The condensation products from 4,6dimethyl-and 2,4,4-trimethylthiosemicatiazides and akdehydes or ketones, as well as those firn 2-methyl-and 2&di&ubstituted thiosemicarbazides and aldehydes have the thiosemica&zzonestructue, while ketones react with 2-methyl-or 2,4-dialkylthiosemicadxzzides to form 1,2,4-triazolidine-3-thiones. Both thiosemicadxuones and 1,2,4-triazolidine-3-thiones in trifuoroacetic acid solution yield 1,3,4-thiadiazolidine-2-iminium salts. Their deprotonation by mridine leads to thiosemicarbazones, including otherwise inaccessible
Zmethyl-and 2,4-diszzbsftuted ketone thiosemicarbazones.
The mass-spectrometric investigation of these compounds also suggestspresence 'of their tautomers in the gas phase.
It was established earlier that thioacylhydrazones exist as 1,3,4-thiadiazolidines', while 1-alkylidene(arylidene)amidrazones undergo cyclization to 1,2,Ctriazoline derivatives under certain conditions2 . Thiosemicarbazones may alternatively cyclize via nitrogen or sulphur nucleophilic attack, but this question has not yet been closely investigated. The low barrier observed for the syn-anti isomerization of thiosemicarbazones was explained by the formation of an intermediate 2-imino-1,3,4-thiadiazoline3 ; the same structure was believed to form from several thiosemicarbazones in acidic medium 4s .
On the contrary, the 1,2,4-triazolidine-3-thione structure was assigned to the product of 2-methyl-4(l-phenylethyl) thiosemicarbazide condensation with acetone6 , as well as for the reaction product from acetophenone @-hydroxyethylhydrazone with KNCS7 .