Novel and efficient synthetic methodologies for nitrogen-containing heterocycles by utilizing a wide range of functionality-containing carbodiimides as vital building blocks are described. The key steps involve the iminophosphorane-mediated aza-Wittig reaction with isocyanates, followed by a variety of ring-forming reactions such as electrocyclization, Diels-Alder reaction, and various intramolecular reactions together with preliminary transformation reaction if necessary.