Enantiopure Alleno-Acetylenic Cage Receptors for Molecular Recognition in Aqueous Medium

The recognition of small molecules in hydrophobic cavities in aqueous medium is a fundamental process in molecular biology. Previously, we described the synthesis and conformational switching of enantiopure alleno-acetylenic cage receptors as well as their application as model systems to study in detail the interplay of space occupancy, conformation, complexation, and chiral recognition in organic solvents. Now, we report the synthesis of three enantiopure alleno-acetylenic cage receptors functionalized with hydrophilic groups at their surface, which allows an extension of the studies to aqueous media. The new receptors show solvent- and guest-induced binary conformational switching between an 'open' and a 'closed' cage conformation. Importantly, the chiroptical properties of the alleno-acetylenic receptors are strongly conformation-dependent, an effect that manifests itself upon guest inclusion. We used electronic circular dichroism titrations to quantitatively study the binding of small organic molecules inside the hydrophobic receptor cavity in methanolic water.