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Chapter-5
Synthesis of highly substituted pyrroles
The reaction of β-amino-α,β-unsaturated esters 91(three atom fragment) with
nitroalkenes (two atom fragment) provides substituted pyrrole involving (3+2)
cyclization with N-C 2 and C 3 -C 4 bonds (Figure-4). 13
Figure-4
Base catalyzed (3+2) cyclizative condensation of α-diketones with amines
substituted with electron withdrawing substituents at α, α’-positions results in the
corresponding pyrroles with the formation of C 2 -C 3 and C 4 -C 5 bonds (Figure-5). 14
Figure-5
The reaction of β-keto esters with α-halo ketones or aldehydes in the presence
of ammonia or primary amine affords pyrroles involving (2+2+1) cyclization with the
formation of N-C 2 , C 3 -C 4 and N-C 5 bonds. (Hantzsch Synthesis). The reaction
proceeds via stabilized enamine intermediate which on C-alkylation and N-alkylation
by α-haloketone leads to the formation of corresponding pyrrole (Figure-6).
Figure-6
The condensation of aliphatic aldehydes or ketones with hydrazine under
strongly acidic conditions affords pyrroles involving (3+3) sigmatropic rearrangement
and cyclization. (Piloty-Robinson Synthesis). This reaction can also be applied for the
preparation of N-substituted pyrroles by condensing ketones with methyl hydrazine or
N,N’-dimethyl hydrazine (Figure-7). 15
R
R
H 2 N NH 2
O + O
R
R
H 3 C
CH 3
H + H +
RH 2 C
N NH H
CH 2 R -NH 3
RH 2 C
R
N
H
R
CH 2 R
Figure-7
Department of Chemistry, Saurashtra University, Rajkot-360005 160