NMR study of tautomerism in natural perylenequinones

The keto–enol tautomerism of the dihydroxyperylenequinone system of a number of natural compounds, cercosporin 1, isocercosporin 2, phleichrome 3, isophleichrome 4, elsinochromes 5–9, cladochrome C 10, and hypocrellin 11 was studied by 1H, 2H and 13C NMR spectroscopy. 4,9-Dihydroxyperylene-3,10-dione and 3,10-dihydroxyperylene-4,9-dione tautomers were recognized as present in fast equilibrium in CDCl3 and [2H6]acetone solutions. The populations of each tautomer were obtained from the coupling between the proton of the hydrogen-bonded OH groups and the adjacent carbon atoms, i.e. J(C3, OH) and J(C4, OH). The most important factors governing the tautomeric equilibrium in these helix-shaped compounds appeared to be the substituent effects, the strength of the intramolecular phenol–quinone hydrogen bond, the distortion from the planarity of the perylenequinone system, solvation and aggregation effects. The strength of the hydrogen bonds, which is related to the distortion from planarity, was evaluated from 1H chemical shifts and primary deuterium isotope effects. Proton shifts of OH groups and isotope effects are linearly correlated. The influence of solvents and concentration on the tautomeric equilibrium was studied in the case of 4, for which a model of vertical stacking-type dimerisation is proposed.