We describe the regioselective cycloaddition of La-2@I-h-C-80 with tetracyanoethylene oxide (TCNEO), which enabled the formation of the corresponding adduct having a tetracyanotetrahydrofuran moiety. X-ray crystallographic analysis revealed that the cycloaddition took place as a [5,6] addition. Along with dynamic swing motion of the metal atoms, the results of this electrochemical study demonstrate that TCNEO addition enhanced the electron-accepting character of La-2@I-h-C-80 and that the first reduction potential of the adduct reached -0.21 V versus the ferrocene/ferrocenium couple, which is the lowest value reported for endohedral metallofullerenes and their derivatives to date.