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N‐Heterocyclic Carbene Induced Cycloaddition Reactions of Indazoles with Acetylenes To Form a New Ring System
Author(s) -
Schmidt Andreas,
Snovydovych Bohdan,
Hemmen Sascha
Publication year - 2008
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200800456
Subject(s) - chemistry , carbene , cycloaddition , betaine , decarboxylation , ring (chemistry) , ylide , deprotonation , medicinal chemistry , mesoionic , azomethine ylide , stereochemistry , indazole , 1,3 dipolar cycloaddition , organic chemistry , catalysis , ion
Thermal decarboxylation of the mesomeric betaine 1,2‐dimethylindazolium‐3‐carboxylate resulted in the formation of the N‐heterocyclic carbene indazol‐3‐ylidene which deprotonated a second molecule of the betaine to give an azomethine ylide. This 1,3‐dipole underwent a cycloaddition/decarboxylation sequence on treatment with ethyl or methyl 3‐phenylpropiolate to give the new ring system 3,5‐dihydro‐2 H ‐pyrrolo[1,2‐ b ]indazole. By contrast, dimethyl or diethyl but‐2‐ynedioate (DMAD, DEAD) as the activated triple bond yielded this ring system with an ester group at the 9b‐position, which originates from the acetylene derivative. Model reactions were carried out to elucidate the mechanisms of these reactions, and some trapping products of the N‐heterocyclic carbene have been characterized. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)