γ-терпінен

What it is

The terpinenes are a group of isomeric monoterpene hydrocarbons that share one molecular formula and carbon framework but differ in the position of their double bonds. In gamma-terpinene the double bonds sit at the 1- and 4-positions of the p-menthane skeleton. Its IUPAC name is 1-methyl-4-propan-2-ylcyclohexa-1,4-diene.

Occurrence

Gamma-terpinene is a major component of essential oils made from citrus fruits. It has been reported in Camellia sinensis and Artemisia thuscula. In tea tree oil as defined by ISO 4730, gamma-terpinene accounts for 14-28% of the oil. Thyme essential oil also contains gamma-terpinene. Oregano essential oil contains it as one of the lesser abundant compounds, beside the main constituents carvacrol and thymol.

Chemistry

In the Lamiaceae, the biosynthesis of thymol and carvacrol starts with the formation of gamma-terpinene from geranyl diphosphate. In the biosynthesis of thymol and carvacrol, cytochrome P450 monooxygenases of the CYP71D subfamily oxidize gamma-terpinene to unstable cyclohexadienol intermediates. A short-chain dehydrogenase/reductase then dehydrogenates these intermediates to ketones, and keto-enol tautomerism leads to the aromatic compounds. In the absence of the dehydrogenase, only p-cymene formed by rearrangement of the intermediates.

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