Corelli developed an example of this type of reaction that employs highly strained alkynes that react selectively with azides. Which side of the array of combining orbitals the two new bonds generated in this type of reaction occur on determines whether these reactions are suprafacial or antarafacial. A retro-version of this type of reaction generates the Criegee intermediate; that example of this type of reaction occurs during ozonolysis. In click chemistry, a reaction of this type named for Huisgen is a popular building block. One well-known example of this class of 
Corelli developed an example of this type of reaction that employs highly strained alkynes that react selectively with azides. Which side of the array of combining orbitals the two new bonds generated in this type of reaction occur on determines whether these reactions are suprafacial or antarafacial. A retro-version of this type of reaction generates the Criegee intermediate; that example of this type of reaction occurs during ozonolysis. In click chemistry, a reaction of this type named for Huisgen is a popular building block. One well-known example of this class of reactions results in the formation of the endo product when a diene and a dienophile combine; the 4-pi plus 2-pi example of this class of reactions is the Diels-Alder reaction. For 10 points, name this class of pericyclic reactions in which two pi bonds are converted to sigma bonds as a ring is formed.
Corelli developed an example of this type of reaction that employs highly strained alkynes that react selectively with azides.
cycloaddition [prompt on pericyclic reactions; prompt on any specific example before it's mentioned, e.g. prompt on Diels-Alder if someone buzzes with that at "dienophile"]