41 Application of a Shift Reagent to the Assignment of the Methyl NMR Resonances of Triterpenes Related to Serratenediol
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Simplification of the NMR spectra of triterpenes related to serratenediol using Eu(DPM)_3 as a shift reagent was examined, and we observed upfield and downfield shifts in the same molecule. This is an example of the importance of the angle dependence term in the equation of McConell and Robertson. The effectiveness of the reagent in aiding the assignment of the methyl resonances in the spectra of the triterpenes was also studied. The shift parameter of a particular methyl resonance in the spectrum of a polyacetoxyl compound will be predicted by the sum of the shift parameter related to the respective complexing sites which is experimentally obtained from the model compound, if the difference in the association probability of the reagent to the complexing site is not observed between the complexing site being present independently and that present together with an additional complexing site. That is not the case and when applied the shift parameter obtained from the model compound to the spectrum of the polyacetoxyl compound, multiplication of the shift parameter by the association coefficient is needed. Although this quantitation method is very appropriate, the close agreement between the calculated and observed shift parameter values suggests that this treatment will facilitate the assignment of the methyl signals in the spectra of the polyfunctional triterpenes.
