Intramolecular reactions amino group replacement

Another type of reaction leading to the incorporation of the Co-C alkyl chain inside the salen ligand set has been developed by Dreos et al. If the dimer 115 was left for a longer time in contact with the reaction mixture, an intramolecular reaction occurred between the chloromethyl group and the equatorial chelate. The unidentate ligands on cobalt can be replaced quantitatively by Af-Melm, leading to a complex 116 in which the / -f -geometry was maintained. If chiral amino acids were used, the diastereoisomers 117 could be separated (Scheme 14). ... 

General acid catalysis is schematized in Fig. 7J,b. Here, an acid A-H increases the polarity of the carbonyl group and, hence, the electrophilicity of the carbonyl C-atom. For entropy reasons, the reaction is greatly facilitated when it is an intramolecular one (Fig. 7J,b2), in other words, when the general acid catalyst is favorably positioned within the molecule itself. Such a mechanism is the one exploited and refined by nature during the evolution of the hydrolases, with the general acid catalyst and the H20 molecule replaced by adequate amino acid side chains, and the enzymatic transition state being de facto a supermolecule (see Chapt. 3). 

Irrespective of whether solution phase or SPPS is practiced, if Boc amino acids are used with monoamino dicarboxylic acids, then the side-chain carboxyl usually is protected as its Bzl ester. Because of the potential for intramolecular succinimide formation with Asp(Bzl) peptides, however, the replacement of Bzl with a cyclohexyl (cHex) ester reduces this undesirable side reaction (14). On the other hand, if Fmoc amino acids are used, then the side-chain carboxyl usually is protected as the Bu ester. Similarly, when using Boc-Ser and Boc-Thr, the hydroxyl function is protected as the Bzl ether. In the case of Tyr, however, the Bzl ether is not stable enough during repeated TFA treatments needed to cleave Boc groups. Therefore, in the case of... 

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