A General Selection Rule for Pericyclic Reactions

In the Conia-ene reaction, a carbonyl compound enolizes and then undergoes an intramolecular ene reaction. The enol fragment serves as the ene component of the reaction, while the olefin fragment serves as the enophile. An example is the reaction of 8-methyl-7-nonen-2-one (92), which yields 2-isopropylcyclopentyl methyl ketone (93, equation 11.41).  

We have developed a different set of selection rules for electrocyclic, sigma-tropic, cycloaddition, and other concerted reactions, but the fundamental principle—the conservation of orbital symmetry—is the same in all cases. Now we will see that all of these reactions can be considered to be variants of cycloaddition reactions. To do so, we must first note that a cr bond can participate in a cycloaddition process, just as can a n bond, with the following provisions  

The participation of a o- bond is suprafacial if the cycloaddition occurs on the inside (overlapping) lobes of the two sp hybrid orbitals or on their outside lobes. 

The (T bond adds antarafacially if one cycloaddition occurs on an inside lobe and one occurs on an outside lobe. 

Alternative representations for electrocyclic reactions considered as cycloadditions. 

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