Unprotected Galactosamine as a Dynamic Key for a Cyclochiral Lock

The discrimination of d-galactosamine (G), representative of the amino-sugar class of compounds, has been probed through nano-ESI-FT-ICR mass spectrometry by isolating the relevant [C·H·G]⁺ proton-bound complexes with the enantiomers of the cyclochiral resorcin[4]arene C and allowing them to react toward three primary amines (B = EtNH₂, iPrNH₂, and (R)- and (S)-sBuNH₂). The system under investigation presents several features that help to unveil the behavior of unprotected G in such a supramolecular architecture: (i) the hydrophobic derivatization of the C convex side forces the polar guest G to be coordinated by the cyclochiral concave region; (ii) protonated d-galactosamine exists as an anomeric mixture, dynamically interconverting throughout the experimental time-window; and (iii) different basicities of B allow the experiment to subtly tune the reactivity of the [C·H·G]⁺ complexes. Three [C·H·G]⁺ aggregate-types were found to exist, differing in both their origin and reactivity. The most reactive adducts ([C·H·G]_ESI⁺), generated in the electrospray environment, undergo a G-to-B ligand exchange in competition with a partial isomerization to the unreactive [C·H·G]_GAS⁺-type complexes. Finally, the poorly reactive [C·H·G]_SOL⁺ aggregates are formed in solution over an hours-long time scale. A cyclochirality effect on the reactivity was found to depend on the considered [C·H·G]⁺ aggregate-type.