MOLECULAR RECOGNITION AND PHASE-TRANSFER OF UNDERIVATIZED AMINO-ACIDSBY A FOLDABLE ARTIFICIAL HOST

A novel molecular host 5 for amino acid zwitterions is described. The covalent connection of a chiral bicyclic guanidinium salt as an anchor function for the carboxylate and a triaza-crown ether as an ammonium binding moiety in combination with a strongly hydrophobic silyl ether serves to complex and transfer amino acids from water into dichloromet hane with unprecedented efficiency. Quantitation of the extraction pro cess by radiometry revealed a clean 1:1 stoichiometry and gives eviden ce for the zwitterion as the species undergoing phase transfer. Even s mall hydrophilic (Ser, Gly) but no charged amino acids were extracted better by factors up to 3000-fold than by a previously reported host o f similar design (Galan, A.; Andreu, D.; Echavarren, A. M.; Prados, P. ; de Mendoza, J. J. Am. Chem. Sec. 1992, 114, 1511-1512). Some enantio selection in the transfer of phenylalanine was observed (40% ee).