1,3‐Dipolar Cycloadditions of a Carhonyl‐ylide with 1,3‐Thiazole‐5(4__H__)‐thiones and Thioketones
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 adduets trans‐3a, cis‐3a, and 13ain 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 CS group of 1a. Surprisingly, 13a proved not to be a regioisomeric cycloadduct of 1a and 2b, but an isomer formed via cleavage of the OC(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 (4__H__)‐thione (1b, Scheme 3). The thermolysis of 4b in p‐xylene at 130° in the presence of adamantine–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 9__H‐xanthene‐9‐thione (12) yielded a single 1:1 adduct in each case (Schemes). 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 carbonylylide 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).