title: Transannular cyclization of (4S,5S)-germacrone-4,5-epoxide into guaiane and secoguaiane-type sesquiterpenes.
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.
authors.affiliation: Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan. kuroyanagi@kph.bunri-u.ac.jp
authors.forename: Masanori
authors.initials: M
authors.lastname: Kuroyanagi
authors.forename: Osamu
authors.initials: O
authors.lastname: Shirota
authors.forename: Setsuko
authors.initials: S
authors.lastname: Sekita
authors.forename: Takahisa
authors.initials: T
authors.lastname: Nakane
mesh_terms: D030024:Curcuma / Q000737:chemistry*; D015394:Molecular Structure; D045788:Sesquiterpenes, Germacrane / Q000737:chemistry* / Q000302:isolation & purification; D045789:Sesquiterpenes, Guaiane / Q000138:chemical synthesis*
chemical_list: C572475:(4S,5S)-germacrone-4,5-epoxide; D045788:Sesquiterpenes, Germacrane; D045789:Sesquiterpenes, Guaiane; C048393:germacrone