Now, Paul et al. demonstrate a different method to obtain these structures, by mixing secondary or primary alcohols with vinyl sulfonium ions in the presence of quinuclidine, a photocatalyst (either iridium complex {Ir[dF(CF3)ppy]2(dtbpy)}PF6, where dF(CF3)ppy and dtbpy are ligands 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine and 4,4′-di-tert-butyl-2,2′-dipyridyl, respectively, or 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene, also known as 4CzIPN) and a base under blue LED light irradiation (pictured). Under photoirradiation, the photocatalyst–quinuclidine system generates a radical in the α-position to the alcohol group, which is nucleophilic enough to add onto the vinyl sulfonium ion. This generates a radical cation which is reduced to a sulfonium ylide, closing the photoredox cycle. This ylide is a good leaving group that can be intramolecularly attacked by the hydroxyl group, generating the oxetane.
The researchers demonstrate application of their methodology to a variety of primary and secondary aliphatic, linear and cyclic alcohols with different functional groups. Reaction with cyclic alcohols and also more complex structures, such as adamantol, leads to the formation of spirocycles. They also demonstrate oxetane formation with more complex alcohols such as pregnenolone or galactose; the reported methodology improves the synthesis of an oxetane-containing steroid with important biological activity in one step from testosterone. The researchers also show that the reaction works with other alkenyl sulfonium ions, albeit with modified conditions and with moderate yields, due to other competing pathways.
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