We developed a general method for the selective photochemical homo- and heterodimerization of cinnamic acid derivatives with the use of commercially available 1,8-dihydroxynaphthalene as a covalent template. A variety of symmetrical and unsymmetrical β-truxinic acids were obtained in high yields and as single diastereomers. The use of a template not only provides the alignment of the two olefins with suitable proximity (<4.2 Å) but also allows the heterodimerization of two different cinnamic acids, leading to unsymmetrical β-truxinic acid products.