Molecular dynamics and NMR analysis of the configurational ¹³C assignment of epimeric 22,23-epoxides of stigmasterol

The determination of the stereochemistry of brasinosteroid analogs with 22,23-epoxide groups can be easily achieved by means of ¹³C NMR spectroscopy. Here, we provide a rationalization of the ¹³C chemical shift pattern found in 22R,23R- and 22S,23S-epoxides of stigmasterol, based on the analysis of γ effects. (22S,23S)and (22R,23R)-3β-acetoxystigmast- 22,23-epoxy-5,6β-diol were used in the study as model compounds. Our methodology starts with a conformational search by means of molecular dynamics and NMR (NOE contacts) spectroscopy, which is followed by the analysis of the different γ interactions affecting the chemical shift of interest. We demonstrate that the differences between the ¹³C chemical shift patterns of 22R,23R and 22S,23S isomers arise from γ effects as the result of diverging local conformations around the C₁₇-C₂₀ and C₂₀-C₂₂ bonds.