Synthesis and properties of new lipophilic macrotricyclic cylindrical cryptands

Cylindrical cryptands 2a-c, in which two 1,7-dioxa-4,10-diazacyclododecane rings are connected by two equally substituted propylene bridges, have been obtained in appreciable yields by a 'one-pot' synthesis. The assembling of the macrotricyclic structure is likely driven by the template effect of metal cations. These compounds, both as free receptors or as complexes, exist as cis and trans diastereoisomers, which do not interconvert and have been separated and characterized by X-ray analysis. The extraction constants (K(e)) of cryptands 2 for alkali picrates under CHCl3/H2O and solid/liquid two-phase conditions have been measured by UV-vis spectrophotometry. The complexation behavior of cryptands 2 has been rationalized analyzing the preorganization of binding sites in the minimum energy conformations obtained by molecular mechanics calculations. Minimum energy conformations have been calculated also for the previously reported cryptands 1 and have been compared with those of 2. Results fit reasonably well with those of X-ray structures.