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-substit uted, 2,2-disubstituted and 5-substituted derivatives have been made b y flash vacuum pyrolysis (FVP) of an appropriate alkylsulfanylmethylen e derivative of Meldrum's acid 3 or 4. These compounds are readily obt ained, either by reaction of methoxymethylene Meldrum's acid with alky lthiols in refluxing acetonitrile, or via the bis(methylsulfanyl) comp ound 18 by known procedures. The pyrolysis proceeds by a hydrogen-tran sfer-cyclisation mechanism in which there is extensive loss of configu ration of a chiral centre at the reaction site. The NMR and mass spect ra of the Meldrum's acid precursors and the mass spectra of the 3; hyd roxythiophenes [thiophen-3(2H)-ones] are briefly discussed.