Activated esters, of carboxylic acids

Another general method for the synthesis of these macrocycles is the reaction of amines with (active) esters of carboxylic acids leading to oxoderivatives of the cycles. The amides can be reduced to amines or directly used for further transformations or complexation of metal ions. Two cyclens 55 and 56, as intermediates for the synthesis of bifunctional DOTA derivatives, were obtained by condensation shown in Scheme 10 between appropriate diamine-amide and active ester of AT-BOC-iminodiacetic acid (BOC = /-butoxycarbonyl), followed by deprotection and reduction <2003NMB581>. 

---