Regioselective C3-H Alkylation of Imidazopyridines with Donor-Acceptor Cyclopropanes

Lewis Acid-Enabled Chemodivergent Cycloadditions of Donor-Acceptor Cyclopropanes with Indoline-2-Thiones

Lewis acid-enabled reactions of donor-acceptor cyclopropanes (DACs) with indoline-2-thiones are reported. The reaction exhibits tunable annulation depending on the Lewis acid and the substituent at N1 of the indoline-2-thiones. With AlCl3 as the Lewis acid and 1-isopropylindoline-2-thiones as reactants, a direct ring opening with DACs, followed by intramolecular nucleophilic addition/dehydration takes place leading to the formation of dihydro-2H-thiepino[2,3-b]indoles in moderate to good yields. Using Yb(OTf)3 as promoter and 1-unsubstituted indoline-2-thiones as reactants, a (3 + 2) cycloaddition with DACs accompanied by sulfur rearrangement nucleophilic addition/dehydration takes place to give 3-indolyl-4,5-dihydrothiophenes in moderate yields. In addition, the synthetic transformation of 3-indolyl-4,5-dihydrothiophene to sulfone and indole-based axially chiral scaffolds further extends the synthetic utility and structural complexity.

Iodine-mediated regioselective C-3 sulfenylation using elemental sulfur and arylhydrazine hydrochloride to access 3-sulfenylated imidazo[1,2-a]pyridines

A transition metal-free protocol for the regioselective C-3 sulfenylation of imidazo[1,2-a]pyridines has been explored using arylhydrazine hydrochloride and elemental sulfur, in the presence of molecular iodine and DABCO via C(sp2)-H functionalization to achieve structurally diverse 3-sulfenylated imidazo[1,2-a]pyridines in reasonably high yields.

Directed regioselective arylation of imidazo[1,2-a]pyridine-3-carboxamides using Rh(III) catalysis

In contrast to previously reported free-radical pathways to functionalize imidazo[1,2-a]pyridines at the C-5 centre, directing group approaches are rare. Herein, we demonstrate a rhodium(III) catalyzed efficient and regioselective strategy for directed C-5 functionalization of imidazo[1,2-a]pyridines using N-methoxyamide as a directing group. This methodology facilitates directed arylation without the necessity for pre-functionalization. It also allows for gram-scale synthesis and post-functionalization.

Bortoluzzi A, Sandjo LP, Braga AL, F. de Assis F. Highly Enantioselective Lewis Acid Catalyzed Conjugate Addition of Imidazo[1,2-a]pyridines to α,β-Unsaturated 2-Acylimidazoles under Mild Conditions

A highly enantioselective protocol for the conjugate addition of 2-arylimidazo[1,2-a]pyridines and other imidazo derivatives to α,β-unsaturated 2-acylimidazoles is described. The method uses a previously reported chiral-at-metal rhodium catalyst and provides the corresponding adducts in yields of 25-98% with enantioselectivities up to er > 99:1. Additionally, the transformation proceeds under mild conditions using ethanol as the solvent at room temperature.

Hydro-phosphorothiolation of Styrene and Cyclopropane with S-Hydrogen Phosphorothioates under Ambient Conditions

A metal-free hexafluoroisopropanol-mediated hydro-phosphorothiolation of styrenes and donor-acceptor cyclopropanes with S-hydrogen phosphorothioates in a Markovnikov fashion has been developed under ambient reaction conditions to afford a library of S-alkyl phosphorothioates. Notably, this strategy provides a simple and efficient way to produce biologically significant kitazin and iprobenfos derivatives. Mechanistic studies disclose that the reaction proceeds through a carbocation intermediate.

Iron-catalyzed cascade C-C/C-O bond formation of 2,4-dienals with donor-acceptor cyclopropanes: access to functionalized hexahydrocyclopentapyrans

Iron-catalyzed cascade C-C and C-O bond formation of 2,4-dienals with donor-acceptor cyclopropanes (DACs) has been developed to furnish hexahydrocyclopentapyrans. Optically active DACs can be coupled stereospecifically (>97% ee). Chirality transfer, use of iron-catalysis and substrate scope are the salient practical features.