The review examines methods for the preparation of compounds containing the 3H-pyrrolo[2,3-c]quinoline fragment, which have been published mainly over the past 10–15 years.

New types of (hydroxymethyl)phosphonic and (methylene)diphosphonic acids bearing heterocyclic fragments were synthesized by addition of tris(trimethylsilyl) phosphite to N-formyl derivatives of five-membered nitrogen heterocycles.

A simple and effective method on the basis of Fischer reaction in polyphosphoric acid is proposed for the synthesis of previously unknown heterocyclic structures that contain the 5-chloro-1H-pyrrolo[2,3-b]pyridine system. This method can be used for the synthesis of 3-substituted and 2,3-disubstituted 5-chloro-7-azaindoles with alkyl and aryl substituents.

Convenient methods for the synthesis of functionalized phosphonic acids containing piperidine moieties via the reaction of tris(trimethylsilyl) phosphite with N-substituted 4-piperidone derivatives have been developed.

A simple and convenient procedure for the preparation of some unknown 2,3‐disubstituted 5‐iodo‐1 H ‐pyrrolo[2,3‐ b ]pyridines from readily available starting materials by Fischer indole cyclization in polyphosphoric acid is described. The present methodology provides an alternative synthetic approach to the synthesis of 5‐iodo‐7‐azaindole scaffold. All synthesized compounds were characterized by IR, MS, 1 H and 13 C NMR, and elemental analysis.

\ The new functionalized hydroxymethylphosphonic and methylenediphosphonic acids are synthesized via unique reaction of tris(trimethylsilyl) phosphite and N-formyl derivatives of five-membered aza-heterocycles at the presence of effective catalyst-trimethylsilyl triflate.

AbstractThe simple and cheap synthesis is general for different alkyl or aryl ketones, but it is not suitable for methyl ketones and N‐substituted piperidin‐4‐ones.

A simple synthesis of some hard-to-reach heterocycles containing 2,3-disubstituted 5-bromo-1H-pyrrolo[2,3-b]pyridine framework by Fis.her indole cyclization in polyphosphoric acid has been developed. A particularly valuable feature of this synthetic route is the possibility to build a 5-bromo-7-azaindole scaffold with alkyl or aryl substituents at positions 2 and 3 from available starting materials.

We present the results of our study towards the synthesis of 2,3-disubstituted 7-azaindoles by cyclization of 2-pyridylhydrazones of aliphatic and alkylaromatic ketones under the conditions of Fischer reaction, catalyzed by polyphosphoric acid.