Syntheses and properties of 1H-pyrrolo[2,3-b]pyridines

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Abstract
Five different routes for the preparation of 1H-pyrrolo[2,3-b]pyridines have been investigated. A number of 2-, 3-, and 4-alkyl and -aryl substituted derivatives were prepared by two of these methods which involved modifications of Madelung- and Fischer-syntheses of indoles. The 1H-pyrrolo[2,3-b]pyridines are shown to undergo nitration, nitrosation, bromination, iodination, and reaction with Mannich bases predominantly at the 3-position although one example of nitration at the 2-position has also been found. Di-3-(1H-pyrrolo[2,3-b]pyridyl)methanes are formed by reaction with aldehydes, and treatment of 2-phenyl-1H-pyrrolo[2,3-b]pyridine with nitrosobenzene yields 2-phenyl-3-phenylimino-3H-pyrrolo[2,3-b]pyridine. A further example of a derivative of this isomeric 3H-system is 3-diazo-2-phenyl-3H-pyrrolo[2,3-b]pyridine which is formed from the corresponding amine by basification of the diazonium salt. 1-Substituted Grignard derivatives yield 3-iodo-compounds on treatment with hydrogen peroxide but only 1-acyl derivatives with acyl chlorides. Treatment of 2-phenyl-1H-pyrrolo[2,3-b]pyridine with chloroform and alkali caused ring-expansion to a 1,8-naphthyridine. A number of unexpected products have been isolated both in the syntheses of the 1H-pyrrolo[2,3-b]pyridines and in their reactions with electrophiles. I.r., n.m.r., and mass spectra have been used to establish all structures.

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