Conformational Pseudopolymorphism and Solid-State CPMAS NMR Studies for Determination of Solvent-Dependent Solution-State Conformational Preferences for (−)-Scopolamine Hydrobromide/Hydrochloride Salts

Abstract

Crystalline (−)-scopolamine hydrohalide (bromide and chloride) salts exist in two conformational families for the tropate ester moiety: the compact conformation (hydrates, phenyl ring underneath the scopine moiety), and the extended conformation (anhydrates, phenyl ring ca. antiperiplanar to oxiranyl O-atom). CPMAS 13C NMR solid-state spectra of anhydrates and hydrates are different. In both tertiary and quaternary scopolammonium salts, phenyl rings are immobile in compact conformation crystals having ca. 2.5 Å close lateral neighbors on both faces of the planar moiety. Phenyl rings undergo a π-flip in extended conformation crystals having ca. 3.0 Å close lateral neighbors on either face of the aromatic plane. Due to different OC−Cα−CH2OH dihedral angles [synperiplanar (eclipsed, in compact) and (+)-synclinal (gauche, in extended)], the methylol carbon chemical shifts of crystalline tertiary and quaternary scopolammonium salts and analogues are diagnostic markers for the tropate ester solid-state conformation [δCH2OH downfield from alkoxy δC(3) for extended conformations and upfield of δC(3) in compact conformations]. These solid-state model Δδ relationships were used to deconvolute weighted time-averaged chemical shifts in different solvents: e.g., one axial N-methyl scopolammonium bromide phenyl-ring face is solvent exposed in the compact conformation predominating in D2O medium, while both faces are solvent accessible in the extended major conformational contributor in CD2Cl2 solution.