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![Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions](/_next/image?url=https%3A%2F%2Fcloudfront.jove.com%2FCDNSource%2Fteasers%2F60786.jpg&w=3840&q=50)
Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
Published on: July 17, 2020
Transannular cyclization of (4S,5S)-germacrone-4,5-epoxide into guaiane and secoguaiane-type sesquiterpenes
Masanori Kuroyanagi1, Osamu Shirota, Setsuko Sekita
1Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan. kuroyanagi@kph.bunri-u.ac.jp
Abstract:
Germacrone (1) and (4S,5S)-germacrone-4,5-epoxide (2) were isolated, along with guaiane and secoguaiane-type sesquiterpenes, from Curcuma aromatica plants. Compound 2 was derived from 1 and cyclized through transannular (T-A) reactions into various guaiane and secoguaiane-type sesquiterpenes in C. aromatica. The cyclization reaction of 2 was initiated by protonation at an epoxide oxygen atom, followed by cleavage of the epoxide ring and the formation of a C-C bond between C-1 and C-5 to give guaiane-type derivatives. Acidic and thermal treatments of 2 produced twelve sesquiterpenes having guaiane and secoguaiane skeletons. The structures of these products were elucidated by spectral methods, including 2D-NMR spectroscopy. Most were identified as sesquiterpenes isolated from C. aromatica as natural products. The T-A cyclization of 2 occurred via two transition states, a cross conformation and a parallel conformation. The mechanism of the T-A cyclization reaction of 2 is discussed.
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