Cahn-Ingold notation

The absolute configuration of tetrahydrofolate (in the Cahn-Ingold notation, 91) is S at C6 (92, 93), but if dihydrofolate is bound in the same orientation as methotrexate, the configuration should be R. When dihydrofolate (or tetrahydrofolate) is bound in the enzyme in the same way as methotrexate, the hydrogen atom that is transferred to NADPH points in the wrong direction. Thus, dihydrofolate and methotrexate must bind to the enzyme with different orientations as established by X-ray crystallographic studies. The binding of folate and methotrexate are shown in Fig. 38. 

Absolute Configuration According to the Cahn-Ingold-Prelog Notational System... 

The Cahn-Ingold-Prelog R-S notation has been extended to chiral allenes and other molecules that have a chiral ity axis Such compounds are so infrequently encountered however we will not cover the rules for specifying their stereochemistry in this text... 

Fischer projections and d-l notation have proved to be so helpful m representing carbohydrate stereochemistry that the chemical and biochemical literature is replete with their use To read that literature you need to be acquainted with these devices as well as the more modern Cahn-Ingold-Prelog R S system... 

Absolute Configuration Using Cahn-Ingold-Prelog Notation (Table 7 1, p 269)... 

Cahn-Ingold-Prelog notation (Section 7.6) System for specifying absolute configuration as / or S on the basis of the order in which atoms or groups are attached to a chirality center. Groups are ranked in order of precedence according to rules based on atomic number. 

A general method of stereochemical notation based on the ligand priority sequence rule of Cahn, Ingold, and Prelog (CIP),8,9 developed at Chemical Abstracts Service,0 1 has been in use in Chemical Abstracts Chemical Substance Indexes for mononuclear coordination compounds since 1972. 

The enantiomers shown are related as a right-hand and left-hand screw, respectively. Chiral allenes are examples of a small group of molecules that are chiral, but don t have a chirality center. What they do have is a chirality axis, which in the case of 2,3-pentadiene is a line passing through the three carbons of the allene unit (carbons 2, 3, and 4). The Cahn-Ingold-Prelog R-S notation has been extended to chiral allenes and other molecules that have a chirality axis. Such compounds are so infrequently encountered, however, we will not cover the rules for specifying their stereochemistry in this text. 

Applying the Cahn-Ingold-Prelog notational system described in Section 7.6 to the product, the order of decreasing precedence is... 

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