SENSORY IMPORTANCE AND MECHANISM OF PHOTOCHEMICAL CONVERSION OF CARVONE TO CARVONECAMPHOR IN ETHANOL-WATER MIXTURES

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 k(O2) similar to 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 n pi singlet state of carvone is suggested to be responsible for the photocycloaddition reaction of carvone.