Chemical reactions induced by light have been utilized for synthesizing highly strained, thermodynamically unstable compounds, which are inaccessible through non-photochemical methods. Photochemical cycloaddition reactions, especially those leading to the formation of four-membered rings, constitute a convenient route to compounds that are difficult to prepare by other methods. Although the theory of cycloaddition reactions is encountered in advanced texts on organic chemistry most practical illustrations of the theory restrict themselves to [4+2] thermally mediated Diels-Alder cycloadditions. In this article, the authors present a [2+2] cycloaddition that is relatively easy to carry out using sunlight for photochemical mediation. (Contains 1 table and 5 notes.)