title: Low-temperature 1H and 13C NMR spectra of N-substituted 1,2,3,4-tetrahydropyrazino[1,2-a]indoles.
abstract: The temperature-dependent (1)H and (13)C NMR spectra of 2-(2-butynyl)-10-methyl-1,2,3,4-tetrahydropyrazino[1,2-a]indole (4) (as a representative example of 1-9) in CFCl(3) + CD(2)Cl(2) solution are described and discussed. Below 183 K, the hexahydropyrazine ring inversions become slow on the NMR time-scale and 4 exists in principle as two conformational diastereomers. In fact, only one was observed with the N-2 substituent in an equatorial position as shown by a low-temperature NOESY experiment. The energy barrier for conformational interchange was calculated from NMR data to be 8.3 kcal mol(-1) (1 kcal = 4.184 kJ), in agreement with quantum chemical calculations. Unambiguous assignments for all proton and carbon resonances of 1-9 were made using 1D (APT, DEPT, NOE difference) and 2D (COSY, NOESY, gHMQC, gHMBC) NMR techniques.
authors.affiliation: Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, USA. katrizky@chem.ufl.edu
authors.forename: Alan R
authors.initials: AR
authors.lastname: Katritzky
authors.forename: Novruz G
authors.initials: NG
authors.lastname: Akhmedov
authors.forename: Evgeniy M
authors.initials: EM
authors.lastname: Myshakin
authors.forename: Akhilesh K
authors.initials: AK
authors.lastname: Verma
authors.forename: C
authors.initials: C
authors.lastname: Dennis Hall
mesh_terms: D002247:Carbon Isotopes / Q000737:chemistry*; D003080:Cold Temperature; D007211:Indoles / Q000737:chemistry*; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods*; D011522:Protons; D011719:Pyrazines / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
chemical_list: C490865:1,2,3,4-Tetrahydropyrazino(1,2-a)indole; D002247:Carbon Isotopes; D007211:Indoles; D011522:Protons; D011719:Pyrazines; D009584:Nitrogen