Biocatalysis in Biphasic Systems Oxynitrilases

In the last decades, cyanohydrins have become versatile chiral building blocks, not only for laboratory synthesis, but also for a range of pharmaceuticals and agrochemicals. Several methods for the enantioselective preparation of these compounds have been published [1, 2]. The most important synthetic approaches are catalysis by oxynitrilases, also termed hydroxynitrile lyases (HNLs), wording used in this chapter, [3] and by transition metal complexes [4], whereas the relevance of cyclic dipeptides as catalysts is decreasing [2]. 

HNLs are found in over 3000 higher plant species, like Rosaceae, Euphorbiaceae, Linaceae and Filitaceae, in several bacteria, and in some insects [5, 6]. For cyanohydrin synthesis, mainly five HNLs have been used the (R)-selective HNLs from Prunus amygdalus (PaHNL) and Linurn usitatissimum (l.ul INI.) and the (S)-selective ones from Hevea brasiliensis (HfeHNL), Manihot esculenta (MeHNL), and Sorghum bicolor (SfcHNL) [1]. 

The first synthesis of mandelonitrile using hydroxynitrile lyases was published by Rosenthaler in 1908 [7], but only fifty years later this research field regained attention for synthetic purposes by Becker et al. [8-10]. However, because of the spontaneous chemical addition of HCN in the aqueous and alcoholic medium used, only low enantiomeric excesses ( ) were achieved. In addition, the low solubility of many substrates in the buffer made this reaction unsuitable for laboratory scale and industrial application. 

The breakthrough came with the employment of water-immiscible organic solvents, where the undesired chemical reaction is more or less suppressed 

In this chapter, the use of HNL-catalysis in such systems to provide increased yield and enantiomeric excess is discussed. 

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As well as biocatalysis in neat organic solvents and biphasic systems (fundamentals and synthetic applications), the present volume covers new and promising aspects of non-aqueous enzymology that have emerged in recent years, including biocatalysis with undissolved solid substrates or vaporized compounds, the use of ionic liquids as solvents, and the preparative-scale exploitation of oxynitrilases and dynamic kinetic resolutions . For the sake of completeness and comparison,... 

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