Abstract
Limonene is one of the most abundant naturally occurring cyclic monoterpenes and has recently emerged as a sustainable alternative to petroleum-based solvents as well as a chemical platform for the production of value-added compounds. The biocatalytic epoxidation of both enantiomers of limonene was carried out in the presence of a peroxygenase-containing preparation from oat (Avena sativa) flour. Different reaction profiles were observed depending on the starting enantiomer of limonene, but in both cases the 1,2-monoepoxide was obtained as the main product with excellent diastereoselectivity. Trans-1,2-monoepoxide and cis-1,2-monoepoxide were isolated from the reaction of (R)-limonene and (S)-limonene, respectively, and the reactions were scaled-up to 0.17 M substrate concentration. The process is valuable for operational simplicity, lack of toxic metal catalysts, and cost-effectiveness of the enzymatic source. Pure stereoisomers of 1,2-monoepoxides of limonene constitute a useful starting material for biorenewable polymers, but can be also converted into other chiral derivatives by epoxide ring opening with nucleophiles. As a proof of concept, a tandem protocol for the preparation of enantiopure (1S,2S,4R)-1,2-diol from (R)-limonene and (1R,2R,4S)-1,2-diol from (S)-limonene was developed.
Highlights
Limonene is one of the most abundant naturally occurring cyclic monoterpenes and its (R)-(+)-enantiomer is obtained by extraction from citrus oil, wherein it is present in more than 90%, whereas the antipode (S)-(−)-limonene is one of the major components of oaks and pines essential oils.Beside the traditional use as flavor additive for fragrance and food industries [1], limonene provides a green and sustainable alternative to petroleum-based solvents in chemical extractions [2] and chromatography [3], in surfactant formulations [4], and for cleaning purposes [5]
We reported that the freeze-dried aqueous extract of defatted flour from oat seed (Avena sativa) exhibits peroxygenase activity, being able to catalyze the regio- and stereoselective epoxidation of eicosapentaenoic acid (EPA, 20:5 ω-3) using tert-butyl hydroperoxide (t-BuOOH) as oxygen source [38]
A first reaction was carried out using a (R)-limonene:t-BuOOH 1:2 molar ratio in phosphate buffer at pH 7.5 and the progress of the biocatalytic reaction was followed by GC-MS analysis
Summary
Beside the traditional use as flavor additive for fragrance and food industries [1], limonene provides a green and sustainable alternative to petroleum-based solvents in chemical extractions [2] and chromatography [3], in surfactant formulations [4], and for cleaning purposes [5]. Great potential for medical uses comes from the interesting anticancer and cancer prevention properties exhibited by this monoterpene [8,9]. Growing interest is focused on the use of limonene as a chemical platform [10] for the production of value-added compounds and, in this context, many efforts have been directed to the development of bio-based polymers [11,12,13] for specialized applications and with enhanced biodegradability. Direct diastereomeric separation of Antioxidants 2021, 10, 1462.
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