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Donor-acceptor cyclopropanes as three-carbon components in a [4+3] cycloaddition reaction with 1,3-diphenylisobenzofuran.

Substituted cyclopropanes have found broad application in modern organic synthesis owing to the unique reactivity of the cyclopropane moiety. Cyclopropanes can often be considered as three-carbon analogues of C=C bonds. For example, alkenes and cyclopropanes react with strong electrophiles and various radicals. Both undergo the addition of hydrogen and can be oxidized at the a position. The reactivity of cyclopropanes with electron-withdrawing substituents is similar to that of electron-deficient alkenes. However, the cycloaddition reactions of alkenes and cyclopropanes are quite different. In particular, the thermal reactions of alkenes are represented mainly by [1+2], [3+2], and [4+2] cycloaddition processes; thermal [2+2] cycloaddition occurs in very specific cases only. In contrast, the most well known type of cyclopropane cycloaddition is the [2p + 2s] reaction with alkenes. As this reaction yields cyclopentanes, it can also be considered as a [2+3] cycloaddition. The scope of such [2+3] cycloaddition reactions has been expanded significantly through the use of donor–acceptor cyclopropanes. The presence of both electron-donating and electron-withdrawing substituents on the cyclopropane ring enables cycloaddition to various multiple bonds, including C=C, C=O, C=N, and C N bonds. The [4+3] cycloaddition of cyclopropanes with dienes (Scheme 1) has not been reported previously, although this