Cycloaddition reactions figure prominently in both synthetic and mechanistic
organic chemistry. The current understanding of the underlying principles in this
area has grown from a fruitful interplay between theory and experiment. The monumental
work of Rolf Huisgen and co-workers in the early 1960s led to the general
concept of 1,3-dipolar cycloaddition. Few reactions rival this process in the number
of bonds that undergo transformation during the reaction, producing products
considerably more complex than the reactants. Over the years, this reaction has
developed into a generally useful method for five-membered heterocyclic ring
synthesis, since many 1,3-dipolar species are readily available and react with a wide
variety of dipolarophiles.