Dehydration of an Insoluble Urea Byproduct Enables the Condensation of DCC and Malonic Acid in Flow

A procedure for the preparation of N,N′-dicyclohexylbarbituric acid from DCC and malonic acid is described. Addition of phosphorus oxychloride to the reaction mixture facilitates dehydration of the insoluble byproduct N,N′-dicyclohexyl urea, enabling operation in continuous flow. A development approach based on in situ monitoring of batch reactions was used, which supported screening and determination of reaction conditions at small scale prior to scaleup in flow. Additional mechanistic understanding and control of impurity formation are presented.