Molecular Rearrangements of (−)-Modhephene and (−)-Isocomene to a (−)-Triquinane

The preparation and further rearrangement of (−)-modhephene (1) to a (−)-triquinane 5 has been assessed through acid catalysis. The rearrangement involved protonation, 1,2 σ-bond and methyl shifts, and deprotonation. Monitored experiments by 1H NMR spectroscopy suggested the intermediate (−)-isocomene (3), which was further evidenced when a sample of natural (−)-3 undergoes acid-catalyzed conversion to the (−)-triquinane 5. In addition, deuterated (−)-modhephene (1-d) labeled stereospecifically at the 14β geminal methyl group at C4 was synthesized, through the corresponding chiral deuterated primary alcohol, in 5 steps, starting from natural (−)-14-hydroxymodhephene (8), and rearranged under acid catalysis to elucidate the stereochemical factors that control the methyl shift at this position. The final deuterium-labeled (−)-triquinane, 5-d, obtained from [14-2H1]-1-d was established to have deuterium in the methyl group at C5 by 13C NMR spectroscopy. This stereoselective methyl migration is in accordance with the molecular orbital demand formulated by the quantum chemical calculations performed in the present study.