Stability of Pulegone and Thujone in Ethanolic Solution

by Otmar Froehlich and Takayuki Shibamoto Journal of Agricultural and Food Chemistry 1990


An important paper that addresses the question of whether thujone degrades over time.

“No change in the thujone concentrations was noticed during the storage experiments in 100% ethanol and 30% ethanol at either pH 2.5 or pH 6.5 (Ia-IIId). At pH 11.5 a very rapid epimerization of α-thujone to the 5 times less toxic β-thujone (Rice and Wilson, 1976) took place as reported by Hach et al. (1971) and reached an equilibrium of about 1:2 α-thujone to β-thujone after about 20 h (Figure 8). A comparison of the reaction rates at 20 “C in sunlight and in the dark showed that the reaction was independent of the influence of light. At the different temperatures (100, 20, and 0 “C), nearly no difference in reaction rate was observed.”

What we find in this experiment is the degradation of thujone can be effected via a sufficient pressure of photoirradiation with 310nm UV light. Under that pressure, thujone undergoes decorbonylation to 5-methylene-6-methyl-hept-2-ene. This compound is of interest where an assay of degradation is concerned.

However, note that no degradation was observed in the absence of sufficient photoirradiation pressure, despite exposure to sunlight, wide differences in pH, and temperature ranges from 0-100ºC. If anything, there is a tendency for thujone to epimerize to a ratio of 2:1 β to α, but it appears to be resistant to degradation even in extreme conditions.

Fröhlich Stability of Pulegone and Thujone in Ethanolic Solution

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