Lewis-Acid-Catalyzed Reductive Hydroalkoxylation of Propargylic N-Hydroxylamines Gives Stereoselective Access to Isoxazolidines

MsOH Promoted Divergent Synthesis of 4-Arylidene Isoxazolidines and Isoxazolines from O-Propargyl Hydroxylamines and Aldehydes

Temperature-dependent, Brønsted-acid-mediated divergent synthesis of 4-arylidene isoxazolidines and isoxazolines from O-propargyl hydroxylamines is developed. A series of control and crossover experiments have been carried out, revealing that the formation of 4-arylidene isoxazoline proceeds through an alkyne-oximium cyclization, ring-opening reaction, and subsequent condensation. Synthetic versatility of the developed methodology was highlighted in the synthesis of 3-aminoacrylaldehyde, β-hydroxy ketone, and isoxazole derivatives.

BF3·OEt2 Mediated Stereoselective Synthesis of 1,2-Heterocycle Fused Isoindolone Derivatives via Intramolecular Alkyne Oximium Cyclization of Oxoisoindolidene

In this research we report Lewis acid mediated intramolecular alkyne oximium cyclization cascade on oxoisoindolidene for the stereoselective syntheses of isoxazolidine/oxazinane/oxazepane fused isoindolones bearing three chiral centers. The approach enables the construction of a new C-C and C-O bonds along with an aza-tertiary stereocenter. The developed protocol features a wide range of substrate scope, functional group tolerance, and has been utilized in the synthesis of tetracyclic lactone.

Alkynyl hydroxylamines: key precursors for 1,2-N/O heterocycles

1,2-N/O Heterocycles are privileged scaffolds found in numerous natural products and have been shown to exhibit diverse pharmacological properties. Additionally, these compounds have emerged as important synthons in organic chemistry. Recent years have witnessed immense interest in these heterocycles from both the medicinal and the synthetic chemistry community. In light of the ever-increasing demand for isoxazolidine, isoxazoline, isoxazole, 1,2-oxazinane, 1,2-oxazine and 1,2-oxazepine in drug discovery, this review highlights an overview of recent methods (up to 2024) for the synthesis of these heterocycles starting from alkynyl hydroxylamines. The mechanistic pathways and stereo-electronic factors affecting yield and selectivity of the products are briefly explained to generate the understanding needed for future developments in this domain.

Silver-Catalyzed Markovnikov Addition Hydrofunctionalization of Terminal Alkynes: Synthesis of N-Enoxyimides and Oximes

N-Enoxyimides are versatile and valuable synthetic synthons in modern organic synthesis and catalysis. Herein, a silver-catalyzed hydrooxyimidation of terminal alkynes that enables the synthesis of N-enoxyimides, has been demonstrated. The protocol features are simple, step- and atom-efficient, scalable, and exhibit a broad scope of functional group tolerance under mild conditions. A novel hydrolysis of the N-enoxyimides unexpectedly provides the corresponding aromatic oximes in 96% yield, showing the potential application of this strategy.

Iodine(III)-Mediated Keto-oximation of O-Alkynyl Hydroxylamines: A Route to 3-Acyl Isoxazolines and 1,2-Oxazines

An intramolecular iodine(III)-mediated keto-oximation of O-alkynyl hydroxylamines offers rapid and straightforward access to 3-acyl Δ2-isoxazolines and 1,2-oxazines. This approach features mild, metal-free, and aerobic reaction conditions with good functional group tolerance. Moreover, the synthetic utility of this method is demonstrated by the synthesis of unique structural motifs such as isoxazolidine, 3-vinyl isoxazoline, and 2,5-diphenylpyrazine derivatives by the conversion of 3-acyl Δ2-isoxazolines, thereby showcasing its efficiency and applicability in synthetic chemistry.

Unlocking a reductive hydroalkoxylation cascade for the stereoselective synthesis of cyclic ethers: total synthesis of (±)-isolaurepan and (±)-cis-lauthisan

A Lewis acid-mediated, 5/6/7/8-endo-dig reductive hydroalkoxylation cascade on enynols gives expeditious, diastereoselective access to small and medium ring cyclic ethers with a long aliphatic side chain. The brevity of the approach allowed a 4-step, stereoselective total synthesis of (±)-isolaurepan and (±)-cis-lauthisan.

Harnessing Gold(I)-Catalyzed Hydroamination/1,3-Sulfonyl Migration Cascade for Divergent Synthesis of Isoxazolidine and Isoxazoline from O-Alkynyl Hydroxylamine

Gold-catalyzed 5-exo-dig hydroamination on O-homopropargylic hydroxylamine gave expeditious access to methylene isoxazolidine. Excess catalyst loading led to facile 1,3-sulfonyl migration in a cascade fashion to furnish the isoxazoline. Mechanistic studies using react-IR and NMR as well as crossover experiments indicated that 1,3-sulfonyl group migration is an intramolecular concerted process. The ene-hydroxylamine moiety also underwent dipolar cycloaddition reactions with nitrile oxide and nitrones leading to the first examples of spirocyclic bis-isoxazolidine derivatives.

Total Synthesis of Pyrrolidine and Piperidine Natural Products via TMSOTf-Mediated "5/6-endo-dig" Reductive Hydroamination of Enynyl Amines

Stereoselective syntheses of pyrrolidines and piperidines bearing hydrophobic chains have been achieved through a metal free, Lewis acid-mediated 5/6-endo-dig reductive hydroamination cascade of enynyl amines. The brevity of the developed strategy allowed for the collective stereoselective total synthesis of various alkaloids, including (±)-pyrrolidine cis-225H, (±)-epi-197B, (±)-epi-225C, the family of (+)-solenopsins and (+)-isosolenopsins, and the formal synthesis of (±)-bgugaine and (+)-azimic acid.