D-Aldohexoses

The task of relating carbohydrate configurations to names requires either a world class memory or an easily recalled mnemonic A mnemonic that serves us well here was pop ularized by the husband-wife team of Lours F Fieser and Mary Fieser of Harvard Uni versity m their 1956 textbook Organic Chemistry As with many mnemonics it s not clear who actually invented it and references to this particular one appeared m the chem ical education literature before publication of the Fiesers text The mnemonic has two features (1) a system for setting down all the stereoisomeric d aldohexoses m a logical order and (2) a way to assign the correct name to each one... 

Of the eight stereoisomeric aldopentoses Figure 25 2 shows the Fischer projections of the d enantiomers (d ribose d arabinose d xylose and D lyxose) Likewise Figure 25 2 gives the Fischer projections of the eight D aldohexoses... 

From among the carbohydrates shown in Figure 25 2 choose the D aldohexoses that yield... 

Which of the other six d aldohexoses yield optically active aldaric acids on oxidation, and which yield optically inactive (meso) aldaric acids (See Problem 25.19.)... 

What other d aldohexose gives the same alditol as D-talose ... 

The orientation of the model described above results in a clockwise numbering of the ring atoms. Groups that appear to the right of the modified Fischer projection appear below the plane of the ring those on the left appear above. In the common Haworth representation of the pyranose form of D-aldohexoses, C-6 is above the plane. 

Each series is built up by adding additional CHOH units between carbon 1 and carbon 2 to produce increasing numbers of isomers, the actual number being 2", where n is the number of additional chiral centres. Thus there are eight stereoisomers of the D-aldohexoses (Figure 9.3), eight stereoisomers of... 

The configuration at C-5 is opposite to that of D-(+)-glyceraldehyde. This particular carbohydrate therefore belongs to the l series. Comparing it with the Fischer projection formulas of the eight D-aldohexoses reveals it to be in the mirror image of D-(+)-talose it is l-(—)-talose... 

The stereogenic center at C-2 in the D-aldose becomes. s/r-hybridized in the enediol. The other pairs of D-aldohexoses that form the same enediols are... 

For each D-aldohexose there will be, as one more stereogenic center is added, two aldoheptoses. Altogether, there are 16 D-aldoheptoses. 

Figure 12.8 shows structures and names of the D-aldohexoses. These compounds all have six carbon atoms, have an aldehyde functional group, and have the OH group on C5 pointed to the right. Of these eight compounds, glucose is by far the most common. 

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

Identify all of the D-aldohexoses from Figure 25.1 that, on reduction with NaBH4, give products 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. 

Two of the D-aldohexoses give the same product upon reduction with NaBH4. Show the structures of these aldohexoses and the reduced product formed from them. 

Chair conformations can be drawn by recognizing the differences between the sugar in question and glucose. The following procedure is useful for drawing D-aldohexoses. 

Classify the following monosaccharides. (Examples D-aldohexose, L-ketotetrose.)... 

Digitalin (A) is related to which D-aldohexose Provide a name for A, including the configuration at the anomeric carbon. Predict the products of the reaction of A with (a) CH3OH, H+ catalyst (b) CH3I, Ag20. 

Problem 25.20 Which of the other six d aldohexoses yield optically active aldaric acids on... 

Finally, to form the D-aldohexoses, we must add another carbon atom (bonded to H and OH) Just below the carbonyl of all the aldopentoses. Because there are four D-aldopentoses to begin with, and there are two ways to place the new OH (right or left), there are now eight D-aldohexoses. Each aldohexose now has four stereogenic centers, so there are 2 = 16 possible stereoisomers, or eight pairs of enantiomers. Only the D-enantiomer of each pair is shown in Figure 27.4. 

Of the D-aldohexoses, only D-glucose and D-galactose are common in nature. D-Glucose is by far the most abundant of all D-aldoses. D-Glucose comes from the hydrolysis of starch and cellulose, and D-galactose comes from the hydrolysis of fruit pectins. 

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