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.研究人员设计并合成了一类新型竹脲[4]环番（bambus[4]urils, BU[4]s），其结构单元为带有不同烷基基团的不对称N,N'-二取代甘脲（glycoluril）亚基，并通过核磁共振（NMR）技术与X射线晶体学对其进行了全面表征。结构研究表明，该类大环存在四种非对映异构体：两种具有Sn对称性的非手性大环，以及两种“固有手性”大环。通过X射线光谱分析明确观测到的Bn4Me4BU[4]（化合物5a）中N-取代基的相对“头-尾”排列方式，决定了竹脲大环结构的整体对称性。手性Pr4Me4BU[4]（化合物4b）通过手性高效液相色谱（HPLC）拆分得到其对映异构体；以(R)-联萘酚（(R)-BINOL）作为手性溶剂化试剂，通过1H核磁共振（1H NMR）光谱可清晰观测到全部四组固有手性竹脲对映体，包括两组Pr4Me4BU[4]与两组Bn4Me4BU[4]立体异构体（即4b、4d、5b与5d）。该分析方法可为固有手性穴状主体（cavitands）的对映体纯度检测提供一种快速高效的手段，可作为传统色谱分析方法的补充与拓展。