The photochemistry of a number of arylidenecyclopentanone oxime 0 - acetates and methyl ethers has been investigated. Irradiation of several of these compounds in methanol leads to initial E-Z geometrical isomérisation and ultimately to the formation of nitrogen containing heterocycles via a 671-electron photocyclisation process followed by an elimination. The photocyclisation has been investigated for aryl groups containing both electron withdrawing and electron donating substituents. Cyclisation is not observed in the case of electron withdrawing groups such as 2-(4- cyanobenzylidene)cyclopentanone oxime O-acetate, 2-(3-cyanobenzylidene) cyclopentanone oxime O-acetate, 2-(3-fluorobenzylidene)cyclopentanone oxime O-acetate and 2-(3-chlorobenzylidene)cyclopentanone oxime O-acetate, whereas the presence of electron donating groups such as 2-(3- methoxybenzylidene)cyclopentanone oxime O-acetate and 2-(3- methylbenzylidene)cyclopentanone oxime O-acetate facilitates the reaction. The regiospecificity of the reaction has also been investigated, and in all cases where the photocyclisation proceeds, only one product is isolated. Both acetates and methyl ethers yield the same cyclised photoproducts. The photoreactivity of the arylidenecyclopentanone oxime O-acetates has also been investigated in natural sunlight at the Plataforma Solar de Almería (PSA) in the south of Spain. The photoreactions proceed in the same manner as in the laboratory yielding the desired products. Reasonable conversions are achieved, and the apparatus available at the PSA makes large scale and inexpensive heterocycle syntheses achievable.