2 H ‐Pyrrole Derivatives from an Aza‐Nazarov Reaction Cascade Involving Indole as the Neutral Leaving Group
Abstract Trifluoromethyl‐substituted N ‐indolinyl‐1‐aza‐1,4‐dien‐3‐ones 4 , which are accessible in good yields from indolinylimino esters 6 in a two‐step procedure, undergo a novel type of aza‐Nazarov cyclization if treated with strong acids to give the hitherto unknown 3‐hydroxy‐5 H ‐pyrrole deriv...
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| Published in | European journal of organic chemistry Vol. 2008; no. 21; pp. 3656 - 3667 |
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| Main Authors | , , |
| Format | Journal Article |
| Language | English |
| Published |
01.07.2008
|
| Online Access | Get full text |
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| Summary: | Abstract
Trifluoromethyl‐substituted
N
‐indolinyl‐1‐aza‐1,4‐dien‐3‐ones
4
, which are accessible in good yields from indolinylimino esters
6
in a two‐step procedure, undergo a novel type of aza‐Nazarov cyclization if treated with strong acids to give the hitherto unknown 3‐hydroxy‐5
H
‐pyrrole derivatives
8a
–
p
. The solvent‐free polyphosphoric acid/acyl anhydride system as the acidic reaction medium is especially efficient and requires only short reaction times. According to quantum chemical calculations the key step in the reaction cascade is the cleavage of the N–N bond of the hydrazone fragment of a protonated
N
‐(indolin‐1‐yl)‐1‐aza‐1,4‐dien‐3‐one intermediate such as
10
. This intermediate releases 3
H
‐indole as an unusual, but very efficient neutral leaving group. Several 1‐aza‐1,4‐dien‐3‐ones
4
and some 3‐hydroxy‐5
H
‐pyrrole derivatives
8
were characterized by X‐ray diffraction.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) |
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| ISSN: | 1434-193X 1099-0690 |
| DOI: | 10.1002/ejoc.200800384 |