Enzymatic Aldol Reactions

Aldol Additions. These reactions catalyzed by lyases are perhaps the most synthetically useful enzymatic reactions for carbon—carbon bond formation. Because of the broad synthetic utiUty of this method, the enzymatic aldol reactions have received considerable attention in recent years and have been extensively covered in a number of books and reviews (10,138—140). 

Figure 10.19 Oxidative enzymatic generation of dihydroxyacetone phosphate in situ for stereoselective aldol reactions using DHAP aldolases (a), and extension by pH-controlled, integrated precursor preparation and product liberation (b).

By this concept, a reversible enzymatic aldol reaction generates a mixture of l-threo/erythro aldol diastereomers (133) from which the i-threo isomer is preferentially decomposed by an irreversible decarboxylation to furnish aromatic aminoalcohol (R)-(134) vhth 78% ee in high yield. 

Figure 6.7 Enzymatic coupling to drive aldol reaction equilibrium in the synthetic direction...

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... 

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. 

Scheme 4.15 Examples of promiscuous enzymatic reactions conducted with the oxyanion hole of Candida antarctica lipase B (a) the aldol reaction [104] (b) the conjugate addition reaction (Michael addition) [105] (c) the epoxidation reaction [106],...

Enzymatic synthesis relying on the use of aldolases offers several advantages. As opposed to chemical aldolization, aldolases usually catalyze a stereoselective aldol reaction under mild conditions there is no need for protection of functional groups and no cofactors are required. Moreover, whereas high specificity is reported for the donor substrate, broad flexibility toward the acceptor is generally observed. Finally, aldolases herein discussed do not use phosphorylated substrates, contrary to phosphoenolpyruvate-dependent aldolases involved in vivo in the biosynthetic pathway, such as KDO synthetase or DAHP synthetase [18,19]. 

Glycosyl radicals can also be used to mimic the enzymatic aldol reaction between phosphoenolpyruvate and carbohydrates [5], Whereas respective ionic reactions fail in the absence of enzymes, alkenes 4-6 are suitable synthons for pyruvate in radical C-C bond-forming reactions (Scheme 3). 

The formation of covalent substrate-catalyst adducts might occur, e.g., by single-step Lewis-acid-Lewis-base interaction or by multi-step reactions such as the formation of enamines from aldehydes and secondary amines. The catalysis of aldol reactions by formation of the donor enamine is a striking example of common mechanisms in enzymatic catalysis and organocatalysis - in class-I aldolases lysine provides the catalytically active amine group whereas typical organocatalysts for this purpose are secondary amines, the most simple being proline (Scheme 2.2). 

It is worth noting that, in a similar manner to enzymatic conversions with type I or II aldolases, a direct asymmetric aldol reaction was achieved when L-proline was used as catalyst. Accordingly, the use of enol derivatives of the ketone component is not necessary, i.e. ketones (acting as donors) can be used directly without previous modification [72]. So far, most asymmetric catalytic aldol reactions with... 

The mechanism similarities to enzymatic processes In principle, L-proline acts as an enzyme mimic of type I metal-free aldolases. Similar to this enzyme, L-proline catalyzes the direct aldol reaction according to an enamine mechanism. Thus, for the first time a mimic of type I aldolases has been found. The close similarity of... 

In nature, most aldolases are rooted in the sugar metabolic cycle and accept highly functionalized substrates for the aldol reaction. Nevertheless, the scope of enzymatic aldol reactions is limited, since aldolases strictly distinguish between the acceptor and the donor, yielding almost exclusively one product, and is furthermore restricted to only a few different possible natural donors. According to the donor molecules, aldolases are grouped in dihydroxyacetone phosphate-, phosphoenolpyruvate- or pyruvate-, acetaldehyde-, and glycine-dependent aldolases [41]. 

In comparison to other aldolases, DERA has a rather broad substrate range. DERA-catalyzed aldol reactions were used to get an access to key intermediates for epothilones (Fig. 36) [194]. According to retrosynthetic analysis, both fragments of the molecule could be obtained from aldol building blocks, and two out of seven stereocenters were established enzymatically. For the southern part of epothilone A,... 

More effort was therefore invested in the application of synthetic methodologies for these alkaloids and some straightforward chemo-enzymatic approaches were recently developed [150]. An enzyme-catalysed aldol reaction was again a crucial step in that synthetic route and is strongly reminiscent of Wong s research strategy relating to the chemo-enzymatic synthesis of pyrrolizidines mentioned earlier. 

Fig. 24 (a, b) Chemo-enzymatic process for synthesis of tetrahydroxylated pyrrolizidines 1-epi-alexine, australine and 3-epi-australine utilising dihydroxyacetone phosphate (DHAP), stereospecific aldol reaction catalysed by fructose-1.6-diphosphate aldolase (FDPA) and acid phosphatase (Pase) [149]... 

The capability of L-proline - as a simple amino acid from the chiral pool - to act like an enzyme has been shown by List, Lemer und Barbas III [4] for one of the most important organic asymmetric transformations, namely the catalytic aldol reaction [5]. In addition, all the above-mentioned requirements have been fulfilled. In the described experiments the conversion of acetone with an aldehyde resulted in the formation of the desired aldol products in satisfying to very good yields and with enantioselectivities of up to 96% ee (Scheme 1) [4], It is noteworthy that, in a similar manner to enzymatic conversions with aldolases of type I or II, a direct asymmetric aldol reaction was achieved when using L-proline as a catalyst. Accordingly the use of enol derivatives of the ketone component is not necessary, that is, ketones (acting as donors) can be used directly without previous modification [6]. So far, most of the asymmetric catalytic aldol reactions with synthetic catalysts require the utilization of enol derivatives [5]. The first direct catalytic asymmetric aldol reaction in the presence of a chiral heterobimetallic catalyst has recently been reported by the Shibasaki group [7]. 

In conclusion, the aldol reaction with L-proline as an enzyme mimic is a successful example for the concept of using simple organic molecules as chiral catalysts. However, this concept is not limited to selected enzymatic reactions, but opens up a general perspective for the asymmetric design of a multitude of catalytic reactions in the presence of organocatalysts [1, 3]. This has been also demonstrated by very recent publications in the field of asymmetric syntheses with amino acids and peptides as catalysts. In the following paragraphs this will be exemplified by selected excellent contributions. 

In PLP-dependent enzymatic reactions, the Schiff base formed by reaction of the substrate with PLP provides an electron sink for stabilization of the negative charge that results from the bond-breaking process required in the reaction (racemization, decarboxylation, aldol reaction, elimination, etc.). The elegant work of Walsh and coworkers provided evidence that, subsequent to Schiff base formation, a common intermediate is formed from several different alanine analogues that are alanine racemase inhibitors. From this they proposed the elimination-Michael addition sequence shown in Figure 14 as the mechanism for inhibition166. 

Acetyl CoA (as an enol) and malonyl CoA are both acylated acetoacetyl CoA. Malonyl CoA is acylated while acetyl CoA by acetyl CoA as an electrophile, but the behaviour of the does the aldol reaction. This could be enzymatic control,... 

Microbes and plants synthesize aromatic compounds to meet their needs of aromatic amino acids (L-Phe, L-Tyr and L-Trp) and vitamins. The biosynthesis of these aromatics [69] starts with the aldol reaction of D-erythrose-4-phosphate (E4P) and phosphoenolpyruvate (PEP), which are both derived from glucose via the central metabolism, into DAHP (see Fig. 8.13). DAHP is subsequently converted, via a number of enzymatic steps, into shikimate (SA) and eventually into chorismate (CHA, see later), which is the common intermediate in the biosynthesis of the aromatic amino acids [70] and vitamins. 

Scheme 5.17. Enzymatic single aldol reactions of remote dialdehydes. BDA = bromoacetalde-hyde dimethyl acetal Pase = phosphatase.

An intramolecular diastereoselective Refor-matsky-type aldol approach was demonstrated by Taylor et al. [47] with an Sm(II)-mediated cy-clization of the chiral bromoacetate 60, resulting in lactone 61, also an intermediate in the synthesis of Schinzer s building block 7. The alcohol oxidation state at C5 in 61 avoided retro-reaction and at the same time was used for induction, with the absolute stereochemistry originating from enzymatic resolution (Scheme II). Direct re.solution of racemic C3 alcohol was also tried with an esterase adapted by directed evolution [48]. In other, somewhat more lengthy routes to CI-C6 building blocks, Shibasaki et al. used a catalytic asymmetric aldol reaction with heterobimetallic asymmetric catalysts [49], and Kalesse et al. used a Sharpless asymmetric epoxidation [50]. 

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