Dependent aldolases

Figure 10.13 Aldol reactions catalyzed in vivo by the four stereocomplementary dihydroxyacetone phosphate-dependent aldolases.

Of the known classes of aldolase, DERA (statin side chain) and pyruvate aldolases (sialic acids) have been shown to be of particular value in API production as they use readily accessible substrates. Glycine-dependent aldolases are another valuable class that allow access to p-hydroxy amino acid derivatives. In contrast, dihydroxy acetone phosphate (DHAP) aldolases, which also access two stereogenic centres simultaneously,... 

Espelt, L., Parella, T., Bujons, J., Solans, C., Joglar, J., Delgado, A. and, Clapes, P., Stereoselective aldol additions catalyzed hy dihydroxyacetone phosphate-dependent aldolases in emulsion systems preparation and structural characterization of linear and cyclic iminopolyols from aminoaldehydes. Chem. Eur. J., 2003, 9, 4887. 

Dihydroxyacetone Phosphate-Dependent Aldolases in the Core of Multi-Step Processes... 

The main group of aldolases from the biocatalytic point of view is, arguably, the one that uses dihydroxyacetone phosphate (DHAP) as donor. Here, we will concentrate on that appHcations in which DHAP-dependent aldolase are part of a multi-enzyme system or, alternatively, on those in which the aldolase-catalyzed reaction is key in a multi-step synthetic pathway. 

DHAP-dependent aldolases produce 2-keto-3,4-dihydroxy adducts with high control of the configuration of the two newly formed stereogenic centers. However, while it can be assumed that the absolute configuration at C3 is independent on the acceptor used in the reaction, the configuration of the stereocenter generated from the addition to the aldehyde (C4 position) in some cases may depend on the structure and stereochemistry of the acceptor [6]. 

Scheme 4.2 Complementary stereochemistry of DHAP-dependent aldolases.

The main drawback of the DHAP-dependent aldolases is their strict specificity for the donor substrate. Apart from the scope limitation that this fact represents, DHAP is expensive to be used stoichiometrically in high-scale synthesis, and labile at neutral and basic pH, and therefore its effective concentration decreases over time in enzymatic reaction media, hindering the overall yield of the aldol reaction. In addition, due to the presence of a phosphate group in both DHAP and the... 

DHAP-dependent aldolases Pyruvate dependent aldolases Deoxyribose 5 -phosphate aldolase —h... 

When DHAP-dependent aldolases are used as catalyst of the aldol reaction, a phosphorylated azido or amino polyhydroxyketone is obtained. The phosphate may be cleaved enzymatically or reductively cleaved under the hydrogenation conditions of the next step in which the azide is reduced to the amine. Intramolecular imine formation occurs spontaneously when the azide is reduced. The intramolecular reductive amination is the second key step of the aldolase-mediated synthesis of iminocyclitols. In general, delivery of hydrogen onto five- and six-membered ring imines occurs from the face opposite to the C4 hydroxyl group. 

Rare or unnatural monosaccharides have many useful applications as nonnutritive sweeteners, glycosidase inhibitors and so on. For example, L-glucose and L-fructose are known to be low-calorie sweeteners. In addition, rare or unnatural monosaccharides are potentially useful as chiral building blocks for the synthesis of biologically active compounds. Therefore, these compounds have been important targets for the development of enzymatic synthesis based in the use of DHAP-dependent aldolases alone or in combination with isomerases. Fessner et al. showed that rare ketose-1-phosphates could be reached not only by aldol addition catalyzed by DHAP-dependent aldolases, but by enzymatic isomerization/ phosphorylation of aldoses [35]. Thus, for example, L-fructose can be prepared... 

DHAP-Dependent Aldolases in the Synthesis of Natural Products... 

DHAP-dependent aldolases have also been used as key step in the synthesis of several complex natural products starting from achiral precursors. Thus, the sex pheromone (+)-exo-brevicomin can be synthesized in a multi-step route starting with the stereospecific aldol addition between DHAP and 5-oxohexanal or its 5-dithiane-protected analog catalyzed by FBPA from rabbit muscle ( RAMA ) as the key step by which the absolute configuration of the target is estabUshed (Scheme 4.16) [40]. 

DHAP-dependent aldolases have also been used in the synthesis of the C3-C9 fragment of aspicilin [46] and of the C12-C20 fragment of amphotericin [47]. 

Fructose-6-Phosphate Aldolase An Alternative to DHAP-Dependent Aldolases ... 

Although fructose-6-phosphate aldolase (FSA) does not belong to the DHAP-dependent aldolases group, it deserves to be mentioned in this chapter as it can be considered as an alternative to those enzymes, or at least, an alternative to FBPA. FSA was described for the first time by Schiirmann and Sprenger in E. coli K-12 strain MG1655 [50]. The enzyme is a class I aldolase with a homodecameric... 

The use of this enzyme in multi-step synthesis is relatively recent. Clapes et al. have reported the first example of FSA-mediated synthesis of iminocyclitols [53]. The synthetic strategy is similar to the one previously described for DHAP-dependent aldolases without the need for the dephosphorylation step. AldoUc reaction of DHA with N-Cbz-3-aminopropanal catalyzed by FSA followed by selective catalytic reductive aminahon furnishes the naturally occurring imino-sugar D-fagomine (Scheme 4.22). 

FSA is an interesting novel tool in chemoenzymatic synthesis. It use greatly simplifies the enzymatic procedures based on the use of DffAP-dependent aldolases since it is employs non-phosphorylated donors and, as a consequence, the reaction products do not require a subsequent dephosphorylation step. Unfortu-natly, only one stereoconfiguration of the aldol adduct is accessible. 

Scheme 6.27 Transketolase versus DHAP-dependent aldolase.

In contrast to transketolase and the DHAP-dependent aldolases, deoxyribose aldolase (DERA) catalyzes the aldol reaction with the simple aldehyde, acetaldehyde. In vivo it catalyzes the formation of 2-deoxyribose-5-phosphate, the building block of DNA, from acetaldehyde and D-glyceraldehyde-3-phosphate, but in vitro it can catalyze the aldol reaction of acetaldehyde with other non-phosphorylated aldehydes. The example shown in Scheme 6.28 involves a tandem aldol reaction... 

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