Concern for the environment has been driving major changes in the coatings industry.

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.

A metal-free Hantzsch ester-mediated synthesis of cyclopentenylketones as well as γ-hetarylketones starting from ketocyclopropanes under eco-friendly conditions was developed. The versatility of the developed conditions is shown by reacting ketocyclopropanes in both a formal [3 + 2] cycloaddition with terminal alkynes (further investigated using theoretical calculations) and a radical C−C-coupling with cyanopyridines. The newly developed methodologies were later on utilized as a downstream reaction for photogenerated cyclopropanes combining UV and visible light photochemistry. Following this procedure, a UV-driven Norrish−Yang-type reaction induces the ring strain of the intermediates, which serves as activation energy for the subsequent ring transformation.

A synthesis of highly substituted 2,4‐diacylpyrroles through a Cu‐catalyzed dimerization of acylazirines generated in situ by a photochemical valence isomerization is described. The shown methodology allows the use of simple precursors and a readily available copper(II)‐catalyst. Since the reaction is best performed at elevated temperatures, a method to adjust the temperature of the reaction mixture during this dual light and metal‐induced process was established. Additionally, mechanistic studies of the reaction were performed in order to provide a deeper understanding of the chemistry of 2‐acylazirines.