Chiral chemical shift reagents

Chiral chemical shift reagents for NMR analysis are also useful, and so are optical methods. 

Achiral lanthanide shifting reagents may be used to enhance the anisochrony of diastereomeric mixtures to facilitate their quantitative analysis. Chiral lanthanide shift reagents are much more commonly used to quantitatively analyze enantiomer compositions. Sometimes it may be necessary to chemically convert the enantiomer mixtures to their derivatives in order to get reasonable peak separation with chiral chemical shift reagents. 

Scheme 1-3. Determining enantiomer composition with chiral chemical shift reagent 18.

Fig. 8. Below 100 MHz proton nuclear magnetic resonance spectrum of the trimethyl-silyl region of 5-(+)-benzyl-a-OH.

The same principle is involved in the use of chiral lanthanide chemical shift reagents for the determination of enantiotopic purity [44]. 

Analysis of Reagent Purity chemical purities are accessed by mp and NMR optical purities are deduced by forming diastereo-meric derivatives, or by derivatization and analysis in the presence of chiral NMR shift reagents. 

It has been long appreciated that a chiral environment may differentiate any physical property of enantiomeric molecules. NMR spectroscopy is a sensitive probe for the occurrence of interactions between chiral molecules [4]. NMR spectra of enantiomers in an achiral medium are identical because enantiotopic groups display the same values of NMR parameters. Enantiodifferentiation of the spectral parameters (chemical shifts, spin-spin coupling constants, relaxation rates) requires the use of a chiral medium, such as CyDs, that converts the mixture of enantiomers into a mixture of diastereomeric complexes. Other types of chiral systems used in NMR spectroscopy include chiral lanthanide chemical shift reagents [61, 62] and chiral liquid crystals [63, 64). These approaches can be combined. For example, CyD as a chiral solvating medium was used for chiral recognition in the analysis of residual quadrupolar splittings in an achiral lyotropic liquid crystal [65]. 

Hetero Diels-Alder reactions are a good entry for the preparation of intermediates for total synthesis of sugars. The lanthanide catalysts allow to work with labile reactants and adducts with an excellent stereo-chemical control. Moreover Danishefsky found (56) that a chiral europium shift reagent (Eu(hfc) ) is able to catalyze asymmetric cycloaddition (eq. 40] ). ... 

Determination of the enantiomeric composition of natural semiochemicals by NMR spectroscopy using a chiral lanthanide shift reagent is a rapid, accurate, and ion-destmctive technique. It is also a quantitative method for detection of adulteration of essential oils and oleoresins by synthetic racemic chemicals. 

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