Oxazepine Synthesis by Copper-Catalyzed Intermolecular Cascade Reactions between O-Propargylic Oximes and Dipolarophiles

Consecutive O-S/N-S Bond Cleavage in Gold-Catalyzed Rearrangement Reactions of Alkynyl N-Sulfinylimines

Gold-catalyzed reactions of alkynyl N-sulfinylimines were used to produce the corresponding 2H-azirines possessing sulfenyl and acyl groups at the 3-position of the azirine ring in good to excellent yields. These reactions involved internal transfer of the sulfinyl oxygen atom to form a thiooxime intermediate tethered to an α-oxo gold carbene moiety. Subsequent insertion of the carbene into the N-S bond resulted in ring construction.

Copper-Catalyzed Cycloisomerization of Unactivated Allene-Tethered O-Propargyl Oximes: A Domino Reaction Sequence toward the Synthesis of Hexahydropyrrolo[3,4-b]azepin-5(4H)-ones

A novel copper-catalyzed cycloisomerization of unactivated allene-tethered O-propargyl oximes has been developed for the synthesis of hexahydropyrrolo[3,4-b]azepin-5(4H)-ones. This one-pot domino reaction proceeds via a [2,3]-sigmatropic rearrangement, a [3 + 2] cycloaddition, and another [3,3]-sigmatropic rearrangement. The methodology offers a practical and straightforward route for the rapid assembly of both ring components of the fused bicyclic motifs from acyclic precursors by simultaneously forming four new bonds (a C═O, a C═N, and two C-C bonds) in a single step.

Divergent Gold Catalysis: Unlocking Molecular Diversity through Catalyst Control

The catalyst-directed divergent synthesis, commonly termed as "divergent catalysis", has emerged as a promising technique as it allows chartering of structurally distinct products from common substrates simply by modulating the catalyst system. In this regard, gold complexes emerged as powerful catalysts as they offer unique reactivity profiles as compared to various other transition metal catalysts, primarily due to their salient electronic and geometrical features. Owing to the tunable soft π-acidic nature, gold catalysts not only evolved as superior contenders for catalyzing the reactions of alkynes, alkenes, and allenes but also, more intriguingly, have been found to provide divergent reaction pathways over other π-acid catalysts such as Ag, Pt, Pd, Rh, Cu, In, Sc, Hg, Zn, etc. The recent past has witnessed a renaissance in such examples wherein, by choosing gold catalysts over other transition metal catalysts or by fine-tuning the ligands, counteranions or oxidation states of the gold catalyst itself, a complete reactivity switch was observed. However, reviews documenting such examples are sporadic; as a result, most of the reports of this kind remained scattered in the literature, thereby hampering further development of this burgeoning field. By conceptualizing the idea of "Divergent Gold Catalysis (DGC)", this review aims to consolidate all such reports and provide a unified approach necessary to pave the way for further advancement of this exciting area. Based on the factors governing the divergence in product formation, an explicit classification of DGC has been provided. To gain a fundamental understanding of the divergence in observed reactivities and selectivities, the review is accompanied by mechanistic insights at appropriate places.

Exo-Cyclization: Intermolecular Methylene Transfer Sequence in Au-Catalyzed Reactions of O-Homopropargylic Oximes

The Au-catalyzed reactions of O-homopropaylic oximes afforded the 3-alkenylated isoxazolines in good to high yields. The mechanistic studies suggest that the reaction proceeds through an exo-cyclization followed by intermolecular methylene group transfer. In addition, oligomeric species of the starting material were observed in the reaction mixture by mass spectra, supporting our proposed mechanism, which proceeds through a repeated intermolecular C-C bond forming process.

Efficient Synthesis of O-tert-Propargylic Oximes via Nicholas Reaction

A synthetic protocol to access O-tert-propargylic oximes derived from tertiary propargylic alcohols was established via Nicholas reaction. Thus, BF3·OEt2-mediated reaction between the dicobalt hexacarbonyl complex of tert-propargylic alcohols and p-nitrobenzaldoxime followed by decomplexation with cerium(IV) ammonium nitrate afforded the corresponding O-tert-propargylic oximes in good to high yields. The obtained O-tert-propargylic oximes were effectively converted into heterocycles, such as four-membered cyclic nitrones, oxazepines, and isoxazolines, by using π-Lewis acidic catalysts.

Au-catalyzed skeletal rearrangement of O-propargylic oximes via N-O bond cleavage with the aid of a Brønsted base cocatalyst

Au-catalyzed skeletal rearrangement reaction of O-propargylic oxime proceeded via N–O bond cleavage with the aid of a Brønsted base cocatalyst.

O-Propargylic oximes that possess an electron-withdrawing aryl group on the oxime moiety undergo Au-catalyzed skeletal rearrangements via N–O bond cleavage to afford the corresponding 2H-1,3-oxazine derivatives. Our studies show that the inclusion of a Brønsted base cocatalyst not only accelerates the reaction but also switches pathways of the skeletal rearrangement reaction, realizing divergent synthesis of heterocyclic compounds. Computational studies indicate that the elimination of propargylic proton in the cyclized vinylgold intermediate is rate-determining and both electron-withdrawing substituents at the oxime moiety and base cocatalyst facilitate the proton elimination. Moreover, the protodeauration process proceeds stepwise involving N–O bond cleavage followed by recyclization to construct the oxazine core.

Cationic cobalt-catalyzed [1,3]-rearrangement of N-alkoxycarbonyloxyanilines

A cationic cobalt catalyst efficiently promoted the reaction of N-alkoxycarbonyloxyanilines at 30 °C, affording the corresponding ortho-aminophenols in good to high yields. As reported previously, our mechanistic studies including oxygen-18 labelling experiments indicate that the rearrangement of the alkoxycarbonyloxy group proceeds in [1,3]-manner. In this article, we discuss the overall picture of the cobalt-catalysed [1,3]-rearrangement reaction including details of the reaction conditions and substrate scope.

Cobalt Ion Promoted Redox Cascade: A Route to Spiro Oxazine-Oxazepine Derivatives and a Dinuclear Cobalt(III) Complex of an N-(1,4-Naphthoquinone)-o-aminophenol Derivative

The study discloses that the redox activity of N-(1,4-naphthoquinone)-o-aminophenol derivatives (L^RH₂) containing a (phenol)-NH-(1,4-naphthoquinone) fragment is notably different from that of a (phenol)-NH-(phenol) precursor. The former is a platform for a redox cascade. L^RH₂ is redox noninnocent and exists in Cat-N-(1,4-naphthoquinone)(2-) (L^R 2-) and SQ-N-(1,4-naphthoquinone) (L^R •-) states in the complexes. Reactions of L^RH₂ with cobalt(II) salts in MeOH in air promote a cascade affording spiro oxazine-oxazepine derivatives (^OXL^R) in good yields, when R = H, Me, ^tBu. Spiro oxazine-oxazepine derivatives are bioactive, and such a molecule has so far not been isolated by a schematic route. In this context this cascade is significant. Dimerization of L^RH₂ → ^OXL^R in MeOH is a (6H⁺ + 6e) oxidation reaction and is composed of formations of four covalent bonds and 6-exo-trig and 7-endo-trig cyclization based on C-O coupling reactions, where MeOH is the source of a proton and the ester function. It was established that the active cascade precursor is [(L^Me •-)Co^IIICl₂] (A). Notably, formation of a spiro derivative was not detected in CH₃CN and the reaction ends up furnishing A. The route of the reaction is tunable by R, when R = NO₂, it is a (2e + 4H⁺) oxidation reaction affording a dinuclear L^R 2- complex of cobalt(III) of the type [(L^NO2 2-)₂Co^III₂(OMe)₂(H₂O)₂] (1) in good yields. No cascade occurs with zinc(II) ion even in MeOH and produces a L^Me •- complex of type [(L^Me •-)Zn^IICl₂] (2). The intermediate A and 2 exhibit strong EPR signals at g = 2.008 and 1.999, confrming the existence of L^Me •- coordinated to low-spin cobalt(III) and zinc(II) ions. The intermediates of L^RH₂ → ^OXL^R conversion were analyzed by ESI mass spectrometry. The molecular geometries of ^OXL^R and 1 were confirmed by X-ray crystallography, and the spectral features were elucidated by TD DFT calculations.

Borylated oximes: versatile building blocks for organic synthesis

Herein, we demonstrate the synthesis and functionalization of α-boryl aldoximes from α-boryl aldehydes, with no sign of C-to-N boryl migration. Selective modification of the oxime functionality enables access to a wide range of borylated compounds, such as borylated heterocycles and N-acetoxyamides. By reducing the α-boryl aldoximes, MIDA deprotection yields the corresponding β-boryl hydroxylamines. As part of this study, we also demonstrate the utility of the boryl aldoxime motif in peptide conjugation.

Copper-Catalyzed Oxidative Cyclization of 2-Amino-azaarenes with Lignin Models: Synthesis of 3-Phenoxy Imidazo Heterocycles

A catalytic oxidative cyclization of 2-aminopyridines or 2-aminobenzothiazole with 2-phenoxyacetophenones (a kind of lignin platform compound) was developed, efficiently providing valuable 3-phenoxy imidazo[1,2-a]pyridines or 3-phenoxy benzo[d]imidazo[2,1-b]thiazoles. The reaction was realized under oxygen by simply using inexpensive CuI as the catalyst.

K2CO3-Mediated Cyclization and Rearrangement of γ,δ-Alkynyl Oximes To Form Pyridols

A novel K₂CO₃-mediated cyclization and rearrangement of γ,δ-alkynyl oximes for the synthesis of pyridols is described. The process accomplishes an efficient [1,3] rearrangement of the O-vinyl oxime intermediate which is in situ generated from the intramolecular nucleophilic addition of γ,δ-alkynyl oximes. The reaction employs readily accessible starting materials, tolerates a wide range of functional groups, and gives a variety of synthetically challenging pyridols in good yields.

Efficient synthesis of multisubstituted 2-alkenylpyridines via 2,3-rearrangement of O-homoallenylic oximes

O-Homoallenylic α,β-unsaturated oximes were efficiently converted to the corresponding 2-alkenylpyridine derivatives by microwave irradiation.

Copper-catalyzed cascade transformation of O-propargylic oximes with sulfonyl azides to α,β-unsaturated N-acylamidines

Copper-catalyzed cascade transformations of O-propargylic oximes and sulfonyl azides were carried out to efficiently afford α,β-unsaturated N-acylamidines. The reaction involved the intramolecular attack of the oxime group to the ketenimine moiety that was generated in situ, followed by the cleavage of the N-O bond.

Acid-catalyzed formal cycloaddition of α,β-unsaturated carbonyls with epoxides: dioxepines or acetals?

It has been recently reported that the reaction of α,β-unsaturated carbonyl derivatives with epoxides in the presence of a homogeneous acid catalyst readily delivers the corresponding dioxepines via formal (4 + 3) cycloaddition. We report herein that the same apparent reactivity can be triggered via heterogeneous catalysis. Characterization of products by means of NMR correlation experiments and DFT modeling revealed, however, that products are the acetals of the unsaturated reagent rather than the desired heterocycles.

N-Allenylnitrone acts as 2-azadiene in the Cu-catalyzed cascade reaction of O-propargylic oximes with azodicarboxylates

Cu-catalyzed reaction between O-propargylic oximes and azodicarboxylates proceeded via 2,3-rearrangement followed by [4 + 2] cycloaddition, in which N-allenylnitrone acts as 2-azadiene.