Haworth structural formulas

In the 1920s, Haworth and his school proposed the terms furanose and pyranose for the two forms. Haworth also introduced the Haworth depiction for writing structural formulae, a convention that was soon widely followed. 

For the depiction of structural formulas of hexofuranoses, a combination of a three-dimensional, Haworth-perspective tetrahydrofuran ring with a Fischer projection of the C-5-C-6 side-chain is commonly used, as exemplified by formulas 3 and 6. With the formal closure of the second ring and formation of a 2,6-dioxabicyclo[3.3.0]octane system, however, the depiction of the C-6-C-3 ring, as in formula 7, also assumes three-dimensional geometry, and this does not correspond to the Fischer projection rule.11 Consequently, structural representations of such bicyclic molecules should be as close as possible to the actual steric situation, as shown by structures 4 and 8. 

The cyclic forms adopted by the hexoses and pentoses can be depicted as symmetrical ring structures called Haworth projection formulae, which give a better representation of the spatial arrangement of the functional groups with respect to one another. The nomenclature is based on the simplest organic compounds exhibiting a similar five- or six-membered ring... 

Fig. 1. Structural representations of sucrose (a) Haworth perspective formula, and (b) conformational structure of sucrose in solid crystals. Adapted from...

Figure 2.2 Structural formulae of a- and /3-lactose, (a) Fischer projection, (b) Haworth projection and (c) conformational formula.

Monosaccharides commonly form internal hemiacetals or hemiketals, in which the aldehyde or ketone group joins with a hydroxyl group of the same molecule, creating a cyclic structure this can be represented as a Haworth perspective formula. The carbon atom originally found in the aldehyde or ketone group (the anomeric carbon) can assume either of two configurations, a and /3, which are interconvertible by mutarotation. In the linear form, which is in equilibrium with the cyclized forms, the anomeric carbon is easily oxidized. 

Haworth structures are easy to draw and unambiguous in depicting configurations,14 but they also do not show the spatial relationships of groups attached to other rings correctly. For this reason conformational formulas of the type described in Section 2 and shown in Fig. 4-4 are used most often in this book. 

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... 

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. 

In the stable forms of the polyacetates of the aldohexopyranosyl and aldohexofuranosyl halides, the halogen atom at Cl is trans to the group at the ring junction when the structural formulas are written according to the Haworth convention, (XIV) to (XVII). 

Haworth perspective formulas. A method for representing cyclic chemical structures (sugars) in order to define the configuration around each chiral center. 

Haworth projection formulas of anomers of D-glucopyranose. The thick line of the structure projects out toward the observer, and the upper edge (thin line) projects behind the plane of the paper. 

The end-group method was applied to the determination of the molecular weight of cellulose by Haworth and others. The structural formula for cellulose is discussed in Chapter 3. The two end groups have the structures... 

Fig. 1. Chemical structures for simple sialic acids in different views, (a) S-amino-S.S-dideoxy-D-gfycero-D-ga/acro-non-2-ulosonic acid (Neu, open chain, Fischer projection formula) (b) 5-acetaraido-3,5-dideoxy-D-g/> cero-a-D-gfl/acto-non-2-ulopyranosonic acid (a-Neu5Ac, Fischer projection formula, note that C7 is the anomeric reference atom) (c) a-Neu5Ac (Haworth projection formula) (d) a-Neu5Ac ( 5 chair conformation) (e) 3-deoxy-D-g/ycero-3-D-gfl/acto-non-2-ulopyranosonic acid (P-Kdn, 5 chair conformation). Note that the D-notation is part of the trivial name.

Haworth structures more closely depict proper bond angles and lengths than do Fischer representations. To convert from the traditional Fischer formula of a D-pen-tose or D-hexose to a Haworth formula, the following steps should be followed ... 

CONFORMATIONAL STRUCTURES Although Haworth projection formulas are often used to represent carbohydrate structure, they are oversimplifications. Bond angle analysis and X-ray analysis demonstrate that conformational formulas are more accurate representations of monosaccharide structure (Figure 7.10). Conformational structures are more accurate because they illustrate the puckered nature of sugar rings. 

In Figure 17.6 a new type of structural formula, called a Haworth projection, is presented. Although on first inspection it appears complicated, it is quite simple to derive a Haworth projection from a structural formula, as Example 17.2 shows. 

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. 

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