A series of 11 amino acid-derived stabilised ylides 12â 14 and 16 have been prepared and characterised. In one case, for compound 17, an X-ray structure determination supports formulation of the compounds as phosphonium enolates. Flash vacuum pyrolysis (FVP) of 12 and 13 at 500 °C results in loss of R² 3PO between the ylide function and one carbonyl of the phthalimido group to give products characterised spectroscopically as the pyrroloisoindolediones 18. The identity of these is also supported by the results of ¹³C and ¹⁵N labelling experiments, but owing to their high reactivity complete separation from the phosphine oxide was not generally possible even when Bu3PO rather than Ph3PO was involved. Chromatographic purification of 29, similarly produced by FVP of 14, led to partial hydrolysis, rearrangement and decarboxylation to give 30. FVP of 12 at 750 °C gave the 1-unsubstituted pyrroloisoindolediones 19 in two cases.