Identification of 15-nor-cleroda-3,12-diene in a Dominican amber

An unknown compound dominant in the aliphatic hydrocarbon fraction of the solvent extract of a Dominican amber sample was isolated and purified using column chromatography and preparative gas chromatography. 1D and 2D NMR analyses allowed its structure to be assigned as 15-nor-cleroda-3,12-diene. The bridgehead methyl group (CH3 at C-19) has a 13C chemical shift of 19.6 ppm, typical for a trans-decalin framework (Nogueira, 2001), indicating the diterpenoid has a trans A/B ring structure. ROESY correlations indicate that the methyl groups at C-19, C-20 were axial and the C-17 methyl group was equatorial, while the H-10 and H-8 were axial and on the other side of Ring B. Taken together, a 5R,8R,9S,10R-configuration for the new nor-clerodadiene can be established. The absolute configuration of 5R,8R,9S,10R was further confirmed by comparing the experimental and calculated electronic circular dichroism (ECD) spectra of this diterpenoid. The identified diene appears to originate from a Hymenaea species (Fabaceae). Non-oxidative decarboxylation of cleroda-3,13-diene-15-oic acid (kovalenic acid) present in Hymenaea species or oxidative decarboxylation of 3-cleroden-15-oic acid in Dominican amber are considered to be the two viable pathways leading to the formation of the identified 15-nor-cleroda-3,12-diene, although the former pathway is more likely.