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