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Chapter-5
Synthesis of highly substituted pyrroles
The reaction of benzoin with benzyl aryl ketones in the presence of
ammonium acetate results in the formation of polyaryl substituted pyrroles involving
(2+2+1) cyclization (Figure-8). 16 Figure-8
The condensation of alkyl isocyanoacetates with aldehydes in 2:1 ratio
proceeds to involve an initial conjugated addition followed by an anionic cyclization
with the formation of C 2 -C 3 , C 3 -C 4 and C 4 -C 5 bonds (Figure-9). 17
Figure-9
It is the most general method and involves (4+1)cyclizative condensation of
1,4-dikitones (enolizable) with ammonia or its derivatives with the formation of N-C 2
and N-C 5 bonds 1-6, 18-19 . The reaction is considered to proceed with the successive
attacks of nucleophilic nitrogen on the carbonyl carbon and finally followed by
dehydration (aromatization) for which driving force is provided by the stability of the
resulting pyrrole (Figure-10).
Figure-10
Metal ion assisted amination of 1,4-dienes or diynes with primary amines
followed by cyclization provides N-substituted pyrroles (Figure 11 & 12). 20-21
Figure 11
Department of Chemistry, Saurashtra University, Rajkot-360005 161