An Intramolecular CAr-H•••O=C Hydrogen Bond and the Configuration of Rotenoids.

Over the past half a century, the structure and configuration of the rotenoids, a group of natural products showing multiple promising bioactivities, have been established by interpretation of their NMR and electronic circular dichroism spectra and confirmed by analysis of single-crystal X-ray diffraction data. The chemical shift of the H-6' H NMR resonance has been found to be an indicator of either a or C/D ring system. In the present study, four structures representing the central rings of a -, a -, a dehydro-, and an oxadehydro-rotenoid have been plotted using the program based on X-ray crystal structures reported previously, with the conformations of the C/D ring system, the local bond lengths or interatomic distances, hydrogen bond angles, and the H-6' chemical shift of these compounds presented. It is shown for the first time that a -fused C/D ring system of rotenoids is preferred for the formation of a potential intramolecular C-H•••O=C H-bond, and that such H-bonding results in the H NMR resonance for H-6' being shifted downfield.