of the risks of handling ozone. The enamine product from dimethylformamide-dimethylacetal reacts by photooxidation in virtually quantitative yield to diethyl oxomalonate-hydrate. A more recent patent describes the synthesis of diethyl oxomalonate from the simple precursor diethyl malonate by oxidation with aqueous sodium chlorite (NaClO2) solution at pH 4.4 in 97% yield. The ester is first produced as a hydrate, which is dehydrated by azeotropic distillation with toluene to the final product. Properties Diethyl oxomalonate is a greenish-yellow, low-viscosity, low-odor oil that rapidly forms the dihydrate and crystallizes with water in shape of white prisms. The refractive index is 1.425 (20 °C, 589 nm) to 1.4310 (22 °C, 589 nm). Application Diethyl oxomalonate acts as electron-poor dienophile and can be used as a carbon dioxide equivalent for Diels-Alder reactions with electron-rich 1,3-dienes such as isoprenes or dimethyl butadienes in a [4+2]cycloaddition to the geminal dihydropyran diester. This diester can be hydrolyzed in the alkaline to gem-diacid, halogenated with oxalyl chloride to gem-diacid chloride, with sodium azide transferred to the gem-diacid azide finally degraded in a Curtius Rearrangement to a dihydropyranone. Diethyl oxomalonate reacts in an aldol addition with the morpholinenamine of 3-pentanone to form an α-hydroxy-γ-ketodiester. This ketodiester forms a substituted butenolide with a phosphorus pentoxide/methanesulfonic acid mixture. With guanidines, a functionalized imidazolone is produced in 85% yield. Diethyl oxomalonate is a versatile reactant in the Baylis-Hillman reaction and forms the corresponding multifunctional compounds with acrylates, acrylonitrile, or methyl vinyl ketone catalysed by DABCO. Diethyl oxomalonate reacts with the Grignard compound formed from 1-iodo-2-chloromethylbenzene and isopropylmagnesium chloride to give 2-bis-carboxyethyl-isobenzofuran. Diethyl oxomalonate is added to terminal double bonds of alkenes in an ene reaction to give 1-hydroxy-1-alkylmalonic esters. References Ketoesters Ethyl esters