A Simple [2 + 2] Photocycloaddition Reaction that Proceeds in an NMR Tube Illuminated by Daylight

This work presents a simple and small-scale procedure that demonstrates the use of daylight to promote a photochemical [2 + 2] cycloaddition reaction. Previous related experimental protocols used in teaching laboratories have been focused on the preparation of “Cookson’s ketone” (pentacyclo-[5.4.0.0^2,6.0^3,10.0^5,9]undecane-8,11-dione). These approaches require an initial Diels–Alder reaction of cyclopentadiene and 1,4-benzoquinone, followed by an ultraviolet (UV) light-promoted intramolecular [2 + 2] cycloaddition reaction. This Laboratory Experiment highlights an alternative approach, where an analogue of Cookson’s ketone is readily prepared on a small scale from a stable diene and the photochemical [2 + 2] reaction proceeds in an NMR tube illuminated by daylight. The synthesis and NMR experiments described have been used in an advanced-level university laboratory course as an example of photochemical synthesis and use of 1D and 2D NMR spectroscopy to monitor reaction progress.