Visible‐Light‐Promoted Photocatalytic B−C Coupling via a Boron‐Centered Carboranyl Radical: Facile Synthesis of B(3)‐Arylated o‐Carboranes

A visible‐light‐mediated in situ generation of a boron‐centered carboranyl radical (o‐C₂B₁₀H₁₁ ^. ) has been described. With eosin Y as a photoredox catalyst, 3‐diazonium‐o‐carborane tetrafluoroborate [3‐N₂‐o‐C₂B₁₀H₁₁][BF₄] was converted into the corresponding boron‐centered carboranyl radical intermediate, which can undergo efficient electrophilic substitution reaction with a wide range of (hetero)arenes. This general and simple procedure provides a metal‐free alternative for the synthesis of 3‐(hetero)arylated‐o‐carboranes.     

An Efficient and Convenient Synthesis of Ethyl 1‐(4‐Methoxyphenyl)‐5‐phenyl‐1H‐1,2,3‐triazole‐4‐carboxylate

The “click chemistry” of using organic azides and terminal alkynes is arguably the most efficient and straightforward route to the synthesis of 1,2,3‐triazoles. In this paper, an alternative and direct access to ethyl 1‐(4‐methoxyphenyl)‐5‐phenyl‐1H‐1,2,3‐triazole‐4‐carboxylate is described. Treatment of ethyl diazoacetate with 4‐methoxyaniline derived aryl imines in the presence of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene provided fully substituted 1,2,3‐triazoles in good to high chemical yields. The base‐mediated reaction tolerates various substituted phenyl imines as well as ethyl diazoacetate or the more bulky diazoacetamide. A reasonable mechanism is proposed that involves the addition of an imine nitrogen atom to the terminal nitrogen atom of the diazo compound, followed by aromatization to give the 1,2,3‐triazole. The presence of the 4‐carboxy group is advantageous as it can be easily transformed into other functional groups.     

Combining the Advantages of Tetrazoles and 1,2,3‐Triazoles: 4,5‐Bis(tetrazol‐5‐yl)‐1,2,3‐triazole, 4,5‐Bis(1‐hydroxytetrazol‐5‐yl)‐1,2,3‐triazole,and their Energetic Derivatives

In the development of new energetic materials, the main challenge is the combination of high energy content with chemical and mechanical stability, two properties that are often contradictory. In this study, the syntheses and comprehensive characterizations of 4,5‐bis(tetrazole‐5‐yl)‐1,2,3‐triazole and the novel 4,5‐bis(1‐hydroxytetrazole‐5‐yl)‐1,2,3‐triazole, as well as their energetic properties, are presented, combining the advantages of the more energetic tetrazole and the more stable 1,2,3‐triazole rings. Nitrogen‐rich salts of both compounds were synthesized to investigate their detonation performances and combustion behavior calculated by computer codes for potential application in erosion‐reduced gun propellant mixtures due to their high nitrogen content. The structures of several of the compounds were studied by single‐crystal X‐ray diffraction and, especially in the case of 4,5‐bis(tetrazol‐5‐yl)‐1,2,3‐triazole, revealed the site of deprotonation.     

Synthetic Approaches to Trifluoromethoxy‐Substituted Compounds

Because of the unique properties of the trifluoromethoxy group, molecules bearing this moiety will find applications in various fields, particularly in the life sciences. However, despite the great interest in this functional group, only a small number of trifluoromethoxylated molecules are currently synthetically accessible. Over the last few years, several innovative and promising strategies for the synthesis of trifluoromethoxylated compounds have been described. This Minireview discusses these existing methods with a particular focus on more recent advances.     

New Energetic Materials: Functionalized 1‐Ethyl‐5‐aminotetrazoles and 1‐Ethyl‐5‐nitriminotetrazoles

A new way to make a bang : Several functionalized 1‐ethyl‐5‐aminotetrazoles, 1‐ethyl‐5‐nitrimino‐tetrazoles, and copper complexes have been synthesized and their chemical and energetic properties have been comprehensively characterized. The compounds belong to all classes of “energetic materials”: explosives, propellants, and pyrotechnics.

     

Synthetic and DFT Studies Towards a Unified Approach to Phlegmarine Alkaloids: Aza‐Michael Intramolecular Processes Leading to 5‐Oxodecahydroquinolines

A diastereoselective synthesis of cis‐5‐oxodecahydroquinolines is described in which three stereocenters are generated in a one‐pot reaction. The reaction involves a lithium hydroxide‐promoted Robinson annulation/intramolecular aza‐Michael domino process from an achiral acyclic tosylamine‐tethered β‐keto ester. The development and scope of this reaction was facilitated through the use of DFT‐based mechanistic studies, which enabled the observed diastereodivergent course of the azacyclization to be rationalized. The varying stereochemistry and stability of the resulting decahydroquinolines was found to depend on whether a β‐keto ester or ketone were embedded in the substrates undergoing aminocyclization. This synthetic approach gave access not only to both diastereomeric cis‐decahydroquinolines from the same precursor, but also to the corresponding trans isomers, through an epimerization processes of the corresponding N‐unsubstituted cis‐5‐oxodecahydroquinolines. The described methodology provides advanced building‐blocks with the three relative stereochemistries required for the total synthesis of phlegmarine alkaloids.     

Palladium‐Catalyzed CH Activation of N‐Allyl Imines: Regioselective Allylic Alkylations to Deliver Substituted Aza‐1,3‐Dienes

A new mode of activation of an imine via a rare aza‐substituted π‐allyl complex is described. Palladium‐catalyzed C(sp³)? H activation of the N‐allyl imine and the subsequent nucleophilic attack by the α‐alkyl cyanoester produced the 1‐aza‐1,3‐diene as the sole regioisomer. In contrast, nucleophilic attack by the α‐aryl cyanoester exclusively delivered the 2‐aza‐1,3‐diene, which was employed in an inverse‐electron‐demand Diels–Alder reaction for heterobiaryl synthesis.     

Metal‐Free Synthesis of Functional 1‐Substituted‐1,2,3‐Triazoles from Ethenesulfonyl Fluoride and Organic Azides

The boom in growth of 1,4‐disubstituted triazole products, in particular, since the early 2000’s, can be largely attributed to the birth of click chemistry and the discovery of the Cu^I‐catalyzed azide–alkyne cycloaddition (CuAAC). Yet the synthesis of relatively simple, albeit important, 1‐substituted‐1,2,3‐triazoles has been surprisingly more challenging. Reported here is a straightforward and scalable click‐inspired protocol for the synthesis of 1‐substituted‐1,2,3‐triazoles from organic azides and the bench stable acetylene surrogate ethenesulfonyl fluoride (ESF). The new transformation tolerates a wide selection of substrates and proceeds smoothly under metal‐free conditions to give the products in excellent yield. Under controlled acidic conditions, the 1‐substituted‐1,2,3‐triazole products undergo a Michael addition reaction with a second equivalent of ESF to give the unprecedented 1‐substituted triazolium sulfonyl fluoride salts.     

Stereoselective Substrate‐Controlled Asymmetric Syntheses of both 2,5‐cis‐ and 2,5‐trans‐Tetrahydrofuranoid Oxylipids: Stereodivergent Intramolecular Amide Enolate Alkylation

The concise, highly stereoselective, substrate‐controlled asymmetric total syntheses of both 2,5‐cis‐ and 2,5‐trans‐tetrahydrofuranoid nematocidal oxylipids from the Australian brown algae Notheia anomala have been accomplished in a stereodivergent fashion. The highly stereoselective intramolecular amide enolate alkylation strategy provides access to both stereoisomers of the 3‐hydroxy‐2,5‐disubstituted tetrahydrofuran core of these marine natural products through chelate and nonchelate control, which is driven by the C3‐hydroxy protecting group. This approach offers an optional and highly stereoelective access to any of the eight possible stereoisomers of the 2,5‐disubstituted‐3‐oxygenated tetrahydrofuran skeleton, an important structural feature which is present in many biologically active natural products.     

Synthesis of Multifunctionalized 1,2,3,4‐Tetrahydropyridines, 2,3‐Dihydropyridin‐4(1H)‐ones,and Pyridines from Tandem Reactions Initiated by [5+1] Cycloaddition of N‐Formylmethyl‐Substituted Enamides to Isocyanides: Mechanistic Insight and Synthetic Application

Tandem reactions for the efficient synthesis of multifunctionalized 1,2,3,4‐tetrahydropyridines, 2,3‐dihydropyridin‐4(1H)‐ones, and pyridine derivatives have been developed and reaction mechanisms have been investigated. Synthetic cascades are initiated by the Zn(OTf)₂‐mediated [5+1] cycloaddition of N‐formylmethyl‐substituted tertiary enamides to isocyanides, thus leading to the versatile heterocyclic enamino imine intermediates. Interception of the intermediates by diastereoselective reduction of imine functionality with Me₄NBH(OAc)₃ afforded 1,6‐disubstituted trans‐3‐hydroxy‐4‐arylamino‐ or ‐alkylamino‐1,2,3,4‐tetrahydropyridines, whereas acylation of the imino group followed by acidic hydrolysis produced 1,6‐disubstituted 3‐acyloxy‐2,3‐dihydropyridin‐4(1H)‐ones. Aerobic oxidation led to the aromatization followed by intermolecular acyl‐group transfer from the pyridinium nitrogen to the 3‐hydroxy moiety, thereby yielding substituted 3‐acyloxy‐4‐aminopyridines. Synthetic potentials of the resulting products have been demonstrated by expedient and highly stereoselective synthesis of cis,cis‐4,5‐dihydroxy‐2‐phenylpiperidine and trans,trans‐4‐amino‐5‐hydroxy‐2‐phenylpiperidine compounds, which are important in medicinal chemistry, through simple and practical reduction reactions.     

The Renaissance of α‐Methylene‐γ‐butyrolactones: New Synthetic Approaches

The amount of research activity concerning α‐methylene‐γ‐butyrolactones and α‐alkylidene‐γ‐butyrolactones has increased dramatically in recent years. This Review summarizes the structural types, biological activities, and biosynthesis of these compounds, concentrating on publications from the past 10 years. Traditional approaches to α‐methylene‐γ‐butyrolactones and α‐alkylidene‐γ‐butyrolactones are then reviewed together with novel approaches, including those from our own research group, reported more recently.