Aza-Nazarov cyclization

Two type la syntheses of (3-hydroxypyrroles have appeared. An aza-Nazarov cyclization of l-azapenta-l,4-dien-3-ones produced (3-hydroxypyrroles including 2,2 -bipyrroles <06EJO5339>. A second approach to a (3-hydroxypyrrole involved an intramolecular N-H insertion into a rhodium carbene derived from the decomposition of a diazoketone <06JOC5560>. On the other hand, the photochemical decomposition of the diazoketone led to pyrrolidin-2-ones. 

Acylium salts undergo aza-Nazarov cyclization under similar conditions to form varied five-membered IV-heterocycles.339 Aryl-tethered pyrrolinones and dihydro-pyridones [Eq. (5.132)] were induced to cyclize with triflic acid to afford tri- and tetracyclic products.340... 

Smooth aza-Nazarov cyclization to form dihydroisoindolones is observed with TfOH where CE3COOH is totally ineffectual. Eormation of dicationic superelectrophilic species is apparently important for overcoming the energy barrier of the cyclization in such cases. ... 

The preparation of fnnctionalized pyrroles has been accomplished through titanium-mediated reactions (Scheme 3.74) [79]. The first step in the process involves the generation of a metaUocycle from the reaction of a diyne with the titanium reagent that was generated in solntion. Once formed, treatment of the titanacycle with an activated imine followed by hydrolysis generated the pyrrole. The overall reaction can be described as an imino aza-Nazarov cyclization, and the process generated good to excellent yields of the 3-aminopyrroles. 

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