Stable C-60-fused tetrahydropitridazine derivatives were synthesized through the hetero-Diels-Alder cycloaddition of C-60 with 1,2-diaza-1,3-butadienes, which were generated in situ by the thermal extrusion of sulfur dioxide from 2,5-dihydro-1,2,3-thiadiazole- 1,1-dioxides. (c) 2006 Elsevier Ltd. All rights reserved.