NMR studies of diterpene alkaloids

A study of the NMR spectra of 44 diterpene alkaloids and derivatives is reported. A comparison of the NMR data for the C(4) Me group in a series of derivatives of atisine and veatchine provides support for the previous proposal regarding the conformation of ring E. Furthermore, the dependency of the chemical shift of the C(4) Me protons on the functionality of the N atom is also discussed.