Stereochemistry at Tetrahedral Carbon Atoms

1 Determination of the Permutation Descriptor after Rotation of the Molecule 

In the example, we proceed from the right (rotated molecule) to the left (reference molecule). The first transposition is done with ligands 4 and 2, in order to obtain the fir.st part of the reference. scqttencc 1 2 . Then, only a pcrmtttation of ligands 3 and 4 has to be done to obtain the reference matrix on the left-hand side. Thus, in total, we have executed two transpositions (4 2) and (3 4), 

The two structures in our example are identical and are rotated by only 1 20 h Clearly, rotation of a molecule docs not change its stereochemistry, Thus, the permutation descriptor of both representations should be (+ I). On this basis, we can define an equation where the number of transpositions is correlated with the permutation descriptors in an exponential term (Eq. (9)). 

Once again, in the case of rotation we have had two transpositions, resulting in a permutation descriptor of (-h 1), a result as desired (-1) + L) = (-h 1). 

Two steroids with the same constitution should be checked to see if they are stereoisomers (Eq. (10). 

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