Modular Synthesis of Cyclopropane-Fused N-Heterocycles Enabled by Underexplored Diazo Reagents

α-(N-Alkyl-N-heteroarenium)-α-diazoacetates: synthesis and reactivity of a novel class of 'onium' diazo compounds

Treatment of alkyl α-(N-heteroaryl)-α-diazoacetates with alkylating reagents affords diazoacetate N-heteroarenium salts. These novel 'onium' diazo compounds are mostly yellow solids, displaying increased thermal and acid stability. Their tetrafluoroborates undergo rhodium catalyzed [2 + 1] and Doyle-Kirmse reactions under mild conditions, suggesting the N-quaternization an effective means of elimination of N-coordination caused catalyst toxicity.

Skeletal and Mechanistic Diversity in Ir-Catalyzed Cycloisomerizations of Allene-Tethered Pyrroles and Indoles

Pyrroles and indoles bearing N-allenyl tethers participate in a variety of iridium-catalyzed cycloisomerization processes initiated by a C-H activation step, to deliver a diversity of synthetically relevant azaheterocyclic products. By appropriate selection of the ancillary ligand and the substitution pattern of the allene, the reactions can diverge from simple intramolecular hydrocarbonations to tandem processes involving intriguing mechanistic issues. Accordingly, a wide range of heterocyclic structures ranging from dihydro-indolizines and pyridoindoles to tetrahydroindolizines, as well as cyclopropane-fused tetrahydroindolizines can be obtained. Moreover, by using chiral ligands, these cascade processes can be carried out in an enantioselective manner. DFT studies provide insights into the underlying mechanisms and justify the observed chemo- regio- and stereoselectivities.

Organocatalytic Reductive Amination of the Chiral Formylcyclopropanes: Scope and Applications

We developed a sustainable three-component reductive amination protocol for the chemoselective coupling of optically active functionally rich donor-acceptor carbonyl-cyclopropanes with various amines under 10 mol % of diphenyl phosphate in the presence of Hantzsch ester as a hydride source. The catalytic selective reductive C-N coupling has wide advantages like no epimerization, no ring opening, large substrate scope, generating only mono N-alkylation products and simultaneously resulting in chiral cyclopropane-containing amines possessing many applications in the medicinal chemistry. In this article, we have shown the synthetic applications of reductive C-N coupling reaction to make chiral α-carbonyl-cyclopropane containing amines 8, double C-N coupled cyclopropane-amines 10, unusual C-N/C-C coupled cyclopropane-amines 12, chiral tert-butylsulfinamide containing cyclopropanes 14/15, and functionally rich chiral cyclopropane-fused N-heterocycles 16/18/19. Many of these chiral cyclopropane-amines 5-19 can serve as building blocks for the synthesis of drug-like small molecules, natural products, pharmaceuticals, and their analogues.

Photogenerated donor-donor diazo compounds enable facile access to spirocyclopropanes

Prompted by the increasing interest in strained hydrocarbons as potential drug candidates, we developed a simple and efficient photochemical protocol for (spiro)cyclopropanes from bench stable tosylhydrazones and electron poor olefins. This two-step one-pot transformation proceeds by (3+2)-cycloaddition of in situ formed donor-donor diazo compounds, followed by nitrogen extrusion of the Δ1-pyrazoline intermediates. Notably, kinetic analysis enabled the isolation of intermediary spiro-heterocycles.

Photosensitized O2 enables intermolecular alkene cyclopropanation by active methylene compounds

Cyclopropanes are key features in many preclinical, clinical, and commercial drugs, as well as natural products. The most prolific technique for their synthesis is the metal-catalyzed reaction of an alkene with a diazoalkane, a highly energetic reagent requiring stringent safety precautions. Discovery of alternative innocuous reagents remains an ongoing challenge. Herein, we report a simple photoredox-catalyzed intermolecular cyclopropanation of unactivated alkenes with active methylene compounds. The reaction proceeds in neutral solvent under air or dioxygen (O2) with a photoredox catalyst excited by blue light-emitting diode light and an iodine co-catalyst that is either added as molecular iodine or generated in situ from alkyl iodides. Mechanistic investigations indicate that photosensitized O2 plays a vital role in the generation of carbon-centered radicals for both the addition of active methylene compounds to alkenes and the ring closure.

Base-Promoted Intramolecular Addition of Vinyl Cyclopropanecarboxamides to Access Conformationally Restricted Aza[3.1.0]bicycles

3-Azabicyclo[3.1.0]hexanes are common structural components in natural products and bioactive compounds. Traditionally, the metal-mediated cyclopropanation domino reaction of chain enzymes is the most commonly used strategy for the construction of this type of aza[3.1.0]bicycle derivative. In this study, a base-promoted intramolecular addition of alkenes used to deliver conformationally restricted highly substituted aza[3.1.0]bicycles is reported. This reaction was tailor-made for saturated aza[3.1.0] bicycle-containing fused bicyclic compounds that may be applied in the development of concise and divergent total syntheses of bioactive compounds.