Aminopyrroles diazotization

With less electron-rich pyrroles, hydrochloric acid can be used in the diazotization. The diazo compounds were isolated after neutralization with aqueous ammonia [84H(22)2269]. 3-Diazo-2,5-diphenylpyrrole, although efficiently prepared by diazotization of the 3-aminopyrrole, can also be prepared by oxidation of the 3-nitrosopyrrole with dinitrogen trioxide (60JCS3270). Pyrrole-3-carboxylic acid, with buffered nitrous acid, gave the 3-diazo derivative 295 by displacement of the carboxyl group, together with the 3-diazo-2-carboxylic acid 296, obtained by further hydrolysis (62JCS1638) (Scheme 90). 

The free bases are much less stable than aniline, particularly 2-aminopyrroles and -furans which are very easily oxidized or hydrolyzed. 2-Aminofurans substituted with electron-withdrawing groups (e.g., N()2) are known and 3-amino-2-methylfuran is a relatively stable amine which can be acylated and diazotized. 2-Aminothiophene can be diazotized and the resulting diazonium salt coupled with -naphthol. 2,3-Diaminothiophene has been prepared and isolated as the hydrobromide the free base is not stable. 

Observations of the reactions of the few known 2-aminopyrroles are sparse. 2-Amino-4-ethyl-3,5-dimethyl- and 2-amino-3,4-diethyl-5-methyl-pyrrole could not be acetylated, benzoylated or diazotized. It is not clear whether these failures were due to inability to react or to instability . Acetyl and benzoyl derivatives were prepared from 2-amino-1-ethyl- and... 

Success was realized with the proposal of structure 29 (Scheme 9). This material was synthesized by treatment of 1 with p ra-nitrobenzene diazonium chloride (from diazotization of the para-nitroaniline), followed by reduction of the diazo confound 28 with SnCla in AcOH.(25) This 2-aminopyrrole co-eluted with the observed inqjurity, and displayed identical mass spec fragmentation and NMR spectra, confirming its structure as 29. 

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