1,3‐Dipolare Cycloadditionen eines Carbonyl‐ylids mit 1,3‐Thiazol‐5(4 H )‐thionen und Thioketonen

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 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 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 ).