一类非同寻常的二次Lossen重排反应:异羟肟酸的新型解毒分子作用机制

An Unusual Double Lossen Rearrangement Reaction: The Novel Molecular Detoxication Mechanism for Hydroxamic Acids

异羟肟酸作为一种常用的金属络合剂在生物医学领域被广泛使用,先前的研究表明其能有效去除五氯酚的致癌代谢物多卤代醌诱导的毒性.但是,这种解毒效应潜在的化学机制并不清楚.我们最近研究发现苯异羟肟酸(BHA)能显著加速、促进高毒性的四氯对苯醌转化为低毒的二氯二羟基苯醌,其转化速率是自然水解时转化速率的15万倍.我们分离得到了BHA反应后的主要产物,并通过多种分析手段鉴定其为O-苯基甲酰胺基苯异羟肟酸.基于产物鉴定结果及O-18同位素标记实验的研究,我们认为这种苯异羟肟酸的快速解毒反应是一种自杀式亲核攻击反应,同时伴随着出人意料的可在正常生理条件下进行的二次(双重)Lossen重排反应.我们的研究结果可能具有广泛的生物与环境学方面的意义.

Hydroxamic acids have attracted considerable interest recently because of their capacity to inhibit a variety of enzymes such as metalloproteases and lipoxygenase, and transition metal mediated oxidative stress. In our previous work, we found that deferoxamine（a trihydroxamate iron chelator used for the treatment of iron overload diseases） and other hydroxamic acids, but not the classic iron chelating agents such as diethylenetriaminepentaacetic acid（DTPA）, could effectively detoxify the carcinogenic polyhalogenated quinoid metabolites of pentachlorophenol. However, the chemical mechanism underlying such detoxication is not clear. We found that benzohydroxamic acid（BHA, a model hydroxamic acid） could dramatically accelerate the hydrolysis of the highly toxic tetrachloro-1,4-benzoquinone（TCBQ） to its much less toxic product, 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone（DDBQ）, with rate accelerations of up to 150000-fold. No enhancing effect was observed with O-methyl BHA, suggesting free benzohydroxamate anion is essential for the acceleration of TCBQ hydrolysis. The major reaction product of BHA was isolated and identified as O-phenylcarbamyl benzohydroxamate. Based on these data and oxygen-18 isotope-labelling studies, we proposed that suicidal nucleophilic attack coupled with an unexpected double Lossen rearrangement reaction was responsible for this remarkable acceleration of the detoxication reaction： A nucleophilic reaction takes place between the benzohydroxamate anion（Ar C（O）-NH-O-） and TCBQ, first forming an unstable transient intermediate Ar C（O）-NH-O-trichloro-1,4-benzoquinone. Following loss of a proton from nitrogen to form the anionic Ar C（O）-N--O-trichloro-1,4-benzoquinone intermediate, a spontaneous Lossen-type rearrangement leads to the formation of Tr CBQ-O-（at low BHA/TCBQ molar ratios） and phenyl isocyanate. When BHA is in excess, Tr CBQ-O-further reacts with BHA, through a similar reaction intermediate, and a second-step spontaneous Lossen-type rearrangement reaction yields DDBQ and another molecule of phenyl isocyanate. The phenyl isocyanate could react with another molecule of BHA to yield O-phenylcarbamyl benzohydroxamate. This is the first report of an unusually mild and facile Lossen-type rearrangement, which could take place under normal physiological conditions in two consecutive steps. Our findings may have broad biological and environmental implications for future research on hydroxamate pharmaceuticals and polyhalogenated quinoid carcinogens, which are two important classes of compounds of major biomedical and environmental interest.