Non-enolizable thioketones and 1,2-epoxycycloalkanes undergo a Lewis acid catalyzed addition reaction to give 1,3-oxathiolanes. Appropriate reaction conditions are CH 2 Cl 2 as the solvent, BF 3 Β΄Et 2 O as the Lewis acid, and a temperature between Γ 788 and r.t. Under the reaction conditions, the 1,3-oxathiolanes are only moderately stable. They decompose to yield the corresponding epithiocycloalkane and ketone. In general, 1,3dithiolanes are isolated as minor products or, after prolonged reaction, as the main product. These secondary products are formed via the Lewis acid catalyzed reaction of the intermediate epithiocycloalkane and a second molecule of the thioketone. In the reaction of thiobenzophenone and 1,2-epoxycyclohexane, trans-8,8-diphenyl-7,9-dioxabicyclo[4.3.0]nonane is formed in small amounts as an additional side product ( Scheme 12). In all cases, the newly formed heterocycle and the carbocycle are trans-fused. This result is consistent with a nucleophilic ring-opening of the complexed oxirane by the thioketone via inversion of the configuration and subsequent formation of the O(1)ΓC(2) bond of the 1,3-oxathiolane ( Scheme 13). The surprising formation of the fused 1,4-oxathiepan derivative 23 ( Scheme 9) is in accordance with an ionic reaction mechanism (cf. Scheme 15).