Dihydroxyacetone formation

The Pt/C catalyst, compared with Pd/C, showed not only enhanced activity (vide supra) but also reduced selectivity for glyceric acid (only 55% at 90% conversion), favoring dihydroxyacetone formation up to 12%, compared with 8% for the Pd case [48]. The Pt/C catalyst promoted with Bi showed superior yields of dihydroxyacetone (up to 33%), at lower pHs. Glyceric and hydroxypyruvic acids, apparently, are formed as by-product and secondary product, respectively [48], The addition of Bi seems to switch the susceptibility of glycerol oxidation from the primary towards the secondary carbon atoms. 

There are two distinct groups of aldolases. Type I aldolases, found in higher plants and animals, require no metal cofactor and catalyze aldol addition via Schiff base formation between the lysiae S-amino group of the enzyme and a carbonyl group of the substrate. Class II aldolases are found primarily ia microorganisms and utilize a divalent ziac to activate the electrophilic component of the reaction. The most studied aldolases are fmctose-1,6-diphosphate (FDP) enzymes from rabbit muscle, rabbit muscle adolase (RAMA), and a Zn " -containing aldolase from E. coli. In vivo these enzymes catalyze the reversible reaction of D-glyceraldehyde-3-phosphate [591-57-1] (G-3-P) and dihydroxyacetone phosphate [57-04-5] (DHAP). 

The dihydroxyacetone side chain is conveniently protected by forming 17a,20 20,21-bismethylenedioxy compounds (BMD) (92). Formation of llf -ethers as by-products from 11 -hydroxycompounds (91) can be limited by using formalin with a low methanol content, or better with paraformaldehyde as a source of alcohol-free formaldehyde. ... 

The transaldolase functions primarily to make a useful glycolytic substrate from the sedoheptulose-7-phosphate produced by the first transketolase reaction. This reaction (Figure 23.35) is quite similar to the aldolase reaction of glycolysis, involving formation of a Schiff base intermediate between the sedohep-tulose-7-phosphate and an active-site lysine residue (Figure 23.36). Elimination of the erythrose-4-phosphate product leaves an enamine of dihydroxyacetone, which remains stable at the active site (without imine hydrolysis) until the other substrate comes into position. Attack of the enamine carbanion at the carbonyl carbon of glyceraldehyde-3-phosphate is followed by hydrolysis of the Schiff base (imine) to yield the product fructose-6-phosphate. 

Due to mechanistic requirements, most of these enzymes are quite specific for the nucleophilic component, which most often is dihydroxyacetone phosphate (DHAP, 3-hydroxy-2-ox-opropyl phosphate) or pyruvate (2-oxopropanoate), while they allow a reasonable variation of the electrophile, which usually is an aldehyde. Activation of the donor substrate by stereospecific deprotonation is either achieved via imine/enamine formation (type 1 aldolases) or via transition metal ion induced enolization (type 2 aldolases mostly Zn2 )2. The approach of the aldol acceptor occurs stereospecifically following an overall retention mechanism, while facial differentiation of the aldehyde is responsible for the relative stereoselectivity. 

A mixture of dihydroxyacetone and inorganic arsenate can replace DHAP due to the transient formation of a monoarsenate ester which is recognized by the aldolase as a DHAP mimic21. This approach suffers from the high toxicity of arsenate, especially at the relatively high levels (>0.5 M) needed for efficient conversion, and from problems in product isolation. 

Figure 10.18 Enzymatic in situ generation of dihydroxyacetone phosphate from fructose 1,6-bisphosphate (b), with extension to an in vitro artificial metabolism for its preparation from inexpensive sugars alongthe glycolysis cascade (a), and utilization for subsequent stereoselective carbon-carbon bond formation using an aldolase with distinct stereoselectivity (c).

Figure 10.20 Substrate analogs of dihydroxyacetone phosphate accessible by the CPO oxidation method, and spontaneous, reversible formation of arsenate or vanadate analogs of dihydroxyacetone phosphate/n s/tu for enzymatic aldol additions.

Aldolases catalyze asymmetric aldol reactions via either Schiff base formation (type I aldolase) or activation by Zn2+ (type II aldolase) (Figure 1.16). The most common natural donors of aldoalses are dihydroxyacetone phosphate (DHAP), pyruvate/phosphoenolpyruvate (PEP), acetaldehyde and glycine (Figure 1.17) [71], When acetaldehyde is used as the donor, 2-deoxyribose-5-phosphate aldolases (DERAs) are able to catalyze a sequential aldol reaction to form 2,4-didexoyhexoses [72,73]. Aldolases have been used to synthesize a variety of carbohydrates and derivatives, such as azasugars, cyclitols and densely functionalized chiral linear or cyclic molecules [74,75]. 

In the case of L-rhamnulose-1-phosphate aldolase (RhaD), we found that the problem of phosphorylated substrate requirement (dihydroxyactone phosphate (DHAP)) could be overcome by a simple change in buffer. Thus, when using borate buffer, reversible borate ester formation created a viable substrate out of dihydroxyacetone, which is not otherwise accepted by the wild-type enzyme (Figure 6.6) [23]. The process was used in a one-step synthesis of... 

Meyerhof and Schulz86 studied this reaction in trisodium phosphate solution, and regarded it as coming to a triose-hexose equilibrium containing 92 % of hexose. Berl and Feazel67 examined the kinetics of hexose formation from trioses in alkaline solution, and noted that 75-90% of hexulose is formed from DL-glycerose alone, but that the yield is lower (about 60%) when dihydroxyacetone is added in equivalent quantity. Paper chromatog-... 

Aldolase 4.1.2.3 Fructose-1,6-diphosphate Dihydroxyacetone phosphate Formation of a hydrazone... 

Scheme 4.—Formation of Aromatics from Dihydroxyacetone in Acid Solution.

Several other non-nitrogenous products have been identified as products of the Maillard reaction. These include butanol, butanone, butane-dione, and pentane-2,3-dione as well as dihydroxyacetone, glycer-aldehyde, and D-erythrose. Obviously, the same products are present after mild acidic or basic degradation of carbohydrates. Thus, the necessity of an amine or amino acid in the mechanism of their formation is uncertain. 

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