The novel chemistry of 5-methylene-substituted 1,3-dioxane-4,6-diones, namely methylene Meldrum’s acids, was examined. These acylals readily reacted with hydrogen peroxide without any catalyst and base to produce epoxides as good crystalline products. On the other hand, these acylals act as dienophiles for the photoenol of o-methylbenzaldehyde to yield [4+2] cycloadducts. However, the reactions of the photoenols of o-methyl-substituted aromatic ketones, such as 2-methylacetophenone and 2-methylbenzophenone, with the acylals gave adducts which bonded between the β-carbon of the acylals and the 2-methyl carbon of the ketones. The γ-allenyl-substituted alkylidene Meldrum’s acids formed an intramolecular CT complex between the allenyl and vinyl moieties. Direct photolyses gave alkenylidenecyclopropanes via a CT excited state, while acetone-sensitizations gave intramolecular [2+2] cycloadducts.