A form of cycloaddition across the terminal atoms of a fully conjugated system with formation of two new sigma bonds to a single atom of the ("monocentric") reagent. There is formal loss of one pi bond in the substrate and an increase in coordination number of the relevant atom of the reagent. An example is the addition of sulfur dioxide to butadiene:
The reverse of this type of reaction is designated "cheletropic elimination". WOODWARD and HOFFMANN (1969).
