Haworth ring

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. 

The translation of the open-chain structure into the Haworth ring structure is best achieved in a step-by-step procedure, since a direct translation is tricky. D-Glucose is used in this example ... 

Convert this to the five-membered Haworth ring form (use Example 2.10 as a guide). 

Draw the Fisher (open-chain) structures and the most common Haworth (ring) structures of D-glucose, D-fructose, D-galactose, and D-ribose. 

Draw the a-anomeric form of the furanose Haworth ring structure for sugar A. 

No. The reaction catalyzed by phosphoglucose isomerase is a simple isomerization between an aldose and a ketose and involves the open-chain structures of both sugars. Since glucose 6-phosphate and fructose 6-phosphate are both reducing sugars, their Haworth ring structures are in equilibrium with their open-chain forms. This equilibration is very rapid and does not require an additional enzyme. Note that this isomerization reaction is of the same type as that catalyzed by triosephosphate isomerase. 

Derive the stmcture of 3-methylfructofuranoside. (1) Write the stmcture for D-fmctose (2) convert this to the five-member Haworth ring form (Fig. 3-8) and (3) take the (3-anomer and substitute the anomeric hydroxyl with a methyl group. 

To convert a Fischer projection into a Haworth projection, the standard ring is drawn. For a D-sugar, the bottom primary hydroxyl group in the Fischer projection is placed up at C-5 in the Haworth projection. Then all of the substituents to the right in the Fischer projection are placed down on the Haworth ring, and all of the substituents to the left in the Fischer projection are placed up. The following examples illustrate the conversion ... 

Examples of the Fischer and Haworth ring formulae for different sugars are given below ... 

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

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... 

The rules previously mentioned for assignment of a- and /3-configurations can be readily applied to Haworth projection formulas. For the D-sugars, the anomeric hydroxyl group is below the ring in the a-anomer and above the ring in the /3-anomer. For L-sugars, the opposite relationship holds. 

The orientation of the model described above results in a clockwise numbering of the ring atoms. Groups that appear to the right of the modified Fischer projection appear below the plane of the ring those on the left appear above. In the common Haworth representation of the pyranose form of D-aldohexoses, C-6 is above the plane. 

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. 

The Haworth representation implies a planar ring. However, monosaccharides assume conformations that are not planar these may be represented by Haworth conformational formulae. The nomenclature of conformations is described in 2-Carb-7. For example, (5-D-glucopyranose assumes a chair conformation ... 

The variants are distinguished by the locants of those ring atoms that lie outside a reference plane (defined below) and are listed for some examples in Table 1. The locants of ring atoms that lie on the side of the reference plane from which numbering appears clockwise (i.e. the upper side in the normal Haworth representation of furanoses and pyranoses) are written as superscripts and precede the letter those that lie on the other side are written as subscripts and follow the letter. Heteroatoms (e.g. O, S) are indicated by their subscript or superscript atomic symbols. Table 1 gives the notations and Chart III some examples. 

When this paper11 was published (in 1938), the evidence indicated that the hydrofuranol ring compound isolated was the pyranoside form (XLVI). The later work of Haworth, Owen, and Smith,28 which is discussed in section V-l, corrected this impression and proved that the compound actually isolated by Peat and Wiggins was methyl 3,6-anhydro-/3-D-glueofuranoside (XLVIII). This correction does not invalidate the above argument, for it is known that XLVI is unstable and a trace of acid suffices to transform it into the stable furanoside. 

Haworth and Streight7 believe that the stability of difructose anhydride I to acid can be attributed to the presence of the dioxane ring. The similar stability of diheterolevulosan and of difructose anhydride III, both of which contain the dioxane group, agrees with this interpretation. 

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. 

D-glucose and its lactone from 2,3,6-trimethyl-D-glucose provided conclusive proof that the ring system was not of the hexylene oxide type.142 188 The final evidence necessary to characterize the tetramethylglucose in question as a furanose derivative was provided by Haworth, Hirst and Miller,176 who demonstrated that oxidation of the tetramethylglucose with bromine water and of the resulting lactone with nitric acid yielded dimethoxysuccinic acid and oxalic acid, but not i-zyZo-trimethoxyglutaric acid, the absence of which ruled out a pyranose structure. 

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