Production of the pepper aroma compound, (-)-rotundone, by aerial oxidation of α-guaiene.

The aroma link between pepper and wine has recently been elucidated to be due to the important aroma compound rotundone. To date, rotundone is the only known impact odorant with a peppery aroma. Although the concentration found in products of natural origin is small, the odor detection threshold is among the lowest of any natural product yet discovered. We report herein the identification of the first known precursor to rotundone, namely, α-guaiene, and that one mechanism of transformation is simple aerial oxidation.

Corymbolone.

The title compound, C15H24O2 [systematic name: (4S,4aR,6R,8aR)-4a-hy-droxy-4,8a-dimethyl-6-(prop-1-en-2-yl)octahydro-naphthalen-1(2H)-one], features two edge-shared six-membered rings with the hydroxyl and methyl substituents at this bridge being trans. One adopts a flattened chair conformation with the C atoms bearing the carbonyl and methyl substituents lying 0.5227 (16) and 0.6621 (15) Å, respectively, above and below the mean plane through the remaining four C atoms (r.m.s. deviation = 0.0145 Å). The second ring, bearing the prop-1-en-2-yl group, has a chair conformation. Supra-molecular helical chains along the b axis are found in the crystal packing, which are sustained by hy-droxy-carbonyl O-H⋯O hydrogen bonding.

2-[(3S,3aS,5R,8S,8aS)-3,8-Di-methyl-hexa-hydro-1H,4H-3a,8a-ep-oxy-azulen-5-yl]propan-2-ol.

Four independent mol-ecules (A-D) comprise the asymmetric unit of the title compound, C15H26O2, which differ only in the relative orientations of the terminal -C(Me)2OH groups [e.g. the range of Cmethyl-ene-Cmethine-Cquaternary-Ohy-droxy torsion angles is 52.7 (7)-57.1 (6)°, where the Cmethyl-ene atom is bound to an epoxide C atom]. The five-membered rings adopt envelope conformations, with the methyl-ene C atom adjacent to the methine C atom being the flap atom in each case. In each mol-ecule, the conformation of the seven-membered ring is a half-chair, with the Cmethyl-ene-Cmethine bond, flanked by methyl-ene C atoms, being the back of the chair. Supra-molecular helical chains along the b axis are found in the crystal packing, sustained by hy-droxy-epoxide O-H⋯O hydrogen bonding. Mol-ecules of A self-associate into a chain as do those of D. A third independent chain comprising B and C mol-ecules is also formed. The studied crystal is a pseudo-merohedral twin (minor component ca 21%).