1,4-Zwitterionic intermediates formed by cleavage of a cyclobutane ring and their cycloaddition reactions

Cycloaddition reaction is an efficient organic reaction because all of the substrate components are introduced into a product skeleton. Recently, more than six-membered rings have been synthesized by Lewis acid-promoted ring cleavage of a cyclobutane ring followed by addition to an unsaturated bond. Typical examples in the literature of these types of reactions using 1-donor-2-acceptor cyclobutanes and 3-donor cyclobutanones with Lewis acids are presented in this Letter.