Carbohydrates Fischer projections, conventions

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. 

Carbohydrates with more than one chirality center are shown in. Fischer projections by stacking the centers on top of one another. By convention, the carbony] carbon is always placed either at or near the top. Glucose, for... 

Fischer projection, 975-978 carbohydrates and, 977-978 D sugars, 980 i., sugars, 980-981 rotation of, 976 R.S configuration of, 977 conventions for, 975-976 Fishhook arrow, radical reactions and, 139, 240... 

Models with three or more stereogenic centers create new problems. Again, carbohydrate chemists were first to give efficient and clear, though perhaps to the outsider cumbersome solutions. The following carbohydrate convention (see Table 11) is based on the Fischer projection. i.e., it prescribes an all-eclipsed conformation of the backbone and defines the direction of the backbone by the oxidation numbers of the terminal carbon atoms. 

The R and S system is widely accepted for most organic compoimds, but for historical reasons we often use the d and l system for carbohydrates and amino acids. By convention, a Fischer projection is drawn vertically with the... 

Erythro and threo. Descriptors used to distinguish between diastereomers of an acyclic structure having two stereogenic centers. When placed in a Fischer projection using the convention proper for carbohydrates, erythro has the higher priority groups on the same side of the Fischer projection, and threo has them on opposite sides. 

Carbohydrate chemistry uses an important structural convention, the Fischer projection. To understand this chapter, you have to be able to move easily from Fischer projections to conventional structures. The directions are right there in Section 22.2. 

The D and L convention for naming enantiomeric forms of a molecule is discussed in Chapter 23 on the acompanying website it tends to be used for carbohydrates and amino acids. It is rather an old-fashioned system and was developed before X-ray crystallography allowed the determination of the absolute configuration of molecules. For sugars, when the d and l system is used, it is based on the Fischer projection of the molecule and everything is compared to glyceraldehyde (2,3-dihydroxypropanal). 

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 ... 

The d/l convention is an old, confusing, and often incorrect method of specifying configurations at stereo-genic centers as d or l (typeset in small caps in published work), which is used to distinguish between enantiomers of chiral carbohydrates and chiral a-amino acids, and is based on the molecule drawn as a Fischer projection in a specific orientation. This convention was devised in the early years of twentieth century by German organic chemist Emil Fischer, who worked extensively with carbohydrates. 

William Mills described a similar convention to depict the structures of monosaccharides. While the ring atoms of the Haworth projections are oriented perpendicular to the paper, Mills chose to depict the carbon skeleton in the plane of the paper (Fig. 1.5). Although Fischer, Haworth, and Mills projections are useful tools for depicting the structures of carbohydrates, the planar nature of these representations does not provide an accurate picture of the actual geometry of the molecules. In order to understand carbohydrate function and reactivity, recognition of each distinct conformation and the properties associated with it is required [15]. 

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