Formulas Haworth

Structural drawings of carbohydrates of this type are called Haworth formulas, after the British chemist Sir Walter Norman Haworth (St Andrew s University and the University of Birmingham) Early m his career Haworth contributed to the discovery that carbohydrates exist as cyclic hemiacetals rather than m open chain forms Later he col laborated on an efficient synthesis of vitamin C from carbohydrate precursors This was the first chemical synthesis of a vitamin and provided an inexpensive route to its prepa ration on a commercial scale Haworth was a corecipient of the Nobel Prize for chem istry m 1937... 

Orienf fhe Haworth formula of the carbohydrate with the ring oxygen at the back and the anomeric carbon at the right... 

Substituents that are to the right m a Fischer projection are down m the corre spondmg Haworth formula those to the left are up... 

Generating Haworth formulas to show stereochemistry m furanose forms of higher aldoses is slightly more complicated and requires an additional operation Furanose forms of D ribose are frequently encountered building blocks m biologically important organic molecules They result from hemiacetal formation between the aldehyde group and the C 4 hydroxyl... 

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

Next convert the Haworth formula to the chair conformation that has the CH2OH group equatorial... 

Haworth formulas (Section 25 6) Planar representations of furanose and pyranose forms of carbohydrates... 

Substituents that ae to the right in a Fischer projection ae down in the cone-sponding Haworth formula those to the left ae up. ... 

Write Haworth formulas corresponding to the furanose forms... 

Note. In writing Haworth formulae, the H atoms hound to the carbon atoms of the ring are often omitted to avoid crowding of the lettering in the ring. For the sake of clarity, the form with H atoms included is preferred in this document. 

It is sometimes desirable to draw Haworth formulae with the ring in other orientations (see Chart II), when there are bulky substituents to be represented, or when linkages in oligo- or poly-saccharides are to be shown. If the ring is inverted [as in (g)-(l)], the numbering runs counterclockwise. 

Fig. 3.—Formulas Representing KDO (1). [a, Fischer projection-formula (acyclic form) b, Haworth formula (ketopyranose) c, conformational drawing (ketopyranose).]...

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. 

SAMPLE SOLUTION (a) By analogy with the procedure outlined for D-glucose in Figure 25.4, first generate a Haworth formula for p-D-galactopyranose ... 

Fig. 5. Haworth formula indicating structural and configurational features essential for efficient binding of aldohexoses with glucose-6-phosphatase. From Lygre and Nordlie (126). Copyright (1965), Elsevier Publishing Co. Reproduced by permission.

Even though Haworth formulas give a sound representation of the ring structures of sugars, the real structure conformation can be most accurately represented by the chair forms of cyclohexane as shown in... 

Figure 4.4. However, despite the inaccuracy of the Haworth formulas, they are used more frequently than the chair conformation, because they are easier to draw and interpret.

The Haworth formula is more realistically drawn as the following chair conformation ... 

Translating the Haworth formula into a proper conformational depiction requires that a choice be made between the two chair conformations shown. 

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