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

Abstract

Approximate odor thresholds for carvonecamphor, with camphor odor, and for the ester [ethyl 2-[(1R,2R,4R,5S)-1,5-dimethyl-2-bicyclo[2.1.1]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.einstein -1 in 42% 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 80%, and 98 ns in 42% ethanol. Oxygen quenched the triplet excited state with kO 2 ∼3 x 10 9 L.mol -1 .s -1 whereas oxygen, piperylene, and diacetyl did not influence the photochemical conversion. Intramolecular electron transfer from an nπ * singlet state of carvone is suggested to be responsible for the photocycloaddition reaction of carvone.