91Ó°ÊÓ

In a cycloaddition reaction, two pie-bond-containing reactants form a cyclic compound by rearranging their pie electrons and forming two new pie bonds.

The Diels–Alder reaction is the best-known example of a cycloaddition reaction which bare of following types:

• [4 + 2] cycloaddition
• [4 + 2] cycloaddition
• [4 + 2] cycloaddition

The reactionbetween1,3-butadiene and 2-cyclohexenone(concerted reaction)in ultraviolet light will form the 8-vinylbicycloctan-2-one as there is a formation of the small ring (4-membered ring). The ring closure will be suprafacial.

Under photochemical conditions, suprafacial ring closure requires an even number of pie bonds in the reacting system. Thus a concerted reaction will occur, but it will use only one of the pie bonds of 1,3-butadiene shown as follows:

Formation of 8-vinylbicycloctan-2-one