1//-Pyrroles acylation

Pyrrole, 2-acetyl-1 -(2-hydroxyethyl)-5-nitro-cydization, 4, 74 ipso substitution, 4, 243 Pyrrole, 2-acetyl-1-methyl-dipole moment, 4, 194 photocydization reactions with 2,3-dimethylbut-2-ene, 4, 269 Pyrrole, 3-acetyl-4-methyl-Vilsmeier-Haack formylation, 4, 222 Pyrrole, 2-acetyl-3-nitro-reduction, 4, 297 Pyrrole, acyl-basicity, 4, 207 isomerization, 4, 208 oximes... 

To test the generality of this reaction, the indole systems 70a,b were prepared from hexahydro-8-oxopyrrolo[ 1,2-a]indole 67 and tetrahydro-4-oxindole 68 (Scheme 11). These compounds in turn were readily obtained from cyclohexane-1,3-one and the appropriate amino acid salt according to Franck s pyrrole acylation protocol.53 Attempts to directly oxidize 67 or 68 to the hydroxyindole oxidation state using DDQ met with failure despite numerous attempts involving variation in solvent and reaction temperature. To circumvent this limitation, it was reasoned that... 

Heterocycles such as furan and pyrrole can be efficiently acetylated with AAN in the presence of zeolite ZSM-5. The higher yields are % of 2-acetylfuran at 93% conversion of furan and 76% of 2-acetylp5u-role af 78% conversion of pyrrole over ZSM-5(30). On the contrary, the catalytic activity of Y is low due to its higher acidity compared to ZSM-5-type catalysts this confirms that the pyrrole acylation reaction prefers moderate acidic centers. 

For a complete description of the synthetic utility of the Houben-Hoesch reaction as it applies to other aromatic systems, as well as some mechanistic discussions, the reader is directed to two reviews on the subject. Due to the diminished electrophilic reactivity of nitriles compared to other carboxylic acid derivatives as well as the broad number of Friedel-Crafts substrates, most Houben-Hoesch pyrrole acylation reactions are conducted intramolecularly. However, an impressive intermolecular example was delineated by Chang and co-workers in efforts directed toward the synthesis of novel 2-[5-aroylpyrrolo]alkanoic acids, for evaluation of their potential analgesic and anti-inflammatory activities. Treatment of substituted pyrrole 4 and 3-cyanopyridine (5) with acid in dry chloroform resulted in the preparation of 6 in good yield. 

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