Inherently Chiral Bambus[4]urils

A new
class of bambus[4]­urils (BU[4]­s) composed of asymmetric N,N′-disubstituted glycoluril subunits
with different alkyl groups were designed, synthesized, and fully
characterized by NMR techniques and X-ray crystallography. Structural
studies showed that four macrocyclic diastereoisomers are possible:
two Sn symmetric achiral macrocycles and
two macrocycles that are “inherently” chiral. The relative
“head-to-tail” arrangement of the N-substituents in Bn4Me4BU­[4], 5a, clearly observed by X-ray spectroscopy analysis, determines the
overall symmetry of the bambusuril structure. Chiral Pr4Me4BU­[4], 4b, was resolved by chiral high-performance
liquid chromatography (HPLC) into its enantiomers, and all four inherently
chiral bambusuril pairs (two Pr4Me4BU­[4] and
two Bn4Me4BU­[4] stereoisomers, 4b, 4d, 5b, and 5d) were clearly
observed by 1H NMR spectroscopy with the aid of (R)-BINOL as a chiral solvating agent. This latter methodology
provides a rapid and powerful approach for investigating the enantiopurity
of inherently chiral cavitands, which complements and augments the
conventional chromatographic approaches.