Size-tunable trehalose-based nanocavities: synthesis, structure, and inclusion properties of large-ring cyclotrehalans.

An efficient strategy toward the synthesis of large-ring cyclodextrin (CD) analogs alternating alpha,alpha'-trehalose disaccharide subunits and pseudoamide segments (cyclotrehalans, CTs), involving a bimolecular macrocyclization reaction as the key step, is reported. NMR and molecular modeling confirmed that the eight and ten alpha-d-glucopyranoside subunits in tetrameric and pentameric CT homologues (CT4 and CT5, respectively) are magnetically equivalent, as in the gamma and epsilonCD counterparts. Yet, the orientation of the monosaccharide constituents is reversed in CTs as compared with CDs, the beta-face being directed to the inside of the nanometric cavity while the alpha-face remains in contact with the bulk solvent. Molecular mechanics and dynamics experiments revealed that the cyclooligosaccharide architecture in CT4 and CT5 is relatively flexible, which is in contrast to that previously observed for the first members of the CT series (CT2 and CT3 oligomers). Thus, although in their fully expanded conformation their cavity size is close to that of gammaCD, the higher mobility of the pseudoamide bridges as compared with classical glycosidic linkages endows these hosts with induced fitting capabilities toward smaller guests.