With photosubstrate 9a in hand, we examined the proposed stereoselective isomerization to 12a. To this end, solutions of 9a (0.03 M) were irradiated with 350 nm UV light at 35 °C to generate cyclobutane 10a. This reactive species was difficult to isolate; therefore, aliquots of the photolysate were analyzed by 1 H NMR to follow the photocycloaddition. The choice of solvent impacted both reaction efficiency and stereoselectivity (Table ). The best results were obtained in THF (0.1 M); however, MeCN, PhMe, and n-BuOH emerged as alternative solvents. To our delight, photolysis of 9a in THF generated 10a as the main product (9:1 d.r. at C3). The reaction mixture was then removed from UV light, treated with Et3N, and heated to 100 °C for 3 h. This single-operation procedure generated 12a as a crystalline solid in 64% yield from 9a. This allowed the relative stereochemistry of 12a to be conclusively established by single crystal X-ray diffraction.