Regio- and Stereoselectivity of the SiO2-Catalyzed Reaction of Thiocamphor (= 1,7,7-Trimethylbicyclo[2.2.1]heptane-2-thione) with Optically Active Monosubstituted Oxiranes

Abstract

The reactions of the enolizable thioketone (1R,4R)-thiocamphor (= (1R,4R)-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 SnCl4 or SiO2 in anhydrous CH2Cl2 led to two diastereoisomeric spirocyclic 1,3-oxathiolanes 3 and 4 with the Me group at C(5'), as well as the isomeric beta-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 beta-hydroxy thioethers 10, 11, and 12 (Scheme 4). In the presence of HCl, the beta-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 stereo- selectivity via an Sn2-type mechanism.