Synthesis of N-unsubstituted bis[1,2]dithiolo[1,4]thiazines and bis[1,2]dithiolopyrroles

The parent bis[1,2]dithiolo[1,4]thiazine-3,5-dione 8, -3,5-dithione 11, the unsymmetrical 3-oxo-5-thione 10 and the bis[1,2]dithiolopyrrole-3,5-dione 9 are synthesised by acid catalysed cleavage of their various N-benzyl, ethyl, ethoxycarbonylethyl and propanoic acid derivatives. These N-alkyl compounds are prepared in the usual way from the appropriate N-alkyldiisopropylamine and S2Cl2. N-Benzyl derivatives 2 and 5 of the thiazine and pyrrole diones give 8 (100%) and 9 (58%) respectively with conc. H2SO4 in dilute DCM solution, and the N-ethyl thiazine derivatives of the dione 12 and the keto-thione 13 give 8 (89%) and 10 (75%) respectively in conc. H2SO4 at 120 °C. Ethyl 3-(diisopropylamino)propanoate 16 with S2Cl2 gives the three thiazines 17 (30%), 18 (15%) and 19 (13%), and 17 and 19 are converted into the pyrroles 20 (95%) and 21 (90%) respectively by thermal extrusion of sulfur in refluxing xylene. All five ethyl esters, 17–21, are hydrolysed with aqueous acid to 22–26 respectively in 92–100% yield, and the N-propanoic acids 22–25 with hot conc. hydrochloric or hot 80–90% sulfuric acid are dealkylated to the corresponding parent products 8–11. It is also shown that the hydrolysis and dealkylation steps 17→22→8 can be combined in one operation (75%).