Water-driven ligations using cyclic amino squarates: a class of useful SN1-like reactions.

We report a class of water-soluble and -stable cyclic amino squarates that ligate with cysteine or lysine residues without side-products in an entirely aqueous environment. The ligations include addition-elimination reactions that are promoted by water in a way similar to S(N)1 reactions. The structural versatility of the reactants allows the potential recognition of selected amino acid residues on proteins.