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Heravi MM, Mohammadi L. Application of Pauson-Khand reaction in the total synthesis of terpenes. RSC Adv 2021; 11:38325-38373. [PMID: 35493249 PMCID: PMC9044263 DOI: 10.1039/d1ra05673e] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2021] [Accepted: 10/28/2021] [Indexed: 12/28/2022] Open
Abstract
The Pauson-Khand reaction (PKR) is a formal [2 + 2 + 1] cycloaddition involving an alkyne, an alkene and carbon monoxide mediated by a hexacarbonyldicobaltalkyne complex to yield cyclopentenones in a single step. This versatile reaction has become a method of choice for the synthesis of cyclopentenone and its derivatives since its discovery in the early seventies. The aim of this review is to point out the applications of PKR in the total synthesis of terpenes.
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Affiliation(s)
- Majid M Heravi
- Department of Chemistry, School of Physics and Chemistry, Alzahra University Vanak Tehran Iran +98 2188041344 +98 9121329147
| | - Leila Mohammadi
- Department of Chemistry, School of Physics and Chemistry, Alzahra University Vanak Tehran Iran +98 2188041344 +98 9121329147
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2
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Li H, Zhang J, She X. The Total Synthesis of Diquinane-Containing Natural Products. Chemistry 2021; 27:4839-4858. [PMID: 32955141 DOI: 10.1002/chem.202003741] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2020] [Revised: 09/15/2020] [Indexed: 12/18/2022]
Abstract
Diquinane or bicyclo[3.3.0]octane is a conspicuous structural unit existing in the carbo-frameworks of a wide range of natural products such as alkaloids and terpenoids. These diquinane-containing molecules not merely exhibit intriguing architectures, but also showcase a broad spectrum of significant bioactivities, which draw widespread attention from the global synthetic community. During the past decade, with an aim to accomplish the total syntheses of such specified cornucopias of natural products, a variety of elegant strategies for construction of the diquinane ring system have been disclosed. In this Minireview, the achievements on this subject in the timeline from 2010 to June 2020 are demonstrated and it is discussed how the diquinane unit is strategically forged in the context of the specific target structure. In addition, impacts of the selected works to the field of natural product total synthesis is highlighted and the particular outlook of diquinane-containing natural product synthesis is provided.
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Affiliation(s)
- Huilin Li
- State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, 222 South Tianshui Road, Lanzhou, 730000, Gansu, P. R. China
| | - Jing Zhang
- State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, 222 South Tianshui Road, Lanzhou, 730000, Gansu, P. R. China
| | - Xuegong She
- State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, 222 South Tianshui Road, Lanzhou, 730000, Gansu, P. R. China
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3
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Affiliation(s)
- Naifeng Hu
- State Key Laboratory of Elemento-organic ChemistryCollege of ChemistryNankai University Tianjin 300071 China
| | - Changming Dong
- State Key Laboratory of Elemento-organic ChemistryCollege of ChemistryNankai University Tianjin 300071 China
| | - Cuifang Zhang
- State Key Laboratory of Elemento-organic ChemistryCollege of ChemistryNankai University Tianjin 300071 China
| | - Guangxin Liang
- State Key Laboratory of Elemento-organic ChemistryCollege of ChemistryNankai University Tianjin 300071 China
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4
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Hu N, Dong C, Zhang C, Liang G. Total Synthesis of (-)-Indoxamycins A and B. Angew Chem Int Ed Engl 2019; 58:6659-6662. [PMID: 30835916 DOI: 10.1002/anie.201902043] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2019] [Indexed: 11/07/2022]
Abstract
The concise total syntheses of (-)-indoxamycins A and B is reported. The chemistry features a seven-step preparation of a highly congested [5.5.6] tricyclic advanced common intermediate from a readily available R-carvone derivative. Key steps involve a Pauson-Khand reaction for the rapid construction of a basic scaffold bearing a quaternary carbon, a copper-catalyzed Michael addition for the introduction of another adjacent all-carbon quaternary stereocenter, and a tandem retro-oxa-Michael addition/1,2-addition/oxa-Michael addition for the installation of a trisubstituted olefin side chain. This synthetic strategy allows for easy access to both enantiomers of this family of natural products and their analogues from cost-effective starting material through straightforward chemical transformations.
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Affiliation(s)
- Naifeng Hu
- State Key Laboratory of Elemento-organic Chemistry, College of Chemistry, Nankai University, Tianjin, 300071, China
| | - Changming Dong
- State Key Laboratory of Elemento-organic Chemistry, College of Chemistry, Nankai University, Tianjin, 300071, China
| | - Cuifang Zhang
- State Key Laboratory of Elemento-organic Chemistry, College of Chemistry, Nankai University, Tianjin, 300071, China
| | - Guangxin Liang
- State Key Laboratory of Elemento-organic Chemistry, College of Chemistry, Nankai University, Tianjin, 300071, China
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5
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Huang Z, Huang J, Qu Y, Zhang W, Gong J, Yang Z. Total Syntheses of Crinipellins Enabled by Cobalt‐Mediated and Palladium‐Catalyzed Intramolecular Pauson–Khand Reactions. Angew Chem Int Ed Engl 2018. [DOI: 10.1002/ange.201805143] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Affiliation(s)
- Zhihui Huang
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Jun Huang
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Yongzheng Qu
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Weibin Zhang
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Jianxian Gong
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Zhen Yang
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
- Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education Beijing National Laboratory for Molecular Science College of Chemistry and Molecular Engineering Peking-Tsinghua Center for Life Sciences Peking University Beijing 100871 China
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6
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Huang Z, Huang J, Qu Y, Zhang W, Gong J, Yang Z. Total Syntheses of Crinipellins Enabled by Cobalt‐Mediated and Palladium‐Catalyzed Intramolecular Pauson–Khand Reactions. Angew Chem Int Ed Engl 2018; 57:8744-8748. [DOI: 10.1002/anie.201805143] [Citation(s) in RCA: 32] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2018] [Indexed: 02/03/2023]
Affiliation(s)
- Zhihui Huang
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Jun Huang
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Yongzheng Qu
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Weibin Zhang
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Jianxian Gong
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
| | - Zhen Yang
- State Key Laboratory of Chemical Oncogenomics Key Laboratory of Chemical Genomics Peking University Shenzhen Graduate School Shenzhen 518055 China
- Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education Beijing National Laboratory for Molecular Science College of Chemistry and Molecular Engineering Peking-Tsinghua Center for Life Sciences Peking University Beijing 100871 China
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7
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Zhao N, Yin S, Xie S, Yan H, Ren P, Chen G, Chen F, Xu J. Total Synthesis of Astellatol. Angew Chem Int Ed Engl 2018; 57:3386-3390. [PMID: 29364591 DOI: 10.1002/anie.201800167] [Citation(s) in RCA: 43] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2018] [Indexed: 12/14/2022]
Abstract
A nearly-30-year-old unanswered synthetic puzzle, astellatol, has been solved in an enantiospecific manner. The highly congested pentacyclic skeleton of this rare sesterterpenoid, which possesses a unique bicyclo[4.1.1]octane motif, ten stereocenters, a cyclobutane that contains two quaternary centers, an exo-methylene group, and a sterically encumbered isopropyl trans-hydrindane motif, makes astellatol arguably one of the most challenging targets for sesterterpenoid synthesis. An intramolecular Pauson-Khand reaction was exploited to construct the right-hand side scaffold of this sesterterpenoid. An unprecedented reductive radical 1,6-addition, mediated by SmI2 , forged the cyclobutane motif. Last, a strategic oxidation/reduction step provided not only the decisive solution for the remarkably challenging late-stage transformations, but also a highly valuable unravelling of the notorious issue of trans-hydrindane synthesis. Importantly, the synthesis of astellatol showcases a rapid, scalable strategy to access diverse complex isopropyl trans-hydrindane sesterterpenoids.
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Affiliation(s)
- Nan Zhao
- Department of Chemistry, Southern University of Science and Technology, Shenzhen Grubbs Institute, Shenzhen, Guangdong, China
| | - Shuqiang Yin
- Department of Chemistry, Southern University of Science and Technology, Shenzhen Grubbs Institute, Shenzhen, Guangdong, China
| | - Shengling Xie
- Department of Chemistry, Southern University of Science and Technology, Shenzhen Grubbs Institute, Shenzhen, Guangdong, China
| | - Hao Yan
- Department of Chemistry, Southern University of Science and Technology, Shenzhen Grubbs Institute, Shenzhen, Guangdong, China
| | - Pan Ren
- Department of Chemistry, Southern University of Science and Technology, Shenzhen Grubbs Institute, Shenzhen, Guangdong, China
| | - Gui Chen
- Department of Chemistry, Southern University of Science and Technology, Shenzhen Grubbs Institute, Shenzhen, Guangdong, China
| | - Fang Chen
- Department of Chemistry, Southern University of Science and Technology, Shenzhen Grubbs Institute, Shenzhen, Guangdong, China
| | - Jing Xu
- Department of Chemistry, Southern University of Science and Technology, Shenzhen Grubbs Institute, Shenzhen, Guangdong, China
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8
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Zhao N, Yin S, Xie S, Yan H, Ren P, Chen G, Chen F, Xu J. Total Synthesis of Astellatol. Angew Chem Int Ed Engl 2018. [DOI: 10.1002/ange.201800167] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
- Nan Zhao
- Department of Chemistry; Southern University of Science and Technology; Shenzhen Grubbs Institute; Shenzhen Guangdong China
| | - Shuqiang Yin
- Department of Chemistry; Southern University of Science and Technology; Shenzhen Grubbs Institute; Shenzhen Guangdong China
| | - Shengling Xie
- Department of Chemistry; Southern University of Science and Technology; Shenzhen Grubbs Institute; Shenzhen Guangdong China
| | - Hao Yan
- Department of Chemistry; Southern University of Science and Technology; Shenzhen Grubbs Institute; Shenzhen Guangdong China
| | - Pan Ren
- Department of Chemistry; Southern University of Science and Technology; Shenzhen Grubbs Institute; Shenzhen Guangdong China
| | - Gui Chen
- Department of Chemistry; Southern University of Science and Technology; Shenzhen Grubbs Institute; Shenzhen Guangdong China
| | - Fang Chen
- Department of Chemistry; Southern University of Science and Technology; Shenzhen Grubbs Institute; Shenzhen Guangdong China
| | - Jing Xu
- Department of Chemistry; Southern University of Science and Technology; Shenzhen Grubbs Institute; Shenzhen Guangdong China
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9
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Lv C, Yan X, Tu Q, Di Y, Yuan C, Fang X, Ben-David Y, Xia L, Gong J, Shen Y, Yang Z, Hao X. Isolation and Asymmetric Total Synthesis of Perforanoid A. Angew Chem Int Ed Engl 2016. [DOI: 10.1002/ange.201602783] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Affiliation(s)
- Chao Lv
- School of Pharmaceutical Sciences; Shandong University; Jinan Shandong 250012 PR China
| | - Xiaohui Yan
- State Key Laboratory of Phytochemistry and Plant Resources in West China; Kunming Institute of Botany; Chinese Academy of Sciences; Kunming 650204 PR China
| | - Qian Tu
- Laboratory of Chemical Genomics; School of Chemical Biology and Biotechnology; Peking University Shenzhen Graduate School; Shenzhen 518055 China
| | - Yingtong Di
- State Key Laboratory of Phytochemistry and Plant Resources in West China; Kunming Institute of Botany; Chinese Academy of Sciences; Kunming 650204 PR China
| | - Chunmao Yuan
- The Key Laboratory of Chemistry for Natural Products of Guizhou Province and Chinese Academy of Sciences; 550002 China
| | - Xin Fang
- State Key Laboratory of Phytochemistry and Plant Resources in West China; Kunming Institute of Botany; Chinese Academy of Sciences; Kunming 650204 PR China
| | - Yaacove Ben-David
- The Key Laboratory of Chemistry for Natural Products of Guizhou Province and Chinese Academy of Sciences; 550002 China
| | - Lei Xia
- The Key Laboratory of Chemistry for Natural Products of Guizhou Province and Chinese Academy of Sciences; 550002 China
| | - Jianxian Gong
- Laboratory of Chemical Genomics; School of Chemical Biology and Biotechnology; Peking University Shenzhen Graduate School; Shenzhen 518055 China
| | - Yuemao Shen
- School of Pharmaceutical Sciences; Shandong University; Jinan Shandong 250012 PR China
| | - Zhen Yang
- Laboratory of Chemical Genomics; School of Chemical Biology and Biotechnology; Peking University Shenzhen Graduate School; Shenzhen 518055 China
| | - Xiaojiang Hao
- State Key Laboratory of Phytochemistry and Plant Resources in West China; Kunming Institute of Botany; Chinese Academy of Sciences; Kunming 650204 PR China
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10
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Lv C, Yan X, Tu Q, Di Y, Yuan C, Fang X, Ben-David Y, Xia L, Gong J, Shen Y, Yang Z, Hao X. Isolation and Asymmetric Total Synthesis of Perforanoid A. Angew Chem Int Ed Engl 2016; 55:7539-43. [PMID: 27167098 DOI: 10.1002/anie.201602783] [Citation(s) in RCA: 52] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2016] [Indexed: 01/16/2023]
Abstract
A novel limonoid, perforanoid A, was isolated, and an asymmetric total synthesis was achieved in 10 steps. The key steps are chiral tertiary aminonaphthol mediated enantioselective alkenylation of an aldehyde to an allylic alcohol, Pd-catalyzed coupling of the allylic alcohol with vinyl ether to form the γ-lactone ring, and cyclopentenone ring formation through a Rh-catalyzed Pauson-Khand reaction. Preliminary studies show that perforanoid A is cytotoxic towards HEL, K562, and CB3 tumor cell lines.
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Affiliation(s)
- Chao Lv
- School of Pharmaceutical Sciences, Shandong University, Jinan, Shandong, 250012, PR China
| | - Xiaohui Yan
- State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming, 650204, PR China
| | - Qian Tu
- Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen, 518055, China
| | - Yingtong Di
- State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming, 650204, PR China
| | - Chunmao Yuan
- The Key Laboratory of Chemistry for Natural Products of Guizhou Province and Chinese Academy of Sciences, 550002, China
| | - Xin Fang
- State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming, 650204, PR China
| | - Yaacove Ben-David
- The Key Laboratory of Chemistry for Natural Products of Guizhou Province and Chinese Academy of Sciences, 550002, China
| | - Lei Xia
- The Key Laboratory of Chemistry for Natural Products of Guizhou Province and Chinese Academy of Sciences, 550002, China
| | - Jianxian Gong
- Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen, 518055, China
| | - Yuemao Shen
- School of Pharmaceutical Sciences, Shandong University, Jinan, Shandong, 250012, PR China.
| | - Zhen Yang
- Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen, 518055, China.
| | - Xiaojiang Hao
- State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming, 650204, PR China.
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11
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Zhang JJ, You L, Wang YF, Li YH, Liang XT, Zhang B, Yang SL, Su Q, Chen JH, Yang Z. Asymmetric Total Synthesis of Propindilactone G, Part 2: Enantioselective Construction of the Fully Functionalized BCDE Ring System. Chem Asian J 2016; 11:1414-24. [DOI: 10.1002/asia.201600130] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2016] [Indexed: 01/08/2023]
Affiliation(s)
- Jia-Jun Zhang
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
| | - Lin You
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
| | - Yue-Fan Wang
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
| | - Yuan-He Li
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
| | - Xin-Ting Liang
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
| | - Bo Zhang
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
| | - Shou-Liang Yang
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
| | - Qi Su
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
| | - Jia-Hua Chen
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
| | - Zhen Yang
- State Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education; Beijing National Laboratory for Molecular Science (BNLMS); Peking-Tsinghua Center for Life Sciences, and; Department of Chemistry; Peking University; 202 Chengfu Road Beijing 100871 P. R. China
- Laboratory of Chemical Genomics; School of Chemical Biology and Biotechnology, Shenzhen Graduate School; Peking University; Shenzhen 518055 P. R. China
- Key Laboratory of Marine Drugs, Chinese Ministry of Education; School of Medicine and Pharmacy; Ocean University of China; 5 Yushan Road Qingdao P. R. China
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12
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Lopez SA, Pourati M, Gais HJ, Houk KN. How Torsional Effects Cause Attack at Sterically Crowded Concave Faces of Bicyclic Alkenes. J Org Chem 2014; 79:8304-12. [DOI: 10.1021/jo501557z] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Affiliation(s)
- Steven A. Lopez
- Department
of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States
| | - Melika Pourati
- Department
of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States
| | - Hans-Joachim Gais
- Institute
of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
| | - K. N. Houk
- Department
of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States
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