1,3-Oxathiolanes from the reaction of aromatic and enolized thioketones with monosubstituted oxiranes

The reactions of 4,4'-dimethoxythiobenzophenone (1) with (S)-2-methyloxirane ((S)-2) and (R)-2-phenyloxirane ((R)-6) in the presence of a Lewis acid such as BF3.Et2O, ZnCl2, or SiO2 in dry CH2Cl2 led to the corresponding 1:1 adducts, i.e., 1,3-oxathiolanes (S)-3 with Me at C(5), and (S)-7 and (R)-8 with Ph at C(4) and C(5), respectively. A 1:2 adduct, 1,3,6-dioxathiocane (4S,8S)-4 and 1,3-dioxolane (S)-9, respectively, were formed as minor products (Schemes 3 and 5, Tables 1 and 2). Treatment of the 1:1 adduct (S)-3 with (S)-2 and BF3.Et2O gave the 1:2adduct (4S,8S)-4 (Scheme 4). In the case of the enolized thioketone 1,3-diphenylprop-1- ene-2-thiol (10) with (S)-2 and (R)-6 in the presence of SiO2 , the enesulfanyl alcohols (19Z,2S)-11 and (19E,2S)-11, and (19Z,2S)-13, (19E,2S)-13, (19Z,1R)-15, and (19E,1R)-15, respectively, as well as a 1,3-oxathiolane (S)-14 were formed (Schemes 6 and 8). In the presence of HCl, the enesulfanyl alcohols (19Z,2S)-11, (19Z,2S)-13, (19E,2S)-13, (19Z,1R)-15, and (19E,1R)-15 cyclize to give the corresponding 1,3-oxathiolanes (S)-12, (S)-14, and (R)-16, respectively (Schemes 7, 9, and 10). The structures of (19E,2S)-11, (S)-12, and (S)-14 were confirmed by X-ray crystallography (Figs. 1 - 3). These results show that 1,3-oxathiolanes can be prepared directly via the Lewis acid-catalyzed reactions of oxiranes with non-enolizable thioketones, and also in two steps with enolized thioketones. The nucleophilic attack of the thiocarbonyl or enesulfanyl S-atom at the Lewis acid-complexed oxirane ring proceeds with high regio- and stereoselectivity via an SN2-type mechanism