This thesis is divided into four chapters. Chapter one is a review of cyclopentadienes (Cps), and is split by sub-classification of applications in organometallics, applications in organic methodology, synthesis of complex Cps from post carbocycle modification and de novo synthesis of Cp. Chapter two introduces the decarboxylative Claisen rearrangement (dCr) and details investigations of the application of this reaction towards Cp synthesis using three different activating groups. Chapter three focuses on the decarboxylative Claisen rearrangement (dCr) from a computational aspect. It begins with a brief review of the mechanism of the [3,3]-sigmatropic rearrangement and examples of density functional theory (DFT) to rationalise Ireland-Claisen rearrangements. The kinetic data of [3,3]-sigmatropic rearrangement of tosyl malonates is studied and compared to DFT calculations. The method devised is tested on literature dCr reactions and then used predictively to interpret synthetic outcomes. Chapter four provides experimental procedures and characterisation data.