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Free keywords:
13C; 1H; NMR; RDC; conformation; monocrotaline
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 NMR parameters measured in four solvents of different polarity (D2 O, DMSO-d6 , CDCl3 , and C6 D6 ). Anisotropic NMR 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, while a PMMA polymer gel was chosen for CDCl3 and C6 D6 , and a poly-HEMA 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.
