Fullerene derivatives with >CH₂ addends in [6,6]-open or [5,6]-closed configuration are uncommon of fullerene derivatives, but they are readily accessible via treatment of C_s-C₇₀(CF₃)₈ (1) with diazomethane followed by thermolysis or photolysis. Both thermodynamic and kinetic factors favor regioselective addition of diazomethane at the near-equatorial [5,6]-double bond of C_s-C₇₀(CF₃)₈ to give thermally labile pyrazoline intermediate 2. Thermal extrusion of N₂ from 2 is a kinetically controlled process with an orbital symmetry controlled Wodward-Hoffmann-allowed mechanism. It quantitatively yields less thermodynamically favorable [6,6]-open isomer of C₇₀(CF₃)₈[CH₂] homofullerene (3), but the latter turns out to be capable of unexpectedly rapid quantitative phototransformation into the thermodynamically preferable [5,6]-closed methanofullerene isomer 4. The transformation involves the manifold of the triplet states that facilitate the required cleavage of the C_cage-CH₂ bonds.