Continuous-Flow Biocatalytic Process for the Synthesis of the Best Stereoisomers of the Commercial Fragrances Leather Cyclohexanol (4-Isopropylcyclohexanol) and Woody Acetate (4-(Tert-Butyl)Cyclohexyl Acetate)

Abstract

Leather cyclohexanol (4-(isopropyl)cyclohexanol) and woody acetate (4-(tert-butyl)cyclohexyl acetate) are commercialized for functional perfumery applications as mixtures of cis- and trans-isomers. The cis-isomers are more potent odorants than the corresponding trans counterparts, but they are the less favoured products in most of the classical synthetic routes. Known stereoselective routes to cis-4-alkylcyclohexanols are characterized by a high environmental burden and/or troublesome reaction work-up. In this work, we examine the use of commercial alcohol dehydrogenases (ADHs) to produce cis-4-alkylcyclohexanols, including the two derivatives with isopropyl and tert-butyl substituents, by the stereoselective reduction of the corresponding ketones. High conversions and diastereoisomeric excess values were achieved with five of the eighteen tested ADHs. To complete the synthetic approach to woody acetate, Candida antarctica A (CALA) was employed as a catalyst for the enzymatic acetylation of cis-4-(tert-butyl)cyclohexanol. In order to provide a technological upgrade to the production of the most odorous isomers of the two commercial fragrances, we designed a continuous-flow process based on the combination of in-line enzymatic steps with in-line work-up, effectively providing samples of cis-leather cyclohexanol and cis-woody acetate with high diastereoisomeric purity.

Comparison of Four Immobilization Methods for Different Transaminases

“…The activity of enzymes in the presence of different solvents is a crucial issue for (industrial) synthesis. For example, in one study, a substrate had to be solubilized in 20% isopropanol with the enzyme still active [84]. ATA-Bmu was shown to withstand different solvents, such as the use of isopropanol or acetonitrile, and maintain a high activity as a soluble protein [65].…”

Characterization and evaluation of immobilized enzymes for applications in flow reactors

“…Macroscopically, the stability of the enzymes involved in continuous processes can also be maximized through reactor engineering (Figure 2F). This approach is frequently used when synthetic cascades, often chemo-enzymatic cascades, meet stability incompatibilities between the different catalysts required for each reaction step [58]. In this context, the spatial segregation of (bio)catalysts is mandatory.…”

Synthesis of odorants in flow and their applications in perfumery

“…The commonly used concentration of woody acetate ( 54 ) in a perfume is 3–10%. Thus, Brenna and co-workers developed a cis -selective synthesis of 54 via a biocatalytic process in flow ( Scheme 12 ) [ 45 ]. In the first step, a mixture of cyclohexanone 51 , NADH, and isopropanol in an aqueous phosphate buffer (pH ≈ 7) is pumped through a continuously stirred membrane reactor at 30 °C with a residence time of 1 h containing alcohol dehydrogenase (ADH200).…”

“…At 30 °C and with a residence time of 11 min, acetylation of cis -alcohol 52 is mediated. After distillation, cis -woody acetate 54 is obtained in 89% isolated yield (de > 99%) on a gram scale [ 45 ].…”