Different types of cyclopropene-1,2-dicarboxylates 1 have been obtained by photochemical methods from the corresponding pyrazoles 11, 12, or 13. These pyrazoles were synthesized by 1,3-dipolar cycloadditions of alkynes 8 or 9 with either preformed diazoalkanes or diazoalkanes generated photochemically in situ, by use of oxadiazolines as diazoalkane precursors. The numerous substrates have clearly established the scope and limitations of the syntheses of the precursors and the cyclopropenes by the different routes;a] 66 [d] 76 [d] c OCH 2 CF 3 Ϫ Ϫ [e] 36 [d] 54 [d] d OCH 2 CH 2 CH 3 82 90 75 90 e OtBu Ϫ Ϫ [a] 65 [f] 76 [f] f OPh 81 [g] 62 [h] 66 [g] 16 g OCH