Cobalt -amino acid ester chelates synthesis

The rapid aminolysis of cobalt(llI)-chelated glycine esters in aprotic solvents (Scheme 10 N4 = (en)2 or trien, R = Me, Et, R = H, CHR"C02Et) could be of value in peptide synthesis. The cobalt atom acts as both an N-protecting and an activating group. The synthesis of the chelated amino acid esters has presented some difficulties.207 A recent paper208 describes the use of methyl trifluoromethanesulfonate for the alkylation of chelated amino acids using dry trimethyl phosphate... 

The cobalt(III)-promoted hydrolysis of amino acid esters and peptides and the application of cobalt(III) complexes to the synthesis of small peptides has been reviewed. The ability of a metal ion to cooperate with various inter- and intramolecular acids and bases and promote amide hydrolysis has been investigated. The cobalt complexes (5-10) were prepared as potential substrates for amide hydrolysis. Phenolic and carboxylic functional groups were placed within the vicinity of cobalt(III) chelated amides, to provide models for zinc-containing peptidases such as carboxypeplidase A. The incorporation of a phenol group as in (5) and (6) enhanced the rate of base hydrolysis of the amide function by a factor of 10 -fold above that due to the metal alone. Intramolecular catalysis by the carboxyl group in the complexes (5) and (8) was not observed. The results are interpreted in terms of a bifunctional mechanism for tetrahedral intermediate breakdown by phenol. 

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