Bichromatic Photosynthesis of Coumarins by UV Filter‐Enabled Olefin Metathesis

Herein, we report on a two‐step bichromatic synthesis of coumarins involving UV‐A and UV‐C light. The first step is a UV‐A‐photoinduced ruthenium‐catalyzed cross‐metathesis (CM) reaction of 2‐nitrobenzyl‐protected 2‐hydroxystyrenes with acrylates, using an external solution of 1‐pyrenecarboxaldehyde as a UV filter. Irradiation in the absence of the filter permanently inhibits the light‐activated catalyst due to photocleavage of the photolabile protecting group (PPG) and ensuing phenolate chelation to the ruthenium. The simple removal of the external filter after CM allows further photochemical reactions with UV‐C light to achieve more complex architectures, such as the coumarins presented in this work.