Urazole synthesis. Part 2: Facilitating N⁴ substitution

The di-tert-butyl-di-p-nitrophenyl ester of hydrazinetetracarboxylic acid was prepared and shown to be useful in the preparation of urazoles (i.e., 1,2,4-triazolidine-3,5-diones), by reaction with a primary amine using either n-BuLi or pyridine as base, depending on the desired N⁴ substituent. With more electronegative N⁴ substituents, pyridine is the preferred base. This work complements our reported urazole synthesis, which introduced the N⁴ substituent early in the sequence and thus did not facilitate variation at N⁴ for library synthesis.