Adaptive Encapsulation of ω-Amino Acids and Their Guanidinium-Amide Congeners

The binding and the encapsulation of the 6-aminohexanoic acid (1) and 11-aminoundecanoic acid (2) are achieved in aqueous solution and in crystalline Pyrene-box cages. Unexpectedly, the amino-guanidinium AG⁺ and the amino acids 1 or 2 are reacting in aqueous solution in the absence and in the presence of Pyrene-box cages. The formation of an amide bond between a carboxylic acid and the amino-guanidine unit under mild acidic conditions in water without the use a coupling reagent is extremely interesting and unexpected. The resulted adducts AG1 and AG2 show adaptive binding behaviors and compressions.