Strain Release Chemistry of Photogenerated Small-Ring Intermediates.

Photochemical processes, such as isomerizations and cycloadditions, have proven to be very useful in the construction of highly strained molecular frameworks. Photoinduced ring strain enables subsequent exergonic reactions which do not require the input of additional chemical energy and provides a variety of attractive synthetic options leading to complex structures. This review covers the progress achieved in the application of sequences combining excitation by ultraviolet light to form strained intermediates, which are further transformed to lower energy products in strain-release reactions. As ring strain is considerable in small ring systems, photogenerated three- and four-membered rings will be covered, mainly focusing on examples from 2000 to May 2020.