Refluxing of Hypocrellin A (or Hypocrellin B) and sodium sulfite in strong alkaline aqueous solution (1% NaOH) with a stream of air blowing through the liquid led to the formation of sodium 14- dehydroxy-15-deacetyl-Hypocrellin A-13-sulfonate (13-So~3Na-DDHA) (in 30% yield) and a red water-soluble polymer. If the reaction of Hypocrellins and sodium sulfite proceeded in mixed solvents of pyridine-water (1:1/v:v) in the presence of CuO, a single water- soluble product 13-SO~3Na-DDHA was obtained in 70% yield. In the latter system, the alternation of the solvent and the oxidant facilitated the production of 13-SO~3Na-DDHA. The ESR study of the reaction system showed that this sufonation reaction was initiated by the heat induced electron transfer between Hypocrellins and SO~3^2^- to produce the semiquinone radical anion and sulfur trioxide radical anion (SO~3^-). The nitromethane aci-anion quenching experiment proved that the key step for the formation of 13-SO~3Na-DDHA was the attack of SO~3^- on Hypocrellins at the 13-position which could be activated under alkaline and higher temperature conditions. The role of the oxidant used was considered to be a quencher of the semiquinone radical anion of HA (or HB) to its parent quinone to increase the relative concentration of HA (or HB) so that the generation of SO~3^- via the electron transfer interaction between Hypocrellins and SO~3^2^- was accelerated.

Key words: ELECTRON SPIN RESONANCE, SULFONATION, OXIDANT, SOLVENTS, HYPOCRELLIN, HYPOCRELLIN A, COPPER OXIDE, REACTION MECHANISM