Haworth projection

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. 

Figure 13-1. o-Glucose. A straight chain form. B a-D-glucose Haworth projection. C a-o-glucose chair form. 

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... 

Figure 9.8 Haworth projection formulae. The ring is considered to be planar with the substituent groups projecting above or below the plane. The thickened lines represent the portion of the ring that is directed out of the paper towards the reader. The alpha and beta anomeric forms are shown with the hydroxyl group at carbon 1 below or above the plane of the ring respectively.

The planar Haworth projection formulae bear little resemblance to the shape of the six-membered pyranoses that actually adopt a non-planar ring conformation comparable to that of cyclohexane. The chair form is the most... 

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. 

Fischer Projection Tollens Projection Haworth Projection Chair Projection... 

P-cellulose Cellulose soluble in 17.5% basic solution but not soluble in 8% caustic solution. Boeseken-Haworth projections Planar hexagonal rings used for simplicity instead of staggered chain forms. 

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

Give an adequately descriptive name of the disaccharide, and draw its Haworth projection formula. 

Figure 20-4 Haworth projection formulas showing the formation and reactions of O-methyl derivatives of glucose. The notation (H.OH) in 20 means that the anomeric configuration is unspecified.

Exercise 20-15 Write Fischer projections, Haworth projections, and sawhorse conformational drawings for the following ... 

Haworth structures are unambiguous in depicting configurations, but even they do not show the true spatial relationship of groups attached to rings. The normal angle between the bonds formed by the saturated carbon atoms (109°) causes the pyranose molecule to pucker into either a chairlike or boatlike conformation. For glucose and most other pyranoses the chair form (fig. 12.6c) predominates. However, we usually display pyranoses by the Haworth projection because it is easier to draw. 

Comparison of the Fischer (a) and Haworth (b) projections for a- and /3-D-glucose. The Haworth projection is a step closer to reality. Chair configurations for the two anomers of D-glucose are the most accurate depiction (c) but they are not always used because of the difficulty in drawing. Note that the largest substituent, —CH2OH, is in an equatorial location in both structures. The differences between the two anomers are shown in color. 

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... 

Two sugars can link to each other by losing water from OHs to form disaccharides. Figure 4.6 shows the Haworth projection formulas of four important disaccharides sucrose, lactose, maltose, and cellobiose, which all have the same molecular formulas, C12H22011. Sucrose and lactose are the most abundant and most important disaccharides of natural origin. Maltose and cellobiose are repeating units of polymeric starch and cellulose, respectively. Disaccharides may hydrolyze to form two monosaccharide molecules. 

First compare each Haworth projection with that of 0-D-glucose. Compound (a) differs in configuration at carbon 3. Compound (b) differs in configuration at carbons 2 and 3. 

Drawn first is a Haworth projection of °=-D-glucopyranose, and then of the solutions to part (a) and (b). 

Draw the cyclic structures (Haworth projection and conformational structure) for a-D-and p-D-glucose and the corresponding methyl glycosides. 

D-Galactose is the C-4 epimer of D-glucose (see Figure 16.1). Therefore, the Haworth projection will be identical to that for D-glucose, except that the C-4 hydroxyl group will be up rather than down. 

The cyclic structures are called Haworth projections. In a Haworth projection, the lower horizontal bond in the ring is understood to be projected towards you, above the plane of the page. This bond is usually in boldface. The upper horizontal bond is understood to be projected away from you, behind the plane of the page. 

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