Hydroxynitrile lyases chiral cyanohydrins

The hydroxynitrile lyase (HNL)-catalyzed addition of HCN to aldehydes is the most important synthesis of non-racemic cyanohydrins. Since now not only (f )-PaHNL from almonds is available in unlimited amounts, but the recombinant (S)-HNLs from cassava (MeHNL) and rubber tree (HbHNL) are also available in giga units, the large-scale productions of non-racemic cyanohydrins have become possible. The synthetic potential of chiral cyanohydrins for the stereoselective preparation of biologically active compounds has been developed during the last 15 years. 

Hydroxynitrile lyases (HNLs or oxynitrilases) catalyze C—C bond-forming reactions between an aldehyde or ketone and cyanide to form enantiopure cyanohydrins (Figure 1.15), which are versatile building blocks for the chiral synthesis of amino acids, hydroxy ketones, hydroxy acids, amines and so on [68], Screening of natural sources has led to the discovery of both... 

Rhodococcus erythropolis NCIMB 11540 has been employed as biocatalyst for the conversion of (R)- or (.S )-cyanohydrins to the corresponding (R)- or (S)-a-hydroxycarboxylic acids with an optical purity of up to >99% enatiomeric excess (ee) [27-29] the chiral cyanohydrins can separately be produced using hydroxynitrile lyase from Hevea braziliensis or from Prunus anygdalis [30]. Using the combined NHase-amidase enzyme system of the Rhodococcus erythropolis NCIMB 11 540, the chiral cyanohydrins were first hydrolyzed to the... 

Chiral cyanohydrins are versatile intermediates in the synthesis of a-hydroxy acids, /3-amino alcohols, amino nitriles, a-hydroxy ketones and aziridines. For the synthesis of enantiopure cyanohydrins, the use of hydroxynitrile lyases is currently the most effective approach.Application of an organic-solvent-free system allows thermodynamically hindered substrates to be converted with moderate to excellent yields. With the use of the highly selective hydroxynitrile lyase from Manihot esculenta, the syntheses of several acetophenone cyanohydrins with excellent enantioselectivities were developed (Figure 8.2). (5)-Acetophenone cyanohydrin was synthesized on a preparative scale. ... 

Scheme 7.23 Synthesis of chiral cyanohydrins from 3-pyridinecarboxyaldehyde in organic solvent using commercially available CLEA particles of hydroxynitrile lyases.

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]. 

The hydroxynitrile lyase (HNL) class of enzymes, also referred to as oxynitrilases, consists of enzymes that catalyze the formation of chiral cyanohydrins by the stereospecific addition of hydrogen cyanide (HCN) to aldehydes and ketones (Scheme 19.36).275 279 These chiral cyanohydrins are versatile synthons, which can be further modified to prepare chiral a-hydroxy acids, a-hydroxy aldehydes and ketones, acyloins, vicinal diols, ethanolamines, and a- and P-amino acids, to name a few.280 Both (R)- and (.S )-selective HNLs have been isolated, usually from plant sources, where their natural substrates play a role in defense mechanisms of the plant through the release of HCN. In addition to there being HNLs with different stereo-preferences, two different classifications have been defined, based on whether the HNL contains a flavin adenine dinucleotide (FAD) co-factor. 

Enantiomerically pure cyanohydrins can easily be modified chemically or enzymatically (Scheme 4.12B), providing access to chiral a-hydroxy acids, a-hydroxy amides, 2-aminoalcohols, and epoxides. Replacement of the hydroxyl functionahty by a better leaving group, such as a sulfonyl moiety (e.g. tosylate), allows the introduction of various other nucleophiles with inversion at the stereocenter [51a]. The structures of some bioactive molecules that have been synthesized using a biotransformation step with a hydroxynitrile lyase are depicted in Scheme 4.12B [51a, 52]. 

Hydroxynitrile lyase enzymes catalyze the asymmetric addition of hydrogen cyanide onto a carbonyl group of an aldehyde or a ketone thus forming a chiral cyanohydrin [1520-1524], a reaction which was used for the first time as long ago as 1908 [1525]. Cyanohydrins are rarely used as products per se, but they represent versatile starting materials for the synthesis of several types of compounds [1526] ... 

With the use of bio catalysts, the preparation of chiral cyanohydrins is possible. (R)- as well as (S)-cyanohydrins are now easily available as a result of the excellent accessibility, the relatively high level of stability and the easy handling of hydroxynitrile lyases (HNLs). An example of the synthesis of (S)-cyanohydrins is shown in Fig. 10.40. The optimization of reaction conditions (solvent, temperature and site-directed mutagenesis) has enabled HNL-catalyzed preparation of optically active cyanohydrins on an industrial scale. 

Hydroxynitrile Lyases for Biocatafytic Synthesis of Chiral Cyanohydrins... 

HYDROXYNITRILE LYASES FOR BIOCATALYTIC SYNTHESIS OF CHIRAL CYANOHYDRINS... 

The application of nitrile-converting enzymes, nitrilases, and hydroxynitrile lyases in the s)mthesis of chiral compounds and cyanohydrins is covered. 

Hydroxynitrile lyases (also known as oxynitrilases) are used for the synthesis of chiral cyanohydrins. Because the hydroxynitrile moiety can be easily converted into a wide range of functional groups (Scheme 28.1), the cyanohydrins represent a versatile building block in total synthesis of natural products. Moreover, both R- and S-selective enzymes are available. They are also straightforward to handle and therefore represent catalysts of choice for syntheses on an industrial scale. Already as early as 1908, they had been used in enantioselective synthesis indeed, the first enantio-selective reaction was performed with an HNL. Consequently, many applications for these enzymes have been developed in natural products synthesis. 

On the other hand, various ( l-cyanohydrins have been prepared using (5)-hydroxy-nitrile lyases from plants (Fig. 34). The (5)-cyanohydrins can be further converted to a-hydroxy acids by acid hydrolysis without racemization [107]. A recent example is the hydroxynitrile lyase from Manihot esculenta, which was cloned in E. coli and used as chiral catalyst for the synthesis of a broad range of optically active a-hydroxynitriles including keto-(5)-cyanohydrins using diisopropyl ether as organic solvent and HCN as cyanide source [112]. Compared to the enzymes from leaves, the overexpressed enzyme in E. coli showed higher enantioselectivity. 

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