Haworth convention

Figure 1.12 The a- and P-anomers of monosaccharides drawn using the Haworth convention. In this convention solid lines represent bonds above the plane of the ring whilst dotted lines are used to indicate bonds below the plane of the ring. Reproduced from G Thomas, Chemistry for Pharmacy and the Life Sciences including Pharmacology and Biomedical Science, 1996, by permission of Prentice Hall, a Pearson Education Company...

In the usual writing of stereochemical formulas of carbohydrates by the Fischer and Haworth conventions, the hydroxyls in a ring are depicted as either cis or trans. It is well known, however, that these formulas do not necessarily give a true representation of the relative positions of the hydroxyls in space. The 6-member rings, and possibly to some extent the 5-member rings, generally are not planar but have a... 

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). 

Troughout this chapter Haworth convention is mainly used and we follow the rule given in Ref. 7) according to which in Haworth formulas the CH2OH group is directed upward for the D-series and downward for the L-series, thus in the D-series the OH (or -OR) group at C-a is directed downward for the a-anomer and upward for the P-anomer. The reverse is true for the L-series... 

Every attempt has been made to present the structures of chemical substances as accurately as possible according to best current practice and recommendations of lUPAC (The International Union of Pure and Applied Chemistry). As much consistency as possible has been aimed at between closely-related structures. For example, all sugars are shown as Haworth formulae, and whenever possible in complex structures the rings are oriented in the standard Haworth convention so that structural comparisons can be quickly made. 

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. 

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. ... 

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]. 

Figure 9.7 Structures of some sugars. (A) L-Fucose (pyranose form). Note that the -CH3 group points downward this indicates l series in the Haworth structural convention. (B) Maltose (a-D-glucopyranosyl-l,4-D-glucopyranose) (C) cellobiose (/3-D-glucopyranosyl-1,4-D-glycopyranose) and (D) sucrose (a-D-glucopyranosyl-l,2-/3-D-fructofuranoside). (Reproduced by permission from Diem K, Lentner C. Scientific Tables. Basel Ciba-Geigy, 1971.)...

Haworth projection A conventional planar representation of a cyclized monosaccharide molecule. The hydroxyls that are represented to the right of the chain in a Fischer projection are shown below the plane in a Haworth projection. 

When drawing either a Haworth projection or a chair conformation, the convention is to place the oxygen atom in the upper, rear-right position, as highlighted in red. Notice that in a cycfic form of D-glucose all substituents can occupy equatorial positions, which renders the compound particularly stable and explains why D-glucose is the most common naturally occurring monosaccharide. 

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