The photolysis of 3H-pyrazoles is a well-known preparative method for cyc1opropenes. l Although some examples of thermal elimination of nitrogen affording cyclopropenes have been reported, 2 the Van Alphen-Huttel rearrangement3v4 is usually the predominant reaction pathway, giving 4H-and/or 1H-pyrazoles according to Scheme I.
![dimethyl 3H-phenanthro[9,10-c]pyrazole-3,3-dicarboxylate Structure](https://image.chemsrc.com/caspic/052/76600-25-4.png)
![methyl 2H-phenanthro[9,10-c]pyrazole-3-carboxylate Structure](https://image.chemsrc.com/caspic/133/54415-28-0.png)
![dimethyl 2H-phenanthro[9,10-c]pyrazole-1,3-dicarboxylate Structure](https://image.chemsrc.com/caspic/128/76600-23-2.png)
![spiro[2-cyclopropene-1,9'-fluorene]-2,3-dimethyldicarboxylate Structure](https://image.chemsrc.com/caspic/407/39500-48-6.png)
![dimethyl 9bH-cyclopenteno[1,2,3-l,m]fluorene-1,2-dicarboxylate Structure](https://image.chemsrc.com/caspic/090/76600-24-3.png)