The photoinduced isomerization of double bonds is a fundamental reaction in photochemistry. While deep UV light is necessary for isomerization of simple olefins, UV-A light is sufficient for α,β-unsaturated enones. For cyclic systems like cyclohept-2-enone (1), this isomerization leads to strained (E)-configured intermediates (2) that are activated to undergo further reactions. A subsequent Diels–Alder reaction furnishes unique trans-configured bi- or tricyclic products (3), depending on the employed diene (Figure 1). The first example of a Diels–Alder reaction of photochemically generated (E)-cycloalk-2-enones was published in 1964. Since then, only selected examples have been reported. Therefore, we turned our attention to this underexplored yet fascinating reaction and evaluated its substrate scope in-depth (Figure 2a). We were pleased to find that this photoinduced Diels–Alder reaction generates a variety of corresponding polycyclic products in good to excellent yields. With modern analytical capabilities, isomeric ratios or stereochemical assignments of products could be precisely determined. In addition, we applied this transformation to the synthesis of the natural product trans-α-himachalene (Figure 2b). With UV/Vis and NMR spectroscopy as well as theoretical calculations, we gained a better understanding of the reaction pathway (Figure 2c). In the future, we want to use chiral Lewis acid catalysts to achieve enantioselective photoinduced Diels–Alder reactions and apply our knowledge to other systems, in which a strained intermediate can be generated photochemically.

Photochemically generated (E)-cycloalk-2-enones in Diels–Alder reactions: Substrate scope, reaction pathway, and synthetic application

ACS Spring 2022

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