SYNTHESIS OF 3-HYDROXYTHIOPHENES AND THIOPHENE-3(2H)-ONES BY PYROLYSIS OF ALKYLSULFANYLMETHYLENE MELDRUMS ACID-DERIVATIVES

3-Hydroxythiophene [thiophen-3(2H)-one] 1 and a range of its 2-substituted, 2,2-disubstituted and 5-substituted derivatives have been made by flash vacuum pyrolysis (FVP) of an appropriate alkylsulfanylmethylene derivative of Meldrum's acid 3 or 4. These compounds are readily obtained, either by reaction of methoxymethylene Meldrum's acid with alkylthiols in refluxing acetonitrile, or via the bis(methylsulfanyl) compound 18 by known procedures. The pyrolysis proceeds by a hydrogen-transfer-cyclisation mechanism in which there is extensive loss of configuration of a chiral centre at the reaction site. The NMR and mass spectra of the Meldrum's acid precursors and the mass spectra of the 3; hydroxythiophenes [thiophen-3(2H)-ones] are briefly discussed.