Effect of the solvent on the conformation of monocrotaline as determined by isotropic and anisotropic NMR parameters

Abstract

The conformation in solution of monocrotaline, a pyrrolizidine alkaloid presenting an eleven-membered macrocyclic diester ring, has been investigated using a combination of isotropic and anisotropic nuclear magnetic resonance parameters measured in four solvents of different polarity (D₂ O, DMSO-d₆ , CDCl₃ , and C₆ D₆ ). Anisotropic nuclear magnetic resonance parameters were measured in different alignment media, based on their compatibility with the solvent of interest: cromoglycate liquid crystal solution was used for D₂ O, whereas a poly (methyl methacrylate) polymer gel was chosen for CDCl₃ and C₆ D₆ , and a poly (hydroxyethyl methacrylate) gel for DMSO-d₆ . Whereas the pyrrolizidine ring shows an E₆ exo-puckered conformation in all of the solvents, the macrocyclic eleven-membered ring adopts different populations of syn-parallel and anti-parallel relative orientation of the carbonyl groups according to the polarity of the solvent.

Keywords: 13C; 1H; NMR; RDC; conformation; monocrotaline.