In p-xylene at 1 50°, 3-phenyloxirane-2,2-dicarbonitrile (4b) and 2-phenyl-3-thia-l-azaspiro[4.4]non-l-ene-4thione (la) gave the three 1 : 1 adducts trans-3a, cis-3a, and 13a in 61, 21, and 3 YO yield, respectively (Scheme 3). The stereoisomers trans-3a and cis3a are the products of a regioselective 1,3-dipolar cycloaddition of carbonylylide 2b, generated thermally by an electrocyclic ring opening of 4b (Scheme6), and the C=S group of la. Surprisingly, 13a proved not to be a regioisomeric cycloadduct of l a and 2h, but an isomer formed via cleavage of the 0-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-phenyI-1 ,3-thiazole-5 ( 415' )thione (lb, 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-tetrainethyIcyclobutane-l,3-thione (1 1) and 9H-xanthene-9-thione (12) yielded a single 1 : 1 adduct in each case (Scheme5). 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:l 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 rrans-3a, have been established by X-ray crystallography (Figs. 1-3 , Table).
