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.