EPR AND ENDOR INVESTIGATIONS OF CHRYSAZIN AND ACLACINOMYCIN-A SEMIQUINONES

Different assignments of the coupling constants to distinct positions of chrysazin are given in the literature. Several methyl derivatives, particularly the hitherto unknown 1,8-dihydroxy-2,3-dimethyl-9,10-anthraquinone, were prepared, and their radical anions were investigated by EPR and ENDOR spectroscopy to achieve a reliable attribution of these values. The data obtained show a remarkable solvent dependence of the spin density distribution. Thus, the discrepancy mentioned above can be explained in terms of intermolecular hydrogen bond formation. Therefore, the requirements for an investigation of the semiquinones of the antitumour drug aclacinomycin A and 7-deoxyaklavinone are fulfilled. The splitting constants of the radical anions are assigned using model compounds, TRIPLE experiments and the solvent dependence. In contrast to the results obtained for daunomycin radicals, the data can be interpreted in terms of an almost rigid half-chair conformation of the A ring.