2-Amino benzamidoxime (ABAO compound 6) swiftly reacts with aldehydes in aqueous solution to form a stable 1,2-dihydroquinazoline-3-oxide (dihydroquinazoline derivative). The reaction involves the formation of a Schiff base as the rate-determining step, followed by rapid intramolecular cyclization. The reaction rate is pH-dependent, indicating that protonated benzamidoxime acts as an internal general acid in Schiff base formation. Substituents on the aromatic ring can increase the basicity of the aromatic amine, accelerating the reaction. The reactive properties of 2-Amino benzamidoxime suggest its potential as a platform for developing new bioconjugation strategies, fluorescent probes, and post-translational diversification of genetically encoded libraries.