Tertiary aromatic amides bearing stereogenic centres ortho to the amide group may adopt two diastereoisomeric conformations which interconvert slowly on the NMR timescale at ambient temperature, and are therefore detectable by NMR. Certain classes of stereogenic centre—particularly sulfoxides, ephedrine-derived oxazolidines, and proline-derived imidazolidines—strongly bias the population of the two conformers. We propose a model, supported by molecular mechanics calculations, which rationalises the sense and magnitude of the conformational selectivity attained in terms of the steric and electronic properties of the controlling centre. The control over conformation may be exploited either by trapping the favoured conformer as an atropisomer, or by using it to relay information about the stereochemistry of the controlling centre.
具有立体中心的第三芳香酰胺在酰胺基团邻位可能采用两种不同的非对映异构体构象,这两种构象在环境温度下在核磁共振(NMR)时间尺度上缓慢互变,因此可通过NMR检测到。某些类型的立体中心,特别是亚
磺酸盐、源自
麻黄碱的
噁唑啉和源自脯
氨酸的
咪唑啉,显著影响两种构象的分布。我们提出了一个模型,并通过分子力学计算支持该模型,合理化了根据控制中心的立体和电子特性所获得的构象选择性的方向和大小。对构象的控制可以通过将偏好的构象捕获为旋转异构体,或通过利用它来传递关于控制中心立体
化学的信息来实现。