INTRODUCTION.
Some years ago [1] was reported a new
series of lipophilic cylindrical cryptands 2a-b in which two
1,7- dioxa-4,10- diazacyclododecane rings are linked together in the 4
and 10 positions by one propylene and one ethylene bridge. These
compounds show the following properties:
- they display three cavities: two lateral (the twelve membered
macrocyclic subunits) and one central which is a twenty membered
macrocycle;
- the lipophilicity is ensured by a benzyl group (2a) or a
long alkyl chain (2b);
- the preorganization of the eight binding sites (four nitrogens and
four oxygens) is dictated by the rigidity of the cage due to the
presence of two short bridges;
- compound 2a forms only mononuclear complexes with alkali
metal cations showing a very high selectivity for Na+ (Table 1).
