Modern synthetic chemistry has evolved to such an extent that exquisite levels of chemo-, regio-, and stereocontrol can be readily achieved within complex, multifunctional molecules. Reaction control in synthetic photochemistry is a much more difficult prospect where the basic bond-forming step is generally controlled by the lifetime of a key excited state, the formation of which is often dominated by complex structural and photophysical issues. Different reaction pathways of a single chromophore (e.g. C=C bond) can be observed by populating either singlet or triplet states.[1] Further limited modes of selection can be achieved by irradiation at userselected wavelengths, [2] through higher exited states from multiphoton absorption, [3] and by feedback-based optical control using evolutionary algorithms and pulse shapers. [4] More recently our research group has demonstrated that photon flux can be a useful parameter in controlling reaction pathways, albeit within the scale limitations imposed by tunable dye lasers.[5]Herein we demonstrate how sensitized and nonsensitized reactions of N-alkenyl maleimides lead to a selective reaction from different bonds (CÀN vs. C=C) within the same molecule. This selection enables either a [5+2] or [2+2] cycloaddition pathway to be chosen, thus providing selective synthesis of complex 7,5-fused azepines or cyclobutanes, respectively.We previously described the intermolecular [2+2] photocycloaddition reactions of tetrahydrophthalimide anhydride 1 (Scheme 1) with alkenol and alkynol partners.[6] We found these reactions to be remarkably efficient, with the [2+2] cycloadducts (e.g. 2) being formed in high yields and with excellent stereoselectivity. In contrast, the attempted intramolecular [2+2] photocycloaddition of the pentenylsubstituted imide 3 yielded the tricyclic azepine 5 exclusively in excellent yield by a [5+2] pathway (attributed to cleavage of the C À N bond). [7] This observation has since proved to be a general trend with maleimide derivatives: [2+2] photocycloaddition is observed with intermolecular reactions and [5+2] photocycloaddition with intramolecular variants.More recently we have focused on exploring the scope of this [5+2] cycloaddition and its application in natural product synthesis.[8] During model studies towards the synthesis of the Stemona alkaloids we observed that an N-alkenyl maleimide displayed atypical [5+2] behavior (Scheme 2). Irradiation of the maleimide system 6 (containing an exocyclic alkene unit) led to a high yield of cycloaddition products, but for the first time the intramolecular [2+2] adduct dominated. Although chemically efficient, the low quantum yields (F [2+2] = 0.018 and F [5+2] = 0.009) illustrate photochemically inefficient bond-forming processes. This observation was initially disregarded as an anomaly; however, recent similar results have