Article abstract of DOI:10.1002/hlca.19970800417

In p-xylene at 150°, 3-phenyloxirane-2,2-dicarbonitrile (4b) and 2-phenyl-3-thia-1-azaspiro[4.4]non-1-ene-4-thione (1a) gave the three 1:1 adducts trans-3a, cis-3a, and 13a in 61, 21, and 3% yield, respectively (Scheme 3). The stereoisomers trans-3a and cis-3a are the products of a regioselective 1,3-dipolar cycloaddition of carbonylylide 2b, generated thermally by an electrocyclic ring opening of 4b (Scheme 6), and the C=S group of 1a. Surprisingly, 13a proved not to be a regioisomeric cycloadduct of 1a and 2b, but an isomer formed via cleavage of the O-C(3) bond of the oxirane 4b. A reaction mechanism rationalizing the formation of 13a is proposed in Scheme 6. Analogous results were obtained from the reaction of 4b and 4,4-dimethyl-2-phenyl-1,3-thiazole-5(4H)-thione (1b, Scheme 3). The thermolysis of 4b in p-xylene at 130° in the presence of adamantane-thione (10) led to two isomeric 1:1 adducts 15 and 16 in a ratio of ca. 2:1, however, in low yield (Scheme 4). Most likely the products are again formed via the two competing reaction mechanisms depicted in Scheme 6. The analogous reactions of 4b with 2,2,4,4-tetramethylcyclobutane-1,3-thione (11) and 9H-xanthene-9-thione (12) yielded a single 1:1 adduct in each case (Scheme 5). In the former case, spirocyclic 1,3-oxathiolane 17, the product of the 1,3-dipolar cycloaddition with 2a corresponding to 3a, was isolated in only 11% yield. It is remarkable that no 2:1 adduct was formed even in the presence of an excess of 4b. In contrast, 4b and 12 reacted smoothly to give 18 in 81% yield; no cycloadduct of the carbonyl-ylide 2a could be detected. The structures of cis-3a, 13a, 15, and 18, as well as the structure of 14, which is a derivative of trans-3a, have been established by X-ray crystallography (Figs. 1-3, Table).