A novel and straightforward method for one-pot synthesis of functionalized 2,3,6,7,9-pentaazabicyclo[3.3.1]nonane derivatives has been developed. The process, which involves ultrasonic irradiation promoted Mannich reaction between 3-oxo-2-arylhydrazonopropanals and ammonia in acetic acid, is catalyzed by N,N-dimethylformamide dimethylacetal (DMF-DMA) and generates the target products in 80–93% yields.