Diels-Alder Reactions Involving Allylidene cyclopropanes

4 Diels-Alder Reactions Involving Allylidene Cyclopropanes 

The formation of the meta-type adducts instead of the ortho ones can be rationalized on the basis of a mechanism involving radicals or ionic intermediates in the transition state (scheme 84). Intermediate structures lb) and 2b) can account for the results usually observed with dienes bearing alkyl groups whereas intermediates such as la) and (2a), in which the strain due to the cyclopropyl ring is 

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