Enzymatic aldol condensation

An enzymatic aldol condensation of dihydroxyacetone phosphate and D,L-glyceraldehyde has reportedly48 given a mixture of L-sorbose l-phos-phate along with D-fructose 1-phosphate. The ketoses may be set free with phosphatase. 

Heteroatom-substituted carbohydrates are efficiently assembled by the enzymatic aldol condensation of DHAP with an appropriately appended aldehyde. Iminocyclitols that are inhibitors of glycosidases, such as deoxynojirimycin and deoxymannojirimycin, are simply prepared by condensation of azo-substituted aldehydes under the FDP protocol followed by dephosphorylation and palladium mediated reductive animation (Scheme 5.18a).39 In addition a number of polyhydroxylated pyrrolidines that are efficient glucosidase inhibitors have been synthesized by this chemo-enzymatic strategy (Scheme 5.18 ).1" 30,40 If the palladium mediated hydrogenation is done in the presence of hydrochloric acid, an amino-sugar intermediate is formed as its hydrochloride salt. Treatment with base then forms polyhydroxylated imines, instead of iminocyclitols (Scheme 5.19).41... 

Mimetics of sialyl Lewis X, the terminal tetrasaccharide of glycoproteins and glycolipids that are known to interact with selectins in the inflammatory process, have been efficiently synthesized through the use of the enzymatic aldol condensation (Scheme 5.26).29 This straightforward approach involved the condensation of mannosyl aldehyde derivatives with DHAP in the presence of DHAP-dependent aldolases. The aldehyde acceptors are generated from mannose by protection of the anomeric center as allyl ether, followed by... 

The pyrrolidines were also prepared by using chemoenzymatic strategy for constructing the required azido-sugars (Scheme 1) Enzymatic aldol condensation of 2-azido-3-hydroxy propanal (35) and DHAP gave a diastereoisomeric mixture of 30 and 31, which... 

This enzyme catalyses the decarboxylation of the ) -ketoacid oxaloacetate, with the same stoichiometry as acetoacetate decarboxylase. The former however, requires a Mn ion for activity and is insensitive to the action of sodium borohydride. This duality of mechanism is not unlike the one observed for enzymatic aldol condensation, where enzymes of Class 1 react by forming Schiff-base intermediates, whereas enzymes of Class II show metal ion requirements [47]. Oxaloacetate decarboxylase from cod also catalyses the reduction by borohydride of the enzymatic reaction product pyruvate. This is evidenced by the accumulation of D-lactate in presence of enzyme, reducing agent, and manganous ions. It has been proposed that both reduction and decarboxylation occur by way of an enzyme-metal ion-substrate complex in which the metal ion acts as an electron sink, thereby stabilizing the enolate ion formed in the decarboxylation reaction [48] ... 

From Achiral Non-carbohydrates. - Racemic methyl 2-acetamido-2-deoxy-threofuranoside 69 was the major epimer formed as indicated in Scheme 19 from the dihydroisoxazole 68, prepared by condensation of nitromethane and chloroacetaldehyde. 2-Amino-2-deoxy-L-erythrono-1,4-lactone 73 was synthesized by enzymatic aldol condensation of 70 and 71 to give a 92 8 mixture of erythro- and rAreo-adducts, from which 72 was obtained by crystallization (Scheme 20). The lactone 74, an intermediate in previous syntheses of A -trifluoroacetyl-L-acosamine and -L-daunosamine (Vol.l4, p.72, ref. 14), was prepared from methyl sorbate as before, but by a rather inefficient route. 

A tandem enzymatic aldol-intramolecular Homer-Wadsworth-Emmons reaction has been used in the synthesis of a cyclitol.310 The key steps are illustrated in Scheme 8.33. The phosphonate aldehyde was condensed with dihydroxyacetone phosphate (DHAP) in water with FDP aldolase to give the aldol adduct, which cyclizes with an intramolecular Horner-Wadsworth-Emmons reaction to give the cyclo-pentene product. The one-pot reaction takes place in aqueous solution at slightly acidic (pH 6.1-6.8) conditions. The aqueous Wittig-type reaction has also been investigated in DNA-templated synthesis.311... 

From this we can conclude that two pKa values can be as much as eight units apart and AG will still be less than 50 kj / mol, low enough to permit rapid enzymatic reactions. However, for transfer of a proton from a C-H bond to a catalytic group, for example, to form an enolate ion for an aldol condensation (Chapter 13), the intrinsic barrier is known to be about 50 kj / mol.141 In this case, to allow rapid enzymatic reaction either the thermodynamic barrier must be very low, as a result of closely matching pKa values, or the enzyme must lower the intrinsic barrier. It may do both. 

The interest in the mechanisms of SchifF base hydrolysis stems largely from the fact that the formation and decomposition of SchifF base linkages play an important role in a variety of enzymatic reactions, for example, carbonyl transfers involving pyridoxal phosphate, aldol condensations, /3-decarboxylations and transaminations. The mechanisms for the formation and hydrolysis of biologically important SchifF bases, and imine intermediates, have been discussed by Bruice and Benkovic (1966) and by Jencks (1969). As the consequence of a number of studies (Jencks, 1959 Cordes and Jencks, 1962, 1963 Reeves, 1962 Koehler et al., 1964), the mechanisms for the hydrolysis of comparatively simple SchifF bases are reasonably well understood. From the results of a comprehensive kinetic investigation, the mechanisms for the hydrolysis of m- and p-substituted benzylidine-l,l-dimethylethylamines in the entire pH range (see, for example, the open circles in Fig. 13) have been discussed in terms of equations (23-26) (Cordes and Jencks, 1963) ... 

Examples include acetal hydrolysis, base-catalyzed aldol condensation, olefin hydroformylation catalyzed by phosphine-substituted cobalt hydrocarbonyls, phosphate transfer in biological systems, enzymatic transamination, adiponitrile synthesis via hydrocyanation, olefin hydrogenation with Wilkinson s catalyst, and osmium tetroxide-catalyzed asymmetric dihydroxylation of olefins. 

The enzymatic aldol reaction represents a useful method for the synthesis of various sugars and sugar-like structures. More than 20 different aldolases have been isolated (see Table 13.1 for examples) and several of these have been cloned and overexpressed. They catalyze the stereospecific aldol condensation of an aldehyde with a ketone donor. Two types of aldolases are known. Type I aldolases, found primarily in animals and higher plants, do not require any cofactor. The x-ray structure of rabbit muscle aldolase (RAMA) indicates that Lys-229 is responsible for Schiff-base formation with dihydroxyacetone phosphate (DHAP) (Scheme 13.7a). Type II aldolases, found primarily in micro-organisms, use Zn as a cofactor, which acts as a Lewis acid enhancing the electrophilicity of the ketone (Scheme 13.7b). In both cases, the aldolases accept a variety of natural (Table 13.1) and non-natural acceptor substrates (Scheme 13.8). 

In order to confirm the structures of solanapyrones, chemical synthesis of these phytotoxins were attempted based on biogenetic consideration [55], The retro synthesis envisaged intramolecular Diels-Alder reaction of the achiral polyketide triene (a), a key intermediate, which is further divided into a pyrone moiety (b) and a diene moiety (c). The moieties a and b were prepared from dehydroacetic acid and hexadienyl acetate, respectively. Aldol condensation of the aldehyde (72) with the dithioacetal (73) gave a dienol, which was further converted to a triene (74). The intramolecular Diels-Alder reaction of 74 in toluene at 170-190 °C for 1 hr in a sealed tube yielded a mixture of the adducts (75) and (76) in a ratio of 1 2. This product ratio depends on the solvents, i.e. in water (1 7), and should be useful in differentiating between artificial and enzymatic reactions in biosynthetic studies. Removal of the thioacetal groups in 75 and 76 yielded solanapyrone A (67) and D (70) in a ratio of 3 5. Though solanapyrone D (70) had not been isolated from the natural resources at this stage, the structure and stereochemistry were confirmed by H NMR spectrum. 

A powerful sulphury aroma compound, 3-mercapto-2-methylpentan-l-ol (51), has recently been identified [13] in a complex process flavouring. It has a low odour threshold of 0.15 mg/L of water. The formation could be traced back to the onions present in the process flavouring and its formation is explained from propanal present in onions via aldol condensation, addition of hydrogen sulphide and enzymatic reduction (Fig. 3.31). 

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. 

N-Acetylneuraminic acid aldolase catalyzes the reversible aldol condensation of pyruvate (23) and N-acetylmannosamine (22 ManNAc) to form N-acetylneuraminic acid (24 NeuAc N-acetyl-5-amino-3,5-dideoxy-D-glycerogalacto-2-nonulopyronic acid Scheme 6).80-83 In vivo the enzyme has a catabolic function and the equilibrium for this reaction is near unity the presence of excess pyruvate can shift this equilibrium. NeuAc and other derivatives of neuraminic acid are termed sialic acids. These compounds are found at the termini of mammalian glycoconjugates and play an important role in cellular recognition.84-89 The production of analogs of NeuAc is a point of great interest to synthetic and medicinal chemists. The enzymatic approach has not been fully explored but it may be a practical alternative to the chemical synthesis of certain sialic acids.89... 

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