Intermolecular Trapping of the Nazarov Intermediate

A noteworthy observation is the lack of [4+3] or [3+2] cycloaddition products when using cyclic dienes to trap the oxyallyl intermediate. Indeed, the use of furan or 1,3-cyclohexadiene 125 gave only alkylation product 126. One can assume that significant steric hindrance between these dienes and the oxyallyl cation in the [4+3]- and [3+2] cycloaddition transition states would prevent these transformations from occurring and favor the Friedel-Crafts alkylation process. 

For review see (a) Habermas, K.L., Denmark, S.E, and Jones, T.K. (1994) The Nazarov cyclization, John Wiley Sons, Inc., New York (b) Pellissier, H. 

---