Selective Enzymatic Synthesis of the Grapefruit Flavor (+)‐Nootkatone

(+)-Nootkatone is a high-value sesquiterpenoid known for its grapefruit-odor impression. Its isolation from natural plant sources suffers from low yields, and chemical syntheses involve carcinogenic or hazardous compounds. Herein, a biocatalytic route for the synthesis of (+)-nootkatone that combines two enzymes in one pot is presented. In the first step, a cytochrome P450 monooxygenase catalyzes the selective allylic hydroxylation of the sesquiterpene (+)-valencene to the intermediate alcohol nootkatol. In the second step, nootkatol is further oxidized to (+)-nootkatone by an alcohol dehydrogenase (ADH). The challenging task of finding a suitable cofactor regeneration system was solved by careful selection of an appropriate cosubstrate for the ADH, which works in a dual-functional mode. After reaction optimization, involving cosolvent and cosubstrate screening, (+)-nootkatone concentrations of up to 360mgL(-1) and a space-time yield of 18mgL(-1)h(-1) were achieved.