Nenitzescu 5-hydroxyindole synthesis mechanism

Kucklaender, U. Mechanism of the Nenitzescu reaction, IV. Synthesis of benzindole derivatives. Liebigs Ann. Chem. 1978,129-139. Kucklaender, U. Mechanism of the Nenitzescu reaction, V. Synthesis of naphthofuran derivatives. Liebigs Ann. Chem. 1978, 140-149. Kucklaender, U., Huehnermann, W. Studies on the mechanism of the Nenitzescu reaction. Synthesis of 6-hydroxyindole derivatives. Arch. Pharm. (Weinheim, Ger.) 1979, 312, 515-526. 

The choice of experimental conditions exerts a major influence on the course of Nenitzescu procedure and thereby determines the structure of the major product. The mechanism demonstrated to be operative for the method can be employed to understand the genesis of certain anomalous products and suggests ways to avoid them, thus increasing the efficiency of the synthesis of 5-hydroxyindoles. 

Only a few indole syntheses make use of building blocks in which the N-atom is not directly bonded to an arene. The Nenitzescu synthesis is of this type. In this synthesis, 1,4-quinones are condensed with 3-aminoacrylic esters to give 5-hydroxyindole-3-carboxylic esters 44. The mechanism of this synthesis has not been completely clarified. It includes a Michael addition (-> 41) and a cyclodehydration (42 43) as well as a redox transformation (41 42 and 43 44) [53] ... 

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