Addition reactions sugar, equilibria

One new twist to a known reaction is intramolecular hemiacetal formation. Carbonyl groups react with nucleophiles in the addition reaction (Chapter 16). When the nucleophile is an alcohol, hemi-acetals are formed, but generally they are not favored at equilibrium. However, when a relatively strain-free ring can be formed in an intramolecular hemiacetal formation, the cyclic form can be favored. Such is the case for aldohexoses (and many other sugars), which exist mainly in the srx-membered pyranose forms (Fig. 22.12). 

A vast range of natural sugars exempMly these cyclic addition products. A typical sugar exists predominantly in the form of a hemiacetal or hemiketal in solution, although this is an equilibrium reaction, and the open chain carbonyl form is always present to a small extent (<1%). The formation of a six-membered cyclic hemiacetal from glucose is achieved by attack of the C-5 hydroxyl onto the protonated carbonyl (conjugate acid). 

In contrast, at [H+] > 0.1 M, the same reaction results in two or three new EPR signals (glso 1.974, 1.971, and 1.966) in addition to the one already mentioned feso= 1.979).68,75 These EPR signals turn out to be consistent with six-coordinated oxo-Cr(V) species. In this situation, the relative intensity of the EPR signal is pH dependent but is independent of the aldohexose/Cr(VI) ratio. In fact, six-coordinated species are dominant at [H+] > 0.75 M. In addition, both species [six- and five-coordinated oxo-Cr(V) complexes] decay at the same rate, meaning that they are in a rapid equilibrium. Scheme 5 shows the complexation chemistry and the observed Cr(V)-sugar redox processes. 

The reaction is catalyzed by specific enzymes, usually termed204 sugar nucleotide pyrophosphorylases. The equilibrium constant is not far from unity, and addition of inorganic pyrophosphatase, an enzyme that converts inorganic pyrophosphate irreversibly into... 

In vivo, pyruvate lyases perform a catabolic function. The synthetically most interesting types are those involved in the degradation of sialic acids or the structurally related octulosonic acid KDO, which are higher sugars typically found in mammalian or bacterial glycoconjugates [62-64], respectively. Also, hexose or pentose catabolism may proceed via pyruvate cleavage from intermediate 2-keto-3-deoxy derivatives which result from dehydration of the corresponding aldonic acids. Since these aldol additions are freely reversible, the often unfavourable equilibrium constants require that reactions in the direction of synthesis have to be driven by an excess of one of the components, preferably pyruvate for economic reasons, in order to achieve a satisfactory conversion. 

Although monosaccharides exist predominantly as hemiacetals, enough aldehyde or ketone is present at equilibrium that the sugars give most of the reactions of these functional groups. In addition, monosaccharides exhibit the reactions of alcohols. Of course, the presence of bodi functional groups may perturb the reactions of either of them. 

Although the exact mechanism of the reaction is difficult to resolve, from our accumulated data we can propose a mechanism which explains the experimental results obtained and enables the prediction of the stereochemistry of the sulfinate formed, based on the steric volume of the amine. The experimental data clearly demonstrate that the diastereoselective formation of a sulfinate is base-dependent. Therefore, the proposed mechanism has to include the effect of the base on the stereocourse of the reaction. Some additional circumstances have to be taken into account (1) the reaction of sulfiny 1 chlorides with alcohols in the presence of bases does not proceed via a sulfine intermediate117 (2) Mislow has proved that the reaction of chiral alcohols with sulfinyl chlorides in the presence of a base is kinetically controlled814,83 and (3) on first inspection, one could imagine the extreme case where DAG, under the influence of the base, reacts with only one of the enantiomeric sulfinyl chlorides. However, this is not the case, because the same yield and ee are obtained when 1.2 or 2.0 equiv of MeSOCl are used. Accordingly, the first step of the process could be an equilibrium reaction involving the sulfinyl chloride and the base. The racemic sulfinamide formed would be the active sulfur species that interacts with the sugar derivative. 

The Bilik reaction applied to 2-ketoses yield 2-hydroxymethyl aldoses in which the tertiary carbon originates from C2 of the ketose and the C2 hydroxyl is on the opposite side to the C3 hydroxyl of the ketose (in the Fischer projection). Thus, o-fructose yield o-hamamelose. The position of equilibrium, however, lies towards the straight-chain sugar, although it can be pulled over somewhat towards the branched-chain aldose by the addition of borate. The mechanism in Figure 6.9 again explains the main reaction, but not the formation of sorbose as a by-product, which probably arises from a metal ion-promoted hydride shift, as there is no isotope exchange with solvent. The Bilik reaction can be applied to the production of l-deoxy-o-xylulose from 2-C-methyl-D-erythrose the reaction is particularly clean and only the two... 

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