A new class of bambusurils (BUs) 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, 5a, clearly observed by X-ray spectroscopy analysis, determines the overall symmetry of the bambusuril structure. Chiral Pr4Me4BU, 4b, was resolved by chiral high-performance liquid chromatography (HPLC) into its enantiomers, and all four inherently chiral bambusuril pairs (two Pr4Me4BU and two Bn4Me4BU 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.
Bibliographical noteFunding Information:
Financial support for this work was provided by the Ministry of Science Technology and Space (MOST grant no. 3-10855). We thank Dr. Natalia Fridman from the crystallographic lab at the Schulich Faculty of Chemistry, Technion, for her valuable contribution to the X-ray crystallographic efforts. We thank Prof. Doron Pappo from the Department of Chemistry, Ben-Gurion University of the Negev, for providing optically pure ( R)-BINOL II – V derivatives for this study. We also thank Prof. N. Gabriel Lemcoff for his valuable comments on this manuscript.
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