Unusual Two-Step Claisen-type Rearrangement Reaction under Physiological Conditions.

Abstract

N-aryl hydroxamic acids, which are best known for their metal-chelating properties in chemical and biomedical research, have been found to markedly detoxify carcinogenic halogenated quinones. However, the exact chemical mechanism underlying such detoxication remains unclear. Here, we show that a very fast reaction took place between N-phenylbenzohydroxamic acid (N-PhBHA) and 2,5-dichloro-1,4-benzoquinone (DCBQ), forming an unexpected new carbon-carbon bonding phenyl-quinone product with high yield. In contrast, no reaction was observed with O-benzoyl N-PhBHA. Analogous results were observed for other N-aryl hydroxamic acids and halogenated quinones, which have an ortho-hydrogen adjacent to the reaction site (DCBQ-type). Interestingly, no free radical intermediates could be detected by both ESR spin-trapping and radical-scavenging methods during the reaction process. Taken together, we proposed that nucleophilic substitution followed by an unusual two-step Claisen-type rearrangement reaction was responsible for the formation of a new C-C bonding compound and the detoxication reaction. This represents the first report of an unusually mild and facile two-step Claisen-type rearrangement, which could take place under normal physiological conditions.