Transannular cyclization of (4S,5S)-germacrone-4,5-epoxide under basic conditions to yield eudesmane-type sesquiterpenes.

Transannular cyclizations of germacrone-4,5-epoxide under acidic and thermal conditions have been reported in our previous study. However, this process gave the different and interesting results under basic conditions. (4S,5S)-Germacrone-4,5-epoxide (1) was treated under basic conditions to yield four products (2-5). Compound 2 was an isomer of 1--(4S,5S,9Z)-4,5-epoxygermacra-7(11),9-dien-8-one--and the remaining three compounds (3-5) were eudesmane-type derivatives. Compounds 4 and 5 are new compounds. The structures of the new compounds were determined using high resolution (HR)-MS, one dimensional (1D)-NMR, 2D-NMR and circular dichroism (CD) spectroscopic data. Products 3-5 had the same carbon skeleton as that of eudesmane-type compounds; however, these compounds showed different arrangement of isoprene units to the natural eudesmane-type sesquiterpenes.