Reactions of substituted 4-diazotetrahydrofurane-3-ones with various acids (pK_a < 3) irrespective of acid strength give rise to elimination of nitrogen with subsequent 1,2-nucleophilic migration of aryl (alkyl) groups from C-5 to C-4 position of heterocyclic core with quantitative formation of regioisomeric 3(2Н)furanones (yields up to 99%). Regioselectivity of this acid-catalyzed process increases with the decreasing of solvent polarity and strongly depends on the nature of substituents in the aryl groups. The suggested mechanism of this reaction via intermediate RN₂ ⁺ cation was confirmed by density functional theory (DFT) calculations at the PBE0/6-31+G(d) functional due to good agreement between calculated and experimental data on the acid stability of diazoketones in different solvents.