Synthesis of Functionalized Oxazolones by a Sequence of Cu(II)- and Au(I)-Catalyzed Transformations

Cycloaddition of Phenyltriazolinedione with Carbazole-Alkynes and Yne-Carbamates to Access Diazacyclobutenes

Previously, we described the synthesis of stable, bicyclic examples of the rather rare diazacyclobutene (DCB) motif by means of a cycloaddition between triazolinediones and electron-rich thiolated alkynes. Here, we report the investigation of the cycloaddition of triazolinediones with related electron-rich yne-carbamates and carbazole-alkynes. Bicyclic DCBs arising from yne-carbamates were isolated in 8-65% yield, while those arising from carbazole-alkynes were isolated in 28-59% yield. Mechanistic studies and characterization of isolable byproducts shed light on the underlying issues leading to poor to moderate yields.

Mechanistic Insights into Oxazolone Synthesis by Bimetallic Au-Pd-Catalyzed Catalysis and Catalyst Design: DFT Investigations

Bimetallic synergistic catalysis is one of the most effective and powerful strategies for the synthesis of oxazolones, an important species in organic synthesis. In this work, the mechanism of AuCl(PMe₃)/AgOTf-Pd(0) ([Au-Pd]) bimetallic catalyst-catalyzed oxazolone synthesis using N-alkynyl carbamates as precursors was studied in detail by DFT calculations and the catalytic performances of a series of bimetallic catalysts were evaluated. The results show that the reaction begins from the [Au]-catalyzed cycloisomerization of N-alkynyl carbamates. After the five-membered intermediate is formed, the [Pd(0)]-catalyzed cycle starts, which contains three steps: oxidation addition, transmetalation, and reductive elimination. The whole reaction belongs to a catalyzed catalysis, and the reductive elimination is the rate-determining step. In the transmetalation process, both the [Pd(0)] catalyst and the ionic bridge are necessary. For the [Au-Pd]-catalyzed process, it is Cl^- as the bridge, not OTf^-. The cheaper metal compound, AgCl(PMe₃), can serve as the alternative of AuCl(PMe₃) to co-catalyze with the [Pd(0)] catalyst for the title reaction.

Efficient Synthesis of 1H-Benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one Derivatives Using Ag2CO3/TFA-Catalyzed 6-endo-dig Cyclization: Reaction Scope and Mechanistic Study

A small library of 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one derivatives was prepared in good to excellent yields, involving a Ag₂CO₃/TFA-catalyzed intramolecular oxacyclization of N-Boc-2-alkynylbenzimidazole substrates. In all experiments, the 6-endo-dig cyclization was exclusively achieved since the possible 5-exo-dig heterocycle was not observed, indicating the high regioselectivity of this process. The scope and limitations of the silver catalyzed 6-endo-dig cyclization of N-Boc-2-alkynylbenzimidazoles as substrates, bearing various substituents, were investigated. While ZnCl₂ has shown limits for alkynes with an aromatic substituent, Ag₂CO₃/TFA demonstrated its effectiveness and compatibility regardless of the nature of the starting alkyne (aliphatic, aromatic or heteroaromatic), providing a practical regioselective access to structurally diverse 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-ones in good yields. Moreover, the rationalization of oxacyclization selectivity in favor of 6-endo-dig over 5-exo-dig was explained by a complementary computational study.

Divergent Pd-catalyzed Functionalization of 4-Oxazolin-2-ones and 4-Methylene-2-oxazolidinones and Synthesis of Heterocyclic-Fused Indoles

Palladium-catalyzed functionalization was presently performed on two building blocks: 4-oxazolin-2-ones and 4-methylene-2-oxazolidinones. Direct Heck arylation of 4-oxazolin-2-ones led to a series of 5-aryl-4-oxazolin-2-ones, including analogues with N-chiral auxiliary, in an almost quantitative yield. The Pd(II)-catalyzed homocoupling reaction of 4-oxazolin-2-ones provided novel heterocyclic across-ring dienes. Meanwhile, the intramolecular cross-coupling of N-aryl-4-methylene-2-oxazolidinones furnished a series of oxazolo[3,4-a]indol-3-ones. Further functionalization of 4-methylene-2-oxazolidinones afforded substituted indoles and heterocyclic-fused indoles with aryl, bromo, carbinol, formyl, and vinyl groups. A computational study was carried out to account for the behavior of the formylated derivatives. The currently developed methodology was applied to a new formal total synthesis of ellipticine.

Deconstructive isomerization of azetidinols via C-C bond cleavage enabled by N-heterocyclic carbene (NHC) catalysis

Herein, we describe an N-heterocyclic carbene (NHC)-catalyzed deconstructive isomerization of azetidinols via an inert C-C bond cleavage. It provides a direct and supplementary pathway to access α-amino ketone and oxazol-2-one derivatives in moderate to good yields. DFT calculation supports the proposed mechanism in which NHC undergoes a concerted proton transfer and ring-opening process. This reaction features non-metal catalysis, simple reaction operation, excellent regioselectivity and gram-scale synthesis.

PIFA-Mediated Tandem Hofmann-Type Rearrangement and Cyclization Reaction of α-Acyl-β-aminoacrylamides: Access to Polysubstituted Oxazol-2(3H)-ones

An efficient and straightforward synthesis of polysubstituted oxazol-2(3H)-ones has been developed via a tandem Hofmann-type rearrangement and cyclization reaction of various α-acyl-β-aminoacrylamides mediated by phenyl iodine(III) bis(trifluoroacetate) (PIFA) in the presence of trifloroacetic acid (TFA). This novel protocol features readily available starting materials, mild reaction conditions, simple execution, high chemoselectivity, good functional group tolerance, and a metal-free oxidation process.

Gold-Catalyzed Annulations with Nucleophilic Nitrenoids Enabled by Heteroatom-Substituted Alkynes

The combination of a nucleophilic nitrene equivalent, a triple bond and a π-acid catalyst has underpinned numerous efficient transformations for the preparation of azacycles. This personal account details our efforts in developing an annulation strategy. Adding a nucleophilic nitrenoid to an activated alkyne can generate carbenoid character that is then quenched by a cyclisation onto the nitrenoid substituent. The use and development of N-acyl and N-heterocyclic pyridinium-N-aminides as 1,3-N,O and 1,3-N,N-dipole equivalents is discussed in the context of oxazole and heterocycle-fused imidazole formation, respectively. The resulting processes are highly efficient, practically straightforward, and tolerate considerable structural and functional group variation. Our use of heteroatom-substituted alkynes as enabling tools for reaction discovery is discussed. The reactivity accessed from the strong donor-like properties of ynamides is complemented by that obtained from alkynyl thioethers, which are emerging as interesting substrates for π-acid catalysis.

Straightforward Synthesis of Indenes by Gold-Catalyzed Intramolecular Hydroalkylation of Ynamides

An original and straightforward entry to polysubstituted indenes from readily available ynamides is reported. Upon reaction with a N-heterocyclic carbene-gold complex under mild conditions, activated keteniminium ions are generated whose unique electrophilicity triggers a [1,5]-hydride shift and a subsequent cyclization. The presence of an endocyclic enamide in the densely functionalized resulting indenes was shown to be especially useful and versatile, offering a range of opportunities for their further postfunctionalization.

Ligand-Enabled Copper-Catalyzed N-Alkynylation of Sulfonamide with Alkynyl Benziodoxolone: Synthesis of Amino Acid-Derived Ynamide

Ynamides are versatile building blocks in organic synthesis. However, the synthesis of amino acid-derived ynamides is difficult but in high demand. Herein, we disclose the copper-catalyzed Csp-N coupling of sulfonamide, including amino acid and peptide derivatives, to give ynamides by using alkynyl benziodoxolones with broad functional group tolerance under mild reaction conditions. The electron-rich bipyridine as a ligand and ethanol as solvent were used for the success of this reaction. The usefulness of the obtained amino acid-derived ynamide as building block was showcased by further derivatization to unique amino acid derivatives. A control experiment to elucidate the mechanistic insight was also described.

Diorganyl Diselenides and Iron(III) Chloride Drive the Regio- and Stereoselectivity in the Selenation of Ynamides

We report here our results on the application of ynamides as substrates in the reactions with diorganyl dichalcogenides and iron(III) chloride to give selectively three different types of compounds: E-α-chloro-β-(organoselenyl)enamides, 4-(organochalcogenyl)oxazolones, and vinyl tosylates. The results reveal that the selectivity in the formation of products was obtained by controlling the functional groups directly bonded to the nitrogen atom of the ynamides. Thus, α-chloro-β-(organoselenyl) enamide derivatives were exclusively obtained when the TsN- and MsN-ynamides were treated with a mixture of diorganyl diselenides (1.0 equiv) and FeCl₃ (3.0 equiv) in dichloroethane (DCE, 3 mL), at room temperature. The 4-(organochalcogenyl)oxazolones were selectively obtained with ynamides having an ester group, directly bonded to the nitrogen atom, upon treatment with a solution of FeCl₃ (1.5 equiv) and diorganyl dichalcogenides (1.0 equiv) in dichloromethane (3 mL) at room temperature. Finally, vinyl tosylates were obtained from ynamides having an ester group, directly bonded to the nitrogen atom, by reaction with p-toluenesulfonic acid. We also studied the application of the prepared compounds as substrates for Suzuki and Sonogashira cross-coupling reactions.

Convergent Synthesis of 2-Oxazolone-4-carboxylates Esters by Reaction of Aldehydes with Ambivalent N-Cbz-α-Tosylglycinate Ester

N-Cbz-α-tosylglycinate ester was combined with aldehydes in a redox-neutral sequence leading to 2-oxazolone-4-carboxylates with high functional group tolerance. While the scope of the method was delineated to primary and secondary aliphatic aldehydes as well as aromatics, no racemization occurred with chiral aldehydes such as Garner's. Hitherto unknown, this process relies on the ambivalent role of N-Cbz-α-tosylglycinate ester acting as a pronucleophile.

Access to (Z)-1,2-Endiamides and 1,1-Endiamides via a Base-Promoted Tandem Reaction

An efficient base-promoted tandem reaction between vinyl 1,1-dichlorides and secondary sulfonamides with ynamide as the key intermediate is described. This method provides a facile approach to (Z)-1,2-endiamide and aryl 1,1-endiamide derivatives via the β-hydroamidation of terminal ynamides and the α-hydroamidation of internal ynamides, respectively. This reaction proceeded through double elimination of vinyl chlorides and double addition of nucleophiles to alkynes. In addition, it features readily available starting materials, mild reaction conditions, a broad substrate scope, a wide functional group tolerance, and an operational convenience.

Organocatalytic Enantioselective Conia-Ene-Type Carbocyclization of Ynamide Cyclohexanones: Regiodivergent Synthesis of Morphans and Normorphans

Described herein is an organocatalytic enantioselective desymmetrizing cycloisomerization of arylsulfonyl-protected ynamide cyclohexanones, representing the first metal-free asymmetric Conia-ene-type carbocyclization. This method allows the highly efficient and atom-economical construction of a range of valuable morphans with wide substrate scope and excellent enantioselectivity (up to 97 % ee). In addition, such a cycloisomerization of alkylsulfonyl-protected ynamide cyclohexanones can lead to the divergent synthesis of normorphans as the main products with high enantioselectivity (up to 90 % ee). Moreover, theoretical calculations are employed to elucidate the origins of regioselectivity and enantioselectivity.

Umpolung Reactivity of Ynamides: An Unconventional [1,3]-Sulfonyl and [1,5]-Sulfinyl Migration Cascade

A regioselective sulfonyl/sulfinyl migration cycloisomerization cascade of alkyne-tethered ynamides is developed in the presence of XPhosgold catalyst. This reaction is the first example of a general [1,3]-sulfonyl migration from the nitrogen center to the β-carbon atom of ynamides, followed by umpolung 5-endo-dig cyclization of the ynamide α-carbon atom to the gold-activated alkyne, and final deaurative [1,5]-sulfinylation. This process allows the synthesis of peripherally decorated unconventional 4-sulfinylated pyrroles with broad scope from N-propargyl-tethered ynamides. In contrast, N-homopropargyl-tethered ynamides undergo intramolecular tetradehydro Diels-Alder reaction to provide 2,3-dihydro-benzo[f]indole derivatives. Control experiments and density-functional theory studies were used to study the reaction pathways.

Selectfluor™-catalyzed oxidative cyclization of ynamides enables facile synthesis of oxazolidine-2,4-diones

A Selectfluor™-catalyzed oxidative cyclization of ynamides employing DMSO as the solvent and oxidant was achieved, affording oxazolidine-2,4-diones in moderate to excellent yields with high chemo-selectivity. The method offers an attractive alternative to existing protocols.

Reactivity of epoxy-ynamides with metal halides: nucleophile (Br/Cl/OH)-assisted tandem intramolecular 5-exo-digor 6-endo-digcyclisation and AgF2-promoted oxidation

Several metal halides (CuBr, LiCl, CuF₂, AgF₂) react with epoxy-ynamides to afford 1,3-oxazolidines, 1,4-oxazines or 1,2-dioxo-enamides.

Reaction of Ynamides with Iminoiodinane-Derived Nitrenes: Formation of Oxazolones and Polyfunctionalized Oxazolidinones

This article describes the reaction of ynamides with metallanitrenes generated in the presence of an iodine(III) oxidant. N-(Boc)-Ynamides are converted to oxazolones via a cyclization reaction. The reaction is mediated by a catalytic dirhodium-bound nitrene species that first behaves as a Lewis acid. The oxazolones can be converted in a one pot manner to functionalized oxazolidinones following a regio- and stereoselective oxyamination reaction with the same nitrene reagent generated in stoichiometric amounts.

PIFA-BF3·OEt2 mediated intramolecular regioselective domino cyclization of ynamides: A novel method for the synthesis of tetrahydroisoquinoline-oxazol-2(3H)-ones

The transition metal-free intramolecular regioselective domino cyclization of N-Boc protected ynamides has been developed to provide the corresponding tetrahydroisoquinoline-oxazo-2(3H)-ones in moderate to good yields.

Highly regioselective gold-catalyzed formal hydration of propargylic gem-difluorides

A highly regioselective gold-catalyzed formal hydration of propargylic gem-difluorides is presented.

Pd/Cu-Catalyzed tandem head-to-tail dimerization/cycloisomerization of terminal ynamides for the synthesis of 5-vinyloxazolones

This is the first Pd/Cu-catalyzed tandem head-to-tail dimerization/cycloisomerization of terminal ynamides for the synthesis of 5-vinyl-oxazolones.

An expedient synthesis of oxazolones using a cellulose supported ionic liquid phase catalyst

A novel cellulose supported ionic liquid phase catalyst has been synthesized and effectively employed as a heterogeneous catalyst in the synthesis of oxazolones.