Generation of new reactive cyclopentenethiones by flash vacuum thermolysis

The flash vacuum thermolysis (FVT) of the tricyclic sulfides 2 and 4 led, u pon elimination of cyclopentadiene by retro-Diels-Alder reaction, to a mixt ure of isomeric allylthio- or propargylthiocyclopentadienes (12, 13 and 16, 17, respectively). Compounds 13 and 17 underwent in turn a thio-Claisen re arrangement giving the reactive 5-alkyl- and 5-allenylcyclopentenethiones 1 1 and 15. The possible retro-ene reaction eliminating propene or allene fro m the allylthio or propargylthio moiety has not been observed. Under FVT, t he tricyclic thiol 3 led similarly to a mixture of cyclopentadienethiols 19 and 20, the latter tautomerizing to cyclopentenethione 21 upon warming up in solution. Precursors 7 and 8, corresponding to 2 and 4 in the cyclo-C-6 series, behav ed quite differently. No thio-Claisen rearrangement occurred, and the allyl thio- and propargylthiocyclohexadienes 22 and 24 evoluted towards stable co mpounds resulting from competitive retro-ene reactions and beta -eliminatio ns. Upon FVT, the cyclobutanic precursor 10 gave quantitatively a mixture o f cyclopentadiene, propene and thiophene, presumably via the cyclobutenic i ntermediate 26.