Higher Sugars

Aldose Dialdose Aldonic acid Uronic acid Aldaric acid Alditol 

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This is the reverse Pasteur or Crabtree effect and is also known as glucose inhibition or cataboHte repression. In the presence of higher sugar concentrations, synthesis of respiratory enzymes such as cytochromes is inhibited. 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

The higher sugars have been identified as subunits in various natural products, and it is now well recognized that they play prominent roles in numerous biological processes. For example, some seven- and eight-carbon-atom sugars are important... 

Scheme 8.32 Kiliani-Fischer synthesis of higher sugars.

The synthesis shown in Fig. 40 provided access to heptoses,55 but according to our definition, this is not a higher sugar synthesis. However, the approach to such derivative was based on the Baylis-Hillman reaction of acyclic sugar-derived aldehydes, a reaction not commonly applied in sugar chemistry and worth to mention in this review. 

The cyanohydrin synthesis of higher sugars, which involves intermediate aldonolactones, allows the introduction of a 14C label in the sugar chain. Thus, for example, L-[5-l4C]arabinose was synthesized (12) from D-xylose, which was first converted, by addition of K14CN and hydrolysis, into D-[ 1-... 

L. Hough and A. C. Richardson, The monosaccharides Pentoses, hexoses and higher sugars, in D. J. Coffey, (Ed.), Rodd s Chemistry of Carbon Compounds, Vol. IF, Elsevier, Amsterdam, 1967, pp. 67-595. Chapter 23. 

Heptoses and higher sugars may similarly be separated, and the identity of D-erythro-c-galacto-nonulose in avocado was determined by comparison of its gas-chromatographic behavior with that of a synthetic sample.336 Higher sugars have been separated, and identified, from avocado,151 Pichi,337 opium poppy,338 and bacterial lipopoly-saccharides.339... 

A synthesis of a higher sugar, 2-deoxy-4-octulose, has been carried out using, as the key step, the Reformatsky reaction of methyl bromoacetate (la) with the /i-D-arabino-hexos-2-ulopyranose derivative 36, which affords the manno-derivative 37 in good d.e. (equation 26)102. The same authors developed analogous diastereoselective syntheses of... 

The two aldotetroses, erythrose and threose, differ from the other aldoses in their behavior.23 Ring formation, to give furanoses, can occur only through the primary hydroxyl group, and is therefore less favored than with the higher sugars. Consequently, considerable proportions of the aldehydo and aldehydrol forms are found in solution. Like all a- and /J-hydroxyaldehydes, the aldehydo form of the aldotetroses readily forms dimers in concentrated solutions of the tetroses, the signals of the dimers are readily visible in their n.m.r. spectra. In the syrupy state, the tetroses consist mainly of dimers, rather than of furanoses they have never been crystallized. 

The direct homologation technique was then extended to the synthesis of various uncommon carbohydrate structures. Thus, higher sugars of the L-series were obtained Starting from 2,3-0-isopropylidene-4-O-benzyl-L-threose (43) [39c] (Scheme 13), and the amino tetrose 47 and pentose 48 were prepared from the a-amino aldehyde 46 derived from L-serine [46a] (Scheme 14). These amino sugars were used as chiral building blocks for the... 

S. Jarosz and B. Fraser-Reid, Synthesis of higher sugars via allytltin derivatives of simple monosaccharides, J. Org. Chem. 54 4011 (1989). 

By means of the cyanohydrin reaction, higher sugars of the heptose. octosc, and nonosc types have been prepared. A monosaccharide such as an aldohexosc may be converted into the next lower monosaccharide, such as an aldopeniosc. by oxidation to the acid, which corresponds to the aldohexose. then treating the calcium salt solution of this acid with a solution or ferrous acetate plus hydrogen peroxide. Carbon dioxide is evolved and aldopentose formed. 

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. 

HI hi acetic acid allows the reduction of jS-peracetates of the higher sugar N-acetylneuraminic acid to the corresponding anomeric deoxy compounds.277 At room temperature this method gave exclusively the a-anomer, whereas at —20 °C a 4 1 a [i ratio resulted. This may be explained by thermodynamic and kinetic protonation of ester enolates generated in situ from anomeric iodide in a manner reminiscent of previous reductions of 2-iodo sugar lactones.278... 

Reaction with sugar lactones. The reaction of sugar lactones with 1 provides a preparative route to higher sugars.1 Example ... 

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