25.4 Configurations of the Aldoses

25.4 • Configurations of the Aldoses

An aldotetrose is a four-carbon sugar with two chirality centers. Thus, there are 2² = 4 possible stereoisomeric aldotetroses, or two D,L pairs of enantiomers named erythrose and threose.

Aldopentoses have three chirality centers and a total of 2³ = 8 possible stereoisomers, or four D,L pairs of enantiomers. These four pairs are called ribose, arabinose, xylose, and lyxose. All except lyxose occur widely in nature. D-Ribose is an important constituent of RNA (ribonucleic acid), L-arabinose is found in many plants, and D-xylose is found in wood.

Aldohexoses have four chirality centers and a total of 2⁴ = 16 possible stereoisomers, or eight D,L pairs of enantiomers. The names of the eight are allose, altrose, glucose, mannose, gulose, idose, galactose, and talose. Only D-glucose, from starch and cellulose, and D-galactose, from gums and fruit pectins, are widely distributed in nature. D-mannose and D-talose also occur naturally but in lesser abundance.

Fischer projections of the four-, five-, and six-carbon D aldoses are shown in Figure 25.4. 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 each of the four D aldopentoses leads to two D aldohexoses (eight total). In addition, each of the D aldoses in Figure 25.4 has an L enantiomer, which is not shown.

Figure 25.4 Configurations of D aldoses. The structures are arranged from left to right so that the –OH groups on C2 alternate right/left (R/L) across each series. Similarly, the –OH groups at C3 alternate two right/two left (2R/2L), the –OH groups at C4 alternate 4R/4L, and the –OH groups at C5 are to the right in all eight (8R). Each D aldose has a corresponding L enantiomer that is not shown.

Louis Fieser of Harvard University long ago suggested a simple procedure for remembering the names and structures of the eight D aldohexoses:

STEP 1
Set up eight Fischer projections with the –CHO group on top and the –CH₂OH group at the bottom.

STEP 2
At C5, place all eight –OH groups to the right (D series).

STEP 3
At C4, alternate four –OH groups to the right and four to the left.

STEP 4
At C3, alternate two –OH groups to the right, two to the left.

STEP 5
At C2, alternate –OH groups right, left, right, left.

STEP 6
Name the eight isomers using the mnemonic “All altruists gladly make gum in gallon tanks.”

The structures of the four D aldopentoses can be generated in a similar way and named by the mnemonic suggested by a Cornell University undergraduate: “RIBs ARe eXtra Lean.”

Worked Example 25.2

Drawing a Fischer Projection

Draw a Fischer projection of L-fructose.

Strategy

Because L-fructose is the enantiomer of D-fructose, simply look at the structure of D-fructose and reverse the configuration at each chirality center.

Solution

Worked Example 25.3

Drawing a Fischer Projection of a Molecular Model

Draw the following aldotetrose as a Fischer projection, and identify it as a D sugar or an L sugar:

Strategy

The Fischer projection of a monosaccharide is drawn vertically, with the carbonyl group at or near the top and the –CH₂OH group at the bottom. The interior –H and –OH are drawn to the sides, pointing out of the page toward the viewer.

Solution

Problem 25-8

Only the D sugars are shown in Figure 25.4. Draw Fischer projections for the following L sugars:

(a)

L-Xylose

(b)

L-Galactose

(c)

L-Allose

Problem 25-9

How many aldoheptoses are there? How many are D sugars, and how many are L sugars?

Problem 25-10

The following model is that of an aldopentose. Draw a Fischer projection of the sugar, name it, and identify it as a D sugar or an L sugar.