For the purpose of doubly functionalizing fullerenes, new tris-annulating reagents 2(n) have been developed. The reagents carry, in one molecule, two cyclopropenone acetals which are connected with an n-carbon methylene tether. Upon thermolysis of 2(n) in the presence of C60, the reagent undergoes [3 + 2] cycloaddition reaction twice in a regio- and stereoselective manner to give Cs and C2 organofullerenes bearing two cyclopentenone acetals. The selectivity varies as the function of the tether structure. The experiments have shown that, in each series of different tether lengths, one can obtain one or two diastereomeric double adducts out of several structural possibilities. The selectivity of the reaction did not conform to the prediction made on the basis of previous knowledge on intermolecular double additions but was found to be correlated to the conformational strain of the tether moiety, which can be estimated by a newly developed "double differential protocol". Systematic studies on the reliability of various computational methods for organofullerenes indicated that certain molecular orbital and molecular mechanics calculations give very reliable structural data while certain others do not.