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

ABSTRACT

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 1H NMR spectra were observed at room temperature and coalesced over 140 °C. It's the first time to determine the atropisomeric preference of diterpenoid alkaloids.