Libraries of permutational isomers

14 Example (Libraries of amidations of a xanthene) Recall the skeleton used in Chapter 3, xanthenetetracarboxylic acid chloride  

In one exception, for proline, the reacting group is an imino (NH) group. To also include this reaction, only one hydrogen atom is included in the reaction scheme  

Bonds to be formed in the reaction are meuked with small dots bonds to be broken 

Initially, the reactants are searched for these substructures. The acid chloride substructure is found four times in the skeleton of M, these are the positions open for substitution . The a-imino acid substructure is found in each amino acid exactly once. Taking into account the numbering (labeling) of the positions open for substitution in the skeleton of M, there are 7 possibilities to substitute y different amino acids to these four positions. Some of these possibilities may be equivalent due to the point 

3 = 4 was the set of (labels of) the positions open for substitution, the set of numbers of the functional groups (C(= 0)CI), while Y indicated a set of amino acids. We assumed that Y = i) Y y, where Y, the set of achiral substituents, is either empty or it consisted of glycine only, while the set of chiral substituents was supposed to consist of pairs y, y of enantiomorphic amino acids. 

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