Studies on the Flash Vacuum Thermolysis of Thiones of Selected N‐, O‐, and S‐Heterocycles

Thermal decomposition of thiones of selected N‐, O‐ and S‐heterocycles under flash vacuum thermolysis conditions was investigated. In the case of six‐membered 4H‐3,1‐benzoxathiin‐4‐thione 6 , the course of the reaction depended on the substitution pattern at C(2) (Scheme 3). Thus, the 2‐unsubstituted derivative 6a led to the unstable product 2 , which upon treatment with MeOH was converted quantitatively into methyl 2‐mercaptobenzoate ( 7 ). The analogous thermolysis of the 2,2‐dimethyl derivative 6b yielded 2‐methyl‐4H‐1‐benzothiopyran‐4‐thione ( 8 ) as a sole product. In the case of thiophthalide derivatives 15 , a thermal rearrangement in the gas phase leading to the corresponding benzoc]thiophen‐1(3H)‐ones 16 in high yields was observed (Scheme 6). Unexpectedly, thionation of 1,3‐oxathiolan‐5‐one 17 with Lawesson's reagent under standard conditions led to 1,2‐dithietane derivative 19 , which, after the gas‐phase thermolysis, underwent a ring enlargement to yield 3H‐1,2‐dithiole 20 (Scheme 7). The six‐membered 4H‐1,3‐benzothiazine‐4‐thione 21 was shown to give three products: phenanthro9,10‐c]‐1,2‐dithiete ( 22 ), 3H‐1,3‐benzodithiole‐3‐thione ( 23 ), and N‐(3H‐1,2‐benzodithiol‐3‐ylidene)prop‐2‐en‐1‐amine ( 24 ) (Scheme 8). The latter is the product of the initial reaction, whereas 22 and 23 are postulated to be formed as secondary products of the conversion of the intermediate 6‐(thioxomethylene)cyclohexa‐2,4‐diene‐1‐thione ( 26 ) (Schemes 9 and 10).