Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction
Cycloadditions: 1,3-Dipolar Cycloadditions
1,3-Dipolar cycloadditions represent synthetically important reactions for the preparation of heterocyclic five-membered rings. They belong into the group of [4+2] cycloadditions according to the Woodward-Hoffmann classification.
The 1,3-dipolar compounds contribute four electrons to the reaction, therefore their structural formulae can only be described by indicating formal charges.
The mechanism of alkene ozonolysis is a sequence of 1,3-dipolar cycloadditions and cycloreversions.