An approach to furolabdanes and their photooxidation derivatives from R-(+)-sclareolide

A synthetic route to furolabdanes from commercially available R-(+)-selareolide is reported, with the specific aim of preparing (12R and 12S,15xi)-12,15-dihydroxylabda-7,13-dien-16,15-olides (3 and 5) and (12R and 12S,16xi)-12,16-dihydroxylabda-7,13-dien-15,16-olides (4 and 6). The key points of our approach are the use of Weinreb's amide 11 to join the furan ring to the terpenic unit. Photooxidation of the furan moiety of compounds 15 and 16, and of their acetates 19 and 20, has been used to built the hydroxybutenolide fragment. In this way the four possible isomers at the butenolide moiety, compounds 3-6, and their C-12 acetyl derivatives 21-24 have been obtained. On the basis of comparison of the spectral data (H-1 NMR) of the synthetic peracetates 25-28 (derived from 21-24) with the reported data for the peracetate 2 (derived from the natural product 1), the relative configuration at carbon C-12 of the natural product has been corrected. Furthermore, the absolute configuration of the natural product 1, considered to belong to the enantio-series, has to be changed to the normal-series on the basis of the optical rotation obtained for the synthetic derivative.