Carbon-13 nuclear magnetic resonance spectra of some aromatic diterpenoids

13C n.m.r. studies of a series of tricarbocyclic ring C aromatic diterpenoids using proton‐noise and single‐frequency off‐resonance decoupling, partially relaxed Fourier transform techniques, shift reagents and specifically labelled derivatives have permitted unequivocal assignments of almost all signals. The shieldings caused by oxygenation of C‐18 and C‐19 and by introduction of various substituents in the aromatic ring are discussed. It is concluded that the 13C n.m.r. data are sufficiently characteristic to allow stereochemical assignments. Some effects of deuterium substitution on the carbon resonances are presented.