Fischer projections of carbohydrates

Aldoses exist almost exclusively as their cyclic hemiacetals very little of the open chain form is present at equilibrium To understand their structures and chemical reac tions we need to be able to translate Fischer projections of carbohydrates into their cyclic hemiacetal forms Consider first cyclic hemiacetal formation m d erythrose To visualize furanose nng formation more clearly redraw the Fischer projection m a form more suited to cyclization being careful to maintain the stereochemistry at each chirality center... 

The Fischer projection of the branched chain carbohydrate D apiose has been presented in Section 25 12... 

Monosaccharides, or simple sugars, are carbohydrates that cannot be hydrolyzed to simpler compounds. Figure 23-1 shows Fischer projections of the monosaccharides glucose and fructose. Glucose is a polyhydroxyaldehyde, and fructose is a polyhydroxy ketone. Polyhydroxyaldehydes are called aldoses (aid- is for aldehyde and -ose is the suffix for a sugar), and polyhydroxyketones are called ketoses (ket- for ketone, and -ose for sugar). 

An alternative definition specifies that the more highly oxidized terminal carbon atom be at the top of the Fischer projection. For carbohydrates and amino acids, there is ordinarily no conflict between these two definitions. For historical notes and an elaboration of the d and l nomenclature system, see Slocum, D. W. Sugarman, D. Tucker, S. P. /. Chem. Educ. 1971,48,597 reference 140a, pp. 88-92. 

Carbohydrates are polyhydroxy aldehydes and ketones that are commonly known as sugars. A monosaccharide is a carbohydrate composed of one sugar unit and a disaccharide is a carbohydrate composed of two monosaccharide units. An oligosaccharide is composed of 2-10 monosaccharide units a polysaccharide is composed of 10 or more monosaccharide units. An aldose is a carbohydrate that contains an aldehyde unit and a ketose is a carbohydrate that contains a ketone unit. Monosaccharides are categorized by the total number of carbons in the structure triose, tetrose, pentose, hexose, etc. The d and 1 configurations of a monosaccharide are based on the Fischer projection of d-glyceraldehyde. A Fischer projection is an older representation of sugars. 

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). 

Carbohydrates Chiral Molecules Fischer Projections of Monosaccharides Haworth Structures of Monosaccharides Chemical Properties of Monosaccharides Disaccharides Polysaccharides... 

We mentioned in Section 7 6 that the d l system of stereochemical notation while outdated for most purposes is still widely used for carbohydrates and amino acids Likewise Fischer projections find their major application m these same two families of compounds... 

Fischer projections and d-l notation are commonly used to describe carbohydrate stereochemistry. The standards are the enantiomers of glycer-aldehyde. 

Because carbohydrates usually have numerous chirality centers, it was recognized long ago that a quick method for representing carbohydrate stereochemistry is needed. In 1891, Emil Fischer suggested a method based on the projection of a tetrahedral carbon atom onto a flat surface. These Fischer projections were soon adopted and are now a standard means of representing stereochemistry at chirality centers, particularly in carbohydrate chemistry. 

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... 

Fischer projections can also be used for compounds with just one stereocenter, as above, but they are usually used to show compounds with multiple stereocenters. You will utilize Fischer projection heavily when you learn about carbohydrates at the end of the course. 

Erythro/threo Terms derived from carbohydrate nomenclature used to describe the relative configuration at adjacent stereocenters. Erythro refers to a configuration with identical or similar substituents on the same side of the vertical chain in Fischer projection. Conversely, a threo isomer has these substituents on opposite sides. These terms came from the nomenclature of two carbohydrate compounds, threose and erythrose (see Fig. 1-35). 

The structures above show some of the fundamental features of carbohydrates. Initially, we have drawn these compounds in the form of Fischer projections, a depiction developed for these compounds to indicate conveniently the stereochemistry at each chiral centre (see Section 3.4.10). The Fischer projection is drawn as a vertical carbon chain with the group of highest oxidation state, i.e. the carbonyl group, closest to the top, and numbering takes place from the topmost carbon. 

For molecules with two stereogenic centers, the traditional descriptors are erythro and threo to which, henceforth, a c (short for carbohydrate) is added for a reason that will become apparent (Table 10). They are unambiguously defined for simple sugars. The backbone is given by the carbon chain the reference conformation is the conformation with all carbon atoms eclipsed. This statement is identical with the requirement of a Fischer projection. If two identical substituents X are located on the same side/opposite sides of the plane traced by the backbone, then the c-erythrojc-threo configuration is assigned. 

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. 

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