D-aldopentoses

Fischer projections of the tour-, five-, and six-carbon d alcloses are shown in Figure 25.3. Starting with D-glyceraldehyde, we can imagine constructing the two d aldotetroses by inserting a new chirality center just below the aldehyde carbon. Each of the two d aldotetroses then leads to two d aldopentoses (four total), and... 

Problem 25.23 Two of the four d aldopentoses yield D-threose on WohJ degradation. What are their structures ... 

Compound A is a D aldopentose that can be oxidized to an optically inactive aldaric acid B. On Kiliani-Fischei chain extension, A is converted into C and D C can be oxidized to an optically active aldaric acid E, but D is oxidized to an optically inactive aldaric acid F. What are the structures of A-F ... 

The preparation of D-threo-2-pentulose ( D-xylulose ) by refluxing D-xylose in pyridine containing 1% of water249 and the transformation of D-arabinose into a mixture of the four D-aldopentoses and two... 

Aldopentoses have three chirality centers. The eight stereoisomers are divided into a set of four D-aldopentoses and an enantiomeric set of four L-aldopentoses. The aldopentoses are named ribose, arabinose, xylose, and lyxose. Fischer projections of the d stereoisomers of the aldopentoses are given in Figure 25.2. Notice that all these diastereomers have the same configuration at C-4 and that this configuration is analogous to that of D-(+)-glyceraldehyde. 

The D-aldopentoses are five-carbon sugars. The most important of these are ri-bose and deoxyribose, because they are used to construct the nucleic acids RNA and DNA. Figure 12.10 gives the names and structures of the D-aldopentoses. 

Optical Activity 3IS D-Sugars and L-Sugars 316 The D-Aldohexoses 317 Fructose 317 D-Aldopentoses 318... 

Identify all of the D-aldopentoses from Figure 25.1 that, on oxidation with nitric acid, give diacids that do not rotate plane-polarized light. 

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. 

A D-aldopentose, X, gives a product that rotates plane-polarized light on reaction with FfN03. Compound X can be prepared from aldotetrose Y by Kiliani-Fischer synthesis. Reaction of Y with HN03 gives a product that rotates plane-polarized light. Show the structures of X and Y. 

To continue forming the family of D-aldoses, we must add another carbon atom (bonded to H and OH) just below the carbonyl of either tetrose. Because there are two D-aldotetroses to begin with, and there are two ways to place the new OH (right or left), there are now four D-aldopentoses D-ribose, D-arabinose, D-xylose, and D-lyxose. Each aldopentose now has three stereogenic centers, so there are 2 = 8 possible stereoisomers, or four pairs of enantiomers. The D-enantiomer of each pair is shown in Figure 27.4. 

Use each fact to determine the relative orientation of the OH groups in the D-aldopentose. 

Fact [1 ] A D-aldopentose A is oxidized to an optically inactive aidaric acid with HNO3. 

Only the aidaric acid from B has a plane of symmetry, making it optically inactive. Thus, B is the correct structure for the D-aldotetrose B, and therefore A is the structure of the D-aldopentose A. 

The reasoning behind the Fischer proof is easier to follow if the eight possible D-aldohexoses are arranged in pairs of epimers at C2. These compounds are labeled 1-8 in Figure 27.9. When organized in this way, each pair of epimers would also be formed as the products of a Kiliani-Fischer synthesis beginning with a particular D-aldopentose (lettered A-D in Figure 27.9). 

The D-aldopentoses and D-aldohexoses needed to illustrate the Fischer proof... 

Which D-aldopentoses ate reduced to optically inactive alditols using NaBH4, CH3OH ... 

Which D-aldopentose is oxidized to an optically active aldaric acid and undergoes the Wohl degradation to yield a D-aldotetrose that is oxidized to an optically active aldaric acid ... 

What other D-aldopentose forms the same alditol as D-arabinose when reduced with NaBH4 in CH3OH ... 

The structures of the four D aldopentoses can be generated in a similar way and can be named by the mnemonic suggested by a Cornell undergraduate Ribs are extra lean. ... 

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 ... 

Which of the eight D aldohexoses give the same aldaric acids as their l enantiomers Which of the other three D aldopentoses gives the same aldaric acid as D-lyxose ... 

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

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