Pentoses and Hexoses

The reaction of 2,3-0-cyclohexylidene-D-glyceraldehyde with the o-hydroxyacetyl anion equivalent (4), prepared from benzyl chloromethyl ether, 2,6-xylyl isocyanide and 

Full experimental details have been published for the synthesis of all eight L-hexoses from 2-butene-l,4-diol by a reiterative two-carbon extension (ycle consisting of four key steps (asymmetric epoxidation, epoxide opening with benzenethiol accompanied by epoxide migration, Pummerer rearrangement, and Wittig olefination) which was outlined in 

The release of free sugars by selective deprotection at C-1 of perbenzylated and peracetylated hexopyranoses is referred to in Chapters 5 and 7, respective.  

Re X9enJ s i, N0.8H4 Ac Oipy jiti, McPBA Iv, AtjO.NaOAc V, Meo fileOH 

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The simple sugars or monosaccharides are polyhydroxy aldehydes or ketones, and belong to Solubility Group II. They are termed tetroses, pentoses, hexoses. etc. according to the number of carbon atoms in the long chain constituting the molecule, and aldoses or ketoses if they are aldehydes or ketones. Most of the monosaccharides that occur in nature are pentoses and hexoses. 

Enantiomerically pure tetroses, pentoses, and hexoses have been synthesized by the following reaction sequence (A.W.M. Lee, 1982 S.Y. Ko, 1983), which is useful as a repetitive two-carbon hotnologi-.ation in total syntheses of higher monosaccharides and other polyhydroxy compounds (1) Wittig reaction of a protected hydroxy aldehyde with (triphenylphosphor-... 

R. Goldberg and J. Tewari, Thermodynamic and transport properties of carbohydrates and their monophosphates The pentoses and hexoses, J. Phys. Chem. Ref. Data, 18 (1989) 809-822. 

D-fructose, and of their optical isomers, a truly remarkable achievement (see Fig. 1). Thus, encouragement was given to the formaldehyde theory. Paper chromatography shows formose to be a complex mixture containing glycolaldehyde, trioses, tetroses, pentoses, and hexoses.62- 63 Schmitz64 re-... 

Simultaneous saccharification and fermentation (SSF) one-stage enzymatic hydrolysis, but the fermentation of pentoses and hexoses takes place in separate process steps. 

Simultaneous saccharification and co-fermentation (SSCF) one-stage enzymatic hydrolysis of cellulose and fermentation of pentoses and hexoses all in one process step. The upstream hydrolysis of the hemicellulose takes place in a separate process step. 

When heated with a strong acid, pentoses and hexoses are dehydrated to form furfural and hydroxymethylfurfural derivatives respectively (Figure 9.20), the aldehyde groups of which will then condense with a phenolic compound to form a coloured product. This reaction forms the basis of some of the oldest qualitative tests for the detection of carbohydrates, e.g. the Molisch test using concentrated sulphuric acid and a-naphthol. 

Commonly used. Gives good separation of pentoses and hexoses. Will resolve glucose and galactose (C)... 

We find also,that ethyl alcohol penetrates rapidly, glycerol more slowly, the tetroses extremely slowly and the pentoses and hexoses not at all. 

The kinetic-acetonation procedure previously reported,107 employing alkyl isopropenyl ethers in N,N-dimethylformamide, and applied to the common pentoses and hexoses, has been extended to various hexuloses, oligosaccharides, and other sugar systems.108 Maltose gave the 4, 6 -monoacetal, isolated as the crystalline hexaacetate, on treatment with isopropenyl methyl ether inN,N-dimethylformamide in the presence of p-toluenesulfonic acid. 

Five peaks are important in the mass spectra of the dialkyl dithioacetals of the deoxy sugars 6 (as well as of the common pentoses and hexoses). These are fragments A, B, and C, the dithioacetal portion [CH(SR)2] resulting from C-l, and the remaining portion of the molecule [M — (CH(SR)j) ]. They result from carbon-carbon bond-cleavage on electron impact, with the production of charged and neutral fragments. 

The first application of GLC in carbohydrate chemistry was to separate fully methylated methyl glycopyranosides of simple pentoses and hexoses (10), and much literature now covers these derivatives (II, 12, 13, 14). Shortly after this work, acetate derivatives were examined by various workers (15, 16), and a little later another development was the use of these acetates with thin-film columns containing liquid phases of high thermal stability such as silicone polymers (e.g., SE-30) and fluoro-alkyl silicone polymers (e.g., QF-1) (17). 

The transformation of pentoses and hexoses into 2-furaldehyde and 5-(hydroxymethyl)-2-furaldehyde, respectively, by the action of acids is a well-known reaction. Professor Bognar was long interested in ascertaining whether this reaction is reversible. With both a theoretical and a practical goal, the Bogn r group then synthesized the dl forms of several important monosaccharides (xylose, ribose, and arabinose) from the aforementioned furan derivatives. 

It is well known that if there is over-hydrolysis in the Feulgen test, the intensity of the color developed is much reduced. Lately, renewed interest in this effect has led to several interpretations being forwarded.69 81 82 The work of Stacey and his coworkers on the properties and reactions of desoxy-pentoses and -hexoses and in particular of 2-desoxy-D- and -L-ribose, indicates that over-hydrolysis of the nucleic acid not only brings it into a more diffusible form, but also changes the desoxypentose... 

Application of the same sequence to (Z)-2, however, is unsatisfactory because asymmetric epoxidation of (Z)-2 proceeds very slowly and with low stereoselectivity. I hc. difficulty has since been resolved by the observation that 7 and 9 are epimerized at C to the corresponding 2,3-t/ireo-aldoscs 10 and 11, respectively, by K2C03 in met lianol. As a result, the four possible D-pentoses can be prepared satisfactorily from (1 1-2. The epimerization is equally effective in the tetrose, pentose, and hexose series.3... 

The form of the initial spectrum is similar for trioses, pentoses, and hexoses, although the spectra of the more rapidly reacting trioses and pentoses broaden to longer wavelength much earlier at a given temperature and water activity. 

Table I shows relative rates of fluorescence development for a triose, pentoses, and hexoses at 80°C, at water activity 1.0.

Table II shows similar relative rates and AFImax measured over saturated aqueous NaCl (water activity 0.76). Limited studies were also made over saturated KI solutions (water activity 0.60). Rate was somewhat greater at the higher water activity, but there was not sufficient difference to warrant further comparative study. The same relative rate order for sugar type occurs over NaCl as over water. Table III reports similar studies over activated silica. The experimental scheme does not permit rigid water activity control at near zero activity, but it is clear that rates are much lower and that the same relative rate order for pentose and hexose sugars is preserved, although the differences are much less pronounced. The triose is relatively less reactive here.

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