Aldopentoses, from aldohexoses

The four possible D-aldopentoses are shown as structures 1 through 4 in Figure 25.4. One of these is D-arabinose. Each of these produces two of the eight possible D-aldohexoses on application of the Kiliani-Fischer synthesis. Note that in each case the configurations of the aldohexoses at carbons 3.4, and 5 are the same as those of the aldopentose at carbons 2, 3, and 4. respectively. If D-arabinose has structure 1, then the two aldohexoses produced from it upon Kiliani-Fischer synthesis, 5 and 6, must be D-glucose and D-mannose. Similarly, if D-arabinose is 2, then D-glucose and D-mannose must be 7 and 8, and so forth. 

The four D aldopentoses and the eight D aldohexoses derived from them by Kiliani-Fischer synthesis are shown in Figure 25.9 (p. 1052), One of the eight aldohexoses is glucose, but which one ... 

The four o aldopentoses and the eight d aldohexoses derived from them by Kiliani-Fischer chain extension. 

The four D aldopentoses and the eight o aldohexoses derived from them by Kilianl-Flscher chain extension. 

Figure 1.6 sets out the structures of the aldohexoses and the aldopentoses from which they can be derived by the Kiliani reaction. Aldohexoses have four non-equivalent asymmetric centres, and hence there are 16 (2" ) stereoisomers, 8 pairs of enantiomers. 

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 reaction is also sensitive because can detect a low quantities like Ipg of fructose. The reaction also is specific because aldohexoses, aldopentoses, and ketopentoses do not interfere, however these compounds do not react even if its concentrations are higher, as Smg.mL-i. The sensitivity of the method is from 1 to 50pg of fructose. A standard solution of fructose is used to build the calibration curve and to get the equation of the linear regression to quantity samples. 

Messineo, L. Musarra, E. (1972). Sensitive spectrophotometric determination of fructose, sucrose, and inulin without interference from aldohexoses, aldopentoses, and ketopentoses. International Journal of Biochemistry, Vol.3, No.l8, p. 691-699, ISSN... 

An order of reactivity was delineated from these experiments simple alcohols > sucrose > aldohexoses > aldopentoses > ketohexoses. It has been established that the participation of the anomeric hydroxyl is a major factor of deactivation mechanism (compare glucose and Me-glucose). Moreover, the sluggish reactivity of cellobiose, which has been chosen as a model for cellulose, is due to the very poor solubility of this substrate in DMAc. 

Note The dipping solution, which can also be used as a spray solution, can be employed with silica gel, kieselguhr, cellulose, RP, NH2 and CN phases. Sugars (exceptions include, for example, fructose, melezitose and raffinose) yield brilliantly colored zones on an almost colorless background when the spray solution is employed. Aldohexoses appear brown, aldopentoses bright red and hexuronic acids orange in color [3]. The detection limit differs for different substances it ranges from 10 ng (pyridoxal) over 100 ng (cinnamaldehyde) up to 2 pg (citral). 

From a structural point of view, the carbohydrate template can have either furan or pyran rings although in some cases open chain structures can be formed. A large variety of aldopentoses (e.g. d- and L-arabinose, D-ribose, D-xylose), aldohexoses (e.g. D-glucose, D-mannose, D-galactose) as well as ketohexoses (e.g. D-fructose, L-sorbose) can be used as scaffolds. 

Emil Fischer s initial studies [1] focused on dithioacetal formation from a variety of simple aldopentoses, aldohexoses, and aldoheptoses. The products were obtained by simply treating the sugar with an excess of ethanethiol in concentrated hydrochloric acid at low temperature ( 0°C). Many of these dithioacetals have low water solubility, crystallize spontaneously from the reaction mixtures, and can be isolated by simple filtration and washing with odd water. Fischer s key paper [1] also outlined most of the major reaction modes of the diethyl dithioacetal derivatives, later developed in detail, and thus paved the way for subsequent studies into this important class of sugar derivative. 

Two simple aldopentoses can be derived structurally from each of the four aldotetroses described, making a total of eight aldopentoses. Therefore, there are 16 aldohexoses. 

A D-aldohexose A is formed from an aldopentose B by the Kiliani-Fischer synthesis. Reduction of A with NaBH4 forms an optically inactive alditol. Oxidation of B forms an optically active aldaric acid. What are the structures of A and B ... 

The 1,6-anhydrohexoses are commonly found in large quantities only in the pyrolysis products of aldohexoses (as well as from polysaccharides [4]), but only small amounts of anhydrasugars (i.e., 1% yield) have been detected from the pyrolysis of ketohexoses [5]. The pyrogram of fructose, for example, reveals the presence of very small amounts of 2,6-anhydrofructofuranose. Aldopentoses pyrolyse in the same manner as their... 

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