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Effect of the solvent on the conformation of monocrotaline as determined by isotropic and anisotropic NMR parameters
Cleyton Marcos de Melo Sousa1,2, Raquel Brandt Giordani3, Wamberto Alristenio Moreira de Almeida3
1Departamento de Química Fundamental, Universidade Federal de Pernambuco, Recife, Pernambuco, Brazil.
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.
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