title: Molecular disorder in (‒)-encecanescin.
abstract: (‒)-Encecanescin (1) has been isolated from the leaves of Eupatorium aschembornianum. Two conformers are present in the crystal structure as a result of molecular disorder. The structure of 1 was established by 1H- and 13C-NMR spectroscopy in CDCl3 solution using 2D NMR techniques (gHSQC, gHMBC and NOESY). A Monte Carlo random search using molecular mechanics followed by the geometry optimization of each minimum energy structure using density functional theory (DFT) calculations at the B3LYP/6-31G* level and a Boltzmann analysis of the total energies generated accurate molecular models describing the conformational behavior of 1. The three most stable conformers 2-4 of compound 1 were reoptimized at the B3LYP/6-311++G(d,p) level of theory using CHCl3 as a solvent. Correlations between the experimental 1H- and 13C-NMR chemical shifts (δexp) have been found, and the GIAO/B3LYP/6-311++G(d,p) calculated magnetic isotropic shielding tensors (σcalc) for conformers 2 and 3, δexp=a+b σcalc, are reported. A good linear relationship between the experimental and calculated NMR data has been obtained for protons and carbon atoms.
authors.affiliation: Laboratorio de Productos Naturales, Área de Química, Departamento de Preparatoria Agrícola AP 74 Oficina de Correos Chapingo, Universidad Autónoma Chapingo, Km. 38.5 Carretera México-Texcoco, Texcoco 56230, Estado de México, Mexico. benijovi@yahoo.com.mx.
authors.forename: Benito
authors.initials: B
authors.lastname: Reyes-Trejo
authors.affiliation: Laboratorio de Productos Naturales, Área de Química, Departamento de Preparatoria Agrícola AP 74 Oficina de Correos Chapingo, Universidad Autónoma Chapingo, Km. 38.5 Carretera México-Texcoco, Texcoco 56230, Estado de México, Mexico. dg_bonita33@yahoo.com.mx.
authors.forename: Diana
authors.initials: D
authors.lastname: Guerra-Ramírez
authors.affiliation: Laboratorio de Productos Naturales, Área de Química, Departamento de Preparatoria Agrícola AP 74 Oficina de Correos Chapingo, Universidad Autónoma Chapingo, Km. 38.5 Carretera México-Texcoco, Texcoco 56230, Estado de México, Mexico. hozuleta_13@hotmail.com.
authors.forename: Holber
authors.initials: H
authors.lastname: Zuleta-Prada
authors.affiliation: Departamento de Química, CINVESTAV-IPN, Apdo. Postal 14-740, México D.F. 07000, Mexico. rsantill@cinvestav.mx.
authors.forename: Rosa
authors.initials: R
authors.lastname: Santillán
authors.affiliation: Escuela Superior de Medicina, Instituto Politécnico Nacional, Plan de San Luis y Díaz Mirón, Colonia Santo Tomás, Delegación Miguel Hidalgo, México D.F. 11340, Mexico. mesmendoza@hotmail.com.
authors.forename: María Elena
authors.initials: ME
authors.lastname: Sánchez-Mendoza
authors.affiliation: Escuela Superior de Medicina, Instituto Politécnico Nacional, Plan de San Luis y Díaz Mirón, Colonia Santo Tomás, Delegación Miguel Hidalgo, México D.F. 11340, Mexico. jearrval@yahoo.com.mx.
authors.forename: Jesús
authors.initials: J
authors.lastname: Arrieta
authors.affiliation: Departamento de Química Orgánica, Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán D.F. 04510, Mexico. linoj@unam.mx.
authors.forename: Lino
authors.initials: L
authors.lastname: Reyes
mesh_terms: D001578:Benzopyrans / Q000737:chemistry* / Q000302:isolation & purification; D002244:Carbon / Q000737:chemistry; D002725:Chloroform; D018360:Crystallography, X-Ray; D036602:Eupatorium / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009010:Monte Carlo Method; D010936:Plant Extracts / Q000737:chemistry; D018515:Plant Leaves / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory; D012997:Solvents; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D001578:Benzopyrans; D010936:Plant Extracts; D011522:Protons; D012997:Solvents; D002244:Carbon; D002725:Chloroform; C066390:encecanescin
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