Facile Synthesis of Disubstituted Isoxazoles from Homopropargylic Alcohol via C═N Bond Formation

Access to Isoxazoles via Photo-oxygenation of Furan Tethered α-Azidoketones

Photocatalyst-free visible light-enabled direct oxygenation of furan-tethered α-azidoketones was studied. The reaction yielded various products depending on the substituents, with isoxazoles forming as the major products. The findings suggest that singlet oxygen was generated during the reaction and reacted with α-azidoketones in a [4 + 2] fashion to yield endoperoxides, which rearranged in multiple ways to generate isoxazoles. Some of the synthesized isoxazoles were evaluated as α-glucosidase inhibitors, and three of them 5bi, 5bj, and 5bl exhibited good activity with IC50 values of 454.57 ± 29.34, 147.84 ± 2.28, and 272.58 ± 42.06 μM, respectively, when compared with the standard drug acarbose (IC50 = 1224.33 ± 126.72 μM).

Tunable Key [3 + 2] and [2 + 1] Cycloaddition of Enaminones and α-Diazo Compounds for the Synthesis of Isomeric Isoxazoles: Metal-Controlled Selectivity

The three-component reactions of enaminones, α-diazo esters/ketones, and t-butyl nitrite (TBN) for the switchable synthesis of isomeric isoxazoles have been realized. The catalysis with Cu(II) salt provides 3,4-disubsituted isoxazoles via [3 + 2] cycloaddition. On the other hand, the catalysis of Ag(I) with identical substrates leads to isomeric isoxazoles with reversed C3 and C4 substitution based on a key [2 + 1] cycloaddition.

One-Pot Synthesis of Isoxazole-Fused Tricyclic Quinazoline Alkaloid Derivatives via Intramolecular Cycloaddition of Propargyl-Substituted Methyl Azaarenes under Metal-Free Conditions

A practical method was developed for the convenient synthesis of isoxazole-fused tricyclic quinazoline alkaloids. This procedure accesses diverse isoxazole-fused tricyclic quinazoline alkaloids and their derivatives via intramolecular cycloaddition of methyl azaarenes with tert-butyl nitrite (TBN). In this method, TBN acts as the radical initiator and the source of N-O. Moreover, this protocol forms new C-N, C-C, and C-O bonds via sequence nitration and annulation in a one-pot process with broad substrate scope and functionalization of natural products.

Metal-Free, Adjustable, and Recyclable Catalytic Systems for the Construction of C-C Bonds by Activating Propargylic Alcohols

Pyrano[3,2-c]coumarin derivatives and C3-substituted 4-hydroxycoumarins as important skeletal structures of active natural products and pharmaceutically relevant molecules have received increasing attention. However, developing an adjustable system for selectively synthesizing them is still a challenging task. Herein, sulfonic acid-functionalized ionic liquid was successfully used as the catalyst for the alkylation of 4-hydroxycoumarin derivatives with secondary aromatic propargylic alcohols using dimethyl carbonate as the green solvent, giving up to 98% yield. On the other hand, protonated imidazole-based ionic liquid-catalyzed cyclization was also selectively achieved with a nearly quantitative yield. Developed metal-free catalytic systems exhibited well adjustable and recyclable properties, avoiding the contamination of metal and halogen, reducing the neutralization after the reaction, and benefiting the separation between the catalyst and the product. New strategies were applied for performing the gram-scale reaction smoothly. The adjustable systems might occur through two different mechanisms involving propargylic or allenic carbocation and hydrogen bonding effects between the catalysts and the substrates.

Palladium-catalyzed cyclizative cross coupling of ynone oximes with 2-haloaryl N-acrylamides for isoxazolyl indoline bis-heterocycles

Metal-catalyzed cyclizative cross-coupling reactions have attracted enormous attention due to their unique cascade nature. We demonstrated, herein, a dual-cyclizative coupling of ynone oxime ethers with acrylamides for the synthesis of methylene-linked isoxazolyl 2-oxindoles. The cascade was triggered by a palladium(II)-catalyzed ynone oxime ether cyclization, which underwent a Heck-type coupling intercepted by an aryl iodide insertion. Control experiments were carried out to understand the mechanism.

Generation of azaarene nitrile oxides from methyl azaarenes and t-BuONO enabling the synthesis of furoxans and 1,2,4-oxadiazoles

A high-efficiency strategy for the synthesis of furoxans and 1,2,4-oxadiazoles has been developed, using tert-butyl nitrite (TBN) as the nitrogen source. Azaarene nitrile oxides were generated in situ via methyl azaarenes functionalization.

Ru-Catalyzed [3 + 2] Cycloaddition of Nitrile Oxides and Electron-Rich Alkynes with Reversed Regioselectivity

Polarity reversal ("umpolung") of a functional group can override its inherent reactivity and lead to distinct bond-forming modes. Herein we describe a rarely studied cycloaddition between nitrile oxides and electron-rich alkynes with reversed regioselectivity, leading to the useful 4-heterosubstituted isoxazoles. The use of a ruthenium catalyst completely overrides the inherent polarity of nitrile oxides. This reversed regioselectivity was also observed for their reactions with a range of electron-deficient alkynes.

Does electrophilic activation of nitroalkanes in polyphosphoric acid involve formation of nitrile oxides?

The mechanistic rationale involving activation of nitroalkanes towards interaction with nucleophilic reagents in the presence of polyphosphoric acid (PPA) was re-evaluated. Could nitrile oxide moieties be formed during this process? This experiment demonstrates that at least in some cases this could happen, as generated nitrile oxides were successfully intercepted as adducts of [3 + 2] cycloadditions.

The mechanistic rationale involving activation of nitroalkanes towards interaction with nucleophilic reagents in the presence of polyphosphoric acid (PPA) was re-evaluated. Could nitrile oxide moieties be formed during this process?

A radical-mediated 1,3,4-trifunctionalization cascade of 1,3-enynes with sulfinates and tert-butyl nitrite: facile access to sulfonyl isoxazoles

An unprecedented indium-promoted three-component 1,3,4-trifunctionalization cascade of 1,3-enynes with sulfinates and tert-butyl nitrite for producing 5-sulfonylisoxazoles via [3+2] annulation is reported.

Synthesis and application of α-carbonyl nitrile oxides

A strategy has been developed to synthesize α-carbonylfuran and isoxazole using tert-butyl nitrite (TBN) as a nitrogen source.

Ethyl 5-[(ethoxycarbonyl)oxy]-5,5-diphenylpent-2-ynoate

The title compound, C22H22O5, crystallizes with two molecules in the asymmetric unit, one of which shows disorder of its ethyl acetate group over two sets of sites in a 0.880 (2):0.120 (2) ratio. The C[triple-bond]C distances in the two molecules are almost the same [1.1939 (16) and 1.199 (2) Å], but the Csp 3—C[triple-bond]C angles differ somewhat [175.92 (12) and 172.53 (16)°]. In the crystal, several weak C—H...O interactions are seen.

Synthesis of 3-Acyl-isoxazoles and Δ2-Isoxazolines from Methyl Ketones, Alkynes or Alkenes, and tert-Butyl Nitrite via a Csp3-H Radical Functionalization/Cycloaddition Cascade

A novel metal-free tandem Csp³-H bond functionalization of ketones and 1,3-dipolar cycloaddition has been developed. An efficient approach to a variety of oxazole and isoxazoline derivatives is demonstrated using the 1,3-dipolar cycloaddition of alkynes and alkenes to nitrile oxides generated by reactions of methyl ketones with tert-butyl nitrite. This new protocol provides access to a variety of isoxazolines with diverse functionalities. An isoxazole generated in this way was found to have significant antifungal activity.

Multiple-functionalizations of terminal alkynes with sodium sulfinates and tert-butyl nitrite: facile synthesis of 2H-azirines

A new, catalyst-free tandem annulation route to 2,2-disulfonyl-2H-azirines via multiple-functionalizations of terminal alkynes with sodium sulfinates and tert-butyl nitrite is described.

1,1-Diphenyl-4-(thiophen-2-yl)but-3-yn-1-ol

The asymmetric unit of the title homopropargyl alcohol, C20H16OS, contains two independent molecules comprising a hydroxy group, a 3-(2-thiophenyl)propargylic moiety and two aromatic rings linked to a central carbon atom. The two unique molecules are linked into a dimer by an O—H...O hydrogen bond. In one molecule, the thiophene ring is disordered over two orientations rotated by 180° with a refined occupancy ratio of 0.575 (4):0.425 (4). The crystal structure is stabilized by O—H...π and C—H...π hydrogen-bond interactions. The crystal studied was a two-component non-merohedral twin, the refined ratio of the twin components being 0.575 (4):0.425 (4).

Homopropargyl alcohol 1,1-diphenylbut-3-yn-1-ol

The asymmetric unit of the title compound, C16H14O, contains one molecule with a central carbon atom having a distorted tetrahedral geometry made of a propargylic fragment, a hydroxy group and two aromatic rings. Directional interactions such as unusual O—H...π contacts are observed between the molecules in the crystal.

Transition-Metal-Free Multicomponent Approach to Stereoenriched Cyclopentyl-isoxazoles through C-C Bond Cleavage

An efficient multicomponent reaction for the synthesis of stereoenriched cyclopentyl-isoxazoles from camphor-derived α-oximes, alkynes, and MeOH is reported. Our method involved a series of cascade transformations, including the in situ generation of an IIII catalyst, which catalyzed the addition of MeOH to a sterically hindered ketone. Oxidation of the oxime, and rearrangement of the α-hydroxyiminium ion generated a nitrile oxide in situ, which, upon [3+2] cycloaddition reaction with an alkyne, delivered the regioselective product. This reaction was very selective for the syn-oxime. This multicomponent approach was also extended to the synthesis of a new glycoconjugate, camphoric ester-isoxazole C-galactoside.

The Divergent Cascade Reactions of Arylalkynols with Homopropargylic Amines or Electron-Deficient Olefins: Access to the Spiro-Isobenzofuran- b-pyrroloquinolines or Bridged-Isobenzofuran Polycycles

Two divergent cascade reactions of arylalkynols with homopropargylic amines or electron-deficient olefins were developed to synthesize the spiro-isobenzofuran- b-pyrroloquinolines or bridged-isobenzofuran heterocycles in good yields, respectively. One reaction actually involved intramolecular 5- endo-dig hydroamination cyclization-protonation of homopropargylic amines to give cycloiminium ions and intramolecular 5- exo-dig hydroalkoxylation cyclization of arylalkynols to generate isobenzofuran with exocyclic double bond, followed by the nontypical Povarov-type reaction in the presence of PtCl₂/FeCl₃ cocatalysts. The other underwent intramolecular hydroalkoxylation cycloisomerization of alkynols with the subsequent normal [4 + 2] cycoaddition with dienophiles. Herein, the arylalkynols acted as both "masked" electron-rich olefins and "masked" electron-rich dienes.

Recent advances in transition-metal-catalyzed reactions of alkynes with isoxazoles

Isoxazoles, as masked 1,3-dicarbonyl equivalents, have proven to be versatile building blocks and pivotal intermediates for the construction of a variety of useful azacycles with molecular complexity. As a result, a range of new reactions have been discovered based on isoxazoles in the past decade. However, the relevant reactions of isoxazoles with alkynes have seldom been explored. In this review, we will focus on the recent progress in the transition-metal-catalyzed formal annulations for the efficient synthesis of N-heterocycles between alkynes and isoxazoles by highlighting their specificity and applicability, and the mechanistic rationale is presented where possible.

Solvent-promoted catalyst-free regioselective N-incorporation multicomponent domino reaction: rapid assembly of π-functionalized [60]fullerene-fused dihydrocarbolines

An unprecedented solvent-promoted N-incorporation multicomponent domino chemistry was developed for the direct construction of π-functionalized [60]fullerene-fused dihydrocarbolines from simple hydrocarbons.

Palladium-Catalyzed One-Pot Conversion of Aldehydes and Ketones into 4-Substituted Homopropargyl Alcohols and 5-En-3-yn-1-ols †

Sequential treatment of 2,3-dichloropropene with magnesium and n-BuLi generated the equivalent of 1,3-dilithiopropyne, which adds regiospecifically to aldehydes and ketones to produce homopropargyl alcohols. The lithium acetylide intermediate formed in this protocol can be further reacted with aromatic and vinyl halides, under palladium catalysis, to produce 4-substituted homopropargyl alcohols and 5-en-3-yn-1-ols, respectively, in one-pot with good overall yields.

A Computational Study on the Addition of HONO to Alkynes toward the Synthesis of Isoxazoles; a Bifurcation, Pseudopericyclic Pathways and a Barrierless Reaction on the Potential Energy Surface

Homopropargyl alcohols react with t-BuONO to form acyloximes which can be oxidatively cyclized to yield ioxazoles. The mechanism for the initial reaction of HONO with alkynes to form acyloximes (e.g., 13c) has been explored at the B3LYP/6-31G(d,p) + ZPVE level of theory. The observed chemoselectivity and regioselectivity are explained via an acid-catalyzed mechanism. Furthermore, the potential energy surface revealed numerous surprising features. The addition of HONO (8) to protonated 1-phenylpropyne (18) is calculated to follow a reaction pathway involving sequential transition states (TS6 and TS8), for which reaction dynamics likely play a role. This reaction pathway can bypass the expected addition product 21 as well as transition state TS8, directly forming the rearranged product 23. Nevertheless, TS8 is key to understanding the potential energy surface; there is a low barrier for the pseudopericylic [1,3]-NO shift, calculated to be only 8.4 kcal/mol above 21. This places TS8 well below TS6, making the valley-ridge inflection point (VRI or bifurcation) and direct formation of 23 possible. The final tautomerization step to the acyloxime can be considered to be a [1,5]-proton shift. However, the rearrangement in the case of 17h to 13c is calculated to be barrierless, arguably because the pathway is pseudopericyclic and exothermic.

Cu-Catalyzed Cascade Annulation of Alkynols with 2-Azidobenzaldehydes: Access to 6H-Isochromeno[4,3-c]quinoline

A copper-catalyzed cascade reaction of alkynols and 2-azidobenzaldehydes has been achieved, giving 6H-isochromeno[4,3-c]quinoline in yields of 40-81%. This reaction provides a novel, concise strategy for rapidly constructing compounds with fused N- and O-containing heterocycles. In contrast to previously reported reactions of alkynols in which the first step is intramolecular cycloisomerization, the first step in this novel reaction of alkynols is entropically unfavorable intermolecular addition. The resulting hemiacetal intermediate then undergoes intramolecular cyclization and aromatization to afford the product.

tert-Butyl nitrite (TBN) as the N atom source for the synthesis of substituted cinnolines with 2-vinylanilines and a relevant mechanism was studied

A green method to synthesize cinnolines by 6π electrocyclic reaction with alkenyl amines and TBN has been developed.

Copper-mediated annulation of 2-(1-arylvinyl) anilines and aryl nitrosos towards 2,3-diaryl-2H-indazoles

A copper-mediated annulation of 2-(1-substituted vinyl)anilines and aryl nitrosos is developed, giving 2,3-diaryl pyrazoles in moderate to good yields.

Co-catalyzed highly selective C(sp3)–H nitration

A Co-catalyzed highly selective C(sp³)–H nitration via a radical pathway was disclosed, overcoming the strong coordination effect of pyrimidine by suppressing C(sp²)–H functionalization as well as proximity C(sp³)–H functionalization.

TEMPO-catalyzed synthesis of 5-substituted isoxazoles from propargylic ketones and TMSN3

A novel and efficient TEMPO-catalyzed synthesis of 5-substituted isoxazoles from propargylic ketones and TMSN₃via the radical mechanism process is described. A plausible reaction mechanism for this process is proposed.

Synthesis of 3,4,5-Trisubstituted Isoxazoles from Morita-Baylis-Hillman Acetates by an NaNO2 /I2 -Mediated Domino Reaction

An efficient NaNO2 /I2 -mediated one-pot transformation of Morita-Baylis-Hillman (MBH) acetates into alkyl 3-nitro-5-(aryl/alkyl)isoxazole-4-carboxylates is described. In a cascade event, initial Michael addition of NaNO2 to the MBH acetate furnishes the allylnitro intermediate which undergoes I2 -catalyzed oxidative α-CH nitration of the nitromethyl subunit followed by [3+2] cycloaddition to afford the title compounds. Structural elaborations of these highly substituted isoxazoles by SN Ar reactions and hydrogenolysis allows access to useful products.