One of the most intriguing aspects of heterocyclic chemistry is the conversion of one heterocyclic system to another.'These processes occur via ring opening of the heterocycle under acidic, basic, thermal, or photochemical conditions followed by recyclization of an acyclic intermediate. Cycloadditions can also provide an approach to heterocyclic ring transformations. Because of its reactivity toward acids, bases, heat, dienophiles, and ...
