Abstract
Chiral auxiliaries are routinely employed in the NMR spectroscopic discrimination of enantiomers in isotropic solutions. Most of the times the presence of impurities, superposition of peaks, enormous line broadening, and complex multiplicity pattern limit the NMR spectroscopic discrimination of enantiomers and determination of their composition. There are a few approaches to unravel the overlap of peaks. A simple approach is to utilize appropriate chiral auxiliary, which induces a large chemical shift difference between the discriminated peaks (Delta delta)(R/S), and minimize or unravel the overlap of peaks. In this direction, several new chiral auxiliaries and new chiral differentiation protocols have been introduced, which are both synthetic and naturally occurring molecules. An alternate spectroscopic approach is to design NMR experiments to circumvent inherent problems, viz., enhancing the spectral resolution, separation of the superimposed spectra of enantiomers, and reduction of the spectral complexity. A large number of NMR experiments, such as two-dimensional selective F-1 decoupling, RES-TOCSY, multiple quantum detection, frequency-selective homodecoupling, band-selective homodecoupling, and broadband homodecoupling called as pure shift NMR, to cite a few, are available for such a purpose. Many of these techniques have simplified the chiral analysis for molecules containing diverse functional groups in the presence of suitable chiral auxiliaries. The present chapter summarizes the work carried out in the authors' laboratory on chiral analysis. This includes the recently introduced new chiral auxiliaries, which are either synthesizable or natural products. They could be either derivatizing agents or solvating agents or the developed three-component protocols, in addition to new NMR experimental methodologies.