Semi-synthesis and structural elucidation of brevicanines A-D, four new C19-diterpenoid alkaloids with rotameric phenomenon from Aconitum brevicalcaratum

Four new C19-diterpenoid alkaloids brevicanines A-D (1-4) with rotameric phenomenon were isolated from Aconitum brevicalcaratum. They all possessed an unusual axial chiral phenyl-quinazoline side chain and their structures were elucidated by extensive spectroscopic analysis and chemical methods. Meanwhile, brevicanines A and B were semi-synthesized from their parent compound scaconine to further confirm their structures. Variable-temperature NMR spectroscopy was also used to investigate the atropisomers of brevicanine A, in which two sets of signals in H-1 NMR spectra were observed at room temperature and coalesced over 140 degrees C. It's the first time to determine the atropisomeric preference of diterpenoid alkaloids.