Metal Complexes with Macrocyclic Ligands. Part XXXII. Reactivity studies of the pendant carboxylic group in a macrocyclic Cu²⁺ complex towards amide formation and its use as a protein‐labelling agent

The Cu 2+ complex of 1 , having a non‐coordinating carboxylic group, can be reacted under the typical conditions of peptide formation with amines such as 2‐methylpropylamine or (pyrid‐2‐yl)methylamine to give, after removal of Cu 2+ with CN − , the amides 4 and 5. The Cu 2+ ion is of paramount importance since it protects the four amino groups of the macrocycle so that the amide condensation can specifically be done with the exogenous amine It is also shown that the Cu 2+ complex of 1 can be covalently attached to bovine serum albumin (BSA), thus opening the possibility to use this compound as a labelling agent for proteins and antibodies.