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Synlett

Current research reports and chronological list of recent articles..




Accounts and Rapid Communications in Synthetic Organic Chemistry: SYNLETT is an international scientific journal reporting research results and trends in synthetic organic chemistry in short personalized reviews and preliminary communications.

The publisher is Thieme. The copyright and publishing rights of specialized products listed below are in this publishing house. This is also responsible for the content shown.

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Additional research articles see Current Chemistry Research Articles. Magazines with similar content (organic chemistry):

 - Beilstein Journal of Organic Chemistry.

 - European Journal of Organic Chemistry.

 - Journal of Organic Chemistry.

 - Journal of Physical Organic Chemistry.

 - Organic Letters.

 - Organic Process Research and Development.

 - Russian Journal of Organic Chemistry.



Synlett - Abstracts



Trithioorthoester Exchange and Metathesis: New Tools for Dynamic Covalent Chemistry

To expand the toolbox of dynamic covalent and systems chemistry, we investigated the acid-catalyzed exchange reaction of trithioorthoesters with thiols. We found that trithioorthoester exchange occurs readily in various solvents in the presence of stoichiometric amounts of strong Brønsted acids or catalytic amounts of certain Lewis acids. The scope of the exchange reaction was explored with various substrates, and conditions were identified that permit clean metathesis reactions between two different trithioorthoesters. One distinct advantage of S,S,S-orthoester exchange over O,O,O-orthoester exchange is that the exchange reaction can kinetically outcompete hydrolysis, thereby making the process less sensitive to residual moisture. We expect that the relatively high stability of the products might be beneficial in future supramolecular receptors or porous materials.
Datum: 18.09.2019


Linked PDF of Table of Contents

Synlett 2019; 30: V-
DOI: 10.1055/s-0039-1690202



© Georg Thieme Verlag Stuttgart · New York

Artikel in Thieme eJournals:
Inhaltsverzeichnis


Datum: 16.09.2019


SYNFORM ISSUE 2019/10

Synlett 2019; 30: A148-A161
DOI: 10.1055/s-0037-1612200



© Georg Thieme Verlag KG Stuttgart · New York

Artikel in Thieme eJournals:
Inhaltsverzeichnis


Datum: 16.09.2019


Efficient Halogenation of 2-Aminopyrazine

2-Aminopyrazine was halogenated with NIS, NCS, and NBS under different reaction conditions. Chlorination and bromination were achieved with good yields by using acetonitrile as the solvent. However, iodination was only obtained in poor yields. Undoubtedly, the best conditions for both mono- and dihalogenation were the use of NBS, acetonitrile, and microwave assistance for short periods. 3,5-Dibromo-2-aminopyrazine is an excellent functionalized starting material for the synthesis of nitrogen heterocycles.
Datum: 11.09.2019


Recent Advances in the Synthesis of Thiadiazoles

Thiadiazole moieties are present in many natural products and pharmaceutical compounds that possess a broad spectrum of biological activities, serving as antidepressant, anxiolytic, antimicrobial, antitubercular, antiinflammatory, antidiabetic, anticancer, antihypertensive, or antifungal drugs. Many excellent methods have been reported for accessing such frameworks. In this review, we summarize advances made within the past ten years in the synthesis of various types of thiadiazole.1 Introduction2 Synthesis of Thiadiazoles2.1 Synthesis of 1,2,3-Thiadiazoles2.1.1 Synthesis of 1,2,3-Thiadiazoles from Diazo/Azide Compounds2.1.2 Synthesis of 1,2,3-Thiadiazoles from Sulfonyl Hydrazines or N-Tosylhydrazones2.2 Synthesis of 1,2,4-Thiadiazoles2.2.1 Synthesis of 1,2,4-Thiadiazoles from Thioamides or their Derivatives2.2.2 Synthesis of 1,2,4-Thiadiazoles from Amidines or 2-Aminopyridines2.3 Synthesis of 1,3,4-Thiadiazoles2.4 Synthesis of 1,2,5-Thiadiazoles3 Conclusion
Datum: 05.09.2019


Radical α-C–H Cyclobutylation of Aniline Derivatives

A catalytic system has been developed for the direct alkylation of α-C–H bonds of aniline derivatives with strained C–C σ-bonds. This method operates through a photoredox mechanism in which oxidative formation of aminoalkyl radical intermediates enables addition to a bicyclobutane derivative, giving rise to α-cyclobutyl N-alkylaniline products. This mild system proceeds through a redox- and proton-neutral mechanism and is operational for a range of substituted arylamine derivatives.
Datum: 03.09.2019


The Trouble with Five: New Synthetic Strategies toward C5 -Symmetric Pillar[5]arenes and Beyond

Rim-differentiated pillar[5]arenes (P[5]s) are intriguing macrocyclic molecular platforms with C 5-symmetry, but their conventional statistical syntheses suffer from low yields and laborious separation. Our group recently reported a ‘pre-oriented’ protocol that results in highly selective production of C 5-symmetric P[5]s among four constitutional isomers. Subsequently, we devised a more general divergent synthetic route starting with a common P[5] precursor with rim differentiation, followed by a series of high-yielding reactions that permit successive transformations of both rims freely. As a result, a variety of rim-differentiated P[5]s can be made to order in gram-scale quantities. This total solution not only populates the list of C 5-symmetric P[5]s, but also enables further design and synthesis of assorted five-fold organic building blocks towards complex supramolecular architectures.
Datum: 03.09.2019


Linked PDF of Table of Contents

Synlett 2019; 30: V-
DOI: 10.1055/s-0039-1689990



© Georg Thieme Verlag Stuttgart · New York

Artikel in Thieme eJournals:
Inhaltsverzeichnis


Datum: 29.08.2019


SYNFORM ISSUE 2019/09

Synlett 2019; 30: A132-A147
DOI: 10.1055/s-0037-1612192



© Georg Thieme Verlag KG Stuttgart · New York

Artikel in Thieme eJournals:
Inhaltsverzeichnis


Datum: 29.08.2019


Samarium Diiodide Catalyzed Radical Cascade Cyclizations that Construct Quaternary Stereocenters

SmI2-catalyzed radical cascade cyclizations were used to generate complex carbocyclic products bearing quaternary stereocenters with high selectivity. Bicyclic scaffolds containing four contiguous stereocenters and one quaternary stereocenter were obtained in excellent yields (up to 99%) and with high diastereocontrol by using 5 mol% of SmI2 at ambient temperature in the absence of co-reductants or additives. Mechanistic studies support a radical relay mechanism.
Datum: 28.08.2019


Dipeptide-Based Phosphonium Salt Catalysis: Application to Enantioselective Synthesis of Fused Tri- and Tetrasubstituted Aziridines

Over the past decades, phase-transfer catalysis (PTC), generally based on numerous chiral quaternary ammonium salts, has been recognized as a powerful and versatile tool for organic synthesis in both industry and academia. In sharp contrast, PTC involving chiral phosphonium salts as the catalysts is insufficiently developed. Recently, our group realized the first enantioselective aza-Darzens reaction for preparing tri- and tetrasubstituted aziridine derivatives under bifunctional phosphonium salt catalysis. This article briefly discusses the recent development in asymmetric reactions (mainly including nucleophilic additions and cyclizations) promoted by chiral quaternary phosphonium salt catalysts. We expect that more catalytic asymmetric reactions will be developed on the basis of such new phase-transfer catalytic systems in the near future.
Datum: 27.08.2019


Functional End Groups in Living Ring-Opening Metathesis Polymerization

Over the last two decades many synthetic methods have been reported to selectively introduce a number of different functional groups at the chain end of a living ring-opening metathesis polymer. In this personal account, I would like to focus on a few such methods developed in my research group over the last several years and how these led to the discovery of catalytic living ring-opening metathesis polymerization, a ring-opening metathesis polymerization method controlled by the polymer end groups. This account consists of the following sections:1 Introduction2 Functionalization of the Propagating Chain End3 Functionalization of the Initiating Chain End4 Polymerization Control by Functional End Groups5 Conclusions
Datum: 19.08.2019


Copper-Catalyzed Radical Bis(trifluoromethylation) of Alkynes and 1,3-Enynes

The copper-catalyzed radical bis(trifluoromethylation) of alkynes and 1,3-enynes is described. With Cu(CH3CN)4BF4 as the catalyst, the reaction of arylalkynes with Togni reagent II and (bpy)Zn(CF3)2at room temperature affords the corresponding 1,2-bis(trifluoromethylated) alkenes in good yields with excellent E-stereoselectivity. Under similar conditions, the reaction of 1,3-enynes provides the corresponding 1,4-bis(trifluoromethylated) allenes exclusively in satisfactory yields. The protocol exhibits broad substrate scope and excellent functional group compatibility.
Datum: 15.08.2019


β-Selective One-Pot Synthesis of Acyl-C-Glycosides via Corey–Seebach Umpolung Reaction

C-Glycosides are commonly used as carbohydrate mimics in drug development due to their stability against enzymatic and chemical hydrolysis. In this Synpacts article we elaborate on our fast and efficient β-selective approach towards protected and unprotected acyl glycosides. Application of a Corey–Seebach umpolung reaction enables the exclusive formation of the β-anomer of aromatic acyl-C-glycosides in good to excellent yields.1 Introduction2 C-Glycosylation of Benzylated Glycosyl Donors3 C-Glycosylation of Silylated Glycosyl Donors4 Conclusion
Datum: 13.08.2019


Ruthenium-Catalyzed Direct Cross-Coupling of Secondary Alcohols to β-Disubstituted Ketones

The β-disubstituted ketone functionality is prevalent in biologically active compounds and in pharmaceuticals. A ruthenium-catalyzed direct synthesis of β-disubstituted ketones by cross-coupling of two different secondary alcohols is reported. This new protocol was applied to the synthesis of variety of β-disubstituted ketones from various cyclic, acyclic, symmetrical, and unsymmetrical secondary alcohols. An amine–amide metal–ligand cooperation in a Ru catalyst facilitates the activation and formation of covalent bonds in selective sequences to provide the products. Kinetic and deuterium-labeling experiments suggested that aliphatic alcohols oxidize faster than benzylic secondary alcohols. A plausible mechanism is proposed on the basis of mechanistic and kinetic studies. Water and H2 are the only byproducts from this selective cross-coupling of secondary alcohols.1 Introduction2 Catalytic Self- or Cross-Coupling of Alcohols and Selectivity Challenges3 Recent Developments in the Synthesis of β-Disubstituted Ketones4 Scope of Ruthenium-Catalyzed Cross-Couplings of Secondary Alcohols5 Mechanistic Studies and Proposed Mechanism6 Conclusion
Datum: 13.08.2019


Magnesium-Catalyzed N2-Regioselective Alkylation of 3-Substituted Pyrazoles

A highly regioselective Mg-catalyzed alkylation of 3-substituted pyrazoles has been developed to provide N2-alkylated regioisomers. Using α-bromoacetates and acetamides as alkylating agents, this new method was applied to a variety of 3-substituted and 3,4-disubstituted pyrazoles to produce the N2-alkylated products with high regioselectivities ranging from 76:24 to 99:1 and 44–90% yields.
Datum: 09.08.2019


Regioselective Radical Alkene Amination Strategies by Using Phosphite-Mediated Deoxygenation

Nitrogen-containing compounds are an essential motif in all disciplines of chemistry and the efficient synthesis of these frameworks is a highly sought-after goal. Presented here is a summary of recent efforts conducted by our group to develop radical-mediated amination strategies for the formal synthesis of primary amines from alkenes with exclusive anti-Markovnikov regioselectivity. We have found that N-hydroxyphthalimide is an effective reagent capable of supplying both the N and H atoms for alkene hydroamination in a group transfer radical addition-type mechanism. Furthermore, allyl-oxyphthalimide derivatives are similarly capable of radical group transfers and allow for the aminoallylation of an external alkene.
Datum: 09.08.2019


Recent Progress in the Copper-Catalyzed Cascade Cyclization Involving Intramolecular Hydroamination of Terminal Alkynes

Transition-metal-catalyzed cascade cyclization initiated by intramolecular alkyne hydroamination has received considerable attention over the past decade and has been widely used in the rapid construction of a diverse array of valuable complex N-heterocycles. Recent progress in this area is reviewed with an emphasis on the cheap and environmentally benign copper-catalyzed cascade cyclization of terminal alkynes.
Datum: 30.07.2019


Emerging Catalyst Control in Cobalt-Catalyzed Oxidative Hydrofunctionalization Reactions

Oxidative functionalization has emerged as an important pathway in Co-catalyzed hydrogen atom transfer (HAT) hydrofunctionalization reactions. Notably, evidence was found for the participation of organometallic intermediates in such radical-polar crossover processes. These findings provide opportunities for catalyst control that was previously absent in HAT catalysis. In this article, we summarize the recent progress towards this direction involving carbon–heteroatom bond-forming intra- and intermolecular reactions, including work from our own group.1 Introduction2 Oxidative Trapping by Solvents and Intramolecular Nucleophiles3 Intermolecular Oxidative Hydrofunctionalization4 Conclusion and Outlook
Datum: 25.07.2019


Selective Functionalization of Aliphatic Amines via Myoglobin-Catalyzed Carbene N–H Insertion

Engineered myoglobins have recently gained attention for their ability to catalyze a variety of abiological carbene transfer reactions including the functionalization of amines via carbene insertion into N–H bonds. However, the scope of myoglobin and other hemoprotein-based biocatalysts in the context of this transformation has been largely limited to aniline derivatives as the amine substrates and ethyl diazoacetate as the carbene donor reagent. In this report, we describe the development of an engineered myoglobin-based catalyst that is useful for promoting carbene N–H insertion reactions across a broad range of substituted benzylamines and α-diazo acetates with high efficiency (82–99% conversion), elevated catalytic turnovers (up to 7,000), and excellent chemoselectivity for the desired single insertion product (up to 99%). The scope of this transformation could be extended to cyclic aliphatic amines. These studies expand the biocatalytic toolbox available for the selective formation of C–N bonds, which are ubiquitous in many natural and synthetic bioactive compounds.
Datum: 11.07.2019


Recent Progress in Equilibrium Acidity Studies of Organocatalysts

This account summarizes our recent work on the pK a scales of some frequently used organocatalysts, especially those of hydrogen-bond-donor catalysts and stronger Brønsted acid catalysts. Most of these pK a values were obtained by the Bordwell overlapping indicator method, which is known to provide high accuracy. Linear free-energy relationships associated with pK a values are discussed in relation to understanding of reaction mechanisms.1 Introduction2 Single Hydrogen-Bonding Donors2.1 Proline-Type Organocatalysts2.2 Cinchona Alkaloids Bearing a Hydrogen-Bonding Donor in the 6′-Position3 Double-Hydrogen-Bonding Donors3.1 Thioureas3.2 Squaramides3.3 BINOLs4 Stronger Brønsted Acids5 N-Heterocyclic Carbenes6 Summary and Outlook
Datum: 10.07.2019


Sustainable Carbene Transfer Reactions with Iron and Light

Carbenes are versatile, highly reactive intermediates with great importance in chemistry. We recently reported on our findings on safe and scalable applications of hazardous diazoacetonitrile using cheap and commercially available iron catalysts in efficient carbene transfer reactions, ranging from cyclopropanation towards C–H functionalization reactions for the synthesis of biologically important building blocks. More lately, we uncovered the reactivity of diazoalkanes under photochemical conditions using visible light and were able to demonstrate a variety of different, metal-free carbene transfer reactions, which now open up new sustainable ways for the construction of small functional molecules.1 Introduction2 Iron-Catalyzed Carbene Transfer Reactions of Diazoacetonitrile3 Metal-free Carbene Transfer Reaction with Visible Light4 Summary
Datum: 26.06.2019


Synthetic Utility of One-Pot Chemoenzymatic Reaction Sequences

In recent years, there has been a rapid and sustained increase in the development and use of one-pot chemoenzymatic reaction processes for the efficient synthesis of high-value molecules. This strategy can provide a number of advantages over traditional synthetic methods, including high levels of selectivity in reactions, mild and sustainable reaction conditions, and the ability to rapidly build molecular complexity in a single reaction vessel. Here, we present several examples of chemoenzymatic one-pot reaction sequences that demonstrate the diversity of transformations that can be incorporated in these processes.
Datum: 06.06.2019


Development of a Hydrazine-Catalyzed Carbonyl-Olefin Metathesis Reaction

Carbonyl-olefin metathesis is a potentially powerful yet underexplored reaction in organic synthesis. In recent years, however, this situation has begun to change, most notably with the introduction of several different catalytic technologies. The development of one of those new strategies, based on hydrazine catalysts and a novel [3+2] paradigm for double bond metathesis, is discussed herein. First, the stage is set with a description of some potential applications of carbonyl-olefin metathesis and a discussion of alternative strategies for this intriguing reaction.1 Introduction2 Potential Applications of Carbonyl-Olefin Metathesis3 Carbonyl-Olefin Metathesis Strategies4 Direct (Type I): Non-Catalytic5 Direct (Type I): Acid-Catalyzed6 Indirect (Type II): Metal Alkylidenes7 Indirect (Type III): Hydrazine-Catalyzed8 Conclusion
Datum: 05.06.2019


Aminoxylation of Thioalkynes through Radical-Polar Crossover

A one-pot procedure for the aminoxylation of thioalkynes for the direct formation of α-functionalized thioesters under mild reaction conditions is reported. A ketenethionium ion is the key intermediate, which is generated in situ by Brønsted acid mediated protonation and undergoes a radical-polar crossover.
Datum: 03.06.2019


Anion Metathesis in Facile Preparation of Olefin Metathesis Catalysts Bearing a Quaternary Ammonium Chloride Tag

Easy and efficient method for preparation of all-chloro ruthenium catalysts bearing a quaternary ammonium tag was developed. The key anion metathesis reaction was made with AgCl, and the facile recycling of silver wastes was demonstrated. The developed method transforms more accessible, yet poorly characterised mixed chloro/iodo Ru complexes into valuable all-chloro catalysts, useful in aqueous metathesis. Practical utility of the developed anion metathesis reaction was demonstrated by preparation of a commercial Ru catalyst, StickyCat™, on a 5 gram scale.
Datum: 20.05.2019


Acetal Metathesis: Mechanistic Insight

The origins and recent applications of acetal metathesis are discussed in the context of synthesizing polyacetals via acetal metathesis polymerization (AMP). A kinetic study of the acid-catalyzed acetal metathesis reaction suggests the rate = k[H+][acetal]2, with MeOCH2OMe and EtOCH2OEt interchanging to yield MeOCH2OEt, achieving the statistical 1:2:1 equilibrium distribution in 4 hours at 80 °C and in 1 hour at 90 °C. Upon heating 1,10-decanediol and diethoxymethane, polydecylene acetal is formed with sequential distillation of ethanol, followed by diethoxymethane. A full mechanism for this polymerization is proposed, which begins with a transacetalization sequence to convert the diol into a bisacetal, followed by acetal metathesis to yield a high-molecular-weight polymer.
Datum: 13.05.2019


Radical Biocatalysis: Using Non-Natural Single Electron Transfer Mechanisms to Access New Enzymatic Functions

Exploiting non-natural reaction mechanisms within native enzymes is an emerging strategy for expanding the synthetic capabilities of biocatalysts. When coupled with modern protein engineering techniques, this approach holds great promise for biocatalysis to address long-standing selectivity and reactivity challenges in chemical synthesis. Controlling the stereochemical outcome of reactions involving radical intermediates, for instance, could benefit from biocatalytic solutions because these reactions are often difficult to control by using existing small molecule catalysts. General strategies for catalyzing non-natural radical reactions within enzyme active sites are, however, undeveloped. In this account, we highlight three distinct strategies developed in our group that exploit non-natural single electron transfer mechanisms to unveil previously unknown radical biocatalytic functions. These strategies allow common oxidoreductases to be used to address the enduring synthetic challenge of asymmetric hydrogen atom transfer.1 Introduction2 Photoinduced Electron Transfer from NADPH3 Ground State Electron Transfer from Flavin Hydroquinone4 Enzymatic Redox Activation in NADPH-Dependent Oxidoreductases5 Conclusion
Datum: 07.05.2019


Iterative Preparation of Platinum Nanoparticles in an Amphiphilic Polymer Matrix: Regulation of Catalytic Activity in Hydrogenation

We demonstrate that iteration of the seeded preparation of platinum nanoparticles dispersed in an amphiphilic polystyrene–poly(ethylene glycol) resin (ARP–Pt) regulates their catalytic activity in the hydrogenation of aromatic compounds in water. The catalytic activity of the fifth generation of ARP–Pt [G5] prepared through four iterations of the seeded preparation was far superior to that of the initial ARP–Pt [G1] in the hydrogenation of aromatic compounds in water.
Datum: 25.04.2019


Consecutive Ring-Expansion Reactions for the Iterative Assembly of Medium-Sized Rings and Macrocycles

Macrocycles and medium-sized rings have important applications in several scientific fields but can be challenging to make using traditional end-to-end cyclization reactions. Ring-expansion methods represent a useful alternative and offer numerous practical benefits. In this Account, we discuss the current state of the art of ring-expansion strategies that have been applied consecutively. Such methods have the power to expedite the design and synthesis of functionalized macro­cycles via the selective, iterative insertion of smaller fragments into ring-enlarged products.1 Introduction2 Insertion Reactions2.1 Transamidation/Transpeptidation2.2 Transesterification2.3 Transthioesterification2.4 Aminyl Radical Cascade2.5 Iterative Synthesis of Lactones2.6 Successive Ring Expansion of β-Ketoesters and Lactams3 Pericyclic Reactions3.1 Sulfur-Mediated Rearrangements3.2 Nitrogen-Mediated Rearrangements4 Fragmentation Reactions5 Conclusions and Future Outlook
Datum: 12.04.2019


Sulfoxide-Directed Iterative Assembly into Oligoarenes

A series of oligoarenes have been synthesized via sulfoxide-based iterative dehydrogenative transformations. By utilizing the sulfinyl moieties as on/off-switchable directing groups, overreactions and/or undesired oligomerizations were completely suppressed. Since the dehydrogenative couplings were not hampered by steric hinderance, sterically encumbered oligoarenes were synthesized.
Datum: 28.03.2019


Thiolate-Initiated Synthesis of Dibenzothiophenes from 2,2′-Bis(methylthio)-1,1′-Biaryl Derivatives through Cleavage of Two Carbon–Sulfur Bonds

A catalytic reaction involving the cleavage of two carbon–sulfur bonds in 2,2′-bis(methylthio)-1,1′-biaryl derivatives is reported. This reaction does not require a transition-metal catalyst and is promoted by a thiolate anion. Notably, based on DFT calculations, the product-forming cyclization step is shown to proceed through a concerted nucleophilic aromatic substitution (CSNAr) mechanism.
Datum: 14.01.2019


 


Category: Current Chemistry Research

Last update: 28.03.2018.






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