Oxynitrilases medium

Initial preparative work with oxynitrilases in neutral aqueous solution [517, 518] was hampered by the fact that under these reaction conditions the enzymatic addition has to compete with a spontaneous chemical reaction which limits enantioselectivity. Major improvements in optical purity of cyanohydrins were achieved by conducting the addition under acidic conditions to suppress the uncatalyzed side reaction [519], or by switching to a water immiscible organic solvent as the reaction medium [520], preferably diisopropyl ether. For the latter case, the enzymes are readily immobilized by physical adsorption onto cellulose. A continuous process has been developed for chiral cyanohydrin synthesis using an enzyme membrane reactor [61]. Acetone cyanhydrin can replace the highly toxic hydrocyanic acid as the cyanide source [521], Inexpensive defatted almond meal has been found to be a convenient substitute for the purified (R)-oxynitrilase without sacrificing enantioselectivity [522-524], Similarly, lyophilized and powered Sorghum bicolor shoots have been successfully tested as an alternative source for the purified (S)-oxynitrilase [525],... 

The Sorghum (S)-oxynitrilase exclusively catalyzes the addition of hydrocyanic acid to aromatic aldehydes with high enantioselectivity, but not to aliphatic aldehydes or ketones [519, 526], In contrast, the Hevea (S)-oxynitrilase was also found to convert aliphatic and a,/ -unsaturated substrates with medium to high selectivity [509, 527]. The stereocomplementary almond (R)-oxynitrilase likewise has a very broad substrate tolerance and accepts both aromatic, aliphatic, and a,/ -unsaturated aldehydes [520, 521, 523, 528, 529] as well as methyl ketones [530] with high enantiomeric excess (Table 9). It is interesting to note that this enzyme will also tolerate sterically hindered substrates such as pivalaldehyde and suitable derivatives 164 which are effective precursors for (R)-pantolactone 165 [531],... 

Among the suitable organic solvents for cyanohydrin synthesis, diisopropyl ether is especially advantageous, since when using it the enzyme activity remains almost constant over a long period of time [139]. In the case of ethyl acetate as reaction medium, a more or less rapid inactivation of the oxynitrilase was observed. However, it seemed to be necessary to immobilise the enzyme to enhance its stability [140], which can be advantageous with respect to reisolation and reusage of the enzyme preparation [2]. 

The (R) -oxynitrilase from the almond Prunus amygdalis has unusually low substrate specificity and gives good yields of a variety of cyanohydrins 126 providing that the reaction medium has two phases to inhibit the uncatalysed addition of HCN to the aldehyde.38... 

When Rosenthaler reported the oxynitrilase-catalyzed hydrocyanation by using almond meal in an aqueous medium in 1908 [53], this contribution not only represented one of the oldest biotransformations in organic chemistry but also the starting... 

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