Nitrilases racemic cyanohydrins

The addition of HCN to aldehydes or ketones produces cyanohydrins (a-hydroxy nitriles). Cyanohydrins racemize under basic conditions through reversible loss of FiCN as illustrated in Figure 6.30. Enantiopure a-hydroxy acids can be obtained via the DKR of racemic cyanohydrins in the presence of an enantioselective nitriletransforming enzyme [86-88]. Many nitrile hydratases are metalloenzymes sensitive to cyanide and a nitrilase is usually used in this biotransformation. The DKR of mandelonitrile has been extended to an industrial process for the manufacture of (R)-mandelic acid [89]. 

One of the most attractive biocatalytic options is the nitrilase-catalysed enantioselective hydrolysis of the racemic cyanohydrin. The hydroxyacid is produced directly without need for protection/deprotection steps and cyanohydrins racemize spontaneously at neutral or... 

However, this route was not developed further because of the amount and resulting cost of the enzyme required to complete the reaction in a reasonable time. Other possible routes with one or more biocatalytic steps included those involving an enantioselective oxynitrilase reaction (Fig. 6). According to the choice of enzyme, it could be possible to form either the (R)- or the (S)-enantiomer. Fig. 7 depicts various routes starting from the racemic cyanohydrin. Nitrilases convert nitriles into the corresponding acids and are sometimes stereospecific. Nitrile hydra-tases convert nitriles into amides, and are also sometimes stereospecific. Ami-dases convert amides into the corresponding acids and are often stereospecific. Screening for enantioselective oxynitrilases [14] and for enantiospecific nitrilases [15] was started, but discontinued when the amidase route (below) was found to be successful. 

Dynamic kinetic resciution of racemic cyanohydrins 25 using recombinant nitrilases toward enantiopure mandeiic and iactic acid derivatives 26 [31]. 

Burk and coworkers have used a variety of nitrilases for the DKR of cyanohydrins [48]. Nitrilases catalyze the hydrolytic conversion of cyanohydrins directly to the corresponding carboxylic acids. Racemization was performed under basic conditions (phosphate buffer, pH 8) through reversible loss of HCN. (R)-Mandelic acid was obtained in high yield (86% yield) and high enantioselectivity (98% ee) after 3 hours (Figure 4.23). 

The group of Burk has used a variety of nitrilases for the DKR of cyanohydrins [23]. Nitrilases catalyze the hydrolytic conversion of cyanohydrins directly to the corresponding carboxylic acid. Racemization was performed under basic... 

Alternatively, enantiopure 2-hydroxycarboxylic acids can be obtained via a dynamic kinetic resolution of the (chemically synthesized) cyanohydrin in the presence of an enantioselective nitrilase (EC 3.5.5.1) (see Figure 16.1, route b). Racemization of the cyanohydrin, via reversible dehydrocyanation, takes place readily at pH 7 or above. The methodology [1] is attractive on account of the mild reaction conditions and is industrially applied in the multiton-scale synthesis of (R)-mandehc acid [2]. 

Enantioselective transformations catalyzed by nitrilases often suffer from poor chiral recognition. Exceptions from this trend are benzaldehyde and phenylac-etaldehyde cyanohydrins. As an additional advantage, these substrates racemize readily at near-neutral pH via reversible loss of hydrogen cyanide representing good starting materials for dynamic kinetic resolution processes. This was demonstrated using 22 substituted phenyl and heteroaryl derivates 25 with two recombinant nitrilases a preparative biotransformation yielded (S)-phenyllactic add 26 in 84% yield and 96% ee on 1 g scale (Scheme 9.7) [31]. 

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