Napelline-type C20-diterpenoid alkaloid iminiums from an aqueous extract of “fu zi”: Solvent-/base-/acid-dependent transformation and equilibration between alcohol iminium and aza acetal forms

Three new napelline-type C20-diterpenoid alkaloids, named aconicarmichinium A and B trifluoroacetates (1 and 2) and aconicarmichinium C chloride (3), were isolated from an aqueous extract of “fu zi”, the lateral roots of Aconitum carmichaelii. Their structures were elucidated by extensive spectroscopic data analysis. Compounds 1–3 represent the first examples of napelline-type C20-diterpenoid alkaloid alcohol iminiums, of which the structures were fully characterized. In addition, transformation and equilibration between the alcohol iminiums (1–3) and the aza acetals 1a–3a were investigated by measurements of the NMR spectra in protic and aprotic deuterium solvents including alkali pyridine-d5, along with evaporation under reduced pressure and gradual additions of TFA, AcOH, and HCl. The results demonstrated that the transformation and equilibration were solvent-, base-, and acid-dependent. Especially, in aqueous biological fluid, these C20-diterpenoid alkaloids would more likely exist as the alcohol iminiums accompanied by anion counterparts in biosystems to increase their solubility, bioavailability, transportations, and functions. The absolute configurations of 1–3 were confirmed by X-ray crystallographic analysis of 2a.

Three napelline-type C20-diterpenoid alkaloid alcohol iminiums, named aconicarmichinium A and B trifluoroacetates (1 and 2) and aconicarmichinium C chloride (3), were isolated from an aqueous extract of “fu zi”, the lateral roots of Aconitum carmichaelii. Their structures were elucidated by extensive spectroscopic analysis. Solvent-, base-, and acid-dependent transformation and equilibration between the alcohol iminiums (1–3) and corresponding aza acetals 1a–3a were found. The absolute configurations of 1–3 were confirmed by X-ray crystallographic analysis of 2a.Figure optionsDownload as PowerPoint slide