Glyceraldehyde Fischer projections

R)-Glyceraldehyde. Fischer projection of, 976 molecular model of, 976, 977 Glyceric acid, structure of. 753 Glycerol, catabolism of, 1132-1133 s/i-Glycerol 3-phosphate, naming of, 1132... 

Fischer projection A method of representing three-dimensional structures in two-dimensional drawings in which the chiral atom(s) lies in the plane of the paper. The two enantiomeric forms of glyceraldehyde are represented as... 

In the Fischer convention, the ermfigurations of other molecules are described by the descriptors d and L, which are assigned comparison with the reference molecule glyceraldehyde. In ertqrloying the Fischer convention, it is convenient to use projection formulas. These are planar representations defined in such a w as to convey three-dimensional structural information. The molecule is oriented with the major carbon chain aligned vertically in such a marmer that the most oxidized terminal carbon is at the top. The vertical bonds at each carbon are directed back, away fiom the viewer, and the horizontal bonds are directed toward the viewer. The D and L forms of glyceraldehyde are shown below with the equivalent Fischer projection formulas. 

A 90° rotation breaks the Fischer convention by exchanging the groups that go into the plane and those that come out. In the following Fischer projections of (jR)-glyceraldehyde, the —H and -OH groups come out of the plane before rotation but go into the plane after a 90° rotation. As a result, the rotated projection represents (S)-glyceraldehyde. 

Figure 25.2 Some naturally occurring D sugars. The -OF group at the chirality center farthest from the carbonyl group has the same configuration as

Fischer projections of the tour-, five-, and six-carbon d alcloses are shown in Figure 25.3. Starting with D-glyceraldehyde, we can imagine constructing the two d aldotetroses by inserting a new chirality center just below the aldehyde carbon. Each of the two d aldotetroses then leads to two d aldopentoses (four total), and... 

The two anomers are designated a or p by comparison of the chiralities at the anomeric centre and at the highest numbered chiral centre. If these are the same (RS convention), the anomer is termed P, or a if they are different. Note that the d and l prefixes are assigned on the basis of the chirality (as depicted in Fischer projections) at the highest numbered chiral centre and its relationship to D-(i )-(+)-glyceraldehyde or l-(5)-(—)-glyceraldehyde (see Section 3.4.10). 

Aldopentoses have three chirality centers. The eight stereoisomers are divided into a set of four D-aldopentoses and an enantiomeric set of four L-aldopentoses. The aldopentoses are named ribose, arabinose, xylose, and lyxose. Fischer projections of the d stereoisomers of the aldopentoses are given in Figure 25.2. Notice that all these diastereomers have the same configuration at C-4 and that this configuration is analogous to that of D-(+)-glyceraldehyde. 

Note The formulas for glyceraldehyde on the left-hand sides of equations 8.2 and 8.3 are written according to the Fischer projection notation. Bonds in the east-west direction come up out of the page and those running north-south go into the page, as represented in the middle formulas in the equations.)... 

Fischer projections are however, unsatisfactory when considering the physical properties and chemical reactivity of monosaccharides for which definitive spatial formulations are necessary. These are given below for D-glyceraldehyde, D-erythrose and D-threose, for which the (/ ,S configuration may be readily assigned at the appropriate chiral carbons. 

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... 

R-(+)-Glyceraldehyde is an aldotriose designated by the following Fischer projection formula ... 

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