1,3‐Oxathiolanes from the Reaction of Aromatic and Enolized Thioketones with Monosubstituted Oxiranes

Abstract

AbstractThe 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 : 2 adduct (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 (1′Z,2S)‐11 and (1′E,2S)‐11, and (1′Z,2S)‐13, (1′E,2S)‐13, (1′Z,1R)‐15, and (1′E,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 (1′Z,2S)‐11, (1′Z,2S)‐13, (1′E,2S)‐13, (1′Z,1R)‐15, and (1′E,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 (1′E,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.