NMR investigations of forskolin. Complete assignment of proton and carbon NMR spectra and conformational analysis

The 1H‐ and 13C NMR spectra of forskolin have been re‐investigated. Assignments are based exclusively on unambiguous carbon–carbon and carbon‐proton correlations derived from two‐dimensional NMR spectra. Stereo‐specific differentiations were deduced from selective proton NOE measurements and the interpretation of vicinal and long‐range proton‐proton scalar couplings. The proton‐proton distances calculated from proton NOE effects fit best with a flattened boat‐like conformation of ring C, which is different from the conformation of forskolin in the solid state.