Chiral catalysts cyanohydrin formation

Keywords. 3-Functionalized ketones, a-Keto acid derivatives. Cinchona modified Pt catalysts. Chiral imprints. Chiral metal surfaces. Chiral polymers. Cyanohydrin formation. Cyclic Dipeptides, Epoxidation catalysts. Heterogeneous catalysts. Hydrogenation catalysts. Modified metal oxides. Polypeptides, Tartrate-modified Nickel catalysts... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

Activation of Me3SiCN by coordination of the Si to lithium BINOL-ate as catalyst has been shown to result in the enantioselective formation of cyanohydrins 73 from aromatic and heteroaromatic aldehydes with 82-98% ee (Scheme 7.15) [71]. (For experimental details see Chapter 14.5.4). Several other groups have used dual activation with a chiral Lewis acid and a non-chiral Lewis base [72]. Asymmetric cyanosilylation of PhCOMe and its congeners has also been reported to occur in the presence of sodium phenyl glycinate as catalyst, with up to 94% ee [73],... 

When certain cyclodipeptides are used as catalysts for the enantioselective formation of cyanohydrins, an autocatalytic improvement of selectivity is observed in the presence of chiral hydrocyanation products [80]. A commercial process for the manufacture of a pyrethroid insecticide involving asymmetric addition of HCN to an aromatic aldehyde in the presence of a cyclic dipeptide has been described [80]. Besides HCN itself, acetone cyanohydrin is also used (usually in the literature referred to as the Nazarov method), which can be activated cata-lytically by certain lanthanide complexes [81]. Acetylcyanation of aldehydes is described with samarium-based catalysts in the presence of isopropenyl acetate cyclohexanone oxime acetate is hydrocyanated with acetone cyanohydrin as the HCN source in the presence of these catalytic systems [82]. 

Several industrial processes using lyases as catalysts have been reported. Perhaps the most prominent lyase-catalyzed process is the production of acrylamide from acrylnitrile. This process is carried out by the Nitto Chemical Company of Japan at a scale of more than 40,000 tons per year. Another example is the use of a fumarase for the production of (5 )-malic acid from fumaric acid. As shown in Fig. 7, a water molecule is added to the double bond in fumarate by means of an addition reaction. The result is a cleavage of the carbon-carbon double bond, and a formation of a new carbon-oxygen bond. A third example is bio-catalytic production of a cyanohydrin from a ketone. This reaction is catalyzed by a lyase called oxynitrilase. It consists of the cleavage of one carbon-oxygen bond, and the addition of a HCN molecule. The chirality of the product is based on the form of the enzyme used (/ -oxynitrilase or 5-oxynitrilase). ... 

Formation of Enantiomerically Enriched Cyanohydrins Employing Chiral Chemical Catalysts and Lipase Catalysed Kinetic Resolution 202... 

The great versatility of binol as a chiral ligand has been extended to the Strecker reaction. Shibasaki38 has developed bifunctional catalysts employing the use of this species. After the reportewd use of an aluminum variation of this catalyst 75 for the asymmetric formation of cyanohydrins, aluminum and gadolinium derivatives of 78 were shown to efficiently catalyze the addition of cyanide to imines. 

Addition to Carbonyls, Imines (Strecker-type Reactions), and Heteroaromatic Rings (Reissert-type Reactions). Cyanohydrin trimethylsilyl ethers are of significant synthetic interest as they can be transformed into a variety of multifunctional intermediates. Aldehydes and ketones can be enantioselectively converted to cyanohydrin trimethylsilyl ethers when treated with cyanotrimethylsilane in the presence of a Lewis acid and a chiral ligand. Enantioselective and/or diastereoselective formation of cyanohydrins and their derivatives has been reported and most of these reactions involve chiral ligands and metal catalysts containing Ti (eq 24), Sm (eq 25), and A1 (eq 26). ... 

Emil Fischer s result involving cyanide additions to carbohydrates had demonstrated the power of diastereoselective synthesis early as the 1890s (Equation 1) [4, 34,162]. The corresponding enantioselective formation of cyanohydrins has been the subject of immense efforts. It has long been appreciated that optically active cyanohydrins are synthetically useful intermediates that can be elaborated into a number of chiral building blocks, such as hydroxy acids. In general, there are three main classes of catalysts for the preparation of chiral cyanohydrins enzymes, cyclic dipeptides, and transition metal complexes [163-166]. 

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