Total assignment of the ¹H and ¹³C chemical shifts for a mixture of <i>cis</i>‐ and <i>trans‐p</i>‐hydroxycinnamoyl esters of taraxerol with the aid of high‐resolution, ¹³C‐detected, ¹³C¹H shift correlation spectra

Abstract

Abstract A mixture of two slowly interconverting isomeric compounds was isolated from Bernardia laurentii and identified as the trans ‐ and cis‐p ‐hydroxycinnamoyl esters of the triterpene taraxerol. Their 13 C and 1 H NMR spectra were totally assigned with the aid of concerted two‐dimensional NMR experiments. Not all assignments could be made using HMQC and HMBC owing to the limited 13 C resolution and severe spectral crowding. However, the assignments could be made unambiguously by HETCOR and FLOCK experiments, relying on the superior 13 C resolution of the 13 C‐detected experiments. Comparison of 13 C spectral assignments for a series of eight pentacyclic triterpenes with identical A and B ring structures but differences in C, D and/or E rings illustrates the risks and limitations of the use of model compounds for spectral assignments.