Diphenylparabanic Acid as a Synthon for the Synthesis of α-Diketones and α-Ketocarboxylic Acids

Diphenylparabanic acid was found to react with >2 equiv of organolithiums at −78 °C to effectively give the corresponding symmetrical α-diketones. However, upon treatment with 1 equiv of organolithium, the parabanic acid gave mainly 5-substituted 5-hydroxyimidazolidine-2,4-diones. On the other hand, Grignard reagents were less reactive toward the parabanic acid at low temperature, and selectively gave the corresponding 5-hydroxyimidazolidine-2,4-diones even if more than 1 equiv of the reagents was used. A tandem process in which the parabanic acid was first reacted with a Grignard reagent and then reacted in one-pot with an organolithium effectively gave the unsymmetrical α-diketone. 5-Substituted 5-hydroxyimidazolidine-2,4-diones were useful as versatile precursors for preparing α-ketocarboxylic acids as well as unsymmetrical α-diketones