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Monitoring site‐specific natural hydrogen isotope fractionation using a novel phosphorus‐containing chiral discriminating reagent
Author(s) -
Hägele Gerhard,
Boisnière Bruno,
Helie Isabelle,
Rabiller Claude,
Martin Gérard J.,
Martin Maryvonne L.
Publication year - 1991
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260290812
Subject(s) - chemistry , reagent , moiety , chemical shift , epimer , enantiomer , proton nmr , nmr spectra database , stereochemistry , carbon 13 nmr , ring (chemistry) , computational chemistry , spectral line , organic chemistry , physics , astronomy
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, C A and C B , which compose reagent C are directly accessible by 31 P NMR. Analysis of 1D and 2D NMR spectra and theoretical simulations enable the 1 H and 13 C chemical shifts of C A and C B to be determined. The chemical shift differences exhibited by the methylenic hydrogens of the ethoxy moiety in C A and C B 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, C A and C B , deduced from energy minimization in a quantum mechanical approach. The relative signs of the chemical shift differences δ R(C) —δ S(C) associated with the pro‐ R and pro‐ S positions in the C A and C B isomers cannot be determined from the proton spectrum. The ring current calculations suggest that δ R(C) —δ S(C) are of opposite signs in the two P‐epimers. This interpretation, and consequently the conformational analysis, are supported by the 2 H 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.