Pyrrolizidine alkaloids from Gaertn and theoretical studies on intramolecular interactions.

Two undescribed pyrrolizidine alkaloids, 13-dehydrosenkirkine () and chloromethylretrorsine (), along with three known analogues, onetine (), retrorsine (), and usaramine -oxide (), were isolated from Gaertn. The structures of the undescribed compounds were elucidated by extensive spectrometric and spectroscopic techniques, including HRESIMS, NMR, calculated C-NMR DP4+ analysis and comparison with experimental and calculated ECD spectra. The undescribed compounds were evaluated for their antitumour activity against HT29, HeLa, and HepG2 cells. In addition, the intramolecular interactions and quadrupolar couplings were revealed by investigating the geometrical and electronic properties of three typical otonecine-type PAs in DFT theory.