Résumé Heterosubstituted methyleneketenes 2d-g are obtained by flash-vacuum pyrolysis of Meldrum's acid derivatives 1d-g. The methoxymethyleneketene 2e is stable for a few hours at room temperature and gives 3e by dimerization; in contrast, the more reactive thiomethoxymethyleneketenes 2f-g are easily converted to 4f-g by intramolecular rearrangement.
