2.1.1 Cyanohydrin Formation/Henry Reaction

Abstract

AbstractEnantiopure cyanohydrins and β-nitro alcohols serve as versatile building blocks for a broad range of chemical and enzymatic reactions, resulting in highly valuable products for many applications. Hydroxynitrile lyases comprise a diverse group of enzymes that catalyze the reversible cleavage of cyanohydrins to carbonyl compounds and hydrogen cyanide. The enzymes have been studied broadly concerning their substrate scope, specificity, structure, and reaction mechanism, and many have been successfully applied and engineered for the synthesis of cyanohydrins from laboratory to industrial scale. Both R- and S-cyanohydrins are accessible from a broad variety of substrates and, in most cases, high yields and enantiopurities can be obtained after enzyme and reaction engineering. Recent progress in the development and optimization of heterologous expression systems make recombinant hydroxynitrile lyases available in the quantities needed for industrial production. The procedures for safe handling of cyanides are also well-defined and established. In addition, some hydroxynitrile lyases are able to catalyze the nonnatural asymmetric Henry reaction. Although the enzyme activities are rather low, the enzymatic synthesis of enantiopure β-nitro alcohols shows promising results.