Ring enlargement and sulfur-transfer processes in SiO2-catalyzed reactions of thiocarbonyl compounds with optically active oxiranes

Abstract

The reactions of 1,3-dioxolane-2-thione (3) with (S)-2-methyloxirane ((S)-1) and with (R)-2-phenyloxirane ((R)-2) in the presence of SiO2 in anhydrous dichloroalkanes led to the optically active spirocyclic 1,3-oxathiolanes 8 with Me at C(7) and 9 with Ph at C(8), respectively (Schemes 2 and 3). The analogous reaction of 1,3-dimethylimidazolidine-2-thione (4a) with (R)-2 yielded stereoselectively (S)-2-phenylthiirane ((S)-10) in 83% yield and 97% ee together with 1,3-dimethylimidazolidin-2-one (11a). In the cases of 3-phenyloxazolidine-2-thione (4b) and 3-phenylthiazolidine-2-thione (4c), the reaction with (RS)-2 yielded the racemic thiirane (RS)-10, and the corresponding carbonyl compounds 11b and 11c (Scheme 4 and Table 1). The analogous reaction of 4a with 1,2-epoxycyclohexane (=7-oxabicyclo[4.1.0]heptane; 7) afforded thiirane 12 and the corresponding carbonyl compound 11a (Scheme 5). On the other hand, the BF3-catalyzed reaction of imidazolidine-2-thione (5) with (RS)-2 yielded the imidazolidine-2-thione derivative 13 almost quantitatively (Scheme 6). In a refluxing xylene solution, 1,3-diacetylimidazolidine-2-thione (6) and (RS)-2 reacted to give two imidazolidine-2-thione derivatives, 13 and 14 (Scheme 7). The structures of 13 and 14 were established by X-ray crystallography (Fig.).