Sensory importance and mechanism of photochemical conversion of carvone to carvonecamphor in ethanol-water mixtures
1995
Refsgaard, H.H.F. | Nielsen, B.R. | Skibsted, L.H.
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 kO2 approximately 3 x 10(9) L/mol/s set whereas oxygen, piperylene, and diacetyl did not influence the photochemical conversion. Intramolecular electron transfer from an npi singlet state of carvone is suggested to be responsible for the photocycloaddition reaction of carvone.
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