Intramolecular cycloadditions nitrile imines

Meier and Heimgartner (208) achieved an intramolecular sydnone-alkene cycloaddition to give adduct 306 in 50% yield. Other tether lengths did not so react, but photolysis of these other sydnones led to novel fused pyrazoles via decarboxylation and subsequent cycloadditions from the subsequent nitrile-imines. 

Over the last 25 years both nitrile ylides and nitrile imines have continued to provide fascinating and synthetically useful chemistry. In both cases, the exploitation of [3 + 2]-cycloaddition chemistry with an increasing range of dipolarophiles has continued as a key route to five-membered heterocycles. The major development of new chemistry, however, has been in the extensive exploration of intramolecular reactions both in cycloaddition chemistry and in the electrocycliza-tion of 1,3-dipoles with extended conjugation. Such chemistry harnesses the unique reactivity of 1,3-dipoles in the synthesis of relatively elaborate structures but does require the design and preparation of quite complex reactants containing both the 1,3-dipole precursor and the dipolarophilic component. However, access to this chemistry is becoming much easier via the application of new synthetic procedures... 

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