Sensory Importance and Mechanism of Photochemical Conversion of Carvone to Carvonecamphor in Ethanol-Water Mixtures

Article abstract of DOI:10.1021/jf00053a011

Approximate odor thresholds for carvonecamphor, with camphor odor, and for the ester hexyl>acetate>, with sweet odor, were determined to be 2200-4100 and 1.8-2.5 ppb, respectively, by gas chromatographic effluent sniffing.The quantum yield of photochemical conversion of carvone to carvonecamphor showed no wavelength dependence but increased with decreasing ethanol content in ethanol-water mixtures: 0.049 +/- 0.008 mol*eistein^-1 in 42percent ethanol (apparent activation energy of 19 kJ*mol^-1) and 0.0033 +/- 0.00015 mol*einstein^-1 in neat ethanol.The quantum yield of further conversion of carvonecamphor to the ester was approximately 0.1 mol*einstein^-1.The triplet lifetime of carvone determined by laser flash spectroscopy in the absence of oxygen increased with decreasing ethanol content: 71 ns in neat, 76 ns in 80percent, and 98 ns in 42percent ethanol.Oxygen quenched the triplet excited state with k_O2 ca.3*10⁹ L*mol^-1*s^-1 whereas oxygen, piperylene, and diacetyl did not influence the photochemical conversion.Intramolecular electron transfer from an n?^* singlet state of carvone is suggested to be responsible for the photocycloaddition reaction of carvone.Keywords: (+)-Carvone; carvonecamphor; ethyl 2-<(1R,2R,4R,5S)-1,5-dimethyl-2-bicyclo<2.1.1>hexyl>acetate; odor thresholds; gas chromatography-olfactometry; photocycloaddition