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

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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, –OCH3, and –C(=O)CH3, 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 Csp2–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 C4sp2–OCH3 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.