Sugars projection formulae

Sometimes the terms erythro and threo are used to specify fee relative configuration of two adjacent stereogenic centers. The terms are derived fom fee sugars erythrose and threose. The terms were originally defined such feat a Fischer projection formula in which two adjacent substituents were on the same side was fee erythro isomer and feat in whidi the substituents were on opposite sides was the threo isomer. 

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 ring system in these dianhydro hexitols is of interest and worthy of some discussion. The formula of isomannide (LXX) based on the Fischer projection formula for sugars does not convey the real character of the molecule and the author has chosen to write these substances as two fused tetrahydrofuran rings. Scale models show this to be a more exact representation. Thus isomannide is written as LXXI, isosorbide as LXXII, and L-isoidide as LXXIII. 

By convention, one of these two forms is designated the d isomer, the other the l isomer. As for other biomolecules with chiral centers, the absolute configurations of sugars are known from x-ray crystallography. To represent three-dimensional sugar structures on paper, we often use Fischer projection formulas (Fig. 7 2). 

When there are several chiral carbons in a molecule, the configuration at one center usually is related directly or indirectly to glyceraldehyde, and the configurations at the other centers are determined relative to the first. Thus in the aldehyde form of the important sugar, (+)-glucose, there are four chiral centers, and so there are 24 = 16 possible stereoisomers. The projection formula of the isomer that corresponds to the aldehyde form of natural glucose... 

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. 

Know the meaning of D- or L-sugar, Fischer projection formula, Haworth formula, epimer. 

Write the Fischer projection formulas for the sugar derivatives named in Example 2.20. 

The stereogenic centers in sugars are often depicted following a different convention than is usually seen for other stereogenic centers. Instead of drawing a tetrahedron with two bonds in the plane, one in front of the plane, and one behind it, the tetrahedron is tipped so that horizontal bonds come forward (drawn on wedges) and vertical bonds go behind (on dashed lines). This structure is then abbreviated by a cross formula, also called a Fischer projection formula. In a Fischer projection formula ... 

D-Sugar (Section 27.2C) A sugar with the hydroxy group on the stereogenic center furthest from the carbonyl on the right side in the Fischer projection formula. 

CONFORMATIONAL STRUCTURES Although Haworth projection formulas are often used to represent carbohydrate structure, they are oversimplifications. Bond angle analysis and X-ray analysis demonstrate that conformational formulas are more accurate representations of monosaccharide structure (Figure 7.10). Conformational structures are more accurate because they illustrate the puckered nature of sugar rings. 

In D-family sugars, the OH on the chiral carbon farthest from the carbonyl group is on the right side in a Fischer projection formula. So both (+)-glucose and (-)-fructose are D-sugars despite their rotation of plane-polarized light in opposite directions. 

Fischer projections choose the cyclic and planar, //-eclipsed conformation of the carbon skeletons of sugars as a basis, then pull the cycle straight to form a line and orient it so that the more highly oxidized end Cl is located at the top. This projection procedure remained alive for almost a century because inherent symmetry properties in carbohydrates become apparent here in the most simple way. Always remember Fischer projection formulas are the short notations of a/l-eclipsed cyclic conformations, although they look linear (Fig. 4.2.1). The terminal chiral center, C5 in hexoses, is usually R- or, in the Fischer notation, D-configured. 

Sugars exist predominantly as cyclic molecules rather than in an open-chain form. Haworth projection formulas are more realistic representations of the cyclic forms of sugars than are Fischer projection formulas. Many stereoisomers are possible for five- and six-carbon sugars, but only a few of the possibilities are encountered frequently in nature. 

Emil Fischer introduced the classical projection formulas for sugars, with a standard orientation (carbon chain vertical, carbonyl group at the top, Scheme 2) as he used models with flexible bonds between the atoms, he could easily stretch his sugar models into a position suitable for projection. 

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