The use of concentrated sunlight in chemical processes is hitherto restricted to provide high temperatures. On the other hand, sunlight can also be used photochemically. In this article we describe results on the application of highly (> 1000 suns) concentrated sunlight in photochemical synthesis of fine chemicals. In a former photochemical investigation with moderately concentrated sunlight (20 suns) on the area of the Plataforma Solar de Almeria, Spain, we used a parabolic reactor. The experiments with highly concentrated sunlight are carried out in a solar furnace. To meet the requirements of a photochemical process in a solar furnace a new type of reactor had to be constructed. The [2+2] cycloaddition reaction of a carbonyl compound to an olefin, using UV-light of the sun, was tested under these conditions. The efficiency of photochemical synthesis in highly and moderately concentrated sunlight is compared.