Chemistry and Photochemistry of 2,6-Bis(2-hydroxybenzilidene)cyclohexanone. An Example of a Compound Following the Anthocyanins Network of Chemical Reactions

The kinetics and thermodynamics of the 2,6-bis­(2-hydroxybenzilidene)­cyclohexanone chemical reactions network was studied at different pH values using NMR, UV–vis, continuous irradiation, and flash photolysis. The chemical behavior of the system partially resembles anthocyanins and their analogue compounds. 2,6-Bis­(2-hydroxybenzilidene)­cyclohexanone exhibits a slow color change from yellow to red styrylflavylium under extreme acidic conditions. The rate constant for this process (5 × 10^–5 s^–1) is pH independent and controlled by the <i>cis–trans</i> isomerization barrier. However, the interesting feature is the appearance of the colorless compound, 7,8-dihydro-6<i>H</i>-chromeno­[3,2-<i>d</i>]­xanthene, isolated from solutions of acid to neutral range, characterized by ¹H NMR and single crystal X-ray diffraction. Light absorption by 2,6-bis­(2-hydroxybenzilidene)­cyclohexanone solutions immediately after preparation exclusively results in <i>cis</i>-isomer as photoproduct, which via hemiketal formation yields (i) red styrylflavylium by dehydration under extremely acidic solutions (pH < 1) and (ii) colorless 7,8-dihydro-6<i>H</i>-chromeno­[3,2-<i>d</i>]­xanthene by cyclization in solutions of acid to neutral range