Enzyme-catalyzed aldol condensation

Kajimoto, T, Liu, K K-C, Pederson, R L, Zhong, Z, Ichikawa, Y, Porco, J A, Wong, C-H, Enzyme-catalyzed aldol condensation for asymmetric synthesis of azasugars synthesis, evaluation, and modeling of glycosidase inhibitors, J. Am. Chem. Soc., 113, 6187-6196, 1991. 

Enzyme catalyzed aldol condensation is a useful strategy for the convergent synthesis of fluorosugars. The fluorinated substrates can be easily prepared with readily available and easy-to-handle fluorinating reagents. With the increasing number of aldolases available, synthesis of carbohydrates and related substances based on this chemo-enzymatic strategy will experience a substantial development in the near future. 

Enzyme-Catalyzed Aldol Condensations Involving Retention of CoNncuRATiON... 

Enzyme-catalyzed Claisen-type condensations are formally similar to enzyme-catalyzed aldol condensations, with the exception that the nucleophilic substrate is an ester or thioester [Eq. (35)] ... 

The enzyme-catalyzed aldol condensation of ca-functionalized C5- and 06-aldehydes with Dhap (dihydroxacetone phosphate) has been used to synthesize unusual Cg- and C9-sugars. An example is given in Scheme 4. Coupling of carbohydrate carboxylic acids such as 21 and 23 with alkanoic acids by mixed... 

Azido-sugars are frequently prepared by reaction of epoxides with azide ion. 3-Azido-3-deoxy-L-threose 68 was synthesized from cu-but-2-ene-l,4-diol 66 via the Sharpless asymmetic epoxidation product 67, and was converted into 6-azido-6-deoxy-L-galocro-heptulose 69 by an enzyme-catalyzed aldol condensation (Scheme 13). 3-Azido-3-deoxy-L-etythrose, and thence 6-azido-6-deoxy-L-g/uco-heptulose were obtained in a similar way via 4-rerf-butyldiphenylsilyoxy-rraiu-but-2-enal. These and two other azido-heptulose isomers made from the enantiomeric 3-azido-3-deoxy-tetroses, were converted to a- and P-l-homonojirimycin and homomannonojirimycin on hydrogenation. Ethyl 3-azido-2,3-dideoxy-D-eryr/iro-pentopyranoside and its 3-C-methyl analogue 71, R=H or Me, were synthesized from crotonaldehyde or 3-methyl-2-... 

An efficient asymmetric total synthesis of L-fructose combines the Sharpless asymmetric dihydroxylation with an enzyme-catalyzed aldol reaction. L-Glyceraldehyde, prepared from acrolein, is condensed to DHAP in a buffered water suspension of lysed cells of KI2 Escherichia coli containing an excess of L-rhamnulose-1-phosphate (Rha) aldolase E. coli raised on L-rhamnose as sole carbon source). The L-fructose phosphate obtained is hydrolyzed to L-fructose with acid phosphatase. Similarly, the RAMA-catalyzed condensation of D-glyceraldehyde with DHAP,... 

V. Enzyme-Catalyzed Aldol- and Claisen-Type Condensations j3-Keto Acid... 

The general class of enzymes catalyzing aldol or retroaldol condensation reactions are the aldolases (Table VI) (156-161). In principle, there are four possible stereochemical routes to product depending on whether C-C bond formation involves retention or inversion of configuration at the methyl or methylene carbon atom a to the ketonic carbonyl (Eq. (30)] ... 

O-Prolected sugar thioformamides have also been prepared in 44-87% yields by tri-n-butyltin hydride reduction of isothiocyanate precursors in ether.156 For the preparation of 5-A-thioacylneuraminic acids (84), a chemoenzymatic route based on the M-acetylneuraminate pyruvate lyase-mediated condensation of the corresponding A -thioacyl-n-mannosamine derivatives (83) and sodium pyruvate has been reported.189 The enzyme-catalyzed aldol reaction was performed at pH 6.8 and afforded the desired compound in 55% yields (Scheme 24). 

FDP Aldolase. The most extensively utilized class of enzymes for monosaccharide synthesis are the aldolases (E.C. sub-class 4.1.2.). This ubiquitous group of enzymes catalyzes reversible aldol reactions in vivo. Two major groups of aldolases exist type I aldolases, found primarily in higher plants and animals, catalyze aldol condensations by means of a Schiff base formed between an enzyme lysine e-amino group and the nucleophilic carbonyl group type II aldolases, found primarily in microorganisms, utilize a divalent zinc to activate the nucleophilic component (79). Approximately 25 aldolases have been identified to date (18),... 

The erythrose 4-phosphate generated in the same step (Scheme 11.7) as the derivative of thiamine diphosphate is then available for enzyme-catalyzed aldol-type condensation with dihydroxyacetone monophosphate just as shown in Scheme 11.5 for the analogous reaction with glyceraldehyde 3-phosphate and using the same fructose-bisphosphate aldolase (EC 4.1.2.13). 

The alkali-catalyzed aldol condensations of lower-carbon sugars as a means of increasing the chain length are of particular interest in view of the importance in biological systems of similar condensations catalyzed by enzymes (aldolases) and also in view of the mechanism of the alkaline rearrangements of sugars (Chapter I). 

As indicated in the Introduction, the condensation of formaldehyde into monosaccharides by basic catalysis has been known since the last century. The synthesis starts with the formation of glycolaldehyde which is a slow reaction and is responsible for the induction period observed in the condensation of formaldehyde to sugars. Once sufficient amounts of glycolaldehyde have been formed, an autocatalytic process ensues which transforms glycolaldehyde into glyceraldehyde and dihydroxyacetone, and then into all the possible tetroses, pentoses and hexoses. The principal mechanism is a base catalyzed aldol condensation, somewhat similar to the enzyme catalyzed biochemical transformations of sugars. A common mineral, kaolinite, has been found to be an efficient catalyst for this reaction. Ribose is indeed one of the important monosaccharides formed in this reaction. 

The total synthesis of sialosides by using the chemoenzymatic approach is as follows [74]. Sialic acid itself can be synthesized from ManNAc, mannose, or their derivatives by sialic acid aldolase enzyme through aldol condensation reaction. If ManNAc is chemically or enzymatically modified at C2, C4—C6 positions, sialic acid has structural modifications at C5, C7-C9 positions, respectively. The sialic acids are subsequently activated by a CMP-siahc acid synthetase to form a CMP-sialic acid, which is the donor used by sialyltransferases. Because CMP-sialic acid is tmstable, the CMP-Neu5Ac synthetase is valuable for the preparative enzymatic synthesis of sialosides. In the last steps, the CMP-sialic acid is transferred to galactose or GalNAc terminated glycosides by sialyltransferases to form structurally defined sialosides. Examples are that Chen and co-workers have recently developed a one-pot multienzyme system for the efficient synthesis of a-sialosides (Table 2) [12,76,79]. In this system, recombinant E. coli K-12 sialic acid aldolase catalyzed the synthesis of sialic acid precursors for... 

Finally, several other animal tissues yield useful enzymes that have been employed in S5mthesis. Pepsin is an important digestive protease from animal stomach whose native role is hydrolyzing amide bonds involving hydrophobic, aromatic amino adds, for example, phenylalanine, tyrosine, and tryptophan. Acylase from pordne kidney sdectivdy hydrolyzes N-acetyl amino adds and is commercially available. It has long been used for kinetic resolutions of amino adds. In addition to hydrolases, other animal enzymes have found important applications in biocatalysis. Rabbit musde aldolase is commerdally awiilable and was shown to catalyze aldol condensations between dihydroxyacetone phosphate and various nonnatural aldehydes by the Whitesides group in 1989 [10]. This seminal report touched off an avalanche of new applications for this and related enzymes in asymmetric synthesis. 

Synthetic peptide dendrimers, catalytic antibodies, RNA catalysts, peptide foldamers as well as other native or modified enzymes with completely different fxmctions were discovered to catalyze carbon-carbon bond formation [15]. 4-Oxalocrotonate tau-tomerase (4-OT) catalyzes in vivo the conversion of 2-hydroxy-2,4-hexadienedioate (136) to 2-oxo-3-hexenedioate (137) (Scheme 10.33a), and it belongs to the catabolic pathway for aromatic hydrocarbons in P. putida mt-2 [200]. This enzyme carries a catalytic amino-terminal proline, which could act as catalyst in the same fashion as the proline mediated by organocatalytic reactions. Initial studies demonstrate that this enzyme was able to catalyze aldol condensations of acetaldehyde to a variety of electrophiles 138 (Scheme 10.33b) [200]. This enzyme was also examined as a potential catalyst for carbon-carbon bond forming Michael-type reactions of acetaldehyde to nitroolefins 139 (Scheme 10.33c) [201,202]. 

Relatively little is known concerning the oxidation of azolium salts. Most of the publications deal with thiazolium salts due to the significant biochemical role of thiamin as a coenzyme in a variety of enzyme-catalyzed decarboxylations and aldol-type condensations. The chemistry of thiamin has been extensively reviewed (83MI1). Depending on the reaction conditions, thio-chrome (197) and the disulfide 198 are formed by oxidation of thiamin (57JA4386). 

Cleavage of Fructose 1,6-Bisphosphate The enzyme fructose 1,6-bisphosphate aldolase, often called simply aldolase, catalyzes a reversible aldol condensation (p. 485). Fructose 1,6-bisphosphate is cleaved to yield two different triose phosphates, glyceraldehyde 3-phosphate, an aldose, and dihydroxyacetone phosphate, a lcetose ... 

MECHANISM FIGURE 22-18 Tryptophan synthase reaction. This enzyme catalyzes a multistep reaction with several types of chemical rearrangements. An aldol cleavage produces indole and glyceraldehyde 3-phosphate this reaction does not require PLP. Dehydration of serine forms a PLP-aminoacrylate intermediate. In steps and this condenses with indole, and the product is hydrolyzed to release tryptophan. These PLP-facilitated transformations occur at the /3 carbon (C-3) of the amino acid, as opposed to the a-carbon reactions described in Figure 18-6. The /3 carbon of serine is attached to the indole ring system. Tryptophan Synthase Mechanism... 

When an ionic organic reaction (the kind catalyzed by most enzymes) occurs a nucleophilic center joins with an electrophilic center. We use arrows to show the movement of pairs of electrons. Tire movement is always away from the nucleophile which can be thought of as "attacking" an electrophilic center. Notice the first step in the second example at right. The unsaturated ketone is polarized initially. However, this is not shown as a separate step. Rather, the flow of electrons from the double bond, between the a- and (1-carbons into the electron-accepting C=0 groups, is coordinated with the attack by the nucleophile. Dotted lines are often used to indicate bonds that will be formed in a reaction step, e.g., in an aldol condensation (right). Dashed or dotted lines are often used to indicate partially formed and partially broken bonds in a transition state, e.g., for the aldol condensation (with prior protonation of the aldehyde). However, do not put arrows on transition state structures. 

Polycarboxylic acid synthases. Several enzymes, including citrate synthase, the key enzyme which catalyzes the first step of the citric acid cycle, promote condensations of acetyl-CoA with ketones (Eq. 13-38). An a-oxo acid is most often the second substrate, and a thioester intermediate (Eq. 13-38) undergoes hydrolysis to release coenzyme A.199 Because the substrate acetyl-CoA is a thioester, the reaction is often described as a Claisen condensation. The same enzyme that catalyzes the condensation of acetyl-CoA with a ketone also catalyzes the second step, the hydrolysis of the CoA thioester. These polycarboxylic acid synthases are important in biosynthesis. They carry out the initial steps in a general chain elongation process (Fig. 17-18). While one function of the thioester group in acetyl-CoA is to activate the methyl hydrogens toward the aldol condensation, the subsequent hydrolysis of the thioester linkage provides for overall irreversibility and "drives" the synthetic reaction. 

The aldol condensation and the reverse cleavage reaction catalyzed by these enzymes both involve a Schiff base. The cleavage reaction is similar to the acetoac-etate decarboxylase mechanism, with the protonated imine being expelled. The condensation reaction illustrates the other function of a Schiff base, the activation of carbon via an enamine (equation 2.40). 

The heat-labile enzyme which catalyzes this aldol condensation was purified eightfold from the extracts of E. coli. By combining this phos-phodesoxyriboaldolase with purified phosphoriboaldolase from yeast, Rackerl 7(b)-108 was able to demonstrate the long sought conversion of D-ribose into desoxy-D-ribose, e. g.,... 

---