Investigation of crystal and solution structures of glaucyrones by X-ray, nuclear magnetic resonance, and ultraviolet methods

Abstract

A single-crystal X-ray study of the black 3,3′-diacetyl-5,5′-bisethoxycarbonylglaucyrone (diethylglaucophanic enol) confirms the enolic structure (1). Abnormally short intermolecular contacts indicate the conjugated molecules to be connected by self charge-transfer interactions above and below the molecular plane, forming a linear but ‘zig-zag’ arrangement through the crystal. ¹H N.m.r. data for the glaucyrones are solvent dependent, the AMX system of bridge protons (and other signals) in dry CDCl₃ readily passing into the slow exchange region if moisture is present: addition of base gives a symmetrical anion (AMA' bridge protons) and this type of spectrum is observed in [²H₆]DMSO. 3′-Acetyl-3,5,5′-trismethoxycarbonylglaucyrone shows related ¹H n.m.r. behaviour, but 3,3′,5,5′-tetrakismethoxycarbonylglaucyrone exists as a 7′-H form (7) in CDCl₃ and as a symmetrical form in [²H₆]DMSO. Electronic spectra in a variety of solvents show the glaucyrones to be highly ionised in polar solvents (blue) and unionised in non-polar solvents (red) with gradations in certain solvent systems. ¹³C N.m.r. assignments for glaucyrones and xanthyrones are based on model pyrones.