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 (CH 3 at C-19) has a 13 C 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 5 R ,8 R ,9 S ,10 R -configuration for the new nor-clerodadiene can be established. The absolute configuration of 5 R ,8 R ,9 S ,10 R 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.