A combination of 1,3-dimethyl-2-phenylbenzimidazoline (DMPBI) and acetic acid has been utilized for photoinduced reductive transformation of a,b-epoxy ketones to b-hydroxy ketones. Study on photoreactions using several proton donors revealed that acetic acid is superior to other proton donors such as HCl, p-TsOH, MeOH, and water. 1,3-Dimethyl-2-phenylbenzimidazolium was produced in the reaction with acetic acid while N-benzoyl-N,N¢-dimethyl-o-phenylenediamine was formed in aqueous solvents. When THF solutions containing aryl carbonyl possessing a,b-epoxy ketones and DMPBI and acetic acid were irradiated (l > 280 nm), b-hydroxy ketones were isolated in good to excellent yields. Photosensitized conditions (l > 340 nm) were employed for the reactions of alkyl carbonyl possessing a,b-epoxy ketones.During several decades of the last century, photoreactions of three-membered ring compounds gained much attention due to their interesting reactivities; among these compounds are epoxides. 1 In particular, photoreaction of a,bepoxy ketones were extensively investigated, which demonstrated that several types of photochemical processes are open to these compounds. 2 Selectivity of the bond cleavage of epoxy rings such as C a -O, C b -O, C a -C b usually depends on the substitution and the nature of the reactive excited states. However, if a single electron is injected to carbonyl substituents to generate their radical anions (ketyl radicals), these intermediates would undergo selective C a -O bond cleavage to give distonic radical anions followed by formal two-proton and single-electron transfer (SET) to become b-hydroxy ketones (Scheme 1). Therefore, irradiation of a,b-epoxy ketones in the presence of electron-donating compounds would become a new way to achieve this synthetically important process. 3 About ten years ago, we first examined this concept by conducting photoreactions of several aromatic epoxy ketones in the presence of triethylamine and other electron donors. 4 About the same time, Cossy and co-workers reported photoreactions of several aliphatic as well as aromatic epoxy ketones with triethylamine. 5 In these experiments, while the yields of hydroxy ketones were modest, several problems were also encountered: other compounds such as b-diketones were the major products in some cases 4a and the reactions could not be completed since prolonged irradiation caused decomposition of bhydroxy ketones. 5 Our subsequent mechanistic study demonstrated that selectivity of the reaction pathway either to produce b-diketones or b-hydroxy ketones depends on the nature of the radical cations of amines used. 6 Now, we report herein a method to achieve photoinduced transformation of a,b-epoxy ketones to b-hydroxy ketones by using a combination of 1,3-dimethyl-2-phenylbenzimidazoline (DMPBI) 7 and acetic acid, which is superior to the previously reported DMPBI-based method. 8Irradiation of trans-1,3-diphenyl-2,3-epoxypropane-1-one (chalcone epoxide) (trans-1a) with DMPBI (1.2-1.5 equivalents) in either aqueous THF or wet benzene ...
