A Cyanide‐free Biocatalytic Process for Synthesis of Complementary Enantiomers of 4‐Chloro‐3‐hydroxybutanenitrile From Allyl Chloride

A biocatalyst used for selective ring scission of (±)‐5‐(chloromethyl)‐4, 5‐dihydroisoxazole to synthesize chiral (R)‐4‐chloro‐3‐hydroxybutanenitrile (90 % ee, 39 % isolated yield) and (S)‐5‐(chloromethyl)‐4, 5‐dihydroisoxazole (99 % ee, 39 % isolated yield) was developed by site‐saturated mutagenesis on aldoxime dehydratase derived from Pseudomonas chlororaphis B23 (OxdA). The positive mutant (OxdA‐L318I, E=68) improved the enantiomeric ratio E by 6‐fold as compared to the wild type enzyme (OxdA‐wild, E=11). The racemic precursor of (±)‐5‐(chloromethyl)‐4, 5‐dihydroisoxazole, used in the reaction, can be synthesized from readily available allyl chloride without utilizing highly toxic cyanide. The enantiopure (S)‐5‐(chloromethyl)‐4, 5‐dihydroisoxazole remaining in the kinetic resolution can be transformed into corresponding chiral (S)‐4‐chloro‐3‐hydroxybutanenitrile without loss of enantiomeric excess by treating it with triethylamine in acetonitrile (99 % ee, 72 % isolated yield) or catalysis of OxdA‐wild enzyme (99 % ee, 88 % isolated yield). An efficient biocatalyst of OxdA‐L318I has been developed for enantioselective N−O bond cleavage of (±)‐5‐(chloromethyl)‐4, 5‐dihydroisoxazole to synthesize (R)‐4‐chloro‐3‐hydroxybutanenitrile by enzyme engineering. The remaining chiral (S)‐5‐(chloromethyl)‐4, 5‐dihydroisoxazole can be transformed into corresponding (S)‐4‐chloro‐3‐hydroxybutanenitrile in alkali condition or under the catalysis of OxdA‐wild type enzyme. This enzymatic method established a cyanide‐free synthetic route to produce complementary enantiomers of 4‐chloro‐3‐hydroxybutanenitrile from readily available allyl chloride.