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9.13.4.2.2.1 Method 1: Substitution Reactions of Mannich Bases

DOI: 10.1055/sos-SD-009-00542

Black, D. StC.Science of Synthesis, (20019539.

The Mannich reaction conveniently provides a method for the introduction of a (dialkylamino)methyl group into the 2-position of the pyrrole ring. The dialkylamino group can be displaced by a range of nucleophiles.[‌476‌,‌477‌] Branched aminoalkylpyrroles can be prepared by the addition of alkyllithium compounds to the iminium salt intermediates of the VilsmeierHaack reaction of pyrroles: these compounds also undergo similar displacement reactions via azafulvene intermediates.[‌478‌] Thus the branched (aminoalkyl)pyrrole 569 can be prepared from pyrrole (1), via the iminium salt 567 and the azafulvene 568, by the addition of tert-butyllithium to the latter intermediate. Treatment of 569 with sodium cyanide in the presence of toluenesulfonic acid leads to the pyrrolecarbonitrile 570 (Scheme 82).[‌478‌] Azide ion has recently been added to the list of nucleophiles capable of displacing trimethylamine from a quaternized pyrrole Mannich base. Thus the ammonium salt 571 can be converted into the (azidomethyl)pyrrole 572, which can serve as a precursor of further useful compounds (Scheme 82).[‌479‌] Another strategy for the introduction of nucleophilic groups on a 2-methylene substituent of a pyrrole is the flash-vacuum pyrolysis of 2-[(dimethylamino)methyl]pyrrole at 650°C: dimethylamine is eliminated and the reactive intermediate is trapped by benzenethiol.[‌480‌]

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