Palladium-catalyzed ring-opening of cyclopropyl benzamides: synthesis of benzo[c]azepine-1-ones via C(sp3)–H functionalization

Palladium-catalyzed C-C bond cleavage of N-cyclopropyl acylhydrazones

Despite their utility as directing groups, the C-C bond cleavage of cyclopropanes utilizing hydrazones has not been explored. Herein, Pd-catalyzed C-C bond cleavage reaction of N-cyclopropyl acylhydrazones, followed by cycloisomerization to yield pyrazoles, has been developed. The protocol enables the synthesis of various α-pyrazole carbonyl compounds, which have a potential of biological activity. Control experiments and DFT calculations suggest that β-carbon elimination of a stable 6-membered chelate palladium complex occurs, generating a conjugated azine as a reaction intermediate for the following cycloisomerization.

Metal-catalysed C-C bond formation at cyclopropanes

Cyclopropanes are important substructures in natural products and pharmaceuticals. Although traditional methods for their incorporation rely on cyclopropanation of an existing scaffold, the advent of transition-metal catalysis has enabled installation of functionalized cyclopropanes using cross-coupling reactions. The unique bonding and structural properties of cyclopropane render it more easily functionalized in transition-metal-catalysed cross-couplings than other C(sp³) substrates. The cyclopropane coupling partner can participate in polar cross-coupling reactions either as a nucleophile (organometallic reagents) or as an electrophile (cyclopropyl halides). More recently, single-electron transformations featuring cyclopropyl radicals have emerged. This Review will provide an overview of transition-metal-catalysed C-C bond formation reactions at cyclopropane, covering both traditional and current strategies, and the benefits and limitations of each.

Access to hexahydroazepinone heterocycles via palladium-catalysed C(sp3)-H alkenylation/ring-opening of cyclopropanes

In this communication, we describe the synthesis of novel hexahydroazepinone derivatives starting from two simple building blocks in presence of a readily available palladium catalyst. The reaction proceeds through a selective C(sp³)-H alkenylation/ring-opening process to obtain the seven-membered ring products in good to excellent yields on a wide variety of substrates under batch, microwave, and continuous flow conditions.

Synthesis of Colibactin Pyrrolidono[3,4-d]pyridones via Regioselective C(sp3)-H Activation

The synthesis of pyrrolidono[3,4-d]pyridones of relevance to putative genotoxic colibactin structures featuring a doubly conjugated 1,6-Michael acceptor system is reported. We investigated and implemented a highly selective Pd-catalyzed C(sp³)-H activation reaction as a key step and further functionalized the pyridone core. Evaluating the role of this structural unit of relevance to colibactin, we found that this structure displayed a high degree of stability toward both acidic conditions and nucleophiles.

Synthesis of trans-2-Substituted Cyclopropylamines from α-Chloroaldehydes

Cyclopropylamines are prevalent in pharmaceuticals and agrochemicals. Herein, we report the synthesis of trans-2-substituted cyclopropylamines in high diastereoselectivity from readily available α-chloroaldehydes. The reaction proceeds via trapping of an electrophilic zinc homoenolate with an amine followed by ring closure to generate the cyclopropylamine. We have also observed that cyclopropylamine cis/trans-isomerization occurs in the presence of zinc halide salts and that this process can be turned off by the addition of a polar aprotic cosolvent.

Site-Selective and Stereoselective C-H Functionalization of N-Cyclopropylamides via a Directed Remote Metalation Strategy

A new methodology for site-selective and stereoselective C-H functionalization of aminocyclopropanes via directed remote lithiation has been developed. Treatment of N-directing group (DG = pivaloyl, tetramethylsuccinimidoyl) arylcyclopropanes with t-BuLi results in a clean β-lithiation and, following quench with electrophiles, leads to a range of cyclopropane derivatives. Sequential double lithiation-methylation to give a dimethylated cyclopropane has been achieved. X-ray, NMR, and computational studies allow rationalization of syn-DG β-deprotonation selectivity via a DG-lithium base coordinated complex.

Access to Cyclopropyl-Fused Azacycles via a Palladium-Catalyzed Direct Alkenylation Strategy

Palladium-catalyzed direct alkenylation of cyclopropyl C-H bonds proceeds in high efficiency. This transformation provides access to novel cyclopropyl-fused azacycles. Ligand studies suggest that bisphosphine monoxide analogues of dppf and rac-BINAP are the active ligand species. Preliminary results support that both BozPhos and IPrMonophos ligands can achieve high enantioinduction for this novel direct alkenylation reaction. To date, this represents the first example of enantioselective C-H functionalization employing a bisphosphine monoxide ligand.

TADDOL-based phosphorus(III)-ligands in enantioselective Pd(0)-catalysed C-H functionalisations

Monodentate TADDOL-derived phosphoramidites and phosphonites are versatile chiral ligands for enantioselective Pd(0)-catalysed C-H functionalisations. They enable highly selective cyclisations to access a wide range of chiral carbo- and heterocycles. The high attractiveness of this ligand class consists in their modular structure, allowing for a quick assembly of a library with variable steric properties. Asymmetric C-H functionalisation methods utilising catalytic systems based on Pd(0) complexes and TADDOL-type ligands are presented and aspects of selectivity are discussed.

Synthesis of Strained γ-Lactams by Palladium(0)-Catalyzed C(sp(3) )-H Alkenylation and Application to Alkaloid Synthesis

A variety of strained α-alkylidene-γ-lactams were synthesized by palladium(0)-catalyzed intramolecular C(sp(3) )-H alkenylation from easily accessible acyclic and monocyclic bromoalkene precursors. These lactams are valuable intermediates for accessing various classes of mono- and bicylic alkaloids containing a pyrrolidine ring, as illustrated with the synthesis of an advanced model of the marine natural product plakoridine A and of the indolizidine alkaloid δ-coniceine.

Enantioselective palladium(0)-catalyzed intramolecular cyclopropane functionalization: access to dihydroquinolones, dihydroisoquinolones and the BMS-791325 ring system

Taddol-based phosphoramidite ligands enable enantioselective palladium(0)-catalyzed C-H arylation of cyclopropanes. The cyclized products are obtained in high yields and enantioselectivities. The reported method provides efficient access to a broad range of synthetically attractive cyclopropyl containing dihydroquinolones and dihydroisoquinolones as well as allows for an efficient enantioselective construction of the 7-membered ring of the cyclopropyl indolobenzazepine core of BMS-791325.

Applications of C-H functionalization logic to cyclobutane synthesis

The application of C-H functionalization logic to target-oriented synthesis provides an exciting new venue for the development of new and useful strategies in organic chemistry. In this article, C-H functionalization reactions are explored as an alternative approach to access pseudodimeric cyclobutane natural products, such as the dictazole and the piperarborenine families. The use of these strategies in a variety of complex settings highlights the subtle geometric, steric, and electronic effects at play in the auxiliary guided C-H functionalization of cyclobutanes.

Cyclization and annulation reactions of nitrogen-substituted cyclopropanes and cyclobutanes

This feature article covers recent progress in cyclization and annulation reactions of aminocyclopropanes and aminocyclobutanes to access nitrogen-rich building blocks.