The isomerization of 3a was explored in detail (Scheme 1). Analysis of a test reaction (hv = 300 nm, 0.1 M in THF) by H NMR revealed initial isomerization of the exocyclic alkene to generate a mixture of (E)-3a and (Z)-3a. These species were present in a 1:1 ratio after 1 h, alongside 4a. After 2 h, (E)-3a was consumed and a mixture of (Z)-3a and 4a persisted until the reaction was complete. We prepared (Z)-3a independently and found that the photochemistry of this isomer was indistinguishable from (E)-3a. Thus, the initial geometry of the exocyclic alkene was inconsequential. In contrast, 4a did not react with 300 nm light, yet reverted to (E)-3a in >95% yield when heated to 150 °C via a thermal (retro) [1,5]-hydride shift. These results indicate that 4a is metastable intermediate that likely accumulates because disruption of the extended p-system prevents re-excitation. This conclusion was supported by analyzing the absorption spectra of (E)-3a, (Z)-3a, and 4a in THF. These data revealed significant hypochromic shifts for (Z)-3a and 4a relative to (E)-3a, which is consistent with impaired conjugation. Notably, the π-system in (Z)-3a is distorted by allylic strain, which also restricts interconversion to the s-trans conformer. This same steric effect may also facilitate a photochemical [1,5]-hydride shift within (Z)-3a to give 4a, a process that requires an otherwise difficult-to-achieve antarafacial transition state. Our next goal was to execute the proposed Heck reaction within 4a. After some experimentation, we identified the combination of Pd 2 dba 3 /P(t-Bu) 3 (1:1 Pd/P) and Cy 2 NMe described by Fu to be the most effective. Exposure of 4a to this catalyst system in dioxane at 85 °C gave 5a in 93% yield. As shown in Scheme 2, the photoisomerization and Heck cyclization were merged into a single operation. This more convenient process afforded 5a in 76% yield from 3a on gram-scale. Moreover, this process was effective across a range of cyclization precursors (3b-3k). Accordingly, C-ring variants 3b-3e with peripheral substituents at every position around the aryl nucleus were tolerated. By design, a single regioisomer of 5 was obtained in every case. In addition, the reaction was effective for electronrich (5a and 5e), non-activated (5b), and electron-deficient (5c and 5d) substrates. This feature allowed us to prepare variants of 5 harboring p-deficient (5f-5h) and p-excessive (5i) heterocyclic rings. Modifications of the A-ring were also investigated. For example, the addition of an alcohol at C1 provided 5j in 2:1 d.r. (61% yield). This selectivity was readily improved to >25:1 d.r. in 5k (75% yield) when a bulky silyl protecting group was included. In contrast, propellane 5l lacking a gemdimethyl group at C4 was a limitation. In this case, photolysis of 3l did not produce isomer 4l. Instead, E®Z isomerization of the exocyclic alkene was the exclusive reaction. This outcome demonstrates the importance of allylic strain introduced by the gem-dimethyl group in the [1,5]-hydride shift process.