Modification and optimization of the bis-picolylamide-based relay protection for carboxylic acids to be cleaved by unusual complexation with Cu2+ salts.

A simple modification of our recently published protection scheme for carboxylic acids as amides resulted in a new protecting group with significantly improved properties. It requires shorter reaction times for deprotection and allows us to replace Cu(OTf)(2) by CuCl(2), indicating at the same time the importance of the nature of the anion of the Cu(2+) source. Since the new scheme fulfills all criteria required for an ideal protection group it should find widespread application in synthetic organic chemistry.