Regio‐ and Stereoselectivity of the SiO 2 ‐Catalyzed Reaction of Thiocamphor (=1,7,7‐Trimethylbicyclo[2.2.1]heptane‐2‐thione) with Optically Active Monosubstituted Oxiranes

The reactions of the enolizable thioketone (1 R ,4 R )‐thiocamphor (=(1 R ,4 R )‐1,7,7‐trimethylbicyclo[2.2.1]heptane‐2‐thione; 1 ) with ( S )‐2‐methyloxirane ( 2 ) in the presence of a Lewis acid such as SnCl 4 or SiO 2 in anhydrous CH 2 Cl 2 led to two diastereoisomeric spirocyclic 1,3‐oxathiolanes 3 and 4 with the Me group at C(5′), as well as the isomeric β ‐hydroxy thioether 5 ( Scheme 2 ). The analogous reactions of 1 with ( RS )‐, ( R )‐, and ( S )‐2‐phenyloxirane ( 7 ) yielded two isomeric spirocyclic 1,3‐oxathiolanes 8 and 9 with Ph at C(4′), an additional isomer 13 bearing the Ph group at C(5′), and three isomeric β ‐hydroxy thioethers 10, 11 , and 12 ( Scheme 4 ). In the presence of HCl, the β ‐hydroxy thioethers 5, 10, 11 , and 12 isomerized to the corresponding 1,3‐oxathiolanes 3 and 4 ( Scheme 3 ), and 8, 9 , and 13 , respectively ( Scheme 5 ). Under similar conditions, an epimerization of 3, 8 , and 9 occurred to yield the corresponding diastereoisomers 4, 14 , and 15 , respectively ( Schemes 3 and 6 ). The structures of 9 and 15 were confirmed by X‐ray crystallography ( Figs. 1 and 2 ). These results show that the Lewis acid‐catalyzed addition of oxiranes to enolizable thioketones proceeds with high regio‐ and stereoselectivity via an S n 2‐type mechanism.