C4H4O2 1,2-Cyclobutanedione 1,2-Dioxin 1,3-Cyclobutanedione 1,4-Dioxin 2-Furanone Cyclobutanedione Cycloprop-2-ene carboxylic acid Diketene Dioxins and dioxin-like compounds Methyl propiolate Tetrolic acid