Fischer projection formulas carbohydrates

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. 

The configuration at C-5 is opposite to that of D-(+)-glyceraldehyde. This particular carbohydrate therefore belongs to the l series. Comparing it with the Fischer projection formulas of the eight D-aldohexoses reveals it to be in the mirror image of D-(+)-talose it is l-(—)-talose... 

Fischer esterification. See Esterification Esters Fischer projection formulas, 271-272, 278, 280, 292, 595 a-amino acids, 1056, 1103 carbohydrates, 973-974, 1007 of meso stereoisomer, 280 tartaric acids, 286... 

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. 

Fischer projection formulas can be used to represent molecules with several stereoisomeric centers, and they are commonly used for carbohydrate molecules. For other types of structures, a common practice is to draw the molecule in an extended conformation with the main chain horizontal. In this arrangement, each tetrahedral carbon has two additional substituents, one facing out and one in. The orientation is specified with solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

Table 17.1 shows the names and Fischer projection formulas for all D-aldotrioses, tetroses, pentoses, and hexoses. Each name consists of three parts. The letter D specifies the configuration at the stereocenter farthest from the carbonyl group. Prefixes, such as rib-, arabin-, and glue-, specify the configurations of all other stereocenters relative to one another. The suffix -ose shows that the compound is a carbohydrate. 

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. 

It was in connection with his studies on carbohydrate stereochemistry that Emil Fischer invented his system of projection formulas. Because we will be using these formulas here, it might be wise for you to review Sections 5.7 through 5.9. Recall that, in a Fischer projection formula, horizontal lines show groups that project above the plane of the paper toward the viewer vertical lines show groups that project below the plane of the paper away from the viewer. Thus, R-(-F)-glyceraldehyde can be represented as... 

The communication of the stereochemical information elucidated for these and related carbohydrates (Chapter 11) is conveniently conveyed graphically by use of the Fischer projection formulas. 

The specification of configurations in diastereomeric species is quite simple, with each chiral center being designated R or S according to the sequence rules when the Cahn-Ingold-Prelog convention is used. An extension of the Fischer convention to systems with more than one asymmetric center that is based on carbohydrate structures and terminology is still used in relatively simple cases. This convention can be illustrated with the same stereoisomeric 2,3,4-trihydroxybutanals just discussed. The 2R,3R and 25,35 isomers are d- and L-erythrose, respectively. The 25,3/ and 2i ,35 isomers are d- and L-threose, respectively. The Fischer projection formulas are shown below ... 

C is correct. The general formula for a carbohydrate is Cn(H,0)n. Since this carbohydrate is in the Fischer projection with the aldehyde or ketone at the top, and the bottom chiral carbon is positioned to the right, it is of the D configuration. The only way to know about polarized light is to use a polarimeter. 

Fischer Projection (or Fischer-Pollens projection) A planar projection formula in which the vertical bonds lie behind the plane of the paper and the horizontal bonds lie above the plane. Used commonly in carbohydrate structures, where each carbon in turn is placed in the proper orientation for planar projection. 

Because Fischer projections must be used with such care, we introduce them now only so that you can understand Fischer projections when you see them in the context of other courses. Our emphasis for most of this book will be on the use of solid and dashed wedges to represent three-dimensional formulas (or chair conformational structures in the case of cyclohexane derivatives), except in Chapter 22 when we will use Fischer projections again in our discussion of carbohydrates. If your instructor wishes to utilize Fischer projections further, you will be so advised. 

---