This thesis describes the design and development of a new methodology for the synthesis of spiroacetals involving a rhodium(II)-catalysed C–H insertion of suitable diazo precur- sors. Three generations of diazo precursors were designed, synthesised and treated with Rh(II) catalysts: The 1st generation precursor consisted of a phenyl substituted tetrahydropyranyl acetal bearing a proximal phenyl diazoketone substituent, which underwent Rh(II)-catalysed C– H insertion to give unexpected spirocyclic orthoesters. DFT calculations showed that these ’anomalous’ products are formed by a mechanism involving hydride transfer to the rhodium carbene followed by intramolecular C–O bond formation.