Carbonyl compounds addition, enzyme-catalyzed

Acyloins (a-hydroxy ketones) are formed enzymatically by a mechanism similar to the classical benzoin condensation. The enzymes that can catalyze reactions of this type arc thiamine dependent. In this sense, the cofactor thiamine pyrophosphate may be regarded as a natural- equivalent of the cyanide catalyst needed for the umpolung step in benzoin condensations. Thus, a suitable carbonyl compound (a -synthon) reacts with thiamine pyrophosphate to form an enzyme-substrate complex that subsequently cleaves to the corresponding a-carbanion (d1-synthon). The latter adds to a carbonyl group resulting in an a-hydroxy ketone after elimination of thiamine pyrophosphate. Stereoselectivity of the addition step (i.e., addition to the Stand Re-face of the carbonyl group, respectively) is achieved by adjustment of a preferred active center conformation. A detailed discussion of the mechanisms involved in thiamine-dependent enzymes, as well as a comparison of the structural similarities, is found in references 1 -4. 

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

The formation of a bond between the carboxylate group derived from bicarbonate and a carbon adjacent to a carbonyl group is indicative of a reaction catalyzed by an enzyme that utilizes biotin as a cofactor (Scheme 16). The recent review by Knowles covers many recent discoveries relating to the role of enzymes in these reactions (43). (Editor s note For additional aspects of biotin-dependent carboxylation, see Chapter 6 by O Leary.) Most biotin-dependent enzymes promote a two-step process in which Ai-carboxybiotin serves as an intermediate in a process involving the exchange of the caiix)xylate group derived from bicarbonate for a proton at the a-carbon of the carbonyl compound (44). 

Many of the observed attributes of enzymes arise by natural selection in order to help the host organism survive and reproduce. Benner et al. have proposed that one such attribute, the stereospecificities of dehydrogenases, has functional significance based on stereochemical arguments (18, 79). The central features of their functional model can be summarized as follows. The stereospecificities of dehydrogenases acting on alcohols are correlated with the equilibrium constant for the alcohol-carbonyl redox reaction as listed in Table IV (18). Enzymes catalyzing reactions where the eq is <10 " ilf transfer the pro-S proton from NADH when is >10"" Af, the pro-R proton is transferred. Thus the more readily reduced carbonyl compounds use the pro-R proton, but the more difficult to reduce carbonyl compounds use the pro-S proton. The proposed correlation is restricted to simple aldehydes and ketones (i.e., without additional chemistry that would influence the equilibrium constant, such as cyclizations of polyols or formation of lactones). The natural substrate of the enzyme must be well... 

Several flavoproteins catalyze the oxidation or reduction of carbon-carbon bonds. Often, the substrates for these enzymes are ct,/3-unsaturated carbonyl compounds. The mechanism used to catalyze these types of reactions involves hydride transfers to or from N 5 of the flavin. In addition, an active site residue is involved in the acid/base chemistry needed for the reaction to occur. Several of these enzymes are involved in cellular energy metabolism. 

In an enzyme-catalyzed addition to a carbonyl compound, only one of the enantiomers is formed. The enzyme can block one face of the carbonyl compound so that it cannot be attacked, or it can position the nucleophile so that it is able to attack the carbonyl group from only one side of the molecule. 

Section 18.15 Enzyme-Catalyzed Additions to a,/3-Unsaturated Carbonyl Compounds 773... 

Hydroxynitrile lyase enzymes catalyze the asymmetric addition of hydrogen cyanide onto a carbonyl group of an aldehyde or a ketone thus forming a chiral cyanohydrin [1520-1524], a reaction which was used for the first time as long ago as 1908 [1525]. Cyanohydrins are rarely used as products per se, but they represent versatile starting materials for the synthesis of several types of compounds [1526] ... 

Enzymes of the hydroxynitrilase dass catalyze the addition of HCN to aldehydes, produdng cyanohydrins. Recendy, the reaction has been extended to a few ketones with modified hydroxynitrilase enzymes. In many cases, these are formed with good optical purities and such reactions are the simplest type of enzyme catalyzed carbon-carbon bond formation. By pairing hydroxynitrile lyases with nitrilases or nitrile hydratases, one-pot, multistep conversions become possible, and this also shifts the equilibrium to favor the addition products. Such concerns are particularly important when applying these catalysts to ketones where the equilibrium generally favors the starting carbonyl compound (Figure 1.17). 

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]. 

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