Cyclic tetraureas with variable flexibility--synthesis, crystal structures and properties.

Macrocyclic molecules containing several amide or urea functions may serve as anion receptors. We describe the synthesis of 32-membered macrocycles, in which four rigid xanthene units (X) and/or diphenyl ether units (D) as flexible analogues are linked via urea groups. All six possible combinations of these units (XXXX, XXXD, XXDD, XDXD, XDDD and DDDD) were synthesized and two examples were characterised by single-crystal X-ray analyses (DDDD and two structures for XXXD).