Background The derivatives of 2-substituted 4H-3,1-benzoxazin-4-one belong to a significant category of heterocyclic compounds, which have shown a wide spectrum of medical and industrial applications. Results A new and effective one-pot method for the synthesis of 2-substituted 4H-3,1-benzoxazin-4-one derivatives is described in this paper. By using the iminium cation from a mixture of cyanuric chloride and dimethylformamide as a cyclizing agent, a series of 2-substituted 4H-3,1-benzoxazin-4-one derivatives was synthesized in high yield under mild conditions and simple workup. Conclusions The iminium cation from a mixture of cyanuric chloride and N,N-dimethylformamide is an effective cyclizing agent for the room temperature one-pot synthesis of 2-substituted 4H-3,1-benzoxazin-4-one derivatives in high yields through a cyclodehydration reaction. Furthermore, the method was performed under mild conditions characterized by simplified pathways and workup, minimized energy, and fewer reaction steps, compared with the previous methods. The proposed method, which is a simpler alternative than the published methods, is applicable for the synthesis of other 2-substituted 4H-3,1-benzoxazin-4-one derivatives.