Synthesis, EPR, ENDOR and TRIPLE Resonance Investigations of Anthracycline-Related Semiquinones

Article abstract of DOI:10.1002/mrc.1260310510

The synthesis of five substituted anthraquinones related to anthracyclines is described.One-electron reduction of these compounds yields the corresponding semiquinones, which were characterized by EPR, ENDOR and TRIPLE resonance spectroscopy.The data indicate a rapid conformational interconversion of the cyclohexene moiety (ring A in the anthracyclines).The low-temperature structures could be assigned to half-chair conformers whose splitting constants and torsion angles were obtained by simple calculations.Introduction of substituents in the 4-position, disturbing the symmetry of the anthraquinone, results in remarkable changes in the spin density distribution, in contrast to published results.A reinterpretation and assignment of known anthraquinone radicals is presented.Key Words: Anthracycline semiquinones, EPR, ENDOR, TRIPLE, Conformation