[2+2]-Photocycloadditions in the Solid State

During the 1960s and 1970s, Schmidt and co-workers studied the solid-state [2+2]-cycloadditions of cinnamic acids thoroughly and systematically.

From the crystallographic investigations, topochemical rules, which connect the configuration of the product and crystal structure of the reactant, were revealed. It is well known that for topochemical [2+2]-photocycloaddition, the distances between the potentially reactive double bonds should be less than ~4.2 Å.