Squaric acid diethylester : a simple and convenient coupling reagent

A new method for the covalent modification of polymers in a two-step process is described. Squaric acid diethylester reacts with primary or secondary aliphatic amines selectively to ester amides. It is shown for the first time that this class of compounds react with polymeric systems containing amine functionalities (e.g. e-amino groups of proteins) in aqueous solution, forming stable squaric acid diamide linkages. Three different monoamides were synthesized and the reaction rates of diamide formation versus hydrolysis were determined. Detailed kinetic studies revealed that unwanted hydrolytic side-reactions are more than ten times slower than the formation of squaric acid diamides