α-Diketones in [4+2]Cycloadditions, Reactions of Dimethyl 2,3-Dioxosuccinate with Enol Ethers

Dimethyl 2,3-dioxosuccinate (1) undergoes cycloaddition with enol ethers. Dihydrodioxine derivates, products of [4+2] cycloaddition, are formed with tetraethoxyethylene and E-1,2-dimethoxyethylene. The stereospecificity of the reaction of 1 with E-1,2-dimethoxyethylene suggests a concerted mechanism for this cycloaddition. The polarized double bond of ethyl vinyl ether reacts with 1 to give 4,5-dihydro-2,3-dimethoxycarbonyl-2,3-epoxy-5-ethoxyfuran in a hitherto unknown reaction of 1,2-diketones