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 block3aand two new acyclic CB[n]-type receptorsP1andP2. P1(9 mM) andP2(11 mM) have moderate aqueous solubility but their host center dot guest complexes are poorly soluble. HostP1does not undergo intermolecular self-association whereasP2does (K-s = 189 +/- 27 M-1).(1) H NMR titrations show thatP1andP2are poor hosts towards hydrophobic (di)cations6-11(P1: K-a = 375-1400 M-1;P2: K-a = 1950-19,800 M-1) compared toTet1andTet2(Tet1: K-a = 3.09 x 10(6)to 4.69 x 10(8) M-1;Tet2: K-a = 4.59 x 10(8)to 1.30 x 10(10) M-1). Molecular modelling shows thatP1andP2exist 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 ofP1andP2do not feature a well-defined central cavity. In the presence of guests,P2adapts its conformation to form 1:1P2 center dot guest complexes; the binding free energy pays the energetic price of conformer selection. This energetically unfavourable conformer selection results in significantly decreased K(a)values ofP1andP2compared toTet1andTet2.