Preparation of fatty epoxy alcohols using oat seed peroxygenase in nonaqueous media

Expanding the Use of Peroxygenase from Oat Flour in Organic Synthesis: Enantioselective Oxidation of Sulfides

Biocatalyzed oxidations are an important target in sustainable synthesis since chemical oxidations often require harsh conditions and metal-based catalysts. A raw peroxygenase-containing enzymatic preparation from oat flour was tested as a biocatalyst for the enantioselective oxidation of sulfides to sulfoxides and the variations of some reaction parameters were evaluated. Under optimal conditions, thioanisole was fully converted into the corresponding (R)-sulfoxide with high optical purity (80% ee) and the same stereopreference was maintained in the oxidation of some other sulfides. Changes in the substituent on the sulfur atom affected the selectivity of the enzyme and the best results were obtained with phenyl methoxymethyl sulfide, which gave the corresponding sulfoxide in 92% ee as exclusive product. The over-oxidation of sulfides to sulfones was instead detected in all the other cases and preferential oxidation of the (S)-enantiomer of the sulfoxide intermediate was observed, albeit with low selectivity. Carrying out the oxidation of thioanisole up to the 29% formation of sulfone led to enhancement of the sulfoxide optical purity (89% ee). The activity in sulfoxidation reactions, in addition to that reported in the epoxidation of different substrates, makes this plant peroxygenase a promising and useful tool in organic synthesis.

Isolation of unsaturated diols after oxidation of conjugated linoleic acid with peroxygenase

Oat seeds are a rich source of peroxygenase, an iron heme enzyme that participates in oxylipin metabolism in plants. An isomer of CLA, 9(Z),11 (E)-octadecadienoic acid (1), believed to have anticarcinogenic activity, was used as a substrate for peroxygenase in an aqueous medium using t-butyl hydroperoxide as the oxidant. After acidification of the reaction medium, the products were extracted with ethyl ether, converted to their methyl esters, and characterized using HPLC. Major products after reaction for 24 h showed resonances from 1H NMR spectroscopy that were further downfield than the expected epoxides and were thought to be diol hydrolysis products. However, analyses by HPLC with atmospheric pressure chemical ionization MS (APCI-MS) of the putative allylic diols or their bis-trimethylsilyl ether derivatives gave incorrect M.W. The M.W. of the diols could be obtained by APCI-MS after removal of unsaturation by hydrogenation or by EI-MS after conversion of the allylic 1,2-diols to cyclic methyl boronic esters. Data from MS in conjunction with analyses using 1H and 13C NMR showed that the methylated products from 1 were methyl 9,10(threo)-dihydroxy-11 (E)-octadecenoate, methyl 9,10(erythro)-dihydroxy-11(E)-octadecenoate, methyl 9,12(erythro)-dihydroxy-10(E)-octadecenoate, and methyl 9,12(threo)-dihydroxy-10(E)-octadecenoate. Solid-phase extraction without prior acidification and conversion of the products to methyl esters allowed identification of the following epoxides: methyl 9,10(Z)-epoxy-11 (E)-octadecenoate (6M), methyl 9,10(E)-epoxy-11 (E)-octadecenoate, and methyl 11,12(E)-epoxy-9(Z)-octadecenoate. At times of up to at least 6 h, 6M accounted for approximately 90% of the epoxide product. Product analysis after the hydrolysis of isolated epoxide 6M showed that hydrolysis of epoxide 6 could largely account for the diol products obtained from the acidified reaction mixtures.

An epoxy alcohol synthase pathway in higher plants: biosynthesis of antifungal trihydroxy oxylipins in leaves of potato

[1-14C]Linoleic acid was incubated with a whole homogenate preparation of potato leaves (Solanum tuberosum L., var. Bintje). The methyl-esterified product was subjected to straight-phase high-performance liquid chromatography and was found to contain four major radioactive oxidation products, i.e., the epoxy alcohols methyl 10(S),11(S)-epoxy-9(S)-hydroxy-12(Z)-octadecenoate (14% of the recovered radioactivity) and methyl 12(R), 13(S)-epoxy-9(S)-hydroxy-10(E)-octadecenoate (14%), and the trihydroxy derivatives methyl 9(S),10(S),11(R)-trihydroxy-12(Z)-octadecenoate (18%)and methyl 9(S), 12(S),13(S)-trihydroxy-10(E)-octadecenoate (30%). The structures and stereochemical configurations of these oxylipins were determined by chemical and spectral methods using the authentic compounds as references. Incubations performed in the presence of glutathione peroxidase revealed that lipoxygenase activity of potato leaves generated the 9- and 13-hydroperoxides of linoleic acid in a ratio of 95:5. Separate incubations of these hydroperoxides showed that linoleic acid 9(S)-hydroperoxide was metabolized into epoxy alcohols by particle-bound epoxy alcohol synthase activity, whereas the 13-hydroperoxide was metabolized into alpha- and gamma-ketols by a particle-bound allene oxide synthase. It was concluded that the main pathway of linoleic acid metabolism in potato leaves involved 9-lipoxygenase-catalyzed oxygenation into linoleic acid 9(S)-hydroperoxide followed by rapid conversion of this hydroperoxide into epoxy alcohols and a slower, epoxide hydrolase-catalyzed conversion of the epoxy alcohols into trihydroxy-octadecenoates. Trihydroxy derivatives of linoleic and linolenic acids have previously been reported to be growth-inhibitory to plant-pathogenic fungi, and a role of the new pathway of linoleic acid oxidation in defense reactions against pathogens is conceivable.