DKR of Cyanohydrins

Several reports on DKR of cyanohydrins have been developed using this methodology The unstable nature of cyanohydrins allows continuous racemization through reversible elimination/addition of HCN under basic conditions. The lipase-catalyzed KR in the presence of an acyl donor yields cyanohydrin acetates, which are not racemized under the reaction conditions. 

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 

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Racemization of some substrates can take place through reversible formation of the substrate via an addition/elimination process. The racemization can be acid or base catalyzed. In this section we vill discuss DKR of cyanohydrins and hemithioacetals. 

Kanerva et al. have also reported DKR of cyanohydrins [47]. In particular, they obtained very good results with C. antartica lipase A (CAL-A) as the catalyst for the KR of a variety of substrates for which other enzymes such as CALB or PS-C do not give good results (Figure 4.22) [47a[. 

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). 

Scheme 4.22 Lipase-catalysed DKR of cyanohydrins using silica-supported BTAH as a racemizing agent.

Scheme 4.29 DKR of cyanohydrins using CALB and ruthenium catalyst.

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]. 

DKR of aldehydes via cyanohydrins One-pot (iPr)20 Acetone cyanohydrin + anion exchange resin Pseudomonas sp. lipase + 29... 

The same CALB preparation was appUed in many dynamic kinetic resolutions combining two types of catalysts with each other. In the presence of homogeneous transition metal catalysts that catalyze the racemization and heterogeneous acids or bases or immobilized transition metals Novozym 435 was not deactivated [1, 26-28]. This is all the more remarkable since the reactions catalyzed by these catalysts include redox reactions at elevated temperatures (>60°C). When Novozym 435 was applied for the enantioselective synthesis of cyanohydrin acetates (10) from aliphatic aldehydes (7), good results were achieved (Scheme 2.2) for this dynamic kinetic resolution (DKR) [29]. Here NaCN is used as the base for the dynamic racemic formation and degradation of the cyanohydrins (6 and 8). 

When Upases are immobiUzed on Celite they can readily be used in dry organic solvents. Pseudomonas cepacia (also named B. cepacia) Upase was immobiUzed in the presence of sucrose on Hyflo super-cel CeUte [36, 40, 41] and used in the first enantioselective synthesis of cyanohydrin acetates via a DKR. Similarly, other successful syntheses of cyanohydrin acetates via DKR were catalyzed by Upases immobiUzed on CeUte [42]. This is in contrast to Novozym 435, which had successfully been used for the DKR of aUphatic cyanohydrin acetates (see Section 2.2.1 and Scheme 2.2) [29]. When Novozym 435 was used for the enantioselective synthesis of mandelonitrile acetate (la) via DKR, the reaction did not proceed [43]. It was... 

This nitrilase dynamic kinetic resolution (DKR) methodology depends on the availability of highly enantioselective biocatalysts that generate a minimum amount of amide. This latter issue may seem trivial and has long been disregarded somewhat, but reports of modest amounts of amide co-products date back to the early days of nitrilase enzymology. Only recently has the subject come under more intense scrutiny [3-5] and has a relationship with the stereochemistry of the nitrile been demonstrated [3, 5]. Hence, we set out to investigate the enantiomer and chemical selectivity of nitrilases in the hydrolysis of a representative set of cyanohydrins. 

Cyanohydrins eliminate HCN under basic conditions, giving the corresponding planar aldehyde or ketone. When combined with an asymmetric reaction, the equilibrium can be used for an efficient in situ racemization of cyanohydrins, leading to a DKR process. For example, chiral secondary cyanohydrins can be acylated by isopropenyl acetate in the presence of lipase and solid base such as anion-exchange resin (OH" form) [8a,b] or silica-supported ammonium hydroxide [8c] (Scheme 5.31). A range of aromatic cyanohydrin acetates can be obtained in high chemical and optical yields, although the efficiency is lower for aliphatic precursors [8a]. The success of DKR is ascribable not only to the stereochemical... 

When the preceding conditions for the enantioselective synthesis of cyanohydrin acetates via DKR were applied to aliphatic substrates, only a kinetic resolution was observed. However, Hanefeld s group has shown that, by... 

In the context of these studies, the same group has demonstrated that the synthesis of cyanohydrin esters via DKR was highly dependent on the carrier of the enzyme. The carrier influenced the amount of water available in the reaction mixture, suppressing or enhancing the undesired hydrolysis of the acyl donor and the final product. Indeed, the DKR proved to be prone to residual water. However, when the hpase was immobilised on cehte as a carrier, the celite absorbed the water and suppressed the water-induced side reactions. Thereby, the enantioselectivity and the reaction times (3 10 days without cehte) for this DKR were improved, enabling a nearly enantiospecific and high-yielding synthesis of mandelonitrile acetate (Scheme 3.28). 

Racemic hydantoins result from the reaction of carbonyl compounds with potassium cyanide and ammonium carbonate or the reaction of the corresponding cyanohydrins with ammonium carbonate (Bucherer-Bergs reaction). Hydantoins racemize readily under basic conditions or in the presence of hydantoin racemase, thus allowing DKR (Figure 6.43). Hydantoinases (EC 3.5.2.2), either isolated enzymes or whole microorganisms, catalyze the hydrolysis of five-substituted... 

For successful DKR two reactions an in situ racemization (krac) and kinetic resolution [k(R) k(S)] must be carefully chosen. The detailed description of all parameters can be found in the literature [26], but in all cases, the racemization reaction must be much faster than the kinetic resolution. It is also important to note that both reactions must proceed under identical conditions. This methodology is highly attractive because the enantiomeric excess of the product is often higher than in the original kinetic resolution. Moreover, the work-up of the reaction is simpler since in an ideal case only the desired enantiomeric product is present in the reaction mixture. This concept is used for preparation of many important classes of organic compounds like natural and nonnatural a-amino acids, a-substituted nitriles and esters, cyanohydrins, 5-alkyl hydantoins, and thiazoUn-5-ones. 

Scheme 2.2 Novozym 435 catalyzes the enantioselective synthesis of aliphatic cyanohydrins via a DKR.

In particular, the use of hydroxynitrile lyase has proved to be a general and reliable method for obtaining a-hydroxy nitriles of both configurations [22]. An interesting approach is the Upase- and baseacyl-cyanohydrin for a synthetic DKR [23]. This is a combination of two reaction systems the dynamic, base-catalyzed equiUbrium between acetone cyanohydrin, acetone, HCN, aldehyde and a racemic cyanohydrin and the lipase-catalyzed enantioselective and irreversible acylation of the hydroxyl group. The combination yields the... 

The nitrilase mediated DKR route to enantiomerically pure 2-hydroxycarboxylic acids is restricted to the (R)-enantiomers because, to our knowledge, no (S)-selec-tive nitrilases for cyanohydrin substrates are commonly available [11]. We reasoned that a fully enzymatic route to the (S)-acids should be possible by combining an (S)-selective oxynitrilase (hydroxynitrile lyase, EC 4.1.2.10, (S)-hydroxynitrile lyase) and a non-selective nitrilase in a bienzymatic cascade (see Figure 16.3). Besides being more environmentally acceptable than chemical hydrolysis, the mild reaction conditions of the combined enzymatic reaction would be compatible with a wide range of hydrolysable groups. 

The stereoselectivity of the hydroxynitrile lyase (HNL) catalysed cyanohydrin formation of monosubstituted cyclic ketones is of general interest for the synthesis of biologically active compounds. In the course of a systematic investigation of the stereoselectivity of HNL-catalysed addition of HCN to a variety of monosubstituted cyclopentanones, Kobler and Effenberger observed a DKR for the addition of HCN to alkyl 2-oxocyclopentanecarboxylates... 

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