Platinum‐catalyzed formal 5+2 and 4+2 annulations of isoxazoles with heterosubstituted alkynes enabled the atom‐economical synthesis of valuable 1,3‐oxazepines and 2,5‐dihydropyridines, respectively. ...Importantly, this Pt catalysis not only led to unique reactivity dramatically divergent from that observed under Au catalysis, but also proceeded via unprecedented α‐imino platinum carbene intermediates.
Gold's deviant relative: Platinum‐catalyzed formal 5+2 and 4+2 annulations of isoxazoles and heterosubstituted alkynes provided valuable 1,3‐oxazepines and 2,5‐dihydropyridines (see scheme). This reactivity deviates dramatically from that observed under gold catalysis and involves the generation of an α‐imino platinum carbene. A computational study provided evidence for the proposed mechanism of this unusual tandem sequence.
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•SMX was degraded by UV/PS system in sea water.•The active sites of SMX were calculated by density functional theory.•Neither trichloromethane nor halate was formed during the ...reaction.•UV/PS is relatively safer than the traditional UV/NaClO disinfection.
Sulfate radical (SO4−) based advanced oxidation processes are effective for the removal of antibiotics including sulfonamides from freshwater. However, few studies were focused on the degradation of sulfonamides in marine aquaculture water and seawater, as well as the impact of transformation products on seafood such as fishes. In this work, sulfamethoxazole (SMX) was treated with UV/persulfate (UV/PS) system in different water samples. Chloride ions (Cl−) had an inhibitory effect possibly due to the consumption and mutual quenching of reactive radicals. A slight promotion by bromide ions (Br−) was observed and should be related not only to the transformation of Br, but also to the properties of the SMX itself. Bicarbonate ions (HCO3–) significantly inhibited degradation as it is a scavenger of SO4− and HO in UV/PS. Sulfate ions (SO42−) showed a negligible impact on SMX degradation. The active sites of SMX were calculated to be the N1 and C7 atoms of the isoxazole ring, as well as the C9, C13 and N3 atoms of the benzene ring. Seven organic transformation products were identified by LC-MS/MS. Bromate (BrO3−) and chloromethane as the potential human carcinogens were not detected from the reaction system of UV/PS with SMX in marine aquaculture water and seawater. Preliminary analysis of toxicity by the EPI Suite model, absorbable organic halogen (AOX) analysis and growth inhibition of Chlorella vulgaris proved that the UV/PS process could be used as a new disinfection method and it is relatively safer than traditional UV/NaClO disinfection.
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Two new gold‐catalyzed annulations of isoxazoles with propiolates have been developed. Most isoxazoles follow an initial O attack on the alkyne to afford a 4+1 annulation product. This process ...results in a remarkable alkyne cleavage of initial propiolates. Unsubstituted isoxazoles proceed through an N attack step to yield formal 2+2+1/4+2 annulation products. These two annulation products arise initially from two seven‐membered heterocyclic intermediates, which then lead to products.
Plan of attack: Two new gold‐catalyzed annulations of isoxazoles with propiolates have been developed, including 4+1 annulations for most isoxazoles, and formal 2+2+1/4+2 annulations for unsubstituted isoxazoles. The different annulations arise initially from different attacks, N or O, by the isoxazoles on the propiolate.
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•SMX was effectively degraded by nonradical activation of PDS in combination with CNT.•Stoichiometric efficiency of SMX oxidized vs PDS consumed in the presence of CNT was ...calculated.•Aniline moiety in SMX molecule was the reactive site for attack by nonradical reactive species.•Degradation of SMX involved hydroxylation, isoxazole ring opening, and sulfonamide NS bond cleavage.•Ecotoxicity of transformation products was estimated by ECOSAR program.
In this work, the transformation of sulfamethoxazole (SMX) by peroxydisulfate (PDS) activated with one multi-walled carbon nanotube (CNT) via a nonradical pathway was examined, and the underlying reaction mechanism was explicated. The nonradical PDS/CNT process exhibited considerable reactivity toward SMX, and the oxidation of SMX was enhanced by the decrease of solution pH. The addition of background inorganic ions (i.e., bicarbonate and chloride ion) showed ignorable impacts on SMX transformation, while natural organic matter resulted in the decrease of SMX oxidation rate. The stoichiometric efficiency of SMX degraded vs PDS consumed in the presence of CNT was obviously higher compared to that obtained in the case of zero-valent iron (a common heterogeneous metal activator for PDS). The oxidation of sub-structural model compounds in the PDS/CNT process unveiled that the main reactive functional group of SMX molecule for attack by nonradical reactive species was aniline moiety, while the isoxazole moiety could also be slowly oxidized. A total of six oxidation products were identified during SMX treatment by the PDS/CNT process, and their generation pathways mainly included hydroxylation, isoxazole ring opening, and sulfonamide NS bond cleavage. Ecotoxicity analysis by ECOSAR showed that the toxicity of these products was generally lower than parent SMX. Consequently, this work demonstrate that the emerging PDS/CNT oxidation technology is likely a prospective candidate for destruction of SMX in water and wastewaters.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Isoxazoles are important five‐membered aromatic heterocycles in organic chemistry. Recently, many exciting advances in the synthesis and functionalization of isoxazoles have been reported. New ...transition metal‐catalyzed reactions have resulted in the development of attractive and highly efficient synthetic approaches to densely functionalized isoxazoles. Complete control of regioselectivity can be achieved on the basis of a judicious choice of metal catalyst and reaction partners using dipolar cycloaddition and cycloisomerization reactions, while more recent studies have focused on the site‐selective functionalization of isoxazoles via CH functionalization. New strategies for the use of isoxazoles as scaffolding templates in asymmetric synthesis have emerged, thus opening new prospects for the synthesis of enantioenriched motifs under the conditions that are orthogonal to other transformations. In this review, recent advances involving the synthesis and reactivity of isoxazoles are summarized. The review covers the period from January 2005 to June 2015.
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Highly strained 2H‐azirines with a tetrasubstituted stereocenter were synthesized by the enantioselective isomerization of isoxazoles with a chiral diene–rhodium catalyst system. The effect of ...ligands and the coordination behavior support the proposed catalytic cycle in which the coordination site is fixed in favor of efficient enantiodiscrimination by a bulky substituent of the ligand. In silico studies also support the existence of a rhodium–imido complex as a key intermediate for enantiodiscrimination.
Coping well under the strain: Highly strained 2H‐azirines with tetrasubstituted stereocenters were synthesized with high enantioselectivity by the N−O bond‐cleaving isomerization of isoxazoles in the presence of a chiral diene–rhodium catalyst (see scheme). This asymmetric ring contraction of isoxazoles proceeded under mild reaction conditions to give 2‐alkoxycarbonyl 2H‐azirines with various substituents, including halogen groups.
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•SMX was decomposed by hydroxylation, hydrolysis and isoxazole ring oxidation.•The main transformation pathway of SMX was affected by initial pH.•Hydroxylation and isoxazole ring ...oxidation were inhibited by NOM.•Hydrolysis TPs formation was promoted by NOM.•Aromaticity and carboxyl content are important indicators of the reactivity of NOM.
As water matrix, inorganic anions and natural organic matter (NOM) may impact the removal of pharmaceuticals not only on the kinetics but also on the transformation products formation. In this study, the effects of initial pH, inorganic anions and NOM on the transformation pathways of sulfamethoxazole (SMX) during photocatalytic decomposition were investigated. Three pathways including hydroxylation, hydrolysis and isoxazole ring oxidation were found during SMX photocatalysis. SMX in deionized water was degraded by hydroxylation principally in alkaline situation, while hydrolysis was the main pathway in acidic condition. Almost all the pathways for SMX decomposition were suppressed when anions were added, with the inhibitory effect followed the order of HPO42− > HCO3− > SO42− > Cl− > H2PO4−. The formation of products with larger energy barrier was affected more significantly when anions were added. The products formation was also impacted by the presence of NOM isolates including “Suwannee River Humic Acid” (SRHA), “Suwannee River Fulvic Acid” (SRFA) and “Suwannee River NOM” (SRNOM). The mechanisms of hydroxylation and isoxazole ring oxidation were inhibited by NOM, with the order of SRFA > SRHA > SRNOM, due to their hydroxyl radical scavenging abilities which were related to their aromaticity degree. Most of the hydrolysis products were favored in the presence NOM, and the enhancement effect was in accordance with the sequence of SRNOM > SRFA > SRHA due to the different photoinductive abilities of the NOM isolates.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A decarboxylative intramolecular aziridination reaction of alkene‐tethered 4H‐isoxazol‐5‐ones with a palladium/phosphine catalyst gave 1‐azabicyclo3.1.0hex‐2‐enes in moderate to high yields (see ...scheme; dba=dibenzylideneacetone). The resulting N‐fused bicyclic aziridines readily reacted with various reagents to afford ring‐opening pyrroline derivatives.
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•Electrochemical mineralization of SMX is performed by using PbO2 electrodes.•Electrochemical degradation of SMX follows pseudo-first-order kinetics.•The nearly complete mineralization of SMX (>95%) ...is achieved by PbO2 electrodes.•A SMX electrochemical mineralization mechanism is proposed.
Electrochemical mineralization of sulfamethoxazole (SMX) was performed in an undivided cell equipped with a Ti/SnO2-Sb/Ce-PbO2 anode and a Ti cathode. The reactions kinetics was determined as a function of applied current density (0.5–40mAcm−2), initial SMX concentration (10–400mgL−1), initial pH (3–11), and electrode distance (3–20mm). Degradation of SMX in contaminated lake water was quicker than that in deionized water. The electrochemical degradation of SMX followed pseudo-first-order kinetics. The nearly complete mineralization of SMX (>95%) was achieved in 60min as the current density was higher than 10mAcm−2. The major mineralization products in aqueous solution were NH4+ and SO42−. The aromatic intermediates including 3-amino-5-methylisoxazole (AMI), p-benzoquinone (BZQ), and sulfanilic acid (SFN) were analyzed and quantified. A possible electrochemical mineralization mechanism of SMX was proposed. Firstly, the sulfone group or isoxazole aromatic ring was attacked by hydroxyl radical (OH), followed by the formation of sub-structures analogues such as SFN and AMI. Subsequently, the OH attacked the different groups of the sub-structures analogues to release NH4+ and SO42−. Furthermore, successive hydroxylation of the formed aromatic intermediates including BZQ and 3, 4, 5-trihydroxy-5-methylisoxazole occurred. The mineralization of intermediates to CO2 and H2O was finally achieved. The nitrogen atom of the isoxazole molecule was converted into NO3−, which was then reduced to gas at the cathode. The economic feasibility was evaluated by determining the energy cost. The results showed that the residence time and energy cost of SMX degradation at the optimal conditions from 100mgL−1 to 1mgL−1 were 32.9–23.0min and 26.3–46.3WhL−1, respectively. This electrochemical technique is expected to be an interesting alternative for the treatment of SMX in wastewater.
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A highly efficient strategy for stereocontrolled construction of potentially bioactive and structurally rigid isoxazole‐fused spirocyclohexane oxindole‐based hexahydroxanthones via an organocatalytic ...domino Michael/Michael cycloaddition of multifunctional chromone‐based oxindole as a 4C building block and 3‐methyl‐4‐nitro‐5‐alkenyl‐isoxazole as an activated Michael acceptor is reported. Complex and skeleton‐diversified hexahydroxanthones bearing five continuous stereocenters including one spiro quaternary carton center were smoothly afforded in up to 92% yield, >20:1 d.r. and >99% ee. Further transformation of the product into other potentially bioactive molecules has also been demonstrated. This method could also forge the libraries of medicinally relevant molecules with skeletal diversity, thus to facilitate the search for new bioactive entities.
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