Diastereoselective 1,3-Dioxolane Formation by Photocatalytic Ring Opening of -Epoxyketones

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a versatile reagent in organic synthesis. Owing to its oxidizing ability and also its relative stability under various reaction conditions cause it to be one of the most used reagents to perform several organic transformations such as oxidation of benzhydrols [1], dehydrogenation [2, 3], deprotection of benzyl ethers [4], anodic oxidation of naphthalenes [5], thiocyanation of aromatic and heteroaromatic compounds [6, 7], and aromatization of 1,4-dihydropyridines [8]. DDQ is also a well-known electron acceptor, and its interaction with a variety of electron donors via the formation of charge transfer (CT) complexes has been the subject of several investigations [9 – 11]. Epoxides, as oxygen-containing three-membered heterocyclic rings, are among the most versatile building blocks in organic synthesis due to their facile formation and higher reactivity toward nucleophiles. Ring opening reactions of epoxides and α-epoxyketones, thermally or photochemically, have attracted considerable interest from both synthetic and mechanistic standpoints. Single electron transfer (SET) induced ring opening of these compounds in the presence of a suitable catalyst or photocatalyst leads to the formation of radical cation intermediates. Nucleophilic attack of suitable species to these intermediates results in the formation of 1,3-dioxolane, trioxolane or ether derivatives [12, 13]. Previously, we have reported the light-induced ring opening of α-epoxyketones 1 in the presence of various photocatalysts such as 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) 2 [14 – 18], 7,7,8,8-tetracyanop-quinodimethane (TCNQ) 3 [19] and N-benzyl-2,4,6triphenylpyridinium tetrafluoroborate (NBTPT) 4 [20] in methanol, acetic acid, acetone and cyclohexanone solutions (Scheme 1). The results obtained in these studies have shown that the diastereoselectivity observed in this reaction depends on the type of photocatalyst and on the nucleophilic power and the steric hindrance of the attacking species. Whereas irradiation of 1a – f in the presence of pyrylium salt 2 in dry acetone resulted in the formation of diastereomeric cisand trans-1,3dioxolanes 5a – f and 6a – f besides the formation of alcoholic by-products 7 and 8 [16], diastereospecific formation of cis-spirodioxolanes 9a – f is observed by carrying out the irradiation in cyclohexanone solution (Scheme 2) [15]. An interesting point is that the irradiation of 1a – f in the presence of TCNQ (3) in acetone resulted in the formation of cis-1,3-dioxolanes 5a – f besides the formation of alcoholic by-products 7 and 8 [19], while by using the photocatalyst 4, the formation of cis-1,3-dioxolanes 5a – f accompanied by the formation of a trace amount of trans-1,3-dioxolanes