Lewis Acid-Promoted Ketene–Alkene [2 + 2] Cycloadditions

Described are the first examples of ketene–alkene
[2 + 2]
cycloadditions promoted by Lewis acids. Notable features of this method
include (1) substantial rate acceleration relative to traditional
thermal reactions, (2) good diastereoselectivities and yields for
the formation of the cyclobutanone products, and (3) inverse diastereoselectivity
compared with related thermal cycloadditions for many examples. These
studies not only provide access to synthetically versatile cyclobutanones
that cannot be prepared by traditional thermal cycloadditions but
also address important mechanistic questions regarding ketene–alkene
[2 + 2] cycloaddition reactions.