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Microwave-assisted 6π-electrocyclization in water
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Yan Wua, Ying-Wu Linb, Wei-Min Hec, *
Chinese Chemical Letters | 2020, 31(12) : 2999 - 3000
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Chinese Chemical Letters | 2020, 31(12): 2999-3000
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Microwave-assisted 6π-electrocyclization in water
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Yan Wua, Ying-Wu Linb, Wei-Min Hec, *
Affiliations
  • a College of Chemistry and Chemical Engineering, Yangtze Normal University, Chongqing 408000, China
  • b School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, China
  • c Department of Chemistry, Hunan University of Science and Engineering, Yongzhou 425100, China
Published: 2020-12-15 doi: 10.1016/j.cclet.2020.09.005
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The recent development of microwave-assisted aqueous synthesis of polyheterocyclic-fused quinoline-2-thiones through 6π-electrocyclization was highlighted.

6π-Electrocyclization  /  Quinoline-2-thiones  /  Microwave-assisted synthesis in water
Yan Wu, Ying-Wu Lin, Wei-Min He. Microwave-assisted 6π-electrocyclization in water[J]. Chinese Chemical Letters, 2020 , 31 (12) : 2999 -3000 . DOI: 10.1016/j.cclet.2020.09.005
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Quinoline-2-thione as a privileged structural motif has been widely found in a wide variety of biologically active molecules, functional materials, and synthetic intermediates. As shown in Fig. 1, quinoline-2-thione derivatives possess remarkable pharma-cological properties, such as mPGES-1 inhibitor, human A3 adenosine receptor antagonists, antiproliferative [1, 2]. In this context, the development of straightforward and efficient methodologies toward quinoline-2-thione derivatives has successfully gained continuous interest. Besides, imidazo[1,2-a]pyridines also occupy an important position in natural products, pharmaceuticals, and optoelectronic materials [3]. From the standpoint of synthetic chemistry, the structural combination of imidazo[1,2-a] pyridines with quinoline-2-thiones in one single molecular framework should be interesting for the discovery of novel functional molecules. However, the synthesis of imidazo[1,2-a] pyridine-fused-quinoline-2-thiones has not been reported until recently.
Very recently, the research group of Chen and Yu at Zhengzhou University developed a microwave-assisted aqueous reaction for the synthesis of polyheterocyclic-fused quinoline-2-thiones through a 6π-electrocyclization strategy (Scheme 1) [4]. This metal- and additive-free approach features the benefits involving catalyst-free, short reaction time, reusable and green solvent system, and facile isolation via simple filtration. The previous studies of 6π-electrocyclization strategies by Saito's group [5] and Quan's group [6] have independently disclosed feasibility to access highly functionalized quinoline-2-thiones. However, volatile solvents and external additives (e.g., 1, 8-diazabicyclo[5. 4. 0]-undec-7-ene, In(OTf)3) were required in these procedures. In this context, the development of an additive-free approach in water should be valuable from the standpoint of green chemistry [7, 8]
In this microwave-assisted reaction, various polyheterocyclic-fused quinoline-2-thiones 3 were obtained in the presence of ortho-heteroaryl anilines 1 (0.2 mmol) and carbon disulfide 2a (0.4 mmol) in H2O (1 mL) at 140 ℃ under irradiation of microwave for 30 min (Scheme 2b). Moreover, the procedure showed broad and excellent functional group compatibility. Overall, 24 examples of polyheterocyclic-fused quinoline-2-thiones were successfully isolated by simple filtration. Moreover, it was found that the heteroaromatic scaffolds including indolizine, benzo[d]imidazo-[2, 1-b]thiazole, imidazo[2, 1-a]isoquinoline, imidazo[1,2-a]quinoline, indole, and pyrrole could also be applied as reaction precursors to realize the cycloaddition to give the desired products (3s-x) (Scheme 2b).
A plausible mechanism of this microwave-assisted cyclization reaction was proposed as shown in Scheme 2c. First, the nucleophilic addition of the amino group in 1a reacted with CS2 to generate the intermediate A. Then the intermediate A was converted into the intermediate B along with releasing H2S. Subsequently, intermediate B via 6π-electrocyclization gave the intermediate C. Finally, the desired product 3a was obtained after the aromatization of intermediate C.
Additionally, the recycling experiments of aqueous media for sustainable synthesis demonstrated that comparably yields were obtained in each recycle of water. This strategy features water as a reused solvent at least five times without significant loss of product yields. Finally, E-factor and EcoScale score were calculated. The excellent scores of E-factor (0.45) and EcoScale penalty points (14.5) demonstrated that this protocol presents great green chemistry and environment benign metrics [9].
In conclusion, the Chen and Yu's group recently developed a microwave-assisted annulation for the first synthesis of a diversity of polyheterocyclic-fused quinoline-2-thione frameworks in water through 6π-electrocyclization without using any catalyst and additive. C(sp2)-N and C(sp2)-C(sp2) bonds were formed in one pot. Significantly, the green solvent water can be recycled at least for five times, and the remarkable E-factor of 0.45 and EcoScale score of 85.5 were observed in this protocol. Such findings will open new routes for the application of 6π-electrocyclization in modern organic synthesis.
Declaration of competing interest
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The authors report no declarations of interest.
References
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[1]
K.J. Liu, J.H. Deng, J. Yang, et al., Green Chem. 22(2020) 433-438.
[2]
Y.F. Si, X.L. Chen, X.Y. Fu, et al., ACS Sustain. Chem. Eng. 8(2020) 10740-10746.
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T. Shi, K. Sun, X.L. Chen, et al., Adv. Synth. Catal. 362(2020) 2143-2149.
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X.Y. Li, Y. Liu, X.L. Chen, et al., Green Chem. 22(2020) 4445-4449.
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T. Otani, S. Kunimatsu, H. Nihei, Y. Abe, T. Saito, Org. Lett. 9(2007) 5513-5516.
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T.L. Wang, X.J. Liu, C.D. Huo, X.C. Wang, Z.J. Quan, Chem. Commun. 54(2018) 499-502.
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X.Y. Yuan, F.L. Zeng, H.L. Zhu, et al., Org. Chem. Front. 7(2020) 1884-1889.
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M.H. Muhammad, X.L. Chen, Y. Liu, et al., ACS Sustain. Chem. Eng. 8(2020) 2682-2687.
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Z. Cao, Q. Zhu, Y.W. Lin, W.M. He, Chin. Chem. Lett. 30(2019) 2132-2138.
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Year 2020 volume 31 Issue 12
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Article Info
doi: 10.1016/j.cclet.2020.09.005
  • Receive Date：2020-07-28
  • Online Date：2026-01-31
  • Published：2020-12-15
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History
  • Received：2020-07-28
  • Revised：2020-08-24
  • Accepted：2020-08-31
Affiliations
    a College of Chemistry and Chemical Engineering, Yangtze Normal University, Chongqing 408000, China
    b School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, China
    c Department of Chemistry, Hunan University of Science and Engineering, Yongzhou 425100, China
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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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