1-Oxa-2,3-cyclohexadiene can be readily generated by treatment of 5-bromo-3,4-dihydro-2H-pyran with potassium tert-butoxide in the presence of “18-crown-6” and can be trapped with dienes or olefins. The remarkable regio- and stereoselectivities with which [4+2] and [2+2] adducts are formed suggests a concerted, non-radical cycloaddition mechanism.
1-Oxa-2,3-环
己二烯很容易通过在“ 18-crown-6”存在下用
叔丁醇钾处理5-
溴-3,4-二氢-2 H-
吡喃而容易生成,并被捕集与二烯或烯烃。形成[4 + 2]和[2 + 2]加合物的显着区域选择性和立体选择性表明了一种协同的,非自由基的环加成机理。