Convenient Method for the 3‐Functionalization of Isoindazoles

Selective functionalization of the 1H-imidazo[1,2-b]pyrazole scaffold. A new potential non-classical isostere of indole and a precursor of push-pull dyes

We report the selective functionalization of the 1H-imidazo[1,2-b]pyrazole scaffold using a Br/Mg-exchange, as well as regioselective magnesiations and zincations with TMP-bases (TMP = 2,2,6,6-tetramethylpiperidyl), followed by trapping reactions with various electrophiles. In addition, we report a fragmentation of the pyrazole ring, giving access to push-pull dyes with a proaromatic (1,3-dihydro-2H-imidazol-2-ylidene)malononitrile core. These functionalization methods were used in the synthesis of an isostere of the indolyl drug pruvanserin. Comparative assays between the original drug and the isostere showed that a substitution of the indole ring with a 1H-imidazo[1,2-b]pyrazole results in a significantly improved solubility in aqueous media.

Surprising Reactivity in NiXantphos/Palladium-Catalyzed α-Arylation of Substituted Cyclopropyl Nitriles

α-Arylations of cyclopropyl and related nitriles provide access to important synthetic intermediates and pharmacophores for biologically active molecules. However, robust methods for coupling of sterically encumbered partners have remained elusive. Through optimization using high-throughput experimentation (HTE), the NiXantphos ligand was found to be effective in the coupling of sterically hindered β-substituted cyclopropyl nitriles with a number of aryl groups and heterocycles, including those containing acidic N-H and O-H bonds.

Direct N-Alkylation and Kemp Elimination Reactions of 1-Sulfonyl-1H-Indazoles

The reactions of 1-sulfonyl-1H-indazoles under basic conditions are discussed, and the direct N-alkylation and Kemp elimination reactions of these compounds are reported. A series of 2-(p-tosylamino)benzonitriles and N-alkyl indazoles were prepared in good yields. Moreover, the 2-(p-tosylamino)benzonitriles could be transformed into a diverse range of important derivatives in a one-pot reaction. This method was successfully applied to the total syntheses of quindolinone and cryptolepinone; quindolinone was prepared in a one-pot reaction from 1-sulfonyl-1H-indazole.

An optimized procedure for direct access to 1H-indazole-3-carboxaldehyde derivatives by nitrosation of indoles

The nitrosation of indoles under slightly acidic conditions and reverse addition conditions leads to the preparation of the corresponding indazole-3-carboxaldehydes in high yields and greatly minimizes side reactions.

Unprotected Indazoles Are Resilient to Ring-Opening Isomerization: A Case Study on Catalytic C-S Couplings in the Presence of Strong Base

Indazoles represent a privileged scaffold in medicinal chemistry. In the presence of strong base, however, N-protected indazoles are prone to an undesirable ring-opening reaction to liberate o-aminobenzonitriles. By employing unprotected indazoles with a free N-H bond, isomerization is averted because the heterocycle is deprotonated in situ. We herein report functional group-tolerant and robust C-S couplings of bromoindazoles with thiols of varying electronic nature in the presence of lithium bis(trimethylsilyl)amide at elevated temperatures.

Rh(III)-Catalyzed Regio- and Chemoselective [4 + 1]-Annulation of Azoxy Compounds with Diazoesters for the Synthesis of 2H-Indazoles: Roles of the Azoxy Oxygen Atom

A Rh(III)-catalyzed tandem C-H alkylation/intramolecular decarboxylative cyclization of azoxy compounds with diazoesters for the synthesis of 3-acyl-2H-indazoles is disclosed. The azoxy instead of the azo group enables a distinct approach for cyclative capture, leading to a [4 + 1]-annulation rather than a classic [4 + 2] manner. The azoxy oxygen atom is traceless after annulation, and further removal from the product is not required. This reaction features a complete regioselectivity for unsymmetrical azoxybenzenes and a compatibility of monoaryldiazene oxides.