Aldopentoses and Aldohexoses

Among the aldopentoses, o-ribose is a component of many biologically important substances, most notably the ribonucleic acids. o-Xylose is very abundant and is isolated by hydrolysis of the polysaccharides present in corncobs and the wood of trees. 

Cellulose Is more abundant than glucose, but each cellulose molecule Is a polysaccharide composed of thousands of glucose units (Section 23.15). Methane may also be more abundant, but most of the methane comes from glucose. 

Of all the monosaccharides, (+)-Y -glucose is the best known, most important, and most abundant. Its formation from carbon dioxide, water, and sunlight is the central theme of photosynthesis. Carbohydrate formation by photosynthesis is estimated to be on the order of 10 tons per year, a source of stored energy utilized, directly or indirectly, by all higher forms of life on the planet. Glucose was isolated from raisins in 1747 and by hydrolysis of starch in 1811. Its structure was determined, in work culminating in 1900, by Emil Fischer. 

Configurations of the d series of aldoses containing three through six carbon atoms. 

PROBLEM 25.3 L-(+)-Arabinose is a naturally occurring l sugar. It Is obtained by acid hydrolysis of the polysaccharide present in mesquite gum. Write a Fischer projection for L-(+)-arabinose. 

D-(+)-Galactose is a constituent of numerous polysaccharides. It is best obtained by acid hydrolysis of lactose (milk sngar), a disaccharide of o-glucose and D-galactose. 

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Conversion of the aldehyde into a nitrile is accomplished by treatment of an aldose with hydroxvlamine to give an oxime (Section 19.8), followed by dehydration of the oxJme with acetic anhydride. The Wohl degradation does not give particularly high yields of chain-shortened aldoses, but the reaction is general for all aldopentoses and aldohexoses. For example, D-galactose is converted by Wohl degradation into n-lyxose. 

Enthalpies of interaction of Ca2+ with several aldopentoses and aldohexoses have been determined calorimetrically (652,653,656-658). Complex formation is generally characterized by the balance between a fairly large favorable enthalpy term and an almost equal but unfavorable entropy term - for ribose both AH and TAS° are — 24 kJ mol-1 a value of +5 cm3 mol 1 for AV° can be understood in terms of electrostriction (658). 

Williams and J. K. N. Jones have shown that the dithioacetals of aldopentoses and aldohexoses may be separated as their acetates.472 In certain cases, however, isomers were not separated (for example, D-altrose and D-talose), but reduction of the dithioacetal to the 1-de-oxyalditol permitted resolution of the acetates. The method is not suitable for quantitative determinations, but it does preserve the... 

The aldopentoses and aldohexoses, which are completely hydroxyl-ated 4-hydroxypentanals and 5-hydroxyhexanals, respectively, contain only a very small proportion (<0.05%) of the aldehydo form at equilibrium (see Table II). The aldehyde content of the 2-deoxyaldoses is only slightly higher. 

Aldopentose and aldohexose may exist in furanose and pyranose forms. The former favors the furanose form while the latter prefers the pyranose form. Would the energetic differences between the two forms as exemplified by the following two pairs of monosaccharides be sufficient to rationalize the preferences. 

The Separation of Aldopentose and Aldohexose Diethyl Dithioacetal Derivatives by Gas-Liquid Partition Chromatography, D. T. Williams and J. K. N. Jones, Can. J. Chem., 44 (1966) 412-415. 

The most abundant aldopentoses and aldohexoses resulting from hydrolysis of polysaccharides in sedimentary organic matter are usually lyxose, arabinose, rhamnose, ribose, xylose, fucose, mannose, galactose... 

The results obtained by Cantor and Peniston became the starting point for the correct interpretation of the polarographic currents of aldopentoses and aldohexoses, and contributed to the discovery (and theoretical treatment) of kinetic currents, which will be considered in the next Section (see p. 135). In contrast to aldopentoses and aldohexoses, the corresponding ketoses exhibit mixed diffusion-kinetic currents, and, for this reason, it is diflBcult to obtain the required data and treat them mathematically. On the other hand, their waves have greater analytical importance than those of the corresponding aldoses. 

The rate constants, kj, for the opening of the hemiacetal ring were calculated by the authors from equation (9) for equilibrium mixtures of the a and p anomers of some aldopentoses and aldohexoses. Assuming that the thickness (8, in cm) of the reaction layer equals the mean distance between two aldose molecules in solution, k values (k/ = kh S) can be calculated, and, from known values of k , k, and D, values of the equilibrium constant K) can also be found by using equation 12). The results are given in Table I. 

All aldoses studied show the same value of the activation energy, AH = 16.5 kcal/mole, for the reaction of the hemiacetal ring and its transformation into the corresponding acyclic form. In the papers mentioned, relations between the observed rate-constants, the activation energies, the activation entropies, and the mechanisms of ring opening, for aldopentoses and aldohexoses, are also discussed. 

There are one, two, three, and four chiral carbons, respectively, in aldotrioses, aldotetroses, aldopentoses, and aldohexoses. There are zero, one, two, and three chiral carbons, respectively, in ketotrioses, ketotetroses, ketopentoses, and ketohexoses. 

As seen in Fig. 3-160, two other compormds belonging to the group of deoxy sugars are eluted prior to aldopentoses and aldohexoses. While fucose (6-deoxy-L-galactose) and other 6-deoxyhexoses (also called methyloses) are abundant in nature in cardio toxic glycosides, 2-deoxy-D-ribose, one of the building blocks of... 

Figures 22.7 and 22.8 show only aldopentoses and aldohexoses, but there are also many keto sugars among the pentoses, the five-carbon carbohydrates, and the hexoses, the six-carbon carbohydrates. Figure 22.9 shows D-fructose, the most common ketohexose.

There are many ways to determine the structures of the remaining aldopentoses and aldohexoses. Here is just one possible sequence. Let s first deal with the three remaining D-aldopentoses, given that we know the structure of D-arabinose. The aldopentose D-ribose gives the same osazone as D-arabinose (Fig. 22.54), which means that these two sugars must differ only in the configuration at C(2).Therefore D-ribose must have the structure shown in Figure 22.54. 

Depending on the number of carbon atoms in the molecule, saccharides are divided to trioses, tetroses, pentoses, hexoses and higher sugars. Compounds with aldehyde functional group are called aldoses (e.g. aldopentoses and aldohexoses) and compounds with a ketone function are called ketoses (such as ketohexoses). 

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