The 1,3-dipolar cycloaddition of an azomethine ylide (Prato reaction) with M3N@I-h-C-80 (denoted as M3N@C-80; M = Sc, Lu, Y, Gd) was carried out to obtain fulleropyrrolidinebis(carboxylic acid) derivatives as scaffolds for the preparation of various functionalized M3N@C-80 materials. The formation of two monoadduct isomers (the [6,6]- and [5,6]-adducts) were detected by HPLC and identified by NMR and vis/NIR spectroscopies. In each Prato reaction with M3N@C-80, the initial addition gave a [6,6]-adduct of the I-h-C-80 cage, and subsequently, a [5,6]-adduct was obtained by complete or partial thermal isomerization via a rearrangement reaction. The reaction rate of the latter thermal conversion of the adducts was dependent on the size of the metal cluster inside C-80, and interestingly, in the reactions of Y3N@C-80 and Gd3N@C-80, this conversion was found to be reversible for the first time. Detailed kinetic studies provided the enthalpy and entropy barriers for the reactions of the adducts of Lu3N@C-80, Y3N@C-80, and Gd3N@C-80. The utility of the obtained Prato adducts was confirmed by preparation of a highly water-soluble Gd3N@C-80 derivative.