Quinones and 1,2-Dicarbonyl Compounds 1,4-Dioxabutadienes

The thermal and photochemical [4 + 2] cycloadditions of o-quinones with olefinic and acetylenic dienophiles have been extensively re viewed and include their 4ir heterodiene Diels-Alder reactions with olefins, vinyl ethers,enamines, selected dienes, diphenyl Iketenimines, ketenes, fulvenes, and selected heterocycles including furan, - benzofuran, - - indoles, azepines, and 1,2-diazepines. The tetrahalo-substituted o-quinones, tetrachloro- and tetrabromo-o-quinone, generally participate in heterodiene [4 + 2] cy-cloadditions at an increased rate over the unsubstituted systems and generally provide higher overall yields of the Diels-Alder products. With simple olefins, the dienophile geometry is maintained in the course of the thermal [4 + 2] cycloadditions [Eq. (52)J,  

Ketene acetals, e.g., 1,1-dimethoxypropene and tetramethoxythylene, generally afford [2 + 2] oxetane cycloadducts with carbonyl compounds including a-diketones. However, the dihydrodioxin [4 + 2] cycloadducts of a-diketones with 1,1-dimethoxypropene and tetramethoxyethylene have been observed in thermal and l wis acid-catalyzed reactions when the [4 + 2] cycloaddition is accompanied by an increase in the stabilization energy of the diketone moiety, e.g., phenanthrene generation [Eq. (53)].220 

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