Napelline-type C_(20)-diterpenoid alkaloid iminiums from an aqueous extract of ‘‘fu zi': Solvent-/base-/acid-dependent transformation and equilibration between alcohol iminium and aza acetal forms

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 deuteriumsolvents 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.