Crude Oat Peroxygenase Enables Sustainable and Selective Oxidation of Cyclic Monoterpenes

The reactivity of a range of cyclic monoterpenes in the presence of a crude peroxygenase-containing preparation from oat flour as a biocatalyst and tert-butyl hydroperoxide as an oxidant was evaluated, with the aim of highlighting the chemo- and stereoselectivity features of the enzyme. Most of the tested terpenes were converted to the corresponding epoxides or related diols via hydrolytic or rearrangement processes. The study on both the enantiomers of cis-carveol revealed that the enzyme catalyzed the formation of an epoxide ring with the same stereopreference, independently from the chirality of the pre-existent stereogenic centers, affording the two diastereoisomeric 1,2-epoxides with (1S,2R) configuration. The use of a commercial mixture of cis/trans (–)-carveol as the substrate, instead, resulted in the exclusive conversion of the cis-isomer and recovery of the unreacted trans-isomer, thus suggesting some influence of the configuration of allylic alcohol group on the recognition ability of the enzyme. A different reactivity of substrate enantiomers was also evidenced in the epoxidation of terpinene-4-ol, while perillaldehyde underwent exclusive oxidation to perillic acid. The biocatalyzed oxidation proceeds with high stereocontrol and offers a sustainable method for valorization of terpenes to fine chemicals, avoiding costs related to enzyme purification and disposal of toxic metal catalysts.