Photochemical [2+2] cycloaddition reactions of alkenes still constitute the most direct and effective method for the construction of cyclobutane scaffold. Despite the remarkable advances in this area, accessing unsymmetrical, multi-substituted cyclobutanes in a regio-, diastereo- and enantioselective manner continues to be a challenge. In this respect, while photodimerization reactions of cinnamic acids have attracted significant attention in the synthetic community for a long time, their selective heterodimerization reactions remained an unsolved problem. In order to solve this longstanding problem, we have recently developed a highly effective method which enables selective homodimerization and heterodimerization reactions of a variety of cinnamic acid derivatives to afford symmetrical and unsymmetrical β-truxinic acid products. The method utilizes 1,8-dihydroxynaphthalene as a covalent template and involves a photochemical [2+2] cycloaddition reaction in the solid state. The reaction tolerates a broad range of electron-rich and electron-deficient aromatic and heteroaromatic groups at the β-position of the α,β-unsaturated carboxylic acid substrates. This way, irradiation with UV light affords [2+2] cycloaddition products as single diastereomers and in high yields (78-99%) which can be detached easily from the template by a straightforward ester hydrolysis step. We have also shown the scalability of the method by performing the photochemical reaction in n-heptane as a suspension. Single-crystal X-ray analyses did not only confirm the relative stereochemistry of the cycloadditon products, but also revealed that the distance between the reacting olefin centers is ca. 3.6 Å, which is well-suited for a photochemical [2+2] cycloaddition in the solid state according to the Schmidt criteria. Our preliminary results on the photodimerization of 2,4-dienoic acids will also be presented.

Access to unsymmetrical cyclobutanes via template-directed, diastereocontrolled photochemical [2+2] cycloadditions

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ACS Fall 2022

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