Structural Studies of Norditerpenoid Alkaloids: Conformation Analysis in Crystal and in Solution States

Conformational analyses are presented of several pharmacologically important norditerpenoid alkaloids (NDAs), in crystal and solution states. Crystal data 8 NDAs (4 free bases 4 salts) were obtained, which crassicauline A, aconitine HCl, methyllycaconitine HClO4 reported for the first time. 1D/2D NMR spectroscopies 7 (5 2 recorded comprehensively. conformations described with a revised cyclohexane ring nomenclature. The A/E-rings NDA-free exist twisted-chair/twisted-chair conformations, leading to examples 1H effect steric compression shown A-ring. NDA salts adopt boat/chair as does protonated synthetic [3.3.1]azabicycle its lattice. However, aqueous solutions at physiological pH, this analogue adopts true chair/true chair conformation where retain their twisted boat/twisted conformations.