Low-temperature oxidation of the phenyl-substituted benzofurans 1 by dimethyldioxirane afforded the rather labile benzofuran epoxides 2, which are in equilibrium with their equally labile quinone methides 3 through valence isomerization. This reversible valence isomerization was established on the basis of the chemical transformations displayed by the benzofuran epoxides 2 and quinone methides 3. These include tetraethylammonium bromide catalyzed rearrangement of epoxide 2a to benzofuran-2-one 4a, dimerization of quinone methide 3c to dibenzofurodioxane 4c, nucleophilic trapping by methanol in the form of the adducts 5, the 1,3-dipolar cycloaddition with tetracyanoethylene (TCNE) to afford 6, and the inverse Diels-Alder reaction to the [4+2] cycloadducts 7 with ethyl vinyl ether. Also styrene gave with epoxide 2a the benzodihydropyran 8a through cycloaddition to its quinone methide valence isomer 3a, while with acetic anhydride the diacetylated adduct 9a was obtained. The driving force for these latter transformations derives from strain relief and aromatization. The labile benzofuran epoxides function as effective alkylating agents, which add nucleophiles at subambient temperatures without acid and base activation.
苯基取代的
苯并呋喃 1 被
二甲基二环氧乙烷低温氧化后,产生了相当易变的
苯并呋喃环氧化物 2,通过价式异构化,这些
环氧化物与同样易变的
醌类甲酯 3 达到了平衡。根据
苯并呋喃环氧化物 2 和
醌类甲化物 3 的
化学变化,可以确定这种可逆的化合价异构化作用。其中包括
环氧化物 2a 在
四乙基溴化铵催化下重排为
苯并呋喃-2-酮 4a,
醌类甲化物 3c 二聚为
二苯并呋喃二恶烷 4c,
甲醇亲核捕获形成加合物 5、与
四氰基乙烯(TCNE)发生 1,3-二极环加成反应生成 6,与
乙烯基乙醚发生反向 Diels-Alder 反应生成 [4+2] 环加成物 7。此外,
苯乙烯与
环氧化物 2a 发生环加成反应,生成苯并二氢
吡喃 8a,而与
醋酸酐发生环加成反应,则生成
二乙酰化加合物 9a。后一种转化的驱动力来自应变释放和芳香化。 易变的
苯并呋喃环氧化物可作为有效的烷基化剂,在亚环境温度下添加亲核物而无需酸碱活化。