An efficient method to construct functionalized 1,4-ketoaldehydes bearing all-carbon α-quaternary centers via regioselective C-C bond activation has been described. The cyclopropanation of bench-stable enaminones with in situ generated diazo reagents from N-tosylhydrazones, followed by selective C-C bond cleavage of the cyclopropane ring, affords 1,4-ketoaldehyde derivatives in good to excellent yields. This method works with broad substrate scope and high regioselectivity.