Complex Polyheterocycles and the Stereochemical Reassignment of Pileamartine A via Aza-Heck Triggered Aryl C-H Functionalization Cascades

Enantioselective (3+2) Annulation of Donor-Acceptor Cyclopropanes with Aldehydes and Ketones Catalyzed by Brønsted Bases

The substituted tetrahydrofuran core is a structural motif in many biologically active and natural compounds. However, the scarcity of enantioselective methods developed towards its synthesis makes this field challenging and attractive to explore. Herein, the first Brønsted-base catalyzed enantioselective (3+2) annulation of donor-acceptor cyclopropanes with aldehydes and ketones affording enantioenriched 2,3,5-substituted tetrahydrofurans is reported. The reaction concept is based on activation of racemic β-cyclopropyl ketones by a chiral bifunctional Brønsted base which catalyzes the (3+2) annulation for a range of aldehydes and ketones. For aldehydes, the annulation furnished tetrahydrofurans in excellent yield, good diastereoselectivity and with excellent enantioselectivity up to >99 % ee. Surprisingly, aromatic aldehydes afforded the cis-2,5-substituted tetrahydrofurans as the major diastereoisomer, while for aliphatic aldehydes the trans-cycloadduct was favored. The reaction also proceeds well for ketones affording spiro tetrahydrofurans in excellent yields and enantioselectivities (up to 99 % ee). Hammett studies have been conducted to elucidate the influence of the electronic nature of benzaldehydes on the stereoselectivity. Based on the diastereochemical outcome for the aldehydes, two reaction paths for aromatic and aliphatic aldehydes are proposed. Finally, two diastereoselective synthetic transformations have been conducted to demonstrate the synthetic potential of the obtained products.

SN2 Reaction at the Amide Nitrogen Center Enables Hydrazide Synthesis

Nucleophilic substitutions are fundamentally important transformations in synthetic organic chemistry. Despite the substantial advances in bimolecular nucleophilic substitutions (SN2) at saturated carbon centers, analogous SN2 reaction at the amide nitrogen atom remains extremely limited. Here we report an SN2 substitution method at the amide nitrogen atom with amine nucleophiles for nitrogen-nitrogen (N-N) bond formation that leads to a novel strategy toward biologically and medicinally important hydrazide derivatives. We found the use of sulfonate-leaving groups at the amide nitrogen atom played a pivotal role in the reaction. This new N-N coupling reaction allows the use of O-tosyl hydroxamates as electrophiles and readily available amines, including acyclic aliphatic amines and saturated N-heterocycles as nucleophiles. The reaction features mild conditions, broad substrate scope (>80 examples), excellent functional group tolerability, and scalability. The method is applicable to late-stage modification of various approved drug molecules, thus enabling complex hydrazide scaffold synthesis.

Enantioselective Synthesis of Isoindolinone by Palladium-Catalyzed Aminoalkynylation of O-Phenyl Hydroxamic Ethers with Alkynes

A highly efficient palladium-catalyzed asymmetric tandem aza-Heck/Sonogashira coupling reaction of O-phenyl hydroxamic ethers with terminal alkynes is described. This protocol enables versatile access to challenging chiral isoindolinone derivatives bearing a quaternary stereogenic center. The palladium-catalyzed aminoalkynylation reaction shows broad functional group tolerance and allows the straightforward preparation of isoindolinones with high efficiency and excellent enantioselectivity under mild conditions. DFT calculations were performed to disclose the reaction mechanism and the origins of the enantioselectivity.

Recent Progress in Asymmetric Domino Intramolecular Cyclization/Cascade Reactions of Substituted Olefins

The domino cyclization/coupling strategy is one of the most effective methods to produce cyclized and multi-functionalized compounds from olefins, which has attracted huge attention from chemists and biochemists especially for its considerable potential of enantiocontrol. Nowadays, more and more studies are developed to achieve difunctionalization of substituted olefins through an asymmetric domino intramolecular cyclization/cascade reaction, which is still an elegant choice to accomplish several synthetic ideas such as complex natural products and drugs. This review surveys the recent advances in this field through reaction type classification. It might serve as useful knowledge desktop for the community and accelerate their research.

Interpreting vibrational circular dichroism spectra: the Cai•factor for absolute configuration with confidence

Vibrational circular dichroism (VCD) spectroscopy can generate the data required for the assignment of absolute configuration, but the spectra are hard to interpret. We have recorded VCD data for thirty pairs of small organic compounds and we use this database to validate a method for the automated analysis of VCD spectra and the assignment of absolute configuration: the Cai•factor (Configuration: absolute information). The analysis of the data demonstrates that the procedure is a reliable and time-efficient method for determination of absolute configuration, which gives both the assignment and a measure of confidence in the outcome, even when the spectra are imperfect. The majority of molecules tested have a high confidence score and all of these have the correct assignment.

Metal- and Base-Free Radical Cascade Cyclization/Hydrolysis of CN-Containing 1,6-Enynes with Ethers to Access Polyheterocycles

A convenient and straightforward approach for the radical cascade cyclization/hydrolysis of CN-containing 1,6-enynes with simple ethers under metal- and base-free conditions is described. This strategy provides a variety of valuable ethers-substituted polyheterocycles via the construction of three C-C bonds, one C=O bond, and two new six-membered rings within a single procedure. The resulting products can smoothly undergo follow-up conversions to various useful scaffolds. The methodology shows excellent functional group tolerance, high step- and atom- economy, and mild reaction conditions, which can be further scaled up to gram quantity in a satisfactory yield.

Palladium-Catalyzed Alkenyl C-H Activation/Diamination toward Tetrahydrocarbazole and Analogs Using Hydroxylamines as Single-Nitrogen Sources

A palladium-catalyzed alkenyl C-H activation/diamination reaction of cycloalkenyl bromoarenes with hydroxylamines is described. A wide range of tetrahydrocarbazoles and analogs has been prepared using fine-tuning bifunctional secondary hydroxylamines as the single-nitrogen sources. Mechanistic investigations suggest that the selective alkenyl C-H activation/diamination cascade process should build the N-heterocycles.

Cyclopropane-Fused N-Heterocycles via Aza-Heck-Triggered C(sp3)-H Functionalization Cascades

Unique examples of aza-Heck-based C(sp³)–H functionalization cascades are described. Under Pd(0)-catalyzed conditions, the aza-Heck-type cyclization of N-(pentafluorobenzoyloxy)carbamates generates alkyl–Pd(II) intermediates that effect C(sp³)–H palladation en route to cyclopropanes. Key factors that control the site selectivity of the cyclopropanation process have been elucidated such that selective access to a wide range of ring- or spiro-fused systems can be achieved.

Cu-Catalyzed Cyclization/Coupling of Alkenyl Aldimines with Arylzinc Reagents: Access to Indole-3-diarylmethanes

We report a Cu(II)-catalyzed cyclization/coupling of alkenyl aldimines with arylzinc reagents to create indole-3-diarylmethane derivatives (Sapkota et al. ChemRxiv 2022, DOI: 10.26434/chemrxiv-2022-d6qn). The current reaction provides a unified modular route from readily available starting materials to indole-3-diarylmethanes in which all three arene cores can be decorated with differential functional substitutions on demand. Since the cyclization/coupling of alkenyl aldimines is unknown to date, the current method widens the scope with regard to both the substrate and product diversity for this class of reaction.

Directed C-H Functionalization of C3-Aldehyde, Ketone, and Acid/Ester-Substituted Free (NH) Indoles with Iodoarenes via a Palladium Catalyst System

Pd(II)-catalyzed C-H arylations of free (NH) indoles including different carbonyl directing groups on C3-position with aryl iodides are demonstrated. Importantly, the reactions are carried out using the same catalyst system without any additional transient directing group (TDG). In this study, the formyl group as a directing group gave the C4-arylated indoles versus C2-arylation. Using this catalyst system, C-H functionalization of 3-acetylindoles provided domino C4-arylation/3,2-carbonyl migration products. This transformation involves the unusual migration of the acetyl group to the C2-position following C4-arylation in one pot. Meanwhile, migration of the acetyl group could be simply controlled and N-protected 3-acetylindoles afforded C4-arylation products without migration of the acetyl group. Functionalization of indole-3-carboxylic acid (or methyl ester) with aryl iodides using the present Pd(II)-catalyst system resulted in decarboxylation followed by the formation of C2-arylated indoles. Based on the control experiments and the literature, plausible mechanisms are proposed. The synthetic utilities of these acetylindole derivatives have also been demonstrated. Remarkably, C4-arylated acetylindoles have allowed the construction of functionalized pityiacitrin (a natural product).

Hydroamination, Aminoboration, and Carboamination with Electrophilic Amination Reagents: Umpolung-Enabled Regio- and Stereoselective Synthesis of N-Containing Molecules from Alkenes and Alkynes

Nitrogen (N) is ubiquitously found in bioactive molecules, pharmaceutical agents, and organic functional materials. Accordingly, development of new C-N bond-forming catalysis has been one of the long-standing research subjects in synthetic organic chemistry. In this Perspective, recent advances in highly selective amination reactions with electrophilic amination reagents are described: by taking advantage of the concept of nitrogen umpolung, otherwise challenging aminofunctionalizations, such as hydroamination, aminoboration, and carboamination, of readily available feedstock-like alkenes and alkynes are possible, giving densely functionalized complex and often chiral alkylamines with high selectivity. The scope, limitations, and reaction mechanism are briefly summarized.

N-Aroyloxycarbamates as switchable nitrogen and oxygen precursor: Ir/Cu controlled divergent C–H functionalization of heteroarenes

Chemodivergent control in the functionalization of nitrogen-directed aromatic C–H bonds has been achieved by a switchable catalyst.

Construction of NH-Unprotected Spiropyrrolidines and Spiroisoindolines by [4+1] Cyclizations of γ-Azidoboronic Acids with Cyclic N-Sulfonylhydrazones

The reactions of N‐sulfonylhydrazones derived from cyclic ketones with γ‐azidopropylboronic acid and 2‐(azidomethyl)phenylboronic acid give rise to spirocyclic pyrrolidines and spiroisoindolines, respectively. The reactions proceed without the need of any transition‐metal catalyst through a domino process that comprises the formation of a Csp³‐C and a Csp³‐N bond of the former hydrazonic carbon. The scope of the reaction has been explored by the preparation of over 50 examples of NH‐unprotected spirocyclic derivatives. Importantly, this methodology could be applied for the preparation of alkaloid steroids from steroid N‐tosylhydrazones.