Synthesis of Polysubstituted Pyrroles via Silver-Catalyzed Oxidative Radical Addition of Cyclopropanols to Imines

Iron-Catalyzed Three-Component Reactions of Cyclopropanols with Alkenes and N-Heteroarenes via Ring-Opening and C-H Functionalization

An iron-catalyzed ring-opening multicomponent reaction of cyclopropyl alcohols with alkenes and N-heteroarenes involving aryl C(sp2)-H functionalization was developed. This protocol facilitates the regioselective introduction of both the β-carbonyl moiety and an N-heteroarene group across the C═C bond of the alkene, thus allowing a straightforward, efficient, and facile access to 5-heteroarene ketones. In this process, this strategy relies on β-carbonyl alkyl radical formation from the ring-opening of cyclopropyl alcohols, addition across C═C bonds, and heteroaryl C(sp2)-H functionalization cascades. This general approach displays excellent selectivity control and broad functional-group tolerance.

Homo-Mannich Reaction of Cyclopropanols: A Versatile Tool for Natural Product Synthesis

ConspectusThe Mannich reaction, involving the nucleophilic addition of an enol(ate) intermediate to an imine or iminium ion, is one of the most widely used synthetic methods for the synthesis of β-amino carbonyl compounds. Nevertheless, the homo-Mannich reaction, which utilizes a homoenolate intermediate as the nucleophilic partner and provides straightforward access to the valuable γ-amino carbonyl compounds, remains underexplored. This can be largely attributed to the difficulties in generation and manipulation of the homoenolate species, despite various homoenolate equivalents that have been developed. Among the homoenolate equivalents developed, cyclopropanol stands out due to its intriguing reactivities endowed by the highly strained cyclopropane. Upon activation by a metal, cyclopropyl alcohol is prone to undergo an endocyclic C(sp3)-C(sp3) bond cleavage to give a homoenolate intermediate or a β-keto radical intermediate, which sets the stage for a diverse range of transformations. This account outlines our recent progress in the development of homo-Mannich reaction of cyclopropanol and its applications in natural product total synthesis. This new methodology can be classified into two subtypes: 1) the homo-Mannich reaction of cyclopropanol with imines or iminium ions and 2) the homo-Mannich-type reaction of cyclopropanol with heteroarenes. Through different ways to generate imines or iminium ions, tandem or sequential reactions of C-H oxidation/homo-Mannich, Bischler-Napieralski/homo-Mannich, and asymmetric allylation/homo-Mannich have been developed, leading to the rapid assembly of core scaffolds of sarpagine, koumine, ibophyllidine, Aspidosperma, Melodinus, and Kopsia alkaloids. Besides the reactions with imines or iminium ions, cyclopropyl alcohol can undergo ring-opening addition to indole and pyrrole rings to deliver core scaffolds of schizozygane and indolizidine alkaloids. Based on these methodology advancements, we have accomplished the asymmetric synthesis of 29 alkaloids belonging to 8 families. In this Account, we present a complete picture of our works concerning synthetic design, method development, and applications in natural product total synthesis. It is anticipated that the development of new methodologies of cyclopropyl alcohol will find broad applications in the realm of natural product synthesis.

The Azobis(isobutyronitrile)-Promoted Oxidative Radical α-Cyanation of Imine under Atmospheric O2

Herein, we report an azobis(isobutyronitrile) (AIBN)-promoted radical α-cyanation of in situ formed imine under atmospheric O2. This oxidative radical addition (ORA) procedure proceeds with the sequential homocleavage of AIBN, extrusion of N2, and capture of O2 toward an O-centered radical, which is converted to a cyano radical by β-scission. Then, the insertion of the cyano radical into the imine C═N bond forms an aminyl radical, leading to α-cyano imine after 1,2-hydrogen atom transfer (HAT) and H abstraction. Such a transition-metal-free procedure features mild reaction conditions and a broad substrate scope, employing molecular O2 as a clean terminal oxidant.

Ring-Opening Coupling of Cyclopropanol with 1,2,3-Triazole for the Synthesis of Fused Triazoles

A radical ring-opening arylation of cyclopropanol with 1,2,3-triazole has been achieved. This synthetic protocol provides straightforward access to a wide range of structurally diverse and chiral 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyridines with high efficiency from readily available chiral cyclopropanols.

Regiocontrol via Electronics: Insights into a Ru-Catalyzed, Cu-Mediated Site-Selective Alkylation of Isoquinolones via a C-C Bond Activation of Cyclopropanols

A site-selective C(3)/C(4)-alkylation of N-pyridylisoquinolones is achieved by employing C-C bond activation of cyclopropanols under Ru(II)-catalyzed/Cu(II)-mediated conditions. The regioisomeric ratios of the products follow directly from the electronic nature of the cyclopropanols and isoquinolones used, with electron-withdrawing groups yielding predominantly the C(3)-alkylated products, whereas the electron-donating groups primarily generate the C(4)-alkylated isomers. Density functional theory calculations and detailed mechanistic investigations suggest the simultaneous existence of the singlet and triplet pathways for the C(3)- and C(4)-product formation. Further transformations of the products evolve the utility of the methodology thereby yielding scaffolds of synthetic relevance.

Copper-catalyzed three-component annulation toward pyrroles via the cleavage of two C-C bonds in 1,3-dicarbonyls

The first copper-catalyzed three-component annulation of α,β-unsaturated ketoximes, 1,3-dicarbonyls and paraformaldehyde has been documented. This novel strategy achieved the two C-C bond cleavage of 1,3-dicarbonyl compounds directly as a single-carbon synthon and provided a new and highly efficient method for the synthesis of 2,3-disubstituted pyrroles in moderate to good yields with broad functional group compatibility.

Lewis Acid-Catalyzed Remote Site-Selective Ring Deconstruction of Cyclobuteno[a]naphthalene-4-ones to Access Unsymmetric 1,1-Diarylated Olefins

A catalytic site-selective ring deconstruction of cyclobuteno[a]naphthalene-4-ones with alcohols is reported, enabling the direct production of a wide range of unsymmetric 1,1-diarylated olefins with good yields and complete regioselectivity. The late-stage application of these resulting terminal olefins demonstrates great possibilities to apply this strategy to complex molecules. The protocol features good functional group compatibility, broad substrate scope, and controllable site selectivity.

Transition-metal-free three-component coupling approach to QUINAP derivatives

The aluminum(III) triflate catalyzed three-component coupling reaction of alkynes, amines and phosphorylated aryl aldehydes to access phosphoryl quinoline derivatives has been developed. The reaction proceeds in a simple system without the use of transition metals, ligands or additives, thus making it attractive for the fast preparation of a variety of new potential N-P bidentate ligands.

Iron-Catalyzed Ring-Opening Reactions of Cyclopropanols with Alkenes and TBHP: Synthesis of 5-Oxo Peroxides

The ring opening of cyclopropanols is rarely used in multicomponent reactions. Herein we report the three-component reaction of cyclopropanols with alkenes and tert-butyl hydroperoxide (TBHP) catalyzed by an iron catalyst. This protocol enables the incorporation of both the β-carbonyl fragment and a peroxy unit across the C═C double bond regioselectively, thus allowing an efficient, facile access to 5-oxo peroxides. Modification of the biologically active molecules and various downstream derivatizations of the peroxides are also demonstrated.

Rhodium(III)-catalyzed oxidative alkylation of N-aryl-7-azaindoles with cyclopropanols

An efficient Rh(iii)-catalyzed C-H oxidative alkylation of N-aryl-7-azaindoles with cyclopropanols by merging tandem C-H and C-C cleavage was developed. This transformation features mild reaction conditions, high regioselectivity, and excellent functional group compatibility. The resulting β-aryl ketone derivatives can be readily transformed into 7-azaindole-containing π-extended polycyclic heteroarenes.