Role of 2‐carboxyl substituent in the tautomerization between two equivalent enol forms of 3‐hydroxyphenalenone

The interconversion between two equivalent enol forms of 3‐hydroxyphenalenone took place predominantly through a diketo form with a free energy of activation of ca 14 kcal mol−1 in DMSO solution determined by 125·8 MHz13C NMR measurement. On the other hand, the corresponding interconversion in 2‐carboxy‐3‐hydroxyphenalenone was not frozen on the time scale of 13C NMR spectroscopy down to −60°C. This acceleration in the interconversion rate of the latter may be interpreted by a double proton switching between the hydrogen‐bonded 2‐carboxyl and 3‐hydroxyenone moieties.