This book describes the thermal fragmentation of heterocycles to give extended cumulenes called iminopropadienones, observed by infrared spectroscopy at 8 K and trapped by preparative flash vacuum pyrolysis (FVP). Chapter 1 describes the generation of iminopropadienones from Meldrum's acid derivatives. Chapter 2 describes the generation of 2,6-diflourophenyl iminopropadienone from 3- (2,6-difluorophenyl)isoxazolo[5,4-d]pyrimidine-4(5H)-one and a comparison with the generation of the same iminopropadienone from the Meldrum's acid derivative. Chapter 3 pyridopyrimidinones and a novel mesoionic compound (pyridopyrimidin-l-ium-2-olate) were used as precursors to generate iminopropadienones through flash vacuum pyrolysis. In Chapter 4, X-ray crystallography as well as theoretical calculations are used to investigate the unusual structural characteristics of Meldrum's acid derivatives used as precursors of iminopropadienones. These molecules were found to have zwitterionic and non-planar structures.