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Mild flash vacuum pyrolysis of the Meldrum's acid derivative (3) gives the first example of a methylenemalonic anhydride (4), which decarboxylates in solution to give the first isolable methyleneketene (5); the structures are supported by I3C n.m.r. spectra and chemical reactions.Since their discovery1 in 1974 methyleneketenes have been of considerable interest to both experimental and theoretical chemists. The zigzag structure (1) of the parent propadienone recently determined by microwave spectroscopy2 indicates that (2) is an important canonical form of this molecule. However, fully optimised single-configuration Hartree-Fock calculations employing extensive Gaussian basis sets and including polarisation functions had failed to predict this deviation from linearity.3 Subsequent inclusion of electron correlation did reproduce the non-linear structure, h ~w e v e r . ~The methyleneketenes studied to date have been obtainable only as fleeting intermediates in the gas phase2 or in low temperature matrices.1.5-6 We now report the first example of a methyleneketene (5), reasonably stable in solution at room temperature, as well as the first recording of a 13C n.m.r. spectrum of a methylketene. The Meldrum's acid derivative (3) was flash pyrolysed at 450 "C/lO-4 Torr (contact time -10-3 s) and the products condensed on a cold finger at -196 "C. After the end of the pyrolysis 1 ml of CDCI3 was condensed onto the cold finger, which was then allowed to warm up, whereby the resulting solution dripped into an n.m.r. tube at -196 "C. After sealing the tube, a 13C n.m.r. spectrum was recorded at -45 "C. Apart from acetone, only one product was present in this sample.? On the basis of the 13C n.m.r. spectrum (Table l), the gated spectrum, and strong i.r. absorptions at 1890 and 1760 cm-1, the compound is identified as N-methyl-2pyrrolidinylidenemalonic anhydride (4). The i .r. absorptions are ca. 70 wavenumbers higher than for 2,2-dimethylmalonic a n h ~d r i d e , ~ demonstrating severe ring strain due to the sp2 hybridised carbon atom.Chemical confirmation of the structure (4) is given by the following observations: (i) warming the sample to -20 "C resulted in the formation of C02 (i.r. 2340 cm-1; 13C n.m.r. 6 124.8) and a new compound, identified below as the methyleneketene (5); (ii) warming the sample to room temperature t Any unreacted starting material condensed before reaching the cold finger and thus was not present in the n.m.r. sample.