The Haworth Projection

In the pyranose form the oxygen at C-5 must be exchanged with the terminal hydroxymethyl group. 

FIGURE 2.19 The Fischer (a), modified Fischer (b), Haworth (c) and simplified Haworth (d) representations of a-D-glucopyranose. 

FIGURE 2.22 Furanose and pyranose structures of all a-D-pentoses and a-D-hexoses. 

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The Haworth formula does not take account of the fact that the pyran ring is not plain, but usually has a chair conformation. In B3, two frequent conformations of D-glucopy-ranose are shown as ball-and-stick models. In the 4 conformation (bottom), most of the OH groups appear vertical to the ring level, as in the Haworth projection (axial or a position). In the slightly more stable " Ci conformation (top), the OH groups take the equatorial or e position. At room temperature, each form can change into the other, as well as into other conformations. 

Haworth structures are unambiguous in depicting configurations, but even they do not show the true spatial relationship of groups attached to rings. The normal angle between the bonds formed by the saturated carbon atoms (109°) causes the pyranose molecule to pucker into either a chairlike or boatlike conformation. For glucose and most other pyranoses the chair form (fig. 12.6c) predominates. However, we usually display pyranoses by the Haworth projection because it is easier to draw. 

Comparison of the Fischer (a) and Haworth (b) projections for a- and /3-D-glucose. The Haworth projection is a step closer to reality. Chair configurations for the two anomers of D-glucose are the most accurate depiction (c) but they are not always used because of the difficulty in drawing. Note that the largest substituent, —CH2OH, is in an equatorial location in both structures. The differences between the two anomers are shown in color. 

A more realistic representation for the hemiacetal ring structure is the Haworth projection formulas. The formulas for a-D-glucose are shown in Figure 4.3. The shorthand form of the Haworth projection eliminates the Hs and indicates OHs by dashes. Five- and six-membered cyclic sugars are called furanose and pyranose, respectively.3... 

Two sugars can link to each other by losing water from OHs to form disaccharides. Figure 4.6 shows the Haworth projection formulas of four important disaccharides sucrose, lactose, maltose, and cellobiose, which all have the same molecular formulas, C12H22011. Sucrose and lactose are the most abundant and most important disaccharides of natural origin. Maltose and cellobiose are repeating units of polymeric starch and cellulose, respectively. Disaccharides may hydrolyze to form two monosaccharide molecules. 

D-Galactose is the C-4 epimer of D-glucose (see Figure 16.1). Therefore, the Haworth projection will be identical to that for D-glucose, except that the C-4 hydroxyl group will be up rather than down. 

In the formula shown below for rodorubicin (a cytostatic agent) convert the groups shown in the Haworth projection into a planar projection of the rings in which the substituents are shown with wedged bonds. 

Populate the ring substituents such that those on the right side of the Fischer projection are on the bottom face of the Haworth projection and those on the left side are on the top. Hydrogen atoms are typically omitted for clarity. 

William Mills described a similar convention to depict the structures of monosaccharides. While the ring atoms of the Haworth projections are oriented perpendicular to the paper, Mills chose to depict the carbon skeleton in the plane of the paper (Fig. 1.5). Although Fischer, Haworth, and Mills projections are useful tools for depicting the structures of carbohydrates, the planar nature of these representations does not provide an accurate picture of the actual geometry of the molecules. In order to understand carbohydrate function and reactivity, recognition of each distinct conformation and the properties associated with it is required [15]. 

The cyclic structure is often drawn initially in the Haworth projection, which depicts the ring as being flat (of course, it is not). The Haworth projection is widely used in biology texts, but most chemists prefer to use the more realistic chair conformation. Figure 23-6 shows the cyclic form of glucose both as a Haworth projection and as a chair conformation. 

Close the ring, and draw the result. Always draw the Haworth projection or chair conformation with the oxygen at the back, right-hand comer, with Cl at the far right. Cl is easily identified because it is the hemiacetal carbon—the only carbon bonded to two oxygens. The hydroxyl group on Cl can be either up or down, as discussed in Section 23-7. 

Draw the Haworth projection for the cyclic structure of D-mannose by laying down the Fischer projection. 

Allose is the C3 epimer of glucose. Draw the cyclic hemiacetal form of D-allose, first in the chair conformation and then in the Haworth projection. 

Using methods similar to Fischer s, the straight-chain form of any monosaccharide can be worked out. As we have seen, however, monosaccharides exist mostly as cyclic pyra-nose or furanose hemiacetals. These hemiacetals are in equilibrium with the open-chain forms, so sugars can react like hemiacetals or like ketones and aldehydes. How can we freeze this equilibrium and determine the optimum ring size for any given sugar Sir Walter Haworth (inventor of the Haworth projection) used some simple chemistry to determine the pyranose structure of glucose in 1926. 

A flat-ring representation of a cyclic sugar. The Haworth projection does not show the axial and equatorial positions of a pyranose, but it does show the cis and trans relationships, (p. 1109)... 

The Haworth projection formulas are neater ways of writing the ring forms shown in the equilibria above and yet preserving the configuration shown at each chiral carbon. It is not difficult to translate the open-chain structure for a monosaccharide into the Haworth ring structure. 

D/L descriptor the OH is to the right on the farthest chiral carbon from the CHO group i.e., C5) in the Fischer projection, and in the Haworth projection, hence D-glucose. 

Open chain monosaccharides (like the Fischer projection above) have n-2 chiral carbons, so 2"-2 isomers (assuming no internal symmetry). Note that sugars can also react internally to produce a variety of rings forms this introduces an extra chiral centre, as you can see in the Haworth projection above. 

Rules of the Fischer Projection Mutarotation The Haworth Projection The Mills Projection The Reeves Projection Conformations of the Six-Membered Rings Conformations of the Five-Membered Rings Conformations of the Seven-Membered Rings Conformations of Fused Rings Steric Factors... 

Given the linear structure of a monosaccharide, draw the Haworth projection of its a- and p-cyclic forms and vice versa. 

Drawing the Haworth Projection of a Monosaccharide from the Structural Formula... 

Look at the two-dimensional structural formula. Note the groups (drawn in blue) to the left of the carbon chain. These are placed above the ring in the Haworth projection. 

Refer to the linear structure of D-galactose. Draw the Haworth projections of ( and fJ-D-galactose. 

Another important hexose is galactose. The linear structure of D-galactose and the Haworth projections of a-D-galactose and p-D-galactose are shown here ... 

The Haworth projection of a D-furanose is viewed edge on, with the ring oxygen away from the viewer. The anomeric carbon is on the right-hand side of the molecule, and the primary alcohol group is drawn up from the back left-hand corner. 

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