Bichromatic Photosynthesis of Coumarins by UV Filter‐Enabled Olefin Metathesis

Abstract

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 2hydroxystyrenes with acrylates, using an external solution of 1-pyrene carboxaldehyde 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.

An Overview of Coumarin as a Versatile and Readily Accessible Scaffold with Broad-Ranging Biological Activities

“…In fact, irradiation with UV-C light started a threereactions chain-photodeprotection of the intermediate, E/Z isomerization and cyclization to form the desired coumarins (Scheme 5). Another example is the bichromatic synthesis of coumarins proposed by Eivgi and collaborators in 2017 [293]. In this work, 2-nitrobenzyl-protected 2-hydroxystyrenes and acrylates underwent a UV-A (380 nm) photoinduced cross metathesis (CM) reaction, in the presence of ruthenium as a catalyst.…”

“…Another example is the bichromatic synthesis of coumarins proposed by Eivgi and collaborators in 2017 [ 293 ]. In this work, 2-nitrobenzyl-protected 2-hydroxystyrenes and acrylates underwent a UV-A (380 nm) photoinduced cross metathesis (CM) reaction, in the presence of ruthenium as a catalyst.…”

Guiding a divergent reaction by photochemical control: bichromatic selective access to levulinates and butenolides

“…To promote the opposite selectivity, we envisioned applying the molecular ‘UV-C filter’ protocol 8 to prevent the photochemically more demanding 1,5-H shift ( vide infra ). Indeed, after screening several UV-C filters and reaction conditions, the irradiation of a DCM solution of E - 3a with phenanthrene afforded 71% isolated yield of the butenolide ( Table 1 , entry 1).…”

“… 7 Recently, we employed UV light absorbing organic molecules in a selective deprotection of photolabile protecting groups and photo-induced cross-metathesis. 8 Moreover, the selective use of light of different frequencies to promote distinct chemical reactions, i.e. chromatic orthogonality, 9 has been applied in synthetic procedures, including the synthesis of a natural peptide product.…”

Sunscreen-Assisted Selective Photochemical Transformations

“…To address this problem, we planned to use the sunscreen methodology and protect the phenol functionality with a PPG. By using a suitable filter molecule, we aimed to selectively activate a photoswitchable metathesis catalyst, exploiting the strong absorption of the ruthenium catalysts in the UV-A region [52]. As a sunscreen, we selected 1-pyrenecarboxaldehyde, which is a dye known for its strong absorption in the UV-A region, and in order to avoid undesired side reactions, the 1-pyrene carboxaldehyde filter was only used externally.…”