¹³C NMR assignments of brassinosteroids by two‐dimensional techniques

Abstract

Abstract 1 H NMR spectra of eight naturally occurring brassinosteroids, plant hormones with a steroid skeleton, and five synthetic analogues were analysed by COSY and long‐range COSY measurements. Based on the 1 H resonances, the 13 C signals were assigned by a combination of CH COSY and long‐range CH COSY spectra detecting 13 C nuclei, or by HMQC and HMBC spectra detecting 1 H nuclei. Although the steroids showed complex 1 H signals around 1–2 ppm, the resonances for the two angular methyl groups and the other methyl groups in the side‐chain could be assigned unambiguously and used as a reference point for the other assignments. Strong correlation peaks between the methyl protons and carbons connected with the protons through two or three bonds were observed in the long‐range CH COSY or the HMBC spectra. These spectra provide valuable information for assigning the 13 C NMR resonances of this class of compounds.