Attempted acid-catalyzed transannular reactions in the cembranoids

Several cembrane diterpenes were treated by various acids under different experimental conditions. All tested compounds were found to be acid sensitive, leading either to local chemical transformations (e.g. opening of epoxides), to transannular reactions and/or to unidentified mixtures. Sarcophine (7), the principal cembrane tested, was found to afford either all types of possible epoxide opening products (11–22), or, by a transannular reaction, when treated with SnCl₄, two tetrahydrooxepine derivatives (23 and 24). The structure determination of the various derivatives of sarcophine was based mainly on the ¹H and ¹³C-NMR spectra and also on several chemical transformations.Flaccidoxide (8), another cembranoid examined, was found to yield, with Zn/Cu couple, the expected deoxygenation product (sarcophytol-B, 32) together with an unexpected transannular reaction product (33). The structure, including the stereochemistry, of the latter THF-derivative of 8 (33) was elucidated by NMR and chemical reactions.