title: Dynamic NMR and ab initio studies of exchange between rotamers of derivatives of octahydrofuro[3,4-f]isoquinoline-7(1H)-carboxylate and tetrahydro-2,5,6(1H)-isoquinolinetricarboxylate.
abstract: The 1H and 13C NMR spectra of methyl-8-(2-furyl)-5-methyl-1,3-dioxo-3,3a,4,6,8,9,9a,9b-octahydrofuro[3,4-f]isoquinoline-7(1H)-carboxylate (1) and trimethyl 8-methyl-3-phenyl-3,4,4a,7-tetrahydro-2,5,6(1H)-isoquinolinetricarboxylate (2) at room temperature displayed doubling of the majority of signals, suggesting the presence of two rotational conformers (rotamers) in a ratio approximately 1:1.2 (in a mixture of CDCl3 and C6D6), approximately 1:1 (in CD2Cl2) and approximately 1:1.4 (in C6D6). On the basis of the temperature-dependent 1H NMR spectra of 1 and 2, the barrier to interconversion of the rotamers was calculated to be approximately 16 kcal mol(-1). The average chemical shifts and spin-spin coupling constants were analyzed for the resolution-enhanced 300 MHz 1H NMR spectrum of 1 at 333 K in CDCl3 solution. From analysis of the spin-spin coupling constants, it is proposed that the nitrogen-containing ring is in a chair conformation with C-2-H equatorial. Low- and room-temperature NOESY experiments in conjunction with theoretical ab initio calculations supported the hypothesis that the two rotamers interchange via rotation about the C-N bond. Spectral assignments of all proton and carbon resonances were made on the basis of one- and two-dimensional NMR experiments (DEPT, DQCOSY, NOESY, HETCOR and gHMBC).
authors.affiliation: Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, USA. nachmed@chem.ufl.edu
authors.forename: Novruz G
authors.initials: NG
authors.lastname: Akhmedov
authors.forename: Evgeniy M
authors.initials: EM
authors.lastname: Myshakin
authors.forename: C Dennis
authors.initials: CD
authors.lastname: Hall
mesh_terms: D002247:Carbon Isotopes; D005663:Furans / Q000737:chemistry*; D006859:Hydrogen; D007546:Isoquinolines / Q000737:chemistry*; D007554:Isotopes; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008968:Molecular Conformation; D012399:Rotation
chemical_list: D002247:Carbon Isotopes; D005663:Furans; D007546:Isoquinolines; D007554:Isotopes; C499117:methyl 8-(2-fuyl)-5-methyl-1,3-dioxo-3,3a,4,6,8,9,9a,9b-octahydrofuro(3,4-f)isoquinoline-7(1H)-carboxylate; C499118:trimethyl 8-methyl-3-phenyl-3,4,4a,7-tetrahydro-2,5,6(1H)-isoquinolinetricarboxylate; D006859:Hydrogen