Indazoles are of considerable interest due to the broad variety of their biological activities. This overview summarizes structures of pharmacologically interesting indazoles published during the last decade, as well as syntheses, reactions, and functionalizations. Recent advances in the chemistry of N‐heterocyclic carbenes of indazole (indazol‐3‐ylidenes) and indazolium salts are also reported.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

1,3-Dimethylimidazolium-2-carboxylate and -4-carboxylate (norzooanemonine), which belong to two distinct classes of heterocyclic mesomeric betaines, undergo thermal decarboxylations to the N-heterocyclic carbenes imidazol-2-ylidene and imidazol-4-ylidene, respectively. These carbenes can be detected by ESI mass spectrometry and can be trapped by isocyanates to imidazolium-amidates, the structure of which was proved by independent syntheses. We performed calculations to characterize the different types of conjugation in the imidazolium-carboxylates.

1,2-Dimethylindazolium-3-carboxylates are pseudocross-conjugated mesomeric betaines (PCCMB) and derivatives of the indazole alkaloid Nigellicin. They decarboxylate on heating to give intermediary N-heterocyclic carbenes of indazole that can be trapped with iso(thio)cyanates to amidates. Alternatively, 1,2-dimethylindazolium-3-amidates can be prepared starting from the corresponding 1H-indazol-3-carboxylic acid which is converted into its chloride, reacted with anilines and deprotonated on anion exchange resin. The heterocumulene moieties of these amidates, which are new representatives of pseudo-cross-conjugated mesomeric betaines, can be exchanged to 3,5-dichlorophenyl isocyanate via the N-heterocyclic carbene.

The acylative dimerisation of 2‐alkenyl‐1,3‐thiazolines 1 gives compounds 3 and 8 upon treatment with trichloroacetyl chloride and trifluoroacetic anhydride, respectively. This reaction is completely diastereoselective, in particular giving only a single double‐bond isomer. The scope of the reaction has been evaluated synthetically, while a computational study has elucidated the mechanism of the reaction and the origin of stereocontrol. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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