Regioselectivity of the 1,3-Oxathiolane Formation in the Lewis Acid-Catalyzed Reaction of Thioketones with Asymmetrically Substituted Oxiranes

The reactions of the aromatic thioketone 4,4'-dimethoxythiobenzophenone (1) with three monosubstituted oxiranes 3a - c in the presence of BF3 . Et2O or SnCl4 in dry CH2Cl2 led to the corresponding 1 : 1 adducts, i.e., 1,3- oxathiolanes 4a-b with R at C(5) and 8c with Ph at C(4). In addition, 1,3-dioxolanes 7a and 7c, and the unexpected 1 : 2 adducts 6a - b were obtained (Scheme 2 and Table 1) . In the case of the aliphatic, nonenolizable thioketone 1,1,3,3-tetramethylindane-2-thione (2) and 3a - c with BF3 . Et2O as catalyst, only 1 : 1 adducts, i.e. 1,3-oxathiolanes 10a - b with R at C(5) and 11a - c with R or Ph at C(4), were formed (Scheme 6 and Table 2). In control experiments, the 1:1 adducts 4a and 4b were treated with 2-methyloxirane (3a) in the presence of BF3 . Et2O to yield the 1:2 adduct 6a and 1:1:1 adduct 9, respectively (Scheme 5). The structures of 6a, 8c, 10a, 11a, and 11c were confirmed by X-ray crystallography (Figs. 1 - 5). The results described in the present paper show that alkyl and aryl substituents have significant influence upon the regioselectivity in the process of the ring opening of the complexed oxirane by the nucleophilic attack of the thiocarbonyl S-atom: the preferred nucleophilic attack occurs at C(3) of alkyl-substituted oxiranes (O-C(3) cleavage) but at C(2) of phenyloxirane (O-C(2) cleavage)