Haworths Perspective Formulas

Fig. 1. Stmctural representations of sucrose (a) Haworth perspective formula, and (b) conformational stmcture of sucrose in soHd crystals. Adapted from...

Disaccharides are named by adding the name of a glycosyl group as a prefix to that of the monosaccharide chosen as parent, as exemplified by a-lactose (a Haworth perspective formula and a conformational formula are shown). 

Haworth perspective formulas like those in Figure 7-7 are commonly used to show the stereochem-... 

FIGURE 7-7 Pyranoses and furanoses. The pyranose forms of o-glucose and the furanose forms of o-fructose are shown here as Haworth perspective formulas. The edges of the ring nearest the reader are represented by bold lines. Hydroxyl groups below the plane of the ring in these Haworth perspectives would appear at the right side of a Fischer projection (compare with Fig. 7-6). Pyran and furan are shown for comparison. 

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

Comment Often it is more convenient as well as more acciurate to represent the cyclic forms of the monosaccharides by a ring diagram. Such a representation is the widely used Haworth perspective formula. 

Example Transformation of a Horizontal Fischer Projection of D-Glucose into a Haworth Perspective Formula. 

Solutions of aldoses at equilibrium contain mixtures of at least two (a and 3) furanoses and two (a and 3) pyranoses. Typically, the two pyranose forms predominate at equilibrium. The anomeric carbon is chiral (C-1 of aldoses and C-2 of ketoses). In the D-series, if the OH on the anomeric carbon is to the right in a Fischer projection or below the plane of a Haworth perspective formula, the isomer is a and it is 3 if the OH is to the left or above. Conversely, in the L-series, the a anomer has the OH at carbon 1 to the left or... 

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 historical aspects of Emil Fischer s fundamental conventions for writing stereoformulas in a plane have been thoroughly delineated.2 It may be mentioned that the perspective formulas generally attributed to W. N. Haworth were actually a revival of J. Bdeseken s cyclic formulas. ... 

Haworth s perspective formulas, III, 18 Heat of combustion, of sucrose, IV, 16 Helianthate, of streptomycin, III, 345, 346... 

In Haworth s perspective formula the plane of the pyranose or furanose ring is assumed to be perpendicular to the plane of paper. The substituents are parallel to this plane and project either above or below the plane of the ring (Fig. 2-6). Hydrogen atoms are indicated by a bar or not at all. 

In order to provide a better picture of the structure and configuration of the glucose molecule, Haworth has proposed a perspective representation. That for a-glucose is shown in formula (IV). 

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