
A cycloaddition is a pericyclic chemical reaction, in which `two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity.` The resulting reaction is a cyclization reaction. Many but not all cycloadditions are concerted. As a class of ad.....
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http://en.wikipedia.org/wiki/Cycloaddition

A reaction in which two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity. The following two systems of notations have been used for the more detailed specification of cycloadditions, of which the second, more recent system (describe...
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http://www.chem.qmul.ac.uk/iupac/gtpoc/CoCy.html

the formation of a cyclic compound by the addition reaction of unsaturated molecules.
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https://www.infoplease.com/dictionary/cycloaddition
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