the structure of fragment A in isochaetoglobosin Db is not right (Fig. 3).
Fig. 3 β-H/C chemical shift values of isochaetoglobosin Db6, verrucarin E9,10 and azamonosporascone.11 |
The 13C-NMR chemical shift values of fragment A (including C-19) in isochaetoglobosin Db (1),6 and fragment B in penochalasins A–C,12 armochaetoblobsin K–M, and R were compared and analyzed (Fig. 4).13 Comparison of the 13C chemical shift values of C-19, C-20, C-21, C-22 and C-23 between isochaetoglobosin Db, penochalasins A–C, armochaetoblobsins K–M, and R revealed that the chemical shift values of these carbons were nearly same. They implied that the pyrrole ring (fragment A) in isochaetoglobosin Db should be reassigned as fragment B (Table 1).
Pos. | 1a | Penochalasin A (CDCl3) | Penochalasin B (CDCl3) | Penochalasin C (CDCl3) | Armochaetoglobsins K (CD3OD) | Armochaetoglobsins L (CDCl3) | Armochaetoglobsins M (CDCl3) | Armochaetoglobsins R (DMSO-d6) |
---|---|---|---|---|---|---|---|---|
19 | 189.3 | 188.47 | 189.49 | 188.04 | 190.5 | 190.6 | 187.6 | 191.1 |
20 | 130.1 | 126.79 | 126.81 | 126.90 | 130.4 | 130.5 | 129.7 | 129.5 |
21 | 113.7 | 114.92 | 114.46 | 115.07 | 116.4 | 116.4 | 114.8 | 114.4 |
22 | 108.0 | 109.48 | 109.47 | 109.17 | 111.3 | 111.2 | 108.9 | 106.4 |
23 | 140.7 | 138.90 | 138.50 | 139.81 | 142.3 | 142.2 | 137.2 | 139.1 |
Coupling constants analysis are also diagnostic about the substitution position on a pyrrole ring. If a pyrrole ring is substituted at C-2 and C-5 such as found in penochalasin C, the coupling constants of H-3/H-4 is, usually, more than 3.0 Hz (3J3,4 > 3.0 Hz); If a pyrrole ring is substituted at C-3 and C-4 such as found in isochaetoglobosin Db, the coupling constants of H-2/H-5 (as W-long-ranged correlation) is at 2.0–3.0 Hz (4J2,5 = 2.0–3.0 Hz).14–19 Analysis of the 1H NMR of isochaetoglobosin Db revealed that the coupling constants of H-21/H-22 was 3.6 Hz, which did not conform to the rule mentioned-above. On the contrary, the coupling constants of H-21/H-22 in penochalasin C was also 3.6 Hz (Fig. 5). These analyses further supported that the fragment A in isochaetoglobosin Db (1) should be assigned as fragment B.
Penochalasins A–C were first isolated from a marine alga symbiotic fungus Penicillium species in 1995, and other analogues including penochalasins D–H, and chaetoglobosin O were later isolated from the same fungus.12,15 It was the first report of chaetoglobosin analogues by possessing a unique pyrrole ring in the macrocylic ring system. From the structural features, the pyrrole ring in penochalasin A (penochalasin B) might be originated from penochalasin E/F (penochalasin H) through the possible intermediate chaetoglobosin C (chaetoglobosin G) by amination and dehydration at C-20, C-21, C-22 and C-23 (Fig. 6).
In 2006, our group isolated five analogues including chaetoglobosins C, E, F, U and penochalasin A from an endophytic fungus Chaetomium globosum IFB-E019.7 Though the structural relationship of these chaetoglobosins were not suggested at that time, the macrocyclic difference in chaetoglobosin C and chaetoglobosin U, penochalasin A implied that the additional cyclopent-2-en-1-one (C-17, C-18, C-19, C-20 and C-21, Fig. S14†) in chaetoglobosin U might be derived from chaetoglobosin C by the intramolecular Michael-addition reaction at C-17 and C-21, whereas the pyrrole ring in penochalasin A could be biosynthesized from chaetoglobosin C by same reactions as those found in Fig. 6.
Recently, Prof Zhang's group also isolated a series of new pyrrole-based chaetoglobosins armochaetoglobins K–R together with other new analogues from Chaetomium globosum (TW1-1).13,20,21 The authors suggested the possible biosynthetic pathway of pyrrole-based chaetoglobosins according to the structural features. When analyzing the structural characteristics, we found the same biosynthetic relationships of these analogues as those found in Fig. 6 and S14.† For example armochaetoglobin X might come from armochaetoglobin U by the intramolecular Michael-addition reaction, which could be originated from isochaetoglobosin J by oxidation and dehydration, whereas the pyrrole ring in armochaetoglobin K might be derived from isochaetoglobosin J by amination and dehydration (Fig. S15†).
Qiu et al. reported two new chaetoglobosin analogues isochaetoglobosin Db and cytoglobosin Ab isolated from an extreme fungus C. globosum SNSHI-5.6 Though the authors did not report known an alogues or possible intermediates from this fungus, according to the structural characteristics, the possible biosynthetic relationship from these chaetoglobosins were suggested, which possesses the same biosynthetic pathway as those found in Fig. 6, and S14–S16.† Thus, the 3,4-substituted pyrrole in isochaetoglobosin Db should be reassigned to be the 2,5-substituted pyrrole in penochalasin C. This result also conforms to the rule summarized in our previous report.
Fortunately, penochalasin C was isolated from an endophytic fungus C. globosum in our lab.22,23 The NMR spectra of penochalasin C were obtained in the same solvent system (DMSO-d6) as that of isochaetoglobosin Db (ESI†). 1H–1H COSY spectrum revealed the correlations H-21, H-22 and 24-NH, and the HMBC correlations from H-21 to C-20, C-22, and C-23, from H-22 to C-20, C-21, and C-23 confirmed a 2,5-substituted pyrrole unit in penochalasin C. The other NMR data including 1H, and 13C data of penochalasin C were the same as those of isochaetoglobosin Db, which further confirmed the conclusion that penochalasin C and isochaetoglobosin Db were the same structure (Table 2).
Position | Isochaetoglobosin Db (DMSO-d6) | Penochalasin C (DMSO-d6) | Penochalasin C (CDCl3) | |||
---|---|---|---|---|---|---|
δH, mult (J in Hz) | δC | δH, mult (J in Hz) | δC | δH, mult (J in Hz) | δC | |
a There are some signal assignments are corrected for isochaetoglobosin Db. | ||||||
1 | 175.3, C | 175.2, C | 169.87, C | |||
2 | 8.16, s | 8.13, s | 5.80, br s | |||
3 | 3.30, m | 53.4, CH | 3.29, m | 53.3, CH | 3.54, dt (10.2, 4.0) | 53.16, CH |
4 | 2.35, m | 51.6, CH | 2.35, m | 51.5, CH | 2.75, t (4.0) | 53.00, CH |
5 | 2.72, m | 31.8, CH | 2.72, m | 31.8, CH | 2.98, qd (6.5, 4.0) | 32.32, CH |
6 | 151.6, C | 151.5, C | 147.92, C | |||
7 | 3.78, m | 69.3, CH | 3.78, dd (6.0, 10.2) | 69.1, CH | 4.02, br d (10.8) | 68.62, CH |
8 | 3.19, m | 47.9, CH | 3.19, t (10.2) | 47.9, CH | 3.05, t (10.0) | 49.82, CH |
9 | 49.5, C | 49.4, C | 49.82, C | |||
10 | 2.95, m | 33.1, CH2 | 2.94, m | 33.1, CH2 | 2.98, dd (14.0, 10.2) | 34.85, CH2 |
2.92, m | 2.94, m | 3.16, dd (14.0, 4.0) | ||||
11 | 0.58, d (6.6) | 13.9, CH3 | 0.59, d (6.6) | 13.8, CH3 | 1.24, d (6.5) | 15.10, CH3 |
12 | 4.86, s | 112.2, CH2 | 4.86, s | 112.1, CH2 | 5.25, s | 114.62, CH2 |
5.16, s | 5.16, s | 5.48, s | ||||
13 | 6.16 dd | 132.0, CH | 6.17 dd (9.6, 15.6) | 131.9, CH | 6.67, ddd (15.5, 10.0, 1.6) | 132.63, CH |
14 | 5.56, m | 135.1, CH | 5.56, m | 135.0, CH | 5.82, ddd (15.5, 11.5, 3.2) | 138.08, CH |
15 | 1.87, m | 41.4, CH2 | 1.87, m | 41.3, CH2 | 2.19 dt (15.5, 11.5); | 41.27, CH2 |
2.43, m | 2.43, m | 2.61 dddd (13.5, 4.8, 3.2, 1.6) | ||||
16 | 2.76, m | 33.3, CH | 2.76, m | 33.2, CH | 2.91, m | 34.09, CH |
17 | 5.29, dd (9.6, 1.5) | 146.1, CH | 5.28, d (9.0) | 145.9, CH | 5.68, dq (9.4, 18) | 142.07, CH |
18 | 135.4, C | 135.2, C | 135.08, C | |||
19 | 189.3, C | 189.1, C | 188.04, C | |||
20 | 130.1, C | 130.0, C | 126.90, C | |||
21 | 6.60, d (3.6) | 113.7, CH | 6.59, dd (2.4, 3.6) | 113.5, CH | 7.02, dd (3.9, 2.7) | 115.07, CH |
22 | 5.65, t (3.3) | 108.0, CH | 5.66, dd (2.4, 3.6) | 107.9, CH | 6.18, dd (3.9, 2.7) | 109.17, CH |
23 | 140.7, C | 140.6, C | 139.81, C | |||
24 | 10.53, br s | 10.52, br s | 10.78, br s | |||
25 | 0.98, d (6.8) | 19.9, CH3 | 0.97, d (6.6) | 19.8, CH3 | 1.10, d (7.0) | 19.78, CH3 |
26 | 1.81, s | 13.2, CH3 | 1.81, s | 13.1, CH3 | 1.95, d (2.0) | 13.68, CH3 |
1′ | 10.91, brs | 10.90, s | 8.21, br s | |||
2′ | 7.16, d (2.2) | 124.6, CH | 7.15, d (1.8) | 124.5, CH | 7.09, d (2.3) | 122.86, CH |
3′ | 110.2, C | 110.1, C | 111.47, C | |||
3′a | 128.2, C | 128.1, C | 129.77, C | |||
4′ | 7.36, d (9.0) | 118.4, CH | 7.35, d (9.0) | 118.3, CH | 7.55, dd (8.0, 1.0) | 118.44, CH |
5′ | 7.05, t (7.1) | 121.3, CH | 7.05, t (7.2) | 121.2, CH | 7.25, td (8.0, 1.0) | 122.60, CH |
6′ | 6.94, t (7.7) | 118.9, CH | 6.94, t (7.8) | 118.7, CH | 7.15, td (8.0, 1.0) | 119.99, CH |
7′ | 7.34, d (9.0) | 111.9, CH | 7.34, d (9.0) | 111.8, CH | 7.40, dd (8.0, 1.0) | 111.62, CH |
7′a | 136.6, C | 136.4, C | 136.51, C | |||
7-OH | 4.87, d (5.9) | 4.81, d (5.4) | 2.00, br s |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra04108d |
This journal is © The Royal Society of Chemistry 2020 |