Diterpenes are a rich source of biologically active molecular scaffolds. Members of this family harboring a fused [6-6-6] carbocyclic nucleus have gained significant attention for their valuable pharmacological profiles. This subgroup of natural products includes abietane, pimarane, and kaurane diterpenes, which all share copalyl diphosphate as a common biosynthetic progenitor (Figure ). Although the skeletal diversity of these diterpenes is localized within the C-ring, each motif is further differentiated through late-stage oxidative tailoring. This added layer of complexity hinders the design of unified synthetic entry to this diverse collection of targets. Recent advances in synthetic biology have streamlined entry to this subgroup from more accessible terpenoid starting points. In contrast, polyene cyclization provides a generalized platform for de novo diterpene synthesis. This bioinspired strategy seeks to reorganize isolated polyenes (1) to polycycles (2) through a series of concerted C-C bond formations. Both cationic and radical reaction modalities are known, as are enantioselective adaptations catalyzed by Lewis acids, organocatalysts, and transition metals. The outcomes of polyene cyclization are predictable and governed by explicit aspects of alkene stereochemistry and electronics. This powerful feature is often exploited in total synthesis; 9c however, it also makes it difficult to extend this strategy to diversified substrates (e.g. 1a-1c) that deviate from biogenic isoprenoid precursors. Inspired by the photobiology of vitamin A, we envisioned a different polyene cyclization strategy. In place of isolated polyene 1, we targeted conjugated b-ionyl derivative 3. Following Buchi's investigations on the photoisomerization of b-ionone to Z-retro-g-ionone, we reasoned that diene 4 could be unveiled from 3 via a photoinduced [1,5]-hydride shift. The bifurcated p-systems in 4 could then be connected via a Heck cyclization, where the position of the C-X bond would guide site-specific incorporation of the aryl ring. We predicted that a 6-exo-trig pathway to generate alkyl metal species A would be favored, thereby providing an avenue to modify the oxidation-state of C20, most directly by cyclization to propellane 5. Fragmentation of this overbred species was viewed as handle for skeletal editing. Moreover, this sequence would allow 5 to be assembled from interchangeable fragments. We expected these features to not only expedite entry to diterpene natural products, but also simplify the design of tailored compositions of matter that are beyond the reach of semi-synthesis. Herein, we report the successful implementation of this programmed polycyclization strategy and highlight its utility for the synthesis of canonical and rearranged abietane diterpenes. We began by examining the photoisomerization of b-ionyl derivatives 3. While the photosensitized geometric isomerization of b-ionyl species is established, 18 positional alkene isomerization via a [1,5]-hydride shift is an underexplored pathway observed upon direct irradiation. Thus, it was not clear if this reaction would be broadly synthetically useful. To address this concern, a set of electronically differentiated photosubstrates 3 were prepared from benzylic alcohols 6 via conversion to aryl phosphonates 7 and olefination with b-cyclocitral. As shown in Table , we were pleased to find that irradiation of 3a (R 1 , R 2 = OMe) with 350 nm light in deoxygenated THF provided Z-diene 4a in 90% yield (entry 1). The reaction time was reduced to 1 h using 300 nm light (entry 2). In contrast, 254 nm light resulted in decomposition (entry 3). Dioxane and MeCN gave similar results to THF (entries 4 and 5); however, the reaction rate slowed as the concentration of 3a increased. Guided by these observations, we developed a general procedure using 300 nm light in dioxane (0.2 M) that provided 4a in 87% yield after 5 h (entry 6). This transformation was remarkably efficient, with no evidence of other products by 1 H NMR. Moreover, these mild conditions were extended to differentiated cyclization substrates 3b (R 1 , R 2 = H) and 3c (R 1 = CF 3 , R 2 = H) to generate 4b (90% yield) and 4c (86% yield) as single isomers, respectively (entries 7 and 8).