Diastereoselectivity in the Staudinger reaction: a useful probe for investigation of nonthermal microwave effects

Unraveling the synthetic strategy, structure activity relationship of azetidinones: Insights into their multidrug and toxin extrusion protein (MATE transporter) inhibition facilitating drug development against MDR

Heterocyclic chemistry gathered a wide audience due to their presence in potential drug candidates and being attractive synthons initiating several retro-syntheses the organic as well as in medicinal chemistry fields. Among them, azetidinones have been a subject of discussion due to their serendipity, curiosity, versatility by Penicillin and Cephalosporins as β-lactam antibiotics. Despite possessing a large margin of biological activities, azetidinones mainly work as antimicrobial, interfering with bacterial cell-wall synthesis blocking transpeptidase. The structure and synthetic strategy of the azetidinone arouse its research interest in drug discovery pipeline. However, the extensive use of antibiotics in the modern era contributes to drug resistance that could only be counterbalanced involving hybridization approach. To explore the potency of the azetidinones, potent compounds found from literatures, have been screened through molecular docking against MATE transporter, targeting for prevailing multidrug resistance. The present review makes a sincere effort to compile the synthetic information and orientations involving hybridization of azetidinones with biologically active scaffolds emphasizing on antimicrobial and anticancer efficacies along with their MATE inhibition, keeping an eye upon the structure activity relationship in a systematic way. The study could motivate the researchers for developing a wide array of β-lactam derivatives more prominently targeting newer pathways to fight MDR.

Microwave-assisted atmospheric alkaline leaching process and leaching kinetics of rare earth melt electrolysis slag

This study focuses on the difficulty of converting fluorinated rare earth elements into hydroxylated rare earth elements in rare earth melt electrolysis slag (RMES) and proposes the use of a microwave-assisted atmospheric alkaline leaching method for the treatment of RMES. The leaching behavior of RMES under microwave-assisted atmospheric alkaline leaching was studied, and the optimal reaction conditions were determined. Under the conditions of a reaction temperature of 150 °C, initial NaOH concentration of 60 %, NaOH-to-slag mass ratio of 4:1, microwave power of 700 W, reaction time of 120 min, and stirring speed of 300 r/min, the conversion rate of fluorinated rare earths reached 99.17 %. The apparent rate equation of the microwave-assisted atmospheric alkaline leaching process was obtained by leaching kinetic analysis, and the apparent activation energy under this process was calculated to be 54.872 kJ/mol, which was 12.458 kJ/mol lower than that achieved when conventional heating was used for leaching (67.33 kJ/mol).

Microwave-mediated stereocontrolled annulations of diazo(aryl)methyl(diaryl)phosphine oxides with pyridinium 1,4-zwitterionic thiolates

Chemoselective annulations of phosphoryl carbenes generated from diazo(aryl)methyl(diaryl)phosphine oxides with pyridinium 1,4-zwitterionic thiolates were performed under microwave irradiation, affording 1-diarylphosphoryl-1H-benzo[c]thiopyran derivatives via [3+3] annulation and indolizine derivatives via ([1+5]-1) annulation with P-Cope elimination as the key step. The annuloselectivity was controlled by the steric hindrance of pyridiniums in pyridinium 1,4-zwitterionic thiolates.

Microwave-specific acceleration of a retro-Diels-Alder reaction

A high-temperature retro-Diels-Alder reaction is accelerated by microwave (MW) heating to rates higher than expected based on Arrhenius kinetics and the measured temperature of the reaction mixture. Observations are consistent with selective MW heating of the polar reactant relative to other, less polar components of the reaction mixture.

Thermal Effect in the Microwave-assisted Aminolysis of Benzoates and Amines

Background:

Microwave selective heating thermal effect is obvious in unimolecular organic reactions. However, it is unclear whether it exists in bimolecular organic reactions under strictly controlled reaction temperature conditions.

Objective:

To determine whether microwave selective heating effect exists in the microwave-assisted bimolecular reactions.

Methods:

Hammett linear relationships in “one-pot” aminolyses of mixed 4-nitrophenyl substituted benzoates with benzylamine and 4-nitrophenyl benzoate with mixed substituted anilines were selected as molecular level probes to explore the thermal effect in the microwave-assisted bimolecular reactions.

Results:

In less polar solvent, there is an obvious “hot spots” effect. “One-pot” aminolyses of mixed 4-nitrophenyl substituted benzoates with benzylamine and 4-nitrophenyl benzoate with mixed substituted anilines were performed in less polar solvent toluene under oil-bath and microwave heating conditions. Generally, slopes of Hammett plots or effect of substituents on reaction rates decrease along with temperature increases under oil-bath heating conditions. Under microwave irradiation conditions, slopes of Hammett plots or effect of substituents on reaction rates decrease in comparison with those under oil-bath heating conditions at the same setting temperature, revealing that higher temperature regions (“hot spots”) still exist in intermolecular organic reactions.

Conclusion:

Microwave selective heating thermal effect still exists in bimolecular organic reactions under strictly controlled reaction temperature conditions, revealing that higher temperature regions (“hot spots”) do exist in intermolecular organic reactions.

Microwave Thermal Effect on Diels-Alder Reaction of Furan and Maleimide

Background:

Higher temperature regions (hot spots) have been observed in organic reactions and are attributed to microwave selective heating. The accumulated heat in reaction systems accelerates certain reactions.

Methods:

The theoretical calculation was applied to select a suitable Diels-Alder reaction as a molecular probe to determine the microwave thermal effect on Diels-Alder reaction, one class of bimolecular reactions. The kinetic investigations were utilized to determine the reaction activation energies and further to calculate the actual reaction temperatures under different microwave conditions from the Arrhenius equation.

Results:

On the basis of the theoretical calculational results, Diels-Alder reaction of furan and maleimide was selected as a molecular probe to determine the microwave thermal effect in Diels- Alder reaction. Their activation energies under thermal conditions were determined from kinetic data by using the Arrhenius equation. The actual reaction temperatures under different microwave conditions were further deduced from their activation energies and the Arrhenius equation.

Conclusion:

Higher temperature regions (hot spots) were observed in Diels-Alder reaction, and they are more obvious in less polar solvents than those in more polar solvents in the microwave irradiated reactions.

New frontiers in the transition-metal-free synthesis of heterocycles from alkynoates: an overview and current status

This review highlights the successful utilization of transition-metal-free approaches for the modular assembly of various heterocycles from alkynoates.

The microwave-assisted syntheses and applications of non-fused single-nitrogen-containing heterocycles

Microwave technology has emerged as a great tool for the efficient synthesis of organic compounds and it provides opportunities for chemists to achieve chemical transformations that tend to be challenging using classical approaches. Additionally, N-heterocycles are well-known for their medicinal/biological significance, along with their applications as excellent building blocks in chemical synthesis. The dominance of N-heterocycles in drug molecules and other pharmacological agents makes them attractive scaffolds, which encourages chemists to develop a wide range of strategies towards the greener synthesis and functionalization of these heterocycles. In this regard, we have collated and discussed literature relating to the microwave-assisted synthesis and the modification of non-(benzo)fused single-nitrogen-containing N-heterocycles from the past decade. The role of the microwave technique and its benefits over the conventional approach have also been emphasized in terms of overall reaction efficiency, reaction time, yield, reduced side-product generation, neat and clean reactions, chemo-/regio-/enantio-selectivity, and the use of mild reagents/reaction conditions to achieve the objectives of green and sustainable chemistry.

Regiospecific synthesis of polysubstituted furans with mono- to tricarboxylates from various sulfonium acylmethylides and acetylenic esters

Polysubstituted furans with mono- to tricarboxylates were prepared in moderate to good yields from various dimethylsulfonium acylmethylides and acetylenic esters.

Ortho-Nitro Effect on the Diastereoselective Control in Sulfa-Staudinger and Staudinger Cycloadditions

The ortho-nitro effect was discovered in sulfa-Staudinger cycloadditions of ethoxycarbonylsulfene with linear imines. When an ortho-nitro group is present at the C-aryl substituents of linear imines, the sulfa-Staudinger cycloadditions deliver cis-β-sultams in considerable amounts, together with the predominant trans-β-sultams. In other cases, the above sulfa-Staudinger cycloadditions give rise to trans-β-sultams exclusively. Further mechanistic rationalization discloses that the ortho-nitro effect is attributed to its strong electron-withdrawing inductive effect. Similarly, the ortho-nitro effect also exists in Staudinger cycloadditions of ethoxycarbonyl ketene with the imines. The current research provides further insights into the diastereoselective control in sulfa-Staudinger and Staudinger cycloadditions.

Understanding MAOS through computational chemistry

Computational chemistry may explain and rationalize the impact of microwave irradiation in organic synthesis.

Sterically controlled diastereoselectivity in thio-Staudinger cycloadditions of alkyl/alkenyl/aryl-substituted thioketenes

Diastereoselectivity in [2 + 2] cycloadditions of alkyl/alkenyl/arylthioketenes and imines, named as thio-Staudinger cycloaddition, was rationalized and verified.

Stereochemistry and mechanistic insights in the [2t + 2i + 2i] annulations of thioketenes and imines

The mechanism and stereochemistry of [2^t + 2ⁱ + 2ⁱ] annulations between one thioketene molecule and two imine molecules have been studied experimentally and theoretically.

Temperature-dependent annuloselectivity and stereochemistry in the reactions of methanesulfonyl sulfene with imines

The annuloselectivity in the reactions of methanesulfonyl sulfene and imines varies with temperature. At 20 °C, the [2^s+ 2ⁱ] annulation occurs exclusively, while at −78 °C, the [2^s+ 2ⁱ+ 2ⁱ] annulation ofN-methyl imines takes place specifically.

Base-switched annuloselectivity in the reactions of ethyl malonyl chloride and imines

The [2 + 2] and [2 + 2 + 2] annuloselectivity in the reactions of ethyl malonyl chloride with imines was controlled by different bases, providing a simple synthesis of β-lactam-3-carboxylates and 2,3-dihydro-1,3-oxazin-4-ones.

Understanding the selectivity in the formation of δ-lactams vs. β-lactams in the Staudinger reactions of chloro-cyan-ketene with unsaturated imines. A DFT study

Selectivity in the formation of δ-lactams vs. β-lactams in the Staudinger reactions of chloro-cyan-ketene with unsaturated imines.