Cycloaddition and Annulation Reactions of Donor–Acceptor Cyclopropanes

Cycloaddition reactions are the most studied reactions of donor–acceptor cyclopropanes in any category. The polarization of the activated carbon-carbon bond of the donor–acceptor cyclopropane (DAC) makes it a perfect synthon for a 1,3-zwitterionic intermediate on activation. So far, the most common activator that has been exploited for the activation of DACs toward formal cycloaddition/annulation reactions is the metal-based Lewis acids. However, a number of methods developed for such cyclization processes also involved metal-free activators. In addition to the 1,3-zwitterionic intermediate, various DACs are also known to serve as precursors for styryl malonates and 1,2-zwitterionic intermediates. Also, there are some examples where DACs act as sources of 1,3- and 1,4-zwitterionic species by using the aryl donor as part of the zwitterionic subunit. Consequently, DACs are known to undergo numerous formal cycloaddition/annulation processes with diverse reacting partners involving C–C and carbon–heteroatom multiple bonds, as well as with many other dipolar species discussed in the domain of this chapter.