Derivatives of benzo[b]furan. Part I. Conformational studies of khellinone and visnaginone

Abstract

The structural analysis of khellinone and visnaginone indicated the planarity of the benzo[b]furan ring system. The oxygen or carbon atoms of the substituents, -OH, -OCH 3, and -C(=O)CH 3, are nearly coplanar with the aromatic ring. The molecular conformation is stabilized by the intramolecular O-H..O hydrogen bond formed between the ortho-substituted hydroxyl and acetyl groups. The intermolecular contacts present in the crystal structure are the C-H..O and C-H..π interactions and the π..π stacking. The energy surface for the internal rotation about the C sp2-O/C bonds in the khellinone and visnaginone molecules has been explored by quantum-chemical calculations. The density functional theory (DFT) methods yielded three stable conformers of khellinone and two of visnaginone obtained by a rotation of the methoxy group about the C4 sp2-OCH 3 bond. The most stable conformation in the gas phase is consistent with the stereochemistry of the molecules adopted in the solid. The secondary minima conformers have energies higher by 0.3-1.8 kcal/mol. The energy of the intramolecular O-H..O interactions, present in the molecular structure of all stable conformers, is around 18 kcal/mol at the DFT level. A complete and reliable assignment of the bands in the IR and Raman spectra of both compounds has been performed. © 2012 Springer Science+Business Media, LLC.