M. Blagoev et al., Stereochemical course of the reaction between thiocarbonyl compounds and oxiranes: Reaction with cis- and trans-2,3-dimethyloxirane, HELV CHIM A, 83(12), 2000, pp. 3163-3178
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).