Conformationally Mobile Acyclic Cucurbit[n]uril-Type Receptors Derived from an S-shaped Methylene Bridged Glycoluril Pentamer.

We report the synthesis of the conformationally mobile S-shaped glycoluril pentamer building block and two new acyclic CB[n]-type receptors and . (9 mM) and (11 mM) have moderate aqueous solubility but their host•guest complexes are poorly soluble. Host does not undergo intermolecular self-association whereas does (K = 189±27 M). H NMR titrations show that and are poor hosts toward hydrophobic (di)cations - (: K = 375-1400 M; : K = 1950-19800 M) compared to and (: K = 3.09 × 10 to 4.69 × 10 M; : K = 4.59 × 10 to 1.30 × 10 M). Molecular modelling shows that and exist as a mixture of three different conformers due to the two S-shaped methylene bridged glycoluril dimer subunits that each possess two different conformations. The lowest energy conformers of and do not feature a well-defined central cavity. In the presence of guests, adapts its conformation to form 1:1 •guest complexes; the binding free energy pays the energetic price of conformer selection. This energetically unfavorable conformer selection results in significantly decreased K values of and compared to and .