Enzymic methods aldol reaction

In most enzyme-catalyzed reactions two or more substrates are involved, such as in the enzyme-catalyzed aldol reaction, the cyanohydrin reaction or enzyme-catalyzed peptide synthesis (examples used before). For many reaction schemes kinetic models have been derived using the steady-state assumption. Some important reaction mechanisms and the corresponding rate equations are summarized in Table 7-1. An approach to the steady-state method and a detailed discussion of the resulting kinetic models is difficult and is not the aim of this chapter. 

Even considering only the example of the proline family of aldol catalysts, it is dear that there will soon be hundreds of cases of organocatalysts described in the literature. Direct, organocatalytic aldol reactions do not yet have the generality of traditional stoichiometric methods, which can offer predictable results for a wide variety of substrates. However, companies already offer to screen substrates against panels of up to 200 enzymes to find the optimum biocatalyst for a reaction, and the same approach could be applied to identify rapidly the best organocatalyst for a process. 

The Aldol reaction is one of the most powerful methods for creating the C-C bond. Typical conditions involve the formation of an enolate, usually with a stoichiometric equivalent of base. Stereoinduction is nsnally accomplished with chiral enolates, aldehydes, or auxiliaries.Nature, however, is much more efficient, having created enzymes that both catalyze the aldol reaction and produce stereospecific product. These enzymes, called aldolases, are of two types. The type II aldolases make use of a zinc enolate. Of interest for this section are the type I aldolases, which make use of enamine intermediates. Sketched in Scheme 6.6 is... 

Catalytic aldol reactions are among the most useful synthetic methods for highly stereo-controlled asymmetric synthesis. In this account we discuss the recent development of a novel synthetic technique which uses tandem enzyme catalysis for the bi-directional chain elongation of simple dialdehydes and related multi-step procedures. The scope and the limitations of multiple one-pot enzymatic C-C bond formations is evaluated for the synthesis of unique and structurally complex carbohydrate-related compounds that may be regarded as metabolically stable mimetics of oligosaccharides and that are thus of interest because of their potential bioactivity. 

The Cilag resolution of the pyridyl amino acid is described in Org. Process Res. Dev. 2001, 5, 23. For an informative comparison of different auxiliary and catalytic methods for the synthesis of a simple chiral carboxylic acid, see Org. Process Res. Dev. 2003, 7, 370. For a leading reference to the use of enzymes to reduce ketones, see the account of the Codexis work on montelukast in Org. Process Res. Dev. 2010, 14, 193. The spectacular synthesis of discodermolide by Novartis using a series of aldol reactions is described in Org. Process Res. Dev. 2004, 8, 92, 101 and 107. 

The FDP-aldolase catalyzed DHAP-addition to glyoxylic acid, examined in detail by Wandrey and Bossow-Berke under continuous flow reaction conditions [18], required the determination of the relative and absolute configuration. Employing the SAMP-/RAMP-hydrazone method, the aldol reaction of 1 with benzyl glyoxylate afforded a doubly protected adduct, which was deprotected with trifluoroacetic acid anhydride (TFAA) and subsequent debenzylation with H2/ Pd-C to give the final product, identical with the enzyme product. The independent chemical synthesis and the enzymatic route are compared in scheme 4. It... 

P. Clapes, 1. loglar, Enzyme-Catalyzed Aldol Additions, in R. Mahrwald (Ed.), Modem Methods in Stereoselective Aldol Reactions, lohn Wiley Sons Ltd, 2013, pp. 475-528. 

Clapes, P. and Joglar, J., Enzyme-catalyzed aldol additions. In Modern Methods in Stereoselective Aldol Reactions, Mahrwald, R., Ed. John Wiley Sons, Ltd Chichester, 2013 pp 475-528. 

Asymmetric C-C bond formation is the most important and most challenging problem in synthetic organic chemistry. In Nature, such reactions are facilitated by lyases, which catalyze the addition of carbonucleophiles to C=0 double bonds in a manner that is classified mechanistically as an aldol addition [1]. Most enzymes that have been investigated lately for synthetic applications include aldolases from carbohydrate, amino acid, or sialic acid metabolism [1, 2]. Because enzymes are active on unprotected substrates under very mild conditions and with high chemo-, regio-, and stereoselectivity, aldolases and related enzymes hold particularly high potential for the synthesis of polyfunctionalized products that are otherwise difficult to prepare and to handle by conventional chemical methods. 

The only useful Cannizzaro reactions involving the use of aldehydes having one or two a-hydrogen atoms are those already described, in which the aldehyde first undergoes an aldol condensation. The direct dismutation of aldehydes of these types has been carried out successfully only by means of enzyme systems or catalytic metals (p. 95). Such reactions do not represent the true Cannizzaro reaction and as yet have found little practical use. The smooth and practically quantitative dismutation of straight-chain aliphatic aldehydes of four to seven carbon atoms under the influence of the enzymes of hog-liver mash 6 suggests that practical applications of this method may be found. 

This chapter focuses on biocatalytic methods that have been demonstrated in the aldol and related reactions. Additionally strategies for enzyme engineering and future challenges faced in this field are addressed. 

Although TA from yeast is commercially available, it has rarely been used in organic synthesis applications, and no detailed study of substrate specificity has yet been performed. This is presumably due to high enzyme cost and also since the reaction equilibrium is near unity, resulting in the formation of a 50 50 mixture of products. In addition the stereochemistry accessible by TA catalysis matches that of FruA DHAP-dependent aldolase and the latter is a more convenient system to work with. In one application, TA was used in the synthesis D-fructose from starch.113 The aldol moiety was transferred from Fru 6-P to D-glyceraldehyde in the final step of this multi-enzyme synthesis of D-fructose (Scheme 5.60). This process was developed because the authors could not identify a phosphatase that was specific for fructose 6-phosphate and TA offered an elegant method to bypass the need for phosphatase treatment. 

Peptide catalysts, peptides that catalyze chemical reactions such as aldol, retro-aldol, and Michael reactions. In contrast to enzymes or catalytic antibodies ( abzymes), small peptides often display limited catalytic activity and substrate specificity. Combinatorial methods combined with reaction-based or catalysis-based high-throughput selection approaches are suited for catalyst optimization [E. Tanaka, Chem. Record 2005, 5, 276]. 

Aldolases are a group of C—C bond forming enzymes with widespread applications. The stereoselective aldol addition reaction catalyzed by aldolases represents an attractive alternative to conventional chiral organic chemistry methods for chemical and pharmaceutical industries. Aldolases are classified according to both their proposed catalytic mechanism and the structure of the donor substrate, their sources and microbial production processes being presented in this chapter. To design appropriate bioreactors for aldol synthesis, the characteristics of aldolase biocatalysts obtained after purification procedures in free and immobilized form are discussed. 

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