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Enzyme catalyzed reaction from almond

S-Cyanohydrins by Selective Cleavage of the R Form in a Racemate. In principle it should be possible to obtain S-cyanohydrins by selective enzymatic decomposition of the R enantiomer in a racemic mixture. This is difficult to achieve on a practical scale because the HCN that is liberated can engage in both enzyme-catalyzed and non-enzyme-catalyzed reactions by which the R enantiomer is again formed. This problem has been elegantly solved by Gotor and co-workers by combining this approach with that of the transcyanation [102]. A mixture of aldehyde and racemic cyanohydrin from a methyl ketone was treated with almond meal. The / -cyanohydrin of the methyl ketone was selectively cleaved and the / -cyanohydrin of the aldehyde was selectively formed by trans-... [Pg.302]

A suitable catalyst for the synthesis of (R)-cyanohydrins is the enzyme (R)-oxynitrilase from bitter almonds. It catalyzes exclusively si-face addition of hydrogen cyanide to benzaldehyde or other aldehydes. A competing non-enzymatic parallel reaction lowers the enantiomeric excess of the product155, 56]. [Pg.200]

Oxynitrilase isolated from almonds contains one FAD molecule per monomer, while the enzyme isolated from other sources does not contain a flavin. " The FAD-containing enzyme from almonds catalyzes both the decomposition of mandelonitrile (I cat = 630s , 25 °C) and the reverse reaction, formation of mandelonitrile... [Pg.94]

For the synthesis of cyanohydrins nature provides the chemist with R- and S-selective enzymes, the hydroxynitrile lyases (HNL) [4-7]. These HNLs are also known as oxynitrilases and their natural function is to catalyze the release of HCN from natural cyanohydrins like mandelonitrile and acetone cyanohydrin. This is a defense reaction of many plants. It occurs if a predator injures the plant cell. The reaction also takes place when we eat almonds. Ironically the benzaldhyde released together with the HCN from the almonds is actually the flavor that attracts us to eat them. [Pg.225]

As mentioned already, the use of organic solvents for the HNL-catalyzed addition of HCN to carbonyl compounds was decisive for many investigations concerning optically active cyanohydrins. Several variations for the practical performance of the HNL-c alyzed preparation of (R)- and (S)-cyanohydrms have been developed in recent years. Instead of pure organic solvents, a biphasic system (water/organic solvent) can be used for the reaction whereby HCN can be prepared in situ from sodium cyanide and acetic acid [20] or by transcyanation with acetone cyanohydrin [21]. It is possible to replace isolated enzymes by whole cells, e.g., by almond and apple meal instead of PaHNL or by Sorghum shoots instead of SbHNL [21,22]. [Pg.323]

Although the natural substrate of ( )-PaHNL from bitter almonds is benzaldehyde, this enzyme also catalyzes the addition of HCN to ketones to give (i )-ketone cyanohydrins (Scheme 3, Table 5) [25]. (i )-Ketone cyanohydrins derived from 2-alkanones are formed in good chemical and excellent optical yields, whereas from 3-alkanones only unsatisfactory chemical and optical yields are obtained. In contrary to the reaction behavior of aldehydes the enantioselectivity of PaHNL-catalyzed HCN addition to ketones in an organic solvent and an aqueous citrate buffer, respectively, is comparable (Table 5) [25]. The variation, working with almond meal instead of isolated (/ )-PaHNL, results also in (i )-ketone cyanohydrins with surprisingly high e.e. values [26]. [Pg.327]


See other pages where Enzyme catalyzed reaction from almond is mentioned: [Pg.298]    [Pg.289]    [Pg.104]    [Pg.312]    [Pg.89]    [Pg.14]    [Pg.76]    [Pg.167]    [Pg.157]    [Pg.24]    [Pg.196]    [Pg.353]    [Pg.321]    [Pg.325]    [Pg.317]    [Pg.201]    [Pg.397]    [Pg.276]    [Pg.62]    [Pg.416]   
See also in sourсe #XX -- [ Pg.457 ]




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