title: Molecular dynamics and NMR analysis of the configurational 13C assignment of epimeric 22,23-epoxides of stigmasterol.
abstract: The determination of the stereochemistry of brasinosteroid analogs with 22,23-epoxide groups can be easily achieved by means of (13)C NMR spectroscopy. Here, we provide a rationalization of the (13)C chemical shift pattern found in 22R,23R- and 22S,23S-epoxides of stigmasterol, based on the analysis of gamma effects. (22S,23S)- and (22R,23R)-3beta-acetoxystigmast-22,23-epoxy-5,6beta-diol were used in the study as model compounds. Our methodology starts with a conformational search by means of molecular dynamics and NMR (NOE contacts) spectroscopy, which is followed by the analysis of the different gamma interactions affecting the chemical shift of interest. We demonstrate that the differences between the (13)C chemical shift patterns of 22R,23R and 22S,23S isomers arise from gamma effects as the result of diverging local conformations around the C17-C20 and C20-C22 bonds.
authors.affiliation: Dpto. de Química Física, Facultad de Química, Universidad de La Habana, La Habana, Cuba.
authors.forename: Reynier
authors.initials: R
authors.lastname: Suardíaz
authors.forename: Mitcheell
authors.initials: M
authors.lastname: Maestre
authors.forename: Ernesto
authors.initials: E
authors.lastname: Suárez
authors.forename: Yamilet
authors.initials: Y
authors.lastname: Bernardo
authors.forename: Esther
authors.initials: E
authors.lastname: Alonso-Becerra
authors.forename: Carlos
authors.initials: C
authors.lastname: Pérez
mesh_terms: D002247:Carbon Isotopes / Q000737:chemistry*; D002725:Chloroform / Q000737:chemistry*; D004852:Epoxy Compounds / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008968:Molecular Conformation; D013237:Stereoisomerism; D013265:Stigmasterol / Q000737:chemistry*
chemical_list: D002247:Carbon Isotopes; D004852:Epoxy Compounds; D002725:Chloroform; D013265:Stigmasterol