竹红菌素的13位磺化反应和反应机理

在通空气条件下,回流竹红菌甲素(HA)(或乙素,HB)和Na~2SO~3的含1%NaOH强碱性水溶性生成14-脱羟基-15-脱乙酰基竹红菌甲素-13-位磺酸钠(13-SO~3Na-DDHA)(产率30%)和另一种红色的水溶性聚合物。当这一反应在吡啶-水(1:1/V:V)混合溶剂中进行,并用CuO作氧化剂时,只得到13-SO~3Na-DDHA一种水溶性产物(70%产率)。在后一体系中,由于改变了溶剂和氧化剂,使13-SO~3Na-DDHA的制备更加便利。反应体系的ESR研究表明,这一磺化反应首先是通过竹红菌素和Na~2SO~3之间热活化的电子转移进行的。电子转移的结果产生竹红菌素半醌负离子自由基和三氧化硫负离子自由基(SO~3^-)。硝基甲烷酸式负离子猝灭实验证明,由于竹红菌素的13位能在碱性、高温下活化,磺化的关键步骤是SO~3^-对其活化了的13位的亲电进攻。氧化剂在这一反应中的作用为:将产生的竹红菌素半醌负离子自由基氧化至其母体醌,增加反应物的相对浓度,同时使竹红菌素与SO~3^2^-之间的电子转移循环进行,加速SO~3^-的产生。

Studies on the sulfonation of hypocrellins at the 13-position and the reaction mechanism

Refluxing of Hypocrellin A (or Hypocrellin B) and sodium sulfite in strong alkalineaqueous solution (1% NaOH) with a stream of air blowing through the liquid led to theformation 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 so-dium 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 lattersystem, the alternation of the solvent and the oxidant facilitated the production of13-SO_3Na-DDHA. The ESR study of the reaction system showed that this sulfonationreaction was initiated by the heat induced electron transfer between Hypocrellins and SO_2~(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 formationof 13-SO_3Na-DDHA was the attack of SO_3 on Hypocrellins at the 13-position which could beactivated under alkaline and higher temperature conditions. The role of the oxidant used was con-sidered to be a quencher of the semiquinone radical anion of HA (or HB) to its parent quinone toincrease the relative concentration of HA (or HB) so that the generation of SO_3~- via the electrontransfer interaction between Hypocrellins and SO_3~(2-) was accelerated.