The reaction of adamantane-2-thione with propiolic acid afforded a novel type of cycloadduct, spiro[adamantane-2,2′-6′H-[1,3]-oxathiin]-6′-one (3a), in quantitative yield. The reaction of thiobenzophenone with propiolic acid gave 2,2-diphenyl-6′H-[1,3]-oxathiin]-6′-one and 4-phenyl-3-thia-3,4-dihydronaphthoic acid in 34% and 35% yields, respectively. The reaction might proceed through a concerted process, as confirmed by kinetics. The reaction of adamantane-2-thione with 2-butynoic acid or phenylpropiolic acid gave the corresponding adducts regioselectively. Interestingly, only one isomer was obtained by the reaction of thiofenchone with propiolic acid, suggesting that the reaction proceeded diastereospecifically. Oxidation of adducts 3 by dimethyldioxirane or m-chloroperoxybenzoic acid gave the corresponding sulfoxides and sulfones. The sulfoxides were thermally decomposed to give disulfide or another type of 1,3-oxathiin-6-one.
