Tautomerism of the natural 1,8-dihydroxy-9,10-anthraquinones chrysophanol, aloe-emodin, and rhein

Existence of chrysophanol, aloe-emodin, and rhein in solutions as primarily the 9,10-anthraquinoid forms was established by quantum-chemical and correlation methods. However, the 1,10-anthraquinoid tautomers are observed in certain media. As a rule, these compounds in alkaline solutions form 1,10-anthraquinoid anions that are stabilized by intramolecular H-bonds and have a single α-hydroxy group. Tautomers exist in both the ground and excited states of the molecule. Quantitative rules that control changes in the ionization and tautomerization parameters of chrysophanol were found. ©2005 Springer Science + Business Media, Inc.