Chirality Fischer projection formulas

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). 

Redraw the perspective drawings a, b, and c as Fischer projection formulas, leaving the configuration at the chiral centers unchanged. Similarly, redraw d and e in perspective, using a staggered sawhorse representation for e. 

The formation of a hemiacetal ring gives rise to a new chiral center in the aldoses, namely, at the C-1 atom. This leads to two C-1 epimers, termed anomers. When the hydroxyl group at C-1 (glycosidic hydroxyl) in the Fischer projection formula is located on the same side as the hemiacetal ring, the anomer is termed the a form in the opposite case it is the j3 form (Fig. 2-5). 

Figure 6 Structures of the four stereoisomers of sphingosine. Sphingosine has two chiral carbon atoms (C-2 and C-3). The Fischer projection formula of each structure is also shown, with C-1 at the top, to illustrate the D/L and erythro/threo stereochemical nomenclature. C-3 has an erythro orthreo configuration as it relates to C-2, depending on whether the similar groups (amino and hydroxy) are on the same or opposite side of the Fischer projection. D versus L refers to the configuration at C-2 relative to the configuration of D-glyceraldehyde versus L-glyceraldehyde.

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. 

Draw a Fischer Projection formula for each of the following compounds. Indicate each of the chiral carbons with asterisks ( ). 

Chiral, definition of, 260 Chiral axis. See Stereogenic axis Chiral center. See Stereogenic center Chiral drugs, 273 Chiral molecules, 259—263, 290 absolute configuration, 267, 292 Fischer projection formulas, 271-272, 278, 280, 292-293... 

Fischer projection formulas a-amino acids, 1056, 1103 carbohydrates, 973-974, 977, 1007 chiral molecules, 271—272, 292 two stereogenic centers, 276—278, 280, 293... 

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. 

Chemists, however, sometimes use formulas called Fischer projections to show three dimensions in chiral molecules such as acyclic carbohydrates. Fischer projection formulas are useful in cases where there are chirality centers at several adjacent carbon atoms, as is often the case in carbohydrates. Use of Fischer projection formulas requires rigjd adherence to certain conventions, however. Used carelessly, these projection formulas can easily lead to incorrect conclusions. 

Write a Fischer projection formula for a tartaric acid isomer that is not chiral. 

Fischer projection (Sections 5.13 and 22.2C) A two-dimensional formula for representing the configuration of a chiral molecule. By convention, Fischer projection formulas are written with the main carbon chain extending from top to bottom with all groups eclipsed. Vertical lines represent bonds that project behind the plane of the page (or that lie in it). Horizontal lines represent bonds that project out of the plane of the page. 

Some use is still made of the terms erythro nd threo to specify the relative configuration of two adjacent chiral atoms. The terms are derived from the sugars erythrose and threose. The terms were originally defined such that a Fischer projection formula in which the two adjacent substituents were on the same side was the erythro isomer and that with the substituents on opposite sides was the threo isomer. 

With the exception of glycine, FI2NCH2COOH, all protein-derived amino acids have at least one stereocenter and therefore are chiral. Figure 18.2 shows Fischer projection formulas for the enantiomers of alanine. The vast majority of carbohydrates in the biological world are of the D series (Section 17.2), whereas the vast majority of a-amino acids in the biological world are of the L-series. 

Chirality in Monosaccharides Fischer Projection Formulas and o,L-Sugars... 

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