Configurational isomers, definition

Bridged binuclear, trinuclear and tetranuclear chelated octahedral structures were examined by Schaffer,17 who used the skew line helical definition for the chirality symbols A and A. The configurational isomers for a tetrakisbidentate, edge-fused-bis-octahedral structure are AA, AA, AA, and AA. 

Fig. (a) Butane (C4H]0) (b) 2-methlypropane (C4H10). Configurational Isomers — Alkenes and Cycloalkanes Definition... 

The term configurational isomer is a historic one that has no real value in modern stereochemistry. It is generally used to encompass enantiomers and diastereomers as isomers (see definitions for these below), but stereochemical isomers is a better term. The term con-... 

The appropriateness and essence of this distinction between absolute and relative configuration in this case may be illustrated by a hypothetical experiment. By definition, the determination of an absolute configuration involves a comparison with a fixed reference standard. Such a standard may be simulated by an enzyme that can discriminate between the isomers 16a and 16c, which differ in the configuration of the central atom. If the reacting isomer 16a is represented by (2/ ,3r,4S)-2,4-dihydroxy-3-((/ )-[2-2H]-propionoxy)glutaric acid, it can be predicted with confidence that the enzyme will also react with the enantiomer... 

It is called an a-amino acid because the amino group is attached to the a (or number 2) carbon atom. To indicate its three-dimensional structure on a flat piece of paper, the bonds that project out of the plane of the paper and up toward the reader are often drawn as elongated triangles, while bonds that lie behind the plane of the paper are shown as dashed lines. The isomer of alanine having the configuration about the a-carbon atom shown in the following structural formulas is called S-alanine or L-alanine. The isomer which is a mirror image of S-alanine is R-alanine or D-alanine. Pairs of R and S compounds (see Section B for definitions) are known as enantiomorphic forms or enantiomers. 

The fact which is significant to us is that the ground states of all isomers for particular numbers and types of atoms will lie on the same potential surface. Which isomer we have at any one time will be determined by which nuclear configuration the molecule possesses. It follows by definition that one isomer can be converted to another by merely surmounting the proper barrier (or barriers). 

An alternative and more rigorous treatment considers C-1 and C-3 as centres of pseudoasymmetry and assigns them the appropriate symbols r and y (actually and see definitions below). (Note that according to this treatment, the cw-isomer is ls, 3s and the trans-isomer lr ,3r i.e., changing the configuration at one centre changes both descriptors.) For a full explanation, see Eliel and Wilen, p. 667. 

Apparently the first definite assignment of a chair form to a six-membered, cyclic-acetal ring was made in order to explain the occurrence of only one stable isomer of methyl 4,6-0-benzylidene-,8-D-glucopyranoside, which will have the conformation L. The stable forms of 4,6-0-benzyli-dene derivatives of methyl /3-D-mannopyranoside, methyl 8-D-allopyrano-side, and methyl S-D-altropyranoside will differ from L only in the configurations at C2 and 03. "... 

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