Carbon‐13 NMR spectra of some pentacyclic triterpenoids with the olean‐12‐ene and 18α‐olean‐12‐ene skeleton

The 13C NMR spectra of some pentacyclic triterpenoids, 3β‐acetoxy‐11‐oxo‐olean‐12‐ene‐30‐oic acid methyl ester, 3β‐acetoxy‐11‐oxo‐olean‐12‐ene‐29‐oic acid methyl ester, the corresponding 11‐desoxo methyl esters, 3β‐acetoxy‐11‐oxo‐18α‐olean‐12‐en‐30‐oic acid methyl ester and 3β‐acetoxy‐11‐oxo‐18α‐olean‐12‐en‐29‐oic acid methyl ester are discussed. The shielding data are interpreted in term of the different orientation of the carbomethoxy group and of the change in configuration at the D/E ring junction and are diagnostically valuable for the differentiation of the mentioned compounds.