1,3-Dipolar cycloaddition

The 1,3-dipolar cycloaddition, also known as the Huisgen cycloaddition or Huisgen reaction, is an organic chemical reaction belonging to the larger class of cycloadditions. It is the reaction between a 1,3-dipole and a dipolarophile, most of which are substituted alkenes, to form a five-membered ring. Rolf Huisgen first saw the prospects of varying the 1,3-dipole and its high value for synthesis of 5-membered heterocycles.

Classes of 1,3-dipoles
Huisgen and others investigated a range of 1,3-dipoles and dipolarophiles. These included:
 * Azides and alkynes react in the Azide alkyne Huisgen cycloaddition.
 * Diazo compounds are 1,3-dipoles in the Diazoalkane 1,3-dipolar cycloaddition.
 * Nitrones react with alkenes to form isoxazolidines.
 * Ozonolysis begins with a 1,3-dipolar cycloaddition of ozone. This is followed by a retro-1,3-dipolar cycloaddition and then a 1,3-dipolar cycloaddition of the fragments.