CONFORMATIONALLY SWITCHED-ON POLYETHER IONOPHORES

The syntheses of two types of conformationally switched podand ionopho res and their ionophoric properties are described. Both feature cycloh exane rings with polyether groups as trans 1,2 substituents. In the '' switched off' forms of the ionophores, the two podand substituents are constrained to a diaxial orientation and cannot chelate a metal ion. In both cases, a chemical reaction, hydrolysis of a ketone acetal, is used to remove the constraint allowing the two podand substituents to achieve a diequatorial orientation. In this conformation, the two dieq uatorial podand substituents can chelate a potassium ion and the ionop horic properties are ''switched on.'' In one case, the chains can be h eld in the diaxial orientation by a large group, an acetyle group deri vatized as the ethylene glycol acetal, in the 4 position. When the siz e of the group is lowered, the polyether groups become equatorial and can complex a potassium ion as evidenced by a conformational change me asured by low temperature NMR spectroscopy. In the second example, an annulated ring holds the cyclohexane ring rigidly in the non-complexin g conformation. When the restraint is removed, complexation can occur as evidenced by transport of potassium picrate through a liquid membra ne (chloroform layer). In both cases, the ionophoric properties are '' switched on'' by hydrolysis of a ketone acetal. (C) 1998 Wiley-Liss, I nc.