C-13 NMR ASSIGNMENTS OF BRASSINOSTEROIDS BY 2-DIMENSIONAL TECHNIQUES

H-1 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 H-1 resonances, the C-13 signals were assigned by a combination of C-H COSY and long-range C-H COSY spectra detecting C-13 nuclei, or by HMQC and HMBC spectra detecting H-1 nuclei. Although the steroids showed complex H-1 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 C-13 NMR resonances of this class of compounds.