General Carbene Routes to Heterocycles

Three principal carbene-based approaches to the construction of heterocycles can be recognized. These include (1) the carbene cyclization reaction, (2) ring contraction or expansion with carbene intermediacy, and (3) cyclization or cycloaddition of unstable primary products of carbene reactions. 

The utility of the above routes for the synthesis of particular heterocyclic systems are considered in the relevant sections of the review. 

---