Lewis acid catalyzed reactions of thioketones with 1,2-epoxycyclohexane and 1,2-epoxycyclopentane

Non-enolizable thioketones and 1,2-epoxycycloalkanes undergo a Lewis acid c atalyzed addition reaction to give 1.3-oxathiolanes. Appropriate reaction c onditions are CH2Cl2 as the solvent, BF3. Et2O as the Lewis acid, and a tem perature between -78 degrees and r.t. Under the reaction conditions, the 1, 3-oxathiolanes are only moderately stable. They decompose to yield the corr esponding epithiocycloalkane and ketone. In general, 1,3-dithiolanes are is olated 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 thioket one. 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 a dditional side product (Scheme 12). in all cases, the newly formed heterocy cle and the carbocycle are trans-fused. This result is consistent with a nu cleophilic ring-opening of the complexed oxirane by the thioketone via inve rsion of the configuration and subsequent formation of the O(1)-C(2) bond o f the 13-oxathiolane (Scheme 13). The surprising formation of the fused 1,4 -oxathiepan derivative 23 (Scheme 9) is in accordance with un ionic reactio n mechanism (cf. Scheme 15).