Pyrazolin-4-ones, ring contractions

Diazocarbonyl compounds readily undergo [3 + 2] cycloaddition to electron-poor alkenes 139). The 1-pyrazolines thus formed usually tautomerize to 2-pyrazolines if there is a hydrogen in an a-position to one of the nitrogen atoms otherwise, thermally induced ring contraction with evolution of nitrogen to give cyclopropanes can occur (Scheme 18). 

Photochemical ring contraction of various substrates, e.g. cyclobutane-1,3-diones and 1 -pyrazolin-4-ones - gives either isolable cyclopropanes or cyclopropanones as intermediates which are usually intercepted as adducts or which fragment to alkenes and carbon monoxide. The photoextrusion process most often leads to fragmentation products. 

---