Aromatic ring current
Analytical Chemistry and Spectroscopy
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The synthesis of a novel chiral discriminating reagent, the ester resulting from the reaction of L-menthylphenylphosphoryl chloride with ethanol, is reported. This reagent, C, is aimed to differentiate the R and S monodeuterated enantiomers of ethanol, which constitutes an important isotopic probe for mechanistic studies and origin inference of natural products. The proportions of the two P-epimeric structures, CA and CB, which compose reagent C are directly accessible by 31P NMR. Analysis of 1D and 2D NMR spectra and theoretical simulations enable the 1H and 13C chemical shifts of CA and CB to be determined. The chemical shift differences exhibited by the methylenic hydrogens of the ethoxy moiety in CA and CB are of the order of 0.3 ppm. These large differences can be mainly attributed to ring current effects. Such effects have been estimated using two different methods on the basis of model conformations of the two epimers, CA and CB, deduced from energy minimization in a quantum mechanical approach. The relative signs of the chemical shift differences δR(C) - δS(C) associated with the pro-R and pro-S positions in the CA and CB isomers cannot be determined from the proton spectrum. The ring current calculations suggest that δR(C) - δS(C) are of opposite signs in the two P-epimers. This interpretation, and consequently the conformational analysis, are supported by the 2H NMR study of a chiral discriminating ester prepared from ethanol selectively enriched in deuterium in the pro-R position by stereospecific exchange reactions involving the enzymatic pair alcohol dehydrogenase-diaphorase. The chiral discriminating reagent, C, was then used for determining the enantiomeric imbalance in ethanol samples obtained from biosynthesis under different conditions, and site-specific fractionation factors were calculated.
Type of Medium: