The reactions of 3H-isobenzofuran-1-thione (1) with (S)-2-methyloxirane (2) and (R)-2-phenyloxirane (6) in the presence of SiO 2 in anhydrous CH 2 Cl 2 led to two pairs of diastereoisomeric spirocyclic 1,3-oxathiolanes, i.e., 3 and 4 with a Me group at C(5'), and 7 and 8 with a Ph group at C(4'), respectively (Schemes 2 and 3). In both cases, 3H-isobenzofuran-1-one ( 5) was formed as a main product. The analogous reactions of 3,4-dihydro-2H-[1]benzopyran-2-thione (9) and 3,4,5,6-tetrahydro-2H-pyran-2-thione ( 14) with 2 and 6 yielded four pairs of the corresponding diastereoisomeric spirocyclic compounds 10 and 11, 12 and 13, 15 and 16, and 18 and 19, respectively (Schemes 4 ± 7). In the reaction of 14 with 6, the 1,3-oxathiolane 20 with a Ph group at C(2) was also formed. The structures of 3, 7, 8, 10, 19, and 20 were established by X-ray crystallography (Figs. 1 ± 4). In contrast to the thiolactones 1, 9, and 14, the thioesters 21a ± 21d did not react with (R)-2-phenyloxirane (6) either in the presence of SiO 2 or under more-drastic conditions with BF 3 ¥ Et 2 O or SnCl 4 (Scheme 8). The results show that spirocyclic 1,3-oxathiolanes can be prepared from thiolactones with oxiranes. The nucleophilic attack of the thiocarbonyl S-atom at the SiO 2 -activated oxirane ring proceeds with high regio-and stereoselectivity via an S N 2-type mechanism.