Leveraging Alkyne-Oximium Cyclization for the Stereoselective Synthesis of Isoxazolidine

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.

Cascade Aza-Prins/Friedel-Crafts Reaction of Homocinnamyloxycarbamate and Aromatic Aldehydes Yielding Aromatic Ring-Annulated Hydrocyclopenta-1,2-oxazinane

The cascade aza-Prins/Friedel-Crafts reaction of homocinnamyloxycarbamate with electron-rich aromatic aldehydes has been successfully established. Most of the aromatic aldehydes react with the carbamate stereoselectively to generate cis-hydroindeno-1,2-oxazinanes. However, the cascade reactions of benzaldehydes bearing two methoxy groups at the meta-positions exhibit a unique stereochemical profile. Furthermore, the cascade reaction of benzaldehyde bearing three methoxy groups at the meta- and para-positions proceeds along with a skeletal rearrangement. Additionally, the aza-Prins/Ritter reaction of cinnamyloxycarbamate with various aldehydes was found during the development of the cascade reaction. In contrast to the cascade aza-Prins/Friedel-Crafts reaction, the aza-Prins/Ritter reaction gave trans-disubstituted isoxazolidines. These stereochemical profiles for both reactions were considered on the basis of several transition-state models.

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.

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.

Brønsted acid-catalyzed annulation reaction of hydroxamic acids: synthesis of cyclopentane-fused isoxazolidines and their benzilic amide rearrangement

Readily available hydroxamic acids were leveraged to access challenging nitrones in the presence of H3PO4 as a Brønsted acid catalyst and engaged in an intramolecular (3+2) annulation reaction to make valuable cyclopentane-fused isoxazolidines with high yields and excellent diastereoselectivity. The products were further utilized in a unique base-promoted benzilic amide rearrangement to provide cyclopentane-fused γ-lactams bearing three contiguous stereocenters as a single diastereomer.

Stereoselective Synthesis of Pyrrolo/Pyrido[2,1-a]isoindoles via Alkyne Iminium Ion Cyclization of Vinylogous Carbamates

An efficient, acid-mediated, intramolecular alkyne iminium ion cyclization of oxoisoindolidene for the diastereoselective synthesis of pyrrolo/pyridoisoindole is described. This protocol features broad substrate scope and easy scalability. An unusual N to C-1,3-alkyl shift is observed with substrates bearing strong electron donating group at the phenyl ring attached to alkyne with concomitant hydration of alkyne to the ketone.

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

Lewis-acid-catalyzed 5-endo-dig reductive hydroalkoxylation cascade on propargylic N-hydroxylamine gave expedient, stereoselective access to isoxazolidine derivatives. The developed method provides a new approach toward the synthesis of isoxazolidine, a biologically privileged scaffold. The synthetic potential of the developed methodology was demonstrated by synthesizing 1,3-aminoalcohol, 4-aminotetrahydropyran, and sedamine natural products.