Stereochemical course of the reaction between thiocarbonyl compounds and oxiranes: Reaction with cis- and trans-2,3-dimethyloxirane

The reactions of thiocarbonyl compounds with cis-2,3-dimethyloxirane (1a) i n CH2Cl2 in the presence of BF3. Et2O or SnCl4 to trans-4,5-dimethyl-1,3-ox athiolanes, whereas with trans-2,3-dimethyloxirane (1b) cis4,5-dimethyl-1,3 -oxathiolanes were formed. With the stronger Lewis acid SnCl4, the formatio n of side-products was also observed. In the case of 1,3-thiazole-5(4H)-thi one 2, these side-products are the corresponding 1,3 thiazol-5(4H)-one 5 an d the 1:2 adduct 8 (Schemes 2-4). Their formation can be rationalized by th e decomposition of the initially formed spirocyclic 1,3-oxathiolane and by a second addition onto the C=N bond of the 1:1 adduct, respectively. The se condary epimerization by inversion of the configuration of the spiro-C-atom (Schemes 5 - 7) can be explained by a Lewis-acid-catalyzed ring opening of the 1,3-oxathiolane ring and subsequent ring closure to the thermodynamica lly more stable isomer (Scheme 12). In the case of 2,2,4,4-tetramethyl-3-th ioxocyclobutanone (20), apart from the expected spirocyclic 1,3-oxathiolane s 21 and 23, dispirocyclic 1:2 adducts were formed by a secondary addition onto the C=O group of the four-membered ring (Schemes 9 and 10).