Fischer projections defined

The aldotetroses are the four stereoisomers of 2 3 4 trihydroxybutanal Fischer pro jections are constructed by orienting the molecule m an eclipsed conformation with the aldehyde group at the top The four carbon atoms define the mam chain of the Fischer projection and are arranged vertically Horizontal bonds are directed outward vertical bonds back... 

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. 

Sometimes the terms erythro and threo are used to specify fee relative configuration of two adjacent stereogenic centers. The terms are derived fom fee sugars erythrose and threose. The terms were originally defined such feat a Fischer projection formula in which two adjacent substituents were on the same side was fee erythro isomer and feat in whidi the substituents were on opposite sides was the threo isomer. 

The (I ,S)-nomenclature still reminds the user of the right and left handed helical pattern arising from Fresnel s 29> interpretation of optical activity. These patterns are characterized by the combination of a translational and a rotational direction. The Ta skeletal symmetry of tetracoordinate systems submits itself to the pictorial models not applicable to other configurational types. The CIP rules may as well be used to define a configurational nomenclature on the basis of the Fischer projection. If one specified that in such a projection of an (R)-... 

Cyclic olefin monomers such as indene, benzofuran, and so on, can give rise to two diisotactic polymers eiythro and threo, illustrated in the Fischer projection in 26 and 27, and to two different disyndiotactic polymers 28 and 29 (Scheme 5), as each monomeric unit is clearly defined and quite distinct from its neighbors (58, 61). For polymers of this type the terminology eiythro and threo is used also. 

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. 

The stereodescriptor erythro indicates that the chlorine atom and the hydroxy group lie on the same side of the main chain in a Fischer projection formula. Since it does not define an absolute configuration, two enantiomers must be considered. The Fischer projection formula, which represents an eclipsed conformation, is simplest first converted into a sawhorse projection (also in its eclipsed conformation) and then one side of the molecule rotated until both the reference groups, the chlorine atom and the hydroxy group, adopt an antiperiplanar arrangement. The required Newman projection formula can then be derived from these formulae. 

An efficient and large scale resolution of methylphenidate (ritalin hydrochloride)using dibenzoyl-tartaric acid has been described (61). Ritalin is marketed for the treatment of children with attention deficient disorder (ADHD). Methylphenidate has two chiral centers and originally was marketed as a mixture of two racemates, 20% OL-threo (29, 30) and 80%DL-erythro (31, 32) (see Fig. 18.12 for the structures of all four isomers). As introduced previously, the erythro-isomer is defined as the case when the main chain of a molecule (drawn vertically in a Fischer projection)has identical or similar substituents at two adjacent non-identical chiral centers on the same side of the chain, whereas the threo isomer has... 

Terms in bold are defined glycoconjugate 238 monosaccharide 238 oligosaccharide 238 disaccharide 238 polysaccharide 238 aldose 239 ketose 239 Fischer projection formulas 240 epimers 240... 

In carbohydrates, the P-anomer has the Ci-hydroxyl or alkoxyl group on the opposite side of a Fischer projection as the substituent ( ) that defines D or L... 

Unfortunately, there is more than one definition for threo- and erythro. Winstein and co-workers gave the following definition of these diastereomers In a compound with two asymmetric carbons that has two common ligands and a third that differs, the isomers that would be meso if the third ligand were identical are erythro diastereomers.An alternative definition is If two asymmetric carbons have only one ligand in common, then the other four ligands are paired in the same commonsense way and isomers that would have equal pairs eclipsed in any conformation are erythro. Eliel, Mislow and their co-workers defined erythro and threo in terms of Fischer projections. The aldol products (see sec. 9.4.A) 68 and 69, and the ester-aldehyde condensation products 70 and 71.30 shown with the erythro and threo notation. 

Figure 3-6. The configuration of heptamers of propylene, each having one isopropyl and one isobutyl end group. The molecule has two equal end groups with respect to configurational analysis (structural analysis), but not with respect to the polymerization (process analysis). The configuration is given in the Fischer projection (see below). The arrows point to pseudoasym-metric carbon atoms. R and S are used in the sense as defined by organic chemistry configurational analysis.

Formally, glucose can be thought of as a hexanal, a 6-carbon aldehyde, that has five hydroxyl groups, one attached to each of the carbons other than the carbonyl. It is often written in the form of a Fischer projection (Structure 1, center structure) to define its stereochemistry. 

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