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The technique previously described as two‐bond heteronuclear selective NOE difference was improved to yield quantitative data on NOE enhancement factors by recording separately the unperturbed and the perturbed FIDs. In this way the E and Z stereoisomers of 4(α‐arylethylidene)‐2‐phenyl‐5(4H)‐oxazolones were easily distinguished. The latter showed positive NOE ehancements on quaternary carbons placed two, three and four bonds away from the irradiated methyl protons, as well as a negative NOE on a nearby protonated carbon.
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Cativiela et al. (1985) studied this question.
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