Abstract
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 BF~3~⋅Et~2~O, ZnCl~2~, or SiO~2~ in dry CH~2~Cl~2~ 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 (4__S__,8__S__)‐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 BF~3~⋅Et~2~O gave the 1 : 2 adduct (4__S__,8__S__)‐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 SiO~2~, the enesulfanyl alcohols (1′Z,2__S__)‐11 and (1′E,2__S__)‐11, and (1′Z,2__S__)‐13, (1′E,2__S__)‐13, (1′Z,1__R__)‐15, and (1′E,1__R__)‐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 (1′Z,2__S__)‐11, (1′Z,2__S__)‐13, (1′E,2__S__)‐13, (1′Z,1__R__)‐15, and (1′E,1__R__)‐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 (1′E,2__S__)‐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 S~n~2‐type mechanism.