Periselectivity cyclopentadiene

Another example of cycloaddition selectivity was observed for the reaction of cyclopentadiene with diphenylketene (Equation (37)). Originally only products of [2+2] cycloaddition were observed and these reactions were considered as highly periselective. 

There is a special kind of site-selectivity which has been called periselectivity. When a conjugated system enters into a reaction, a cycloaddition for example, the whole of the conjugated array of electrons may be mobilized, or a large part of them, or only a small part of them. The Woodward-Hoffmann rules limit the total number of electrons (to 6, 10, 14 etc. in all-suprafacial reactions, for example), but they do not tell us which of 6 or 10 electrons would be preferred if both were feasible. Thus in the reaction of cyclopentadiene (355) and tropone (356), mentioned at the beginning of this book, there is a possibility of a Diels-Alder reaction, leading to 354, but, in fact, an equally allowed, ten-electron reaction is actually observed,121 namely the one leading to the adduct (357). The product is probably not thermodynamically much preferred to the... 

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