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10.13.1.1.1.3.3.1 Method 1: From Benzo-1,4-quinones and Enamines; Nenitzescu Synthesis

DOI: 10.1055/sos-SD-110-00001

Joule, J. A.Science of Synthesis Knowledge Updates, (2010281.

The Nenitzescu synthesis consists of reacting a benzo-1,4-quinone with an enamine, the product indole carrying a 5-hydroxy group (Scheme 109);[‌293‌‌298‌] see also Table 4. When the benzoquinone carries a substituent, mixtures of 6- and 7-substituted indol-5-ols are sometimes obtained; it seems that at whatever stage the 3—3a bond is made, it is made at one of the two positions of the unsubstituted double bond of the benzoquinone. The enamine is generally a 3-aminoacrylate and also generally carries an additional 3-substitutent [e.g., benzo-1,4-quinone (307) with acrylate 308 giving indole 309]. The use of alkyl- and arylaminofumarates leads to indol-5-ols carrying two ester groups on the heterocyclic ring (e.g., 307 with 310 giving 311).[‌298‌] Nenitzescu reactions are carried out in acetone, ethanol, chloroform, and acetic acid; however, nitromethane is strongly endorsed as a suitable solvent.[‌294‌] The use of methyl esters rather than ethyl esters has a favorable effect on yields.[‌294‌] A variation on the Nenitzescu theme is the use of mono-[‌299‌] and bis-imine derivatives of the benzo-1,4-quinone when indol-5-amines can be obtained.[‌300‌,‌301‌]

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