Absolute configuration of labdanes and ent-clerodanes from Chrormolaena pulchella by vibrational circulardichroism

The aerial parts of Chromolaena pulchella biosynthesize two groups of dithierpenes belonging to oppositeenantiomeric series, specifically, the furanoid ent-clerodanes (5R,8R,9S,10R)-(-)-hardwikiic acid (1), methyl(5R,8R,9S,10R)-(-)-hardwikiate (2), (55,8R,9S,10R)-(-)-hautriwaic acid lactone (3), methyl (5R,8R,9S,10R)-(-)-nidoresedate (4) and methyl (8R,9R)-(-)-strictate (5), as well as the labdanes (5S,8R,9R,10S)-(+)-(13E)-labd-13-ene-8,15-diol (6) and (5S,8R,9R,10S)-(+)-isoabienol (7). The absolute configuration of the two groups of diterpenes wasunambiguously assigned by comparison of the vibrational circular dichroism spectra of 3 for ent-clerodanes, and of 7 forlabdaries with their theoretical spectra obtained by density functional theory calculations. The results support a biogeneticproposal to diterpenes found in the studied botanical species.