P. Lankhorst, T. A. Beek, C. Haasnoot
Sep 2, 2010
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Journal
Recueil des Travaux Chimiques des Pays-Bas
Abstract
The complete assignment of the 1H-NMR spectrum of the fully saturated tetracyclic sesquiterpene hydrocarbon longicyclene is presented. The assignment was achieved using phase-sensitive double-quantum filtered COSY at 600 MHz, heteronuclear correlation spectroscopy and COLOC spectroscopy. Accurate vicinal 1H-1H coupling constants involving the seven-membered ring protons were obtained using extensive simulation of the ID spectrum. Molecular mechanics calculations indicate that the title compound may adopt two conformations which differ mainly in the geometry of the seven-membered ring. These geometries are best described as twist-chair-like conformations denoted TC11 and TC8, respectively. The energy of the TC11 conformer is predicted to be slightly lower (4.6 kJ/mol) than the energy of the TC8 conformer. The vicinal 1H-1H coupling constants of these two conformers were predicted from a generalized Karplus equation. The predicted coupling constants agree well with the experimental coupling constants if it is assumed that, in solution, longicyclene is engaged in a conformational equilibrium between TC11 (86%) and TC8 (14%).