2-(Guanidiniocarbonyl)furans as a new class of potential anion hosts: Synthesis and first binding studies

The synthesis of a new class of potential anion hosts, the 2-(guanidiniocarbonyl)furans 5a-d, is presented in this study. The facile decomposition of furans under acidic conditions makes the synthesis of these compounds challenging. First binding studies showed that the (guanidiniocarbonyl)furans are much more acidic (pK(a) approximate to 5.5) than the analogous pyrroles (pK(a) approximate to 7) previously introduced by us for oxoanion binding in aqueous solvents. Hence, anion binding with the furan derivatives occurs only in acidic solutions below pH = 5. Therefore, carboxylates are not bound efficiently, whereas, for example, the less basic hydrogen sulfate is bound by 5b with an association constant of K = 600 m(-1) in aqueous DMSO even in the presence of a large excess of buffer. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).