Photodriven solid-state multiple [2 + 2] cycloaddition strategies for the construction of polycyclobutane derivatives

Solid-state multiple [2 + 2] cycloaddition reactions of polyenes continue to attract attention as a mediate for the synthesis of polycyclobutane derivatives. The solid and crystalline states of polyenes restrict molecular movements of CC bonds and, as a result, contribute to the selective formation of polycyclobutane derivatives through photochemical [2 + 2] cycloaddition reactions. In this review, functionalised polyenes preorganised by suitable templates, such as muconate and 4-pyr-poly-2-ene (ppene) derivatives, are summarised and their multiple [2 + 2] cycloaddition reactions are described, which lead to the generation of polycyclobutane derivatives in the solid state. Single-crystal-to-single-crystal (SCSC) transformations in which the single-crystal character is preserved along the reaction sequence are emphasised, as they provide unambiguous information regarding the structural changes during the reaction.