Inherently Chiral Bambus[4]urils.

A new class of bambus[4]urils (BU[4]s) composed of asymmetric ,'-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 symmetric achiral macrocycles and two macrocycles that are "inherently" chiral. The relative "head-to-tail" arrangement of the -substituents in BnMeBU[4], , clearly observed by X-ray spectroscopy analysis, determines the overall symmetry of the bambusuril structure. Chiral PrMeBU[4], , was resolved by chiral high-performance liquid chromatography (HPLC) into its enantiomers, and all four inherently chiral bambusuril pairs (two PrMeBU[4] and two BnMeBU[4] stereoisomers, , , , and ) were clearly observed by H NMR spectroscopy with the aid of ()-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.