R -Cyanohydrins

The two established Hnls, those from L. usitatissimum and P. amygdalus, have found biocatalytic applications for the production of (i )-cyanohydrins. The former of these Hnls is the least widely applied, the natural substrates being acetone cyanohydrin or (i )-2-butanone cyanohydrin (Table 1) [28]. Although an improved procedure for the purification of this enzyme has been reported [27] it is still only available in limited quantities (from 100 g of seedlings approximately 350 U of enzyme are obtained). It was found that this enzyme transforms a range of aliphatic aldehyde and ketone substrates [27], the latter of which included five-membered cyclic (e.g. 2-methylcyclopentanone) and chlorinated ketone substrates. In contrast, attempts to transform substituted cyclohexanones and 3-methylcyclopentanone failed and it was even found that benzaldehyde deactivated the enzyme. 

Recent work [64] by Kiljunen and Kanerva has been directed towards the search for novel sources of (R)-oxynitrilases which may transform bulky aryl aldehydes. For this purpose whole cell preparations (called meal) from apple seeds and cherry, apricot and plum pips were tested for their (R)-cyanohydrin activity. In this study a comparison of almond and apple meal showed that they possess similar properties for the formation of the (R)-stereogenic centre. However, in certain cases higher enantioselectivity was observed using the apple meal preparation. Additionally, apple meal (R)-Hnl has also been applied to transform ketones into their corresponding cyanohydrins [65] thus creating a wider repertoire of substrates for this latest of (R)-Hnls. Thus it has only recently been shown that apple meal (R)-oxynitrilase is now an additional member of the (R)-Hnl family. 

Also in the (R)-class of Hnls, although of more limited application, is the (R)-mandelonitrile lyase from Phlebodium aureum [35] which catalyses the addition to some aromatic and heterocyclic carbonyls but only poorly to aliphatic carbonyls. 

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Enantioselective addition of hydrogen cyanide to hydroxypivaldehyde (25), catalyzed by (lf)-oxynittilase, afforded (R)-cyanohydrin (26) in good optical yield. Acid-catalyzed hydrolysis followed by cyclization resulted in (R)-pantolactone in 98% ee and 95% yield after one recrystallization (56). 

Cyanohydrin Synthesis. Another synthetically useful enzyme that catalyzes carbon—carbon bond formation is oxynitnlase (EC 4.1.2.10). This enzyme catalyzes the addition of cyanides to various aldehydes that may come either in the form of hydrogen cyanide or acetone cyanohydrin (152—158) (Fig. 7). The reaction constitutes a convenient route for the preparation of a-hydroxy acids and P-amino alcohols. Acetone cyanohydrin [75-86-5] can also be used as the cyanide carrier, and is considered to be superior since it does not involve hazardous gaseous HCN and also virtually eliminates the spontaneous nonenzymatic reaction. (R)-oxynitrilase accepts aromatic (97a,b), straight- (97c,e), and branched-chain aUphatic aldehydes, converting them to (R)-cyanohydrins in very good yields and high enantiomeric purity (Table 10). 

The comparison shows that, although the reaction in ethyl acetate requires longer reaction times, the enantiomeric purity of the (R)-cyanohydrins is appreciably better than for the reaction in ethanol/ water. 

A general method for preparing (/ )-cyanohydrins, derived from ketones, is the (/ )-oxyni-trilase-catalyzed addition of hydrocyanic acid to ketones in an organic solvent30. The (R)-cyanohydrins are obtained with good chemical yields and in high optical purity (Table 3)30. 

Table 3. (R)-Cyanohydrins by Enzymatic Formation from Ketones and Hydrocvamic Acid as well as (7 )-a-Hydroxy-a-methyl Carboxylic Acids by Hydrolysis...

Beta nicotinic acid to 6-hydroxy nicotinic acid 3-substituted pyridine to a building block for imidacloprid (an insecticide) R-Cyanohydrins from benzaldehydes and HCN ... 

R)-Cyanohydrins react with toluenesulfonyl chloride, methanesulfonyl chloride or 4-nitrobenzenesulfonyl chloride without loss of stereochemical purity, and the 2-sulfonyloxy-nitrile reacts with a variety of Sn2 reactions to give a variety of products, such as 2-fluoro nitrile [64], 2-azidonitrile [65] and /V- p h t h a I o yl - p rot e c te d 2-aminonitrile [66], 2-acetoxy nitrile [66], and 2-mercapto nitrile [67]. Hydrogenation of 2-sulfonyloxynitriles with LiAIH4 in good chemical yields and high ee afforded 2-monosubstituted (S)-aziridines [68]. 

Groger, H., Copan, E., Bathuber, A. and Vorlop, K. (2001) Asymmetric synthesis of an (R)-cyanohydrin using enzymes entrapped in lens-shaped gels. Organic Letters, 13, 1969-1971. 

Effenberger, F. and Jager, J. (1997) Synthesis of the adrenergic bronchodilators (R)-terbutaline and (R)-salbutamol from (R)-cyanohydrins. The Journal of Organic Chemistry, 62, 3867-3873. 

Syed, J., Forster, S. and Effenberger, F. (1998) Application of the Blaise reaction stereoselective synthesis of (4R)-tert-butyl 3-amino-4-trimethylsilyloxy-2-alkenoates from (R)-cyanohydrins. Tetrahedron Asymmetry, 9, 805-815. 

Indeed the only conversion where biocatalysis should be seriously considered is the transformation of aldehydes into optically active cyanohydrins1 2. For example, the conversion of aryl aldehydes into the appropriate (R)-cyanohydrins using almond meal may be accomplished in quantitative yield and gives products... 

Complexation of an amino acid derivative with a transition metal to provide a cyanation catalyst has been the subject of investigation for some years. It has been shown that the complex formed on reaction of titanium(IV) ethoxide with the imine (40) produces a catalyst which adds the elements of HCN to a variety of aldehydes to furnish the ( R)-cyanohydrins with high enantioselectivity[117]. Other imines of this general type provide the enantiomeric cyanohydrins from the same range of substrates11171. 

The endo-spiro-OZT could be prepared through a reaction sequence similar to that applied for the exo-epimer, with spiro-aziridine intermediates replacing the key spiro-epoxides (Scheme 18). Cyanohydrin formation from ketones was tried under kinetic or thermodynamic conditions, and only reaction with the d-gluco derived keto sugar offered efficient stereoselectivity, while no selectivity was observed for reaction with the keto sugar obtained from protected D-fructose. The (R) -cyanohydrin was prepared in excellent yield under kinetic conditions (KCN, NaHC03, 0 °C, 10 min) a modified thermodynamic procedure was applied to produce the (S)-epimer in 85% yield (Scheme 18). 

Serianni, A.S., Nunez, H.A. and Barker, R., Cyanohydrin synthesis studies with carbon-13-labeled cyanide. 7. Org. Chem., 1980, 45, 3329. 

Effenberger, E Gutterer, B. Jager, J. Stereoselective synthesis of (IR)- and (lR,25)-l-aryl-2-alkylamino alcohols from (R)-cyanohydrins. Tetrahedron Asymmetry 1997, 8,459 67. 

In these synthesis, the optically active (R)-cyanohydrin is transformed into the corresponding a-hydroxy carboxylic ester and the hydroxyl funchon is achvated by sulfonylahon. The treatment of the corresponding intermediate with tetra-hydrothieno[3,2-c]pyridine stereoselectively yields the (S)-configured clopidogrel (Scheme 10.23). In the second case, a mutant of the recombinant almond (Pmnus amigdalus) (R)-oxynitrilase isoenzyme 5 catalyzes the formation of enantiopure (R)-2-hydroxy-4-phenylbutyronitrile [54]. Reaction of the sulfonylated hydroxyester derivative with the corresponding dipeptide leads to the formation of enalapril or lisinopril (Scheme 10.24). 

ImHNL is a nonglycosylated homodimer (84 kDa) which catalyzes the reversible cleavage of aliphatic (R)-cyanohydrins [34]. This HNL does not require complex protein modification after protein biosynthesis. Thus, expression in prokaryotic Escherichia coli) and eukaryotic hosts Pichia pastoris) is possible [35-37]. However, initial trials to express IwHNL in E. coli were hampered by formation of inclusion bodies [36]. 

Aliphatic aldehydes have been converted to their (R)-cyanohydrins using a bipha-sic system to accommodate hydroxynitrile lyase enzyme (from the Japanese apricot, Prunus mume) as the enantioselective catalyst.251... 

In a second approach hydrocyanic acid was added to hydroxypivaldehyde by use of (R)-selective hydroxynitrile lyase from almonds (PaHNL) [11]. (R)-Cyanohydrin was obtained in 84% yield and 89% ee, and was directly cydized to give crude D-pantolactone by acid-catalyzed hydrolysis. Unfortunately, in contrast with O-protected hydroxy and halogenated pivalaldehydes, the technically available starting compound hydroxypivaldehyde requires use of purified enzyme (and high enzyme loading). 

Scheme 12 Option. The oxynitrilase-catalyzed HCN addition to the aldehyde Xin appeared to offer an attractive prospect presuming that the R-cyanohydrin (XIV) could be formed, and this then converted to dilevalol via intermediates XV and XVI. Although the oxynitrilase-catalyzed formation of chiral aromatic and aliphatic cyanohydrins and their reduction to chiral aminoalcohols has been known for some time,16 the selective reduction of XIV to XV and the likelihood of 100% induction in the reduction of the Schiff base XVI raised many questions.

The k-selective Prunus amygdalus HNL is readily available from almonds. Approximately 5 g of pure enzyme can be isolated from 1 kg of almonds, alternatively crude defatted almond meal has also been used with great success. This enzyme has already been used for almost 100 years and it has successfully been employed for the synthesis of both aromatic and aliphatic R-cyanohydrins (Scheme 5.2) [11]. More recently it has been cloned into Pichea pastoris, guaranteeing unlimited access to it and enabling genetic modifications of this versatile enzyme [12]. 

Scheme 5.2 Application of Prunus amygdalus HNL for the synthesis of R-cyanohydrins.

The Lewis acid-Lewis base bifunctional catalyst 178a, prepared from Ti(Oi-Pr)4 and diol 174 (1 1), realizes highly enantioselective cyanosilylation of a variety of ketones to (R)-cyanohydrin TMS ethers (Scheme 10.241) [645]. The proposed mechanism involves Ti monocyanide complex 178b as the active catalyst this induces reaction of aldehydes with TMSCN by dual activation. Interestingly, the catalyst prepared from Gd(Oi-Pr)3 and 174 (1 2) serves for exclusive formation of (S)-cyanohy-drin TMS ethers [651]. The catalytic activity of the Gd complex is much higher than that of 178a. The results of NMR and ESI-MS analyses indicate that Gd cyanide complex 179 is the active catalyst. It has been proposed that the two Gd cyanide moieties of 179 play different roles - one activates an aldehyde as a Lewis acid and the other reacts with the aldehyde as a cyanide nucleophile. 

Table 2. Synthesis of (R)-cyanohydrins by Prunus amygdalus oxynitrilase catalysed addition of HCN to aldehydes RCHO ...

Lipases have been used to effect the enantioselective esterification of cyanohydrins or the enantioselective hydrolysis of cyanohydrin esters. This works for aldehyde cyanohydrins. Selective (S)-cyanohydrin esterification is effected by the enzyme from Pseudomonas sp. [11], There is also an example of selective (R)-cyanohydrin esterification by Candida cylindracea lipase [12]. Effenberger has shown the feasibility of this approach in principle to produce a number of enantiopure cyanohydrins derived from aldehydes. In situ derivatization with racemization as shown in Fig. 7 is possible, resulting in theoretically 100% yield of the desired enantiomer [13]. Ketone cyanohydrins, which are tertiary alcohols, do not easily undergo this reaction. 

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