The combination of electron-deficient maleate and thiyl radicals will lead to efficient release of the adduct intermediates, leading to a fast and sustained catalysis for cis-trans isomerization. As in the case of acid-catalyzed isomerization, almost no reaction (maximum 2% conversion and yield depending on the pH) was observed when starting from fumaric acid instead of maleic acid (Figure ), and this will indicate that the reaction is controlled by the equilibrium of the regioisomers (thermodynamic control). When the reaction was started with angelic acid instead of maleic acid, the isomerization also proceeded, but a reversible reaction is also noticeable.