Carboxylation and Mitsunobu Reaction of Amines to Give Carbamates: Retention vs Inversion of Configuration Is Substituent-Dependent

Chemical fixation of CO2/CS2 to access iodoallenyl oxazolidinones and allenyl thiazolidine-thiones

Constructing heterocyclic compounds by chemical fixation of CO2/CS2 as a C1 building block is a promising approach. An efficient and environmentally friendly synthetic approach has been developed using CO2/CS2 to prepare complicated allenyl heterocycles with high yields and diastereoselectivities in a metal-free manner under mild conditions. NIS promoted CO2 fixation and the cyclization reaction by exclusive 1,4-syn-addition of 1,3-enynes rather than 1,2-addition or 3,4-addition, while CS2 participated in unique 1,4-syn-hydrothiolation of 1,3-enynes to afford allenyl heterocycles with different reaction patterns.

Microwave-Assisted Electrostatically Enhanced Phenol-Catalyzed Synthesis of Oxazolidinones

An electrostatically enhanced phenol is utilized as a straightforward, sustainable, and potent one-component organocatalyst for the atom-economic transformation of epoxides to oxazolidinones under microwave irradiation. Integrating a positively charged center into phenols over a modular one-step preparation gives rise to a bifunctional system with improved acidity and activity, competent in rapid assembly of epoxides and isocyanates under microwave irradiation in a short reaction time (20-60 min). A careful assessment of the efficacy of various positively charged phenols and anilines and the impact of several factors, such as catalyst loading, temperature, and the kind of nucleophile, on catalytic reactivity were examined. Under neat conditions, this one-component catalytic platform was exploited to prepare more than 40 examples of oxazolidinones from a variety of aryl- and alkyl-substituted epoxides and isocyanates within minutes, where up to 96% yield and high degree of selectivity were attained. DFT calculations to achieve reaction barriers for different catalytic routes were conducted to provide mechanistic understanding and corroborated the experimental findings in which concurrent epoxide ring-opening and isocyanate incorporation were proposed.

Switchable synthesis of cyclic carbamates by carbon dioxide fixation at atmospheric pressure

The base-promoted switchable synthesis of five- and six-membered cyclic carbamates using atmospheric pressure carbon dioxide as the C1 source was developed. The chemoselectivity of products was simply controlled by changing bases and solvents. The reaction proceeds effectively under mild conditions, affording valuable cyclic carbamates. Experimental results and DFT studies revealed the reaction mechanism.

Diethylammonium iodide as catalyst for the metal-free synthesis of 5-aryl-2-oxazolidinones from aziridines and carbon dioxide

The catalytic potential of ammonium halide salts was explored in the coupling reaction of a model aziridine with carbon dioxide, highlighting the superior activity of [NH2Et2]I. Then, working at room temperature, atmospheric CO2 pressure and in the absence of solvent, the [NH2Et2]I-catalyzed synthesis of a series of 5-aryl-2-oxazolidinones was accomplished in good to high yields and excellent selectivity, from 2-aryl-aziridines with N-methyl or N-ethyl groups. NMR studies and DFT calculations outlined the pivotal role of both the diethylammonium cation and the iodide anion. The proposed method represents a convenient choice for obtaining a limited number of valuable molecules for which more complex and more expensive catalytic systems have been reported even in recent years.

Green Synthesis of 2-Oxazolidinones by an Efficient and Recyclable CuBr/Ionic Liquid System via CO2, Propargylic Alcohols, and 2-Aminoethanols

With the aim of profitable conversion of carbon dioxide (CO2) in an efficient, economical, and sustainable manner, we developed a CuBr/ionic liquid (1-butyl-3-methylimidazolium acetate) catalytic system that could efficiently catalyze the three-component reactions of propargylic alcohols, 2-aminoethanols, and CO2 to produce 2-oxazolidinones and α-hydroxy ketones. Remarkably, this catalytic system employed lower metal loading (0.0125–0.5 mol%) but exhibited the highest turnover number (2960) ever reported, demonstrating its excellent activity and sustainability. Moreover, our catalytic system could efficiently work under 1 atm of CO2 pressure and recycle among the metal-catalyzed systems.

Stereoselective Synthesis of Oxazolidin-2-Ones via an Asymmetric Aldol/Curtius Reaction: Concise Total Synthesis of (-)-Cytoxazone

Herein, we are reporting an efficient approach toward the synthesis of 4,5-disubstituted oxazolidin-2-one scaffolds. The developed approach is based on a combination of an asymmetric aldol and a modified Curtius protocol, which uses an effective intramolecular ring closure to rapidly access a range of oxazolidin-2-one building blocks. This strategy also permits a straightforward and concise asymmetric total synthesis of (−)-cytoxazone. Consisting of three steps, this is one of the shortest syntheses reported to date. Ultimately, this convenient platform would provide a promising method for the early phases of drug discovery.

Design, Synthesis, and Biological Evaluation of a Series of Oxazolone Carboxamides as a Novel Class of Acid Ceramidase Inhibitors

Acid ceramidase (AC) is a cysteine hydrolase that plays a crucial role in the metabolism of lysosomal ceramides, important members of the sphingolipid family, a diversified class of bioactive molecules that mediate many biological processes ranging from cell structural integrity, signaling, and cell proliferation to cell death. In the effort to expand the structural diversity of the existing collection of AC inhibitors, a novel class of substituted oxazol-2-one-3-carboxamides were designed and synthesized. Herein, we present the chemical optimization of our initial hits, 2-oxo-4-phenyl-N-(4-phenylbutyl)oxazole-3-carboxamide 8a and 2-oxo-5-phenyl-N-(4-phenylbutyl)oxazole-3-carboxamide 12a, which resulted in the identification of 5-[4-fluoro-2-(1-methyl-4-piperidyl)phenyl]-2-oxo-N-pentyl-oxazole-3-carboxamide 32b as a potent AC inhibitor with optimal physicochemical and metabolic properties, showing target engagement in human neuroblastoma SH-SY5Y cells and a desirable pharmacokinetic profile in mice, following intravenous and oral administration. 32b enriches the arsenal of promising lead compounds that may therefore act as useful pharmacological tools for investigating the potential therapeutic effects of AC inhibition in relevant sphingolipid-mediated disorders.

Completing the β,γ-CXY-dNTP Stereochemical Probe Toolkit: Synthetic Access to the dCTP Diastereomers and 31P and 19F NMR Correlations with Absolute Configurations

Nucleoside 5'-triphosphate (dNTP) analogues in which the β,γ-oxygen is mimicked by a CXY group (β,γ-CXY-dNTPs) have provided information about DNA polymerase catalysis and fidelity. Definition of CXY stereochemistry is important to elucidate precise binding modes. We previously reported the (R)- and (S)-β,γ-CHX-dGTP diastereomers (X = F, Cl), prepared via P,C-dimorpholinamide CHCl (6a, 6b) and CHF (7a, 7b) bisphosphonates (BPs) equipped with an (R)-mandelic acid as a chiral auxiliary, with final deprotection using H₂/Pd. This method also affords the β,γ-CHCl-dTTP (11a, 11b), β,γ-CHF (12a, 12b), and β,γ-CHCl (13a, 13b) dATP diastereomers as documented here, but the reductive deprotection step is not compatible with dCTP or the bromo substituent in β,γ-CHBr-dNTP analogues. To complete assembly of the toolkit, we describe an alternative synthetic strategy featuring ethylbenzylamine or phenylglycine-derived chiral BP synthons incorporating a photolabile protecting group. After acid-catalyzed removal of the (R)-(+)-α-ethylbenzylamine auxiliary, coupling with activated dCMP and photochemical deprotection, the individual diastereomers of β,γ-CHBr- (33a, 33b), β,γ-CHCl- (34a, 34b), β,γ-CHF-dCTP (35a, 35b) were obtained. The β,γ-CH(CH₃)-dATPs (44a, 44b) were obtained using a methyl (R)-(-)-phenylglycinate auxiliary. ³¹P and ¹⁹F NMR Δδ values are correlated with CXY stereochemistry and pK_a2-4 values for 13 CXY-bisphosphonic acids and imidodiphosphonic acid are tabulated.

Bypassing the Inertness of Aziridine/CO2 Systems to Access 5-Aryl-2-Oxazolidinones: Catalyst-Free Synthesis Under Ambient Conditions

The development of sustainable synthetic routes to access valuable oxazolidinones via CO₂ fixation is an active research area, and the aziridine/carbon dioxide coupling has aroused a considerable interest. This reaction features a high activation barrier and thus requires a catalytic system, and may present some other critical issues. Here, the straightforward gram-scale synthesis of a series of 5-aryl-2-oxazolidinones was developed at ambient temperature and atmospheric CO₂ pressure, in the absence of any catalyst/co-catalyst. The key to this innovative procedure consists in the direct transfer of the pre-formed amine/CO₂ adduct (carbamate) to common aziridine precursors (dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation of aziridine) and then CO₂ . The reaction mechanism was investigated by NMR spectroscopy and DFT calculations applied to model cases.

Pivotal Role of the Basic Character of Organic and Salt Catalysts in C-N Bond Forming Reactions of Amines with CO2

Organocatalysts promote a range of C-N bond forming reactions of amines with CO₂ . Herein, we review these reactions and attempt to identify the unifying features of the catalysts that allows them to promote a multitude of seemingly unrelated reactions. Analysis of the literature shows that these reactions predominantly proceed by carbamate salt formation in the form [BaseH][RR'NCOO]. The anion of the carbamate salt acts as a nucleophile in hydrosilane reductions of CO₂ , internal cyclization reactions or after dehydration as an electrophile in the synthesis of urea derivatives. The reactions are enhanced by polar aprotic solvents and can be either promoted or hindered by H-bonding interactions. The predominant role of all types of organic and salt catalysts (including ionic liquids, ILs) is the stabilization of the carbamate salt, mostly by acting as a base. Catalytic enhancement depends on the combination of the amine, the base strength, the solvent, steric factors, ion pairing and H-bonding. A linear relationship between the base strength and the reaction yield has been demonstrated with IL catalysts in the synthesis of formamides and quinazoline-2,4-diones. The role of organocatalysts in the reactions indicates that all bases of sufficient strength should be able to catalyze the reactions. However, a physical limit to the extent of a purely base catalyzed reaction mechanism should exist, which needs to be identified, understood and overcome by synergistic or alternative methods.

Molecular Catalysis for Utilizing CO2 in Fuel Electro-Generation and in Chemical Feedstock

Processes for the conversion of CO2 to valuable chemicals are highly desired as a result of the increasing CO2 levels in the atmosphere and the subsequent elevating global temperature. However, CO2 is thermodynamically and kinetically inert to transformation and, therefore, many efforts were made in the last few decades. Reformation/hydrogenation of CO2 is widely used as a means to access valuable products such as acetic acids, CH4, CH3OH, and CO. The electrochemical reduction of CO2 using hetero- and homogeneous catalysts recently attracted much attention. In particular, molecular CO2 reduction catalysts were widely studied using transition-metal complexes modified with various ligands to understand the relationship between various catalytic properties and the coordination spheres above the metal centers. Concurrently, the coupling of CO2 with various electrophiles under homogeneous conditions is also considered an important approach for recycling CO2 as a renewable C-1 substrate in the chemical industry. This review summarizes some recent advances in the conversion of CO2 into valuable chemicals with particular focus on the metal-catalyzed reductive conversion and functionalization of CO2.

Ionic Liquid-Promoted Three-Component Domino Reaction of Propargyl Alcohols, Carbon Dioxide and 2-Aminoethanols: A Thermodynamically Favorable Synthesis of 2-Oxazolidinones

To circumvent the thermodynamic limitation of the synthesis of oxazolidinones starting from 2-aminoethanols and CO₂ and realize incorporation CO₂ under atmospheric pressure, a protic ionic liquid-facilitated three-component reaction of propargyl alcohols, CO₂ and 2-aminoethanols was developed to produce 2-oxazolidinones along with equal amount of α-hydroxyl ketones. The ionic liquid structure, reaction temperature and reaction time were in detail investigated. And 15 mol% 1,5,7-triazabicylo[4.4.0]dec-5-ene ([TBDH][TFE]) trifluoroethanol was found to be able to synergistically activate the substrate and CO₂, thus catalyzing this cascade reaction under atmospheric CO₂ pressure. By employing this task-specific ionic liquid as sustainable catalyst, 2-aminoethanols with different substituents were successfully transformed to 2-oxazolidinones with moderate to excellent yield after 12 h at 80 °C.

In Situ Generated Zinc(II) Catalyst for Incorporation of CO2 into 2-Oxazolidinones with Propargylic Amines at Atmospheric Pressure

Incorporation of CO₂ into heterocyclic compounds (i.e., 2-oxazolidinones) under mild conditions, especially at atmospheric pressure still remains challenging. The mononuclear Zn^II complex ZnCl₂ (TBD)₂ , where TBD=1,5,7-triazabicyclo[4.4.0]dec-5-ene, in this study was demonstrated as a robust catalyst for the carboxylative cyclization of propargylic amines with CO₂ to exclusively afford various 2-oxazolidinones in excellent yields. Notably, the Zn^II catalytic species is readily generated in situ from ZnCl₂ and TBD without pre-preparation and further isolation. Such a CO₂ fixation protocol could proceed smoothly under atmospheric pressure at mild temperature in an atom economic and environmentally benign manner. ¹³ C NMR and control experiments were performed to explore the possible interaction between Zn^II and the carbon-carbon triple bond of propargylic amine. The dual catalytic role of the Zn catalyst to enhance O-nucleophilicity of the carbamate anion intermediate and activate the carbon-carbon triple bond is proposed based on mechanistic investigations.

Thermodynamically Favorable Synthesis of 2-Oxazolidinones through Silver-Catalyzed Reaction of Propargylic Alcohols, CO2, and 2-Aminoethanols

Development of catalytic routes to incorporate CO2 into carbonyl compounds at mild conditions remains attractive and challenging. Herein, a one-pot three-component cascade reaction of terminal propargylic alcohols, CO2 , and 2-aminoethanols through Ag(I) -based catalysis is reported for the synthesis of carbonyl compounds through C-O/C-N bond formation. This thermodynamically favorable route can be ingeniously regulated to afford a wide range of 2-oxazolidinones along with concurrent production of α-hydroxyl ketone derivatives in excellent yields and selectivity. Preliminary mechanistic studies indicate that such a process proceeds through successive formation of α-alkylidene cyclic carbonate, β-oxopropylcarbamate, and 2-oxazolidinones.

Reaction mechanisms of carbon dioxide, ethylene oxide and amines catalyzed by ionic liquids BmimBr and BmimOAc: a DFT study

The binary ionic liquids BmimBr and BmimOAc have the important electrostatic and hydrogen bond effects on the catalytic conversion of carbon dioxide, ethylene oxide and aniline to 3-phenyl-2-oxazolidionone.

The Mitsunobu reaction in the 21st century

Recent advancements in the condensation of alcohols with pronucleophiles by the Mitsunobu reaction are described.

Synthesis of N,N-Diethylbenzamides via a Non-Classical Mitsunobu Reaction

The use of the Mitsunobu reaction for the synthesis of N,N-diethylbenzamides affords ortho-, meta-, and para-substituted benzamides, containing both electron-donating and electron-withdrawing groups. While the preparation of numerous functional groups has been efficiently demonstrated employing the Mitsunobu reaction, our methodology represents the first application of the Mitsunobu reaction for the construction of benzamides using benzoic acid and amine starting materials. Moreover, this synthetic transformation is believed to proceed via a non-classical mechanism involving the existence of an acyloxyphosphonium ion.

How easy are the syntheses of allenes?

This short review highlights some of the recent important developments on the synthesis of allenes and its applications on the synthesis of some allenic natural products and allenic-based optoelectronic materials.

[11C]GR103545: novel one-pot radiosynthesis with high specific activity

INTRODUCTION

GR103545 is a potent and selective kappa-opioid receptor agonist. Previous studies in non-human primates demonstrated favorable properties of [(11)C]GR103545 as a positron emission tomography tracer for in vivo imaging of cerebral kappa-opioid receptor. Nonetheless, advancement of [(11)C]GR103545 to imaging studies in humans was hampered by difficulties of its multiple-step radiosynthesis, which produces a final product with low specific activity (SA), which in turn could induce undesirable physiological side effects resulting from the mass associated with an injected amount of radioactivity. We report herein an alternative radiosynthesis of [(11)C]GR103545 with higher SA and radiochemical yields.

METHODS

The TRACERLab FXC automated synthesis module was used to carry out the two-step, one-pot procedure. In the first step, the desmethoxycarbonyl precursor was converted to the carbamic acid intermediate desmethyl-GR103545 via transcarboxylation with the zwitterionic carbamic complex, 1,8-diazabicyclo[5.4.0]undec-7-ene-carbon dioxide, in the presence and/or absence of cesium carbonate and tetrabutylammonium triflate. In the second step, the intermediate was radiolabeled at the carboxyl oxygen with [(11)C]methyl trifluoromethanesulfonate to give [(11)C]GR103545.

RESULTS

This novel synthesis produced [(11)C]GR103545 with ≥90% chemical and radiochemical purities and an SA of 290.45±99.9 MBq/nmol at the end of synthesis (n=26). Injectable radioactivity was 1961±814 GBq/μmol with 43 min of average synthesis time from the end of beam.

CONCLUSION

We have developed a practical one-pot method for the routine production of [(11)C]GR103545 with reliably high SA and radiochemical yield, thus allowing the advancement of this radiotracer to imaging applications in humans.