Cu‐free Sonogashira Type Cross‐Coupling of 6‐Halo‐2‐cyclopropyl‐3‐(pyridyl‐3‐ylmethyl) Quinazolin‐4(3H)‐ones as Potential Antimicrobial Agents

C(sp)–C(sp2) bond formation via Sonogashira cross‐coupling reactions on 6‐halo‐2‐cyclopropyl‐3‐(pyridyl‐3‐ylmethyl)quinazolin‐4(3H )‐ones with appropriate alkynes was explored. Optimization of reaction conditions with various catalysts, ligands, bases, and solvents was conducted. The combination of PdCl₂(MeCN)₂ with X‐Phos proved to be the best metal–ligand system for this conversion in the presence of triethylamine (Et₃N) in tetrahydrofuran at room temperature for iodosubstrates, at 80°C for the bromosubstrates in 8 h, and also for the chlorosubstrates in 16 h. We also demonstrated synthesis of a successful diversity‐oriented synthesis library of highly functionalized quinazolinones via Cu‐free Sonogashira coupling of diverse aryl halides and azido‐alkyne (“click”) ligation reactions with substituted azides. The library exhibited significant antimicrobial activity when screened against several microorganisms.     

Protecting‐group‐free palladium‐catalyzed hydroxylation,C–O and C–N coupling of chiral 6‐bromo‐ and 6,6’‐dibromo‐ 1,1’‐binaphthols

A straightforward, protecting‐group‐free protocol for the synthesis of chiral 6‐substituted and 6,6’‐disubstituted binols (binol = 1,1’‐bi‐2‐naphthol) by palladium‐catalyzed hydroxylation, C–N and C–O coupling of chiral 6‐bromo‐ and 6,6’ ‐dibromo‐1,1’‐binaphthols is developed. The protecting group free palladium‐catalyzed hydroxylation, C–O and C–N cross‐coupling protocol affords a straightforward and general method for the synthesis of chiral 6‐substituted and 6,6’‐disubstituted binols with good yields, avoiding the tedious procedures of introduction and removal of protecting groups. Copyright © 2013 John Wiley & Sons, Ltd.     

Bioinspired Oxidative Cyclization of the Geissoschizine Skeleton for the Total Synthesis of (−)‐17‐nor‐Excelsinidine

We report the first total synthesis of (?)‐17‐nor‐excelsinidine, a zwitterionic monoterpene indole alkaloid that displays an unusual N4?C16 connection. Inspired by the postulated biosynthesis, we explored an oxidative coupling approach from the geissoschizine framework to forge the key ammonium–acetate connection. Two strategies allowed us to achieve this goal, namely an intramolecular nucleophilic substitution on a 16‐chlorolactam with the N4 nitrogen atom or a direct I₂‐mediated N4?C16 oxidative coupling from the enolate of geissoschizine.     

C–N Coupling of 1,2‐Dihydro‐2,2,4‐trimethylquinoline Derivatives via a Silver(I)‐Catalyzed Direct Functionalization of a C–H Bond

Two C,N‐linked dimeric 1,2‐dihydro‐2,2,4‐trimethylquinolines, namely 6‐chloro‐1‐(6‐chloro‐1,2‐dihydro‐2,2,4‐trimethylquinolin‐8‐yl)‐1,2‐dihydro‐2,2,4‐trimethylquinoline ( 3a ) and 6‐ethoxy‐1‐(6‐ethoxy‐1,2‐dihydro‐2,2,4‐trimethylquinolin‐8‐yl)‐1,2‐dihydro‐2,2,4‐trimethylquinoline ( 3b ), have been prepared through a silver‐catalyzed dimerization of their corresponding monomers. The effect of different silver salts on the reaction was also investigated, and the obtained results suggest that silver ions effectively catalyzed the formation of a C–N bond under these mild conditions. This represents one of the rare reports on the silver‐catalyzed C–N bond formation through a coupling of a secondary amine and an activated aromatic system, via a direct C–H functionalization. Theoretical studies showed that these dimeric structures favor a conformation in which their monomer units are oriented approximately perpendicular to each other, with an intramolecular hydrogen bond (N–H distance of 2.33 Å) forming between the hydrogen atom of the amine in one of the monomeric units and the tertiary nitrogen atom of the other one.     

Nickel‐ and Photoredox‐Catalyzed Cross‐Coupling Reactions of Aryl Halides with 4‐Alkyl‐1,4‐dihydropyridines as Formal Nucleophilic Alkylation Reagents

A combination of nickel and photoredox catalysts promoted novel cross‐coupling reactions of aryl halides with 4‐alkyl‐1,4‐dihydropyridines. 4‐Alkyl‐1,4‐dihydropyridines act as formal nucleophilic alkylation reagents through a photoredox‐catalyzed carbon–carbon (C?C) bond‐cleavage process. The present strategy provides an alternative to classical carbon‐centered nucleophiles, such as organometallic reagents.     

Naked and Self‐Clickable Propargylic‐Decorated Single‐Chain Nanoparticle Precursors via Redox‐Initiated RAFT Polymerization

Protection of acetylenic monomers is a common practice to avoid parasitic side reactions during polymerization. Herein, we report that redox‐initiated RAFT polymerization allows the direct, room temperature synthesis of a variety of single‐chain nanoparticle precursors (displaying narrow molecular weight dispersity, / = 1.12 –1.37 up to = 100 kDa) containing well‐defined amounts of naked, unprotected acetylenic functional groups available for rapid and quantitative intrachain cross‐linking via metal‐catalyzed carbon–carbon coupling (i.e., C–C “click” chemistry). To illustrate the useful “self‐clickable” character of the new unprotected acetylenic precursors, single‐chain nanoparticles have been prepared for the first time in a facile and highly efficient manner by copper‐catalyzed alkyne homocoupling (i.e., Glaser–Hay coupling) at room temperature under normal air atmosphere.     

Tetraarylethylenes from Diarylmethanones and Hexachlorodisilane: The “Sila‐McMurry” Reaction

Hexachlorodisilane reacts with diarylmethanones (aryl=C₆H₅, 4‐MeC₆H₄, 4‐tBuC₆H₄, 4‐ClC₆H₄, 4‐BrC₆H₄) to furnish the corresponding tetraarylethylenes in good yields. The reductive conversion requires temperatures of about 160 °C and reaction times of 60–72 h. In the initial step, the Lewis‐basic carbonyl groups likely generate low‐valent [SiCl₂] as an analogue of [TiCl₂] in the classical McMurry reaction. The coupling sequence further proceeds via benzopinacolones, which have been isolated as key intermediates.     

ortho‐Phenylene‐Bridged Cyclic Oligopyrroles: Conformational Flexibilities and Optical Properties

ortho‐Phenylene‐bridged cyclic trimeric oligopyrrole C3 and hexameric oligopyrrole C6 were synthesized by Suzuki–Miyaura coupling reactions. The twisted structures of C3 and C6 were unambiguously revealed by X‐ray diffraction analysis. The optical properties of these cyclic oligopyrroles were compared with linear oligopyrrole L3 and cyclic tetramer C4 . The cyclic oligopyrroles exhibited large Stokes shifts and blue fluorescence with high quantum yields in solution and in the solid state. In addition, selective N‐methylation and N‐tolylation of C3 were used to tune the optical and electrochemical properties by changing the molecular twists and conformational flexibilities. Throughout these studies, the structure–property relationship of these cyclic strained oligopyrroles has been illustrated as an interesting molecular motif for novel cyclic π‐conjugated systems.     

Synthesis of Methylene‐Bridged Biscarbazole Alkaloids by using an Ullmann‐type Coupling: First Total Synthesis of Murrastifoline‐C and Murrafoline‐E

We describe the total synthesis of methylene‐bridged biscarbazole alkaloids by using a late‐stage Ullmann‐type coupling of fully functionalised carbazole subunits. The carbazole derivatives were synthesised via a sequence of palladium(0)‐ and palladium(II)‐catalysed coupling reactions. Our approach has provided bismurrayafoline‐A, bismurrayafolinol, chrestifolines B–D, and the first total synthesis of murrastifoline‐C and murrafoline‐E.     

A highly selective synthesis of 1‐substituted (E)‐buta‐1,3‐dienes with 4,4,5,5‐tetramethyl‐2‐vinyl‐1,3,2‐dioxaborolane as building block

Highly selective synthesis of 1‐substituted (E)‐buta‐1,3‐dienes via palladium‐catalyzed Suzuki–Miyaura cross‐coupling of (E)‐alkenyl iodides with 4,4,5,5‐tetramethyl‐2‐vinyl‐1,3,2‐dioxaborolane ( 1 ) is reported. The vinylboronate pinacol ester ( 1 ) acts as a vinyl building block to show high chemoselectivity for the Suzuki–Miyaura pathway versus Heck coupling in the presence of biphasic conditions (Pd(PPh₃)₄, aqueous K₂CO₃, toluene and ethanol). Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis and characterization of novel 3,6‐di[3′,5′‐bis(trifluoromethyl)phenyl]pyromellitic dianhydride for polyimide synthesis

A novel dianhydride monomer, 3,6‐di[3′,5′‐bis(trifluoromethyl)phenyl]pyromellitic dianhydride (12FPMDA), was synthesized via a three‐step process: (1) the preparation of 3,5‐bis(trifluoromethyl)benzene boronic acid (6FBB) and 3,6‐dibromo‐1,2,4,5‐tetramethylbenzene (2B4MB) via Grignard and bromination reactions, respectively; (2) the Suzuki cross‐coupling reaction of 6FBB and 2B4MB, resulting in 3,6‐di[3′,5′‐bis(trifluoromethyl)phenyl]tetramethylbenzene (12F4MB); and (3) the oxidation and cyclodehydration of 12F4MB to afford 12FPMDA. 12FPMDA was then characterized by Fourier transform infrared (FTIR), ¹H NMR, ¹⁹F NMR, elemental analysis, and a melting‐point apparatus, and it was used to prepare polyimides with aromatic diamines such as 1,1‐bis(4‐aminophenyl)‐2,2,2‐trifluoroethane and 4,4′‐diaminodiphenylether. Polyimides were synthesized via a two‐step process: (1) the preparation of poly(amic acid) in p‐chlorophenol with isoquinoline and (2) solution imidization at the reflux temperature for 12 h. They were designed to have molecular weights of 20,000 g/mol via off‐stoichiometry. The resulting polyimides were characterized by FTIR, NMR, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis, and their solubility, solution viscosity, water absorption, coefficients of thermal expansion (CTEs), and dielectric constants were also evaluated. The polyimides exhibited excellent solubility even in acetone and toluene, high glass‐transition temperatures (>311 °C), good thermal stability (>518 °C in air), and well‐controlled molecular weights (19,000–21,000 g/mol). They also provided low CTEs (35–50 ppm/°C), water absorption (1.26–1.35 wt %), and dielectric constants (2.49–2.52). © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4217–4227, 2002     

Microwave‐Promoted Pd‐Catalyzed Synthesis of Dibenzofurans from Ortho‐Arylphenols

ortho‐Aryl phenols, synthesized via protecting group free Suzuki–Miyaura coupling of ortho‐halophenols and arene boronic acids, undergo a cyclization to dibenzofurans via oxidative C–H activation. The reaction proceeds under microwave irradiation in short reaction times using catalytic amounts of Pd(OAc)₂ without additional ligands.     

A Novel Synthesis of 1‐Aryl‐6‐bromo‐3‐(5‐ethylthio‐4‐phenyl‐4H‐1,2,4‐triazol‐3‐yl)‐1H‐pyrazolo[4,3‐b]quinolin‐9(4H)‐ones via Thermal Cyclization of 4‐Azidopyrazoles

2‐Aryl‐hydrazononitriles 3a , 3b , 3c were prepared by coupling 3‐ethylthio‐5‐cyanomethyl‐4‐phenyl‐1,2,4‐triazole ( 1 ) with diazonium salts 2a , 2b , 2c . Reacting 3a , 3b , 3c with both ethyl bromoacetate ( 4a ) and 4‐bromobenzyl bromide ( 4b ) in DMF, in the presence of K₂CO₃, at 80 °C for 3–4 h, gave the corresponding 4‐amino‐pyrazoles 6a , 6b , 6c , 6d , 6e , 6f . Diazotization of 6a , 6b , 6c , 6d , 6e , 6f , followed by reaction with NaN₃, leads to the formation of 4‐azidopyrazoles 8a , 8b , 8c , 8d , 8e , 8f , a new heterocyclic ring system. Interestingly, fusion of 4‐azidopyrazoles 8d , 8e , 8f at temperature higher than their melting points with 5 °C for 2 min did not give the expected fused pyrazolo[4,3‐c]isoxazoles 9 but furnished instead the novel pyrazolo[4,3‐b]quinolinones 10a , 10b , 10c , in high yields.     

Enzyme‐Mediated,Site‐Specific Protein Coupling Strategies for Surface‐Based Binding Assays

Covalent surface immobilization of proteins for binding assays is typically performed non‐specifically via lysine residues. However, receptors that either have lysines near their binding pockets, or whose presence at the sensor surface is electrostatically disfavoured, can be hard to probe. To overcome these limitations and to improve the homogeneity of surface functionalization, we adapted and optimized three different enzymatic coupling strategies (4′‐phosphopantetheinyl transferase, sortase A, and asparaginyl endopeptidase) for biolayer interferometry surface modification. All of these enzymes can be used to site‐specifically and covalently ligate proteins of interest via short recognition sequences. The enzymes function under mild conditions and thus immobilization does not affect the receptors’ functionality. We successfully employed this enzymatic surface functionalization approach to study the binding kinetics of two different receptor–ligand pairs.     

Enzyme‐Mediated,Site‐Specific Protein Coupling Strategies for Surface‐Based Binding Assays

Covalent surface immobilization of proteins for binding assays is typically performed non‐specifically via lysine residues. However, receptors that either have lysines near their binding pockets, or whose presence at the sensor surface is electrostatically disfavoured, can be hard to probe. To overcome these limitations and to improve the homogeneity of surface functionalization, we adapted and optimized three different enzymatic coupling strategies (4′‐phosphopantetheinyl transferase, sortase A, and asparaginyl endopeptidase) for biolayer interferometry surface modification. All of these enzymes can be used to site‐specifically and covalently ligate proteins of interest via short recognition sequences. The enzymes function under mild conditions and thus immobilization does not affect the receptors’ functionality. We successfully employed this enzymatic surface functionalization approach to study the binding kinetics of two different receptor–ligand pairs.     

Nickel‐catalyzed Kumada Cross‐coupling Reaction for the Synthesis of 2,4‐Diarylquinazolines

A series of 2,4‐diarylquinazolines have been successfully synthesized via the Ni‐catalyzed cross‐coupling reaction of quinazoline‐4‐tosylates and aryl Grignard reagents, which provided alternative straightforward approaches for the introduction of aryl groups to quinazolines at C‐4 position.     

On‐Surface Synthesis of Cumulene‐Containing Polymers via Two‐Step Dehalogenative Homocoupling of Dibromomethylene‐Functionalized Tribenzoazulene

Cumulene compounds are notoriously difficult to prepare and study because their reactivity increases dramatically with the increasing number of consecutive double bonds. In this respect, the emerging field of on‐surface synthesis provides exceptional opportunities because it relies on reactions on clean metal substrates under well‐controlled ultrahigh‐vacuum conditions. Here we report the on‐surface synthesis of a polymer linked by cumulene‐like bonds on a Au(111) surface via sequential thermally activated dehalogenative C?C coupling of a tribenzoazulene precursor equipped with two dibromomethylene groups. The structure and electronic properties of the resulting polymer with cumulene‐like pentagon–pentagon and heptagon–heptagon connections have been investigated by means of scanning probe microscopy and spectroscopy methods and X‐ray photoelectron spectroscopy, complemented by density functional theory calculations. Our results provide perspectives for the on‐surface synthesis of cumulene‐containing compounds, as well as protocols relevant to the stepwise fabrication of carbon–carbon bonds on surfaces.     

Transition‐Metal‐Catalyzed Redox‐Neutral and Redox‐Green C–H Bond Functionalization

Transition‐metal‐catalyzed C–H bond functionalization has become one of the most promising strategies to prepare complex molecules from simple precursors. However, the utilization of environmentally unfriendly oxidants in the oxidative C–H bond functionalization reactions reduces their potential applications in organic synthesis. This account describes our recent efforts in the development of a redox‐neutral C–H bond functionalization strategy for direct addition of inert C–H bonds to unsaturated double bonds and a redox‐green C–H bond functionalization strategy for realization of oxidative C–H functionalization with O₂ as the sole oxidant, aiming to circumvent the problems posed by utilizing environmentally unfriendly oxidants. In principle, these redox‐neutral and redox‐green strategies pave the way for establishing new environmentally benign transition‐metal‐catalyzed C–H bond functionalization strategies.     

Diversification of ortho‐Fused Cycloocta‐2,5‐dien‐1‐one Cores and Eight‐ to Six‐Ring Conversion by σ Bond C−C Cleavage

Sequential treatment of 2‐C₆H₄Br(CHO) with LiC≡CR¹ (R¹=SiMe₃, tBu), nBuLi, CuBr?SMe₂ and HC≡CCHClR² [R²=Ph, 4‐CF₃Ph, 3‐CNPh, 4‐(MeO₂C)Ph] at ?50 °C leads to formation of an intermediate carbanion (Z)‐1,2‐C₆H₄{C_A(=O)C≡C_BR¹}{CH=CH(CH^?)R²} ( 4 ). Low temperatures (?50 °C) favour attack at C_B leading to kinetic formation of 6,8‐bicycles containing non‐classical C‐carbanion enolates ( 5 ). Higher temperatures (?10 °C to ambient) and electron‐deficient R² favour retro σ‐bond C?C cleavage regenerating 4 , which subsequently closes on C_A providing 6,6‐bicyclic alkoxides ( 6 ). Computational modelling (CBS‐QB3) indicated that both pathways are viable and of similar energies. Reaction of 6 with H⁺ gave 1,2‐dihydronaphthalen‐1‐ols, or under dehydrating conditions, 2‐aryl‐1‐alkynylnaphthlenes. Enolates 5 react in situ with: H₂O, D₂O, I₂, allylbromide, S₂Me₂, CO₂ and lead to the expected C ‐E derivatives (E=H, D, I, allyl, SMe, CO₂H) in 49–64 % yield directly from intermediate 5 . The parents (E=H; R¹=SiMe₃, tBu; R²=Ph) are versatile starting materials for NaBH₄ and Grignard C=O additions, desilylation (when R¹=SiMe) and oxime formation. The latter allows formation of 6,9‐bicyclics via Beckmann rearrangement. The 6,8‐ring iodides are suitable Suzuki precursors for Pd‐catalysed C?C coupling (81–87 %), whereas the carboxylic acids readily form amides under T3P® conditions (71–95 %).     

5‐Unsubstituted Pyrido[3,2,1‐jk]carbazol‐6‐ones: Syntheses,Substitution, and Cyclization Reactions

The synthesis of the 5‐unsubstituted pyrido[3,2,1‐jk]carbazol‐6‐one 4 can be achieved by the reaction of carbazole ( 1 ) and malonate derivatives, either in a three‐step synthesis via 5‐acetyl‐pyridocarbazolone 3 or in a one‐step reaction from 1 and malonic acid/phosphoryl chloride. The 5‐acetyl derivative 5 can be transformed via a tosylate intermediate to 4‐azido‐pyridocarbazolone 11 , which cyclizes by thermal decomposition to the isoxazolo‐pyrido[3,2,1‐jk]carbazolone 12 . The thermolysis conditions were investigated by DSC. Nitration of pyridocarbazolone 4 and subsequent introduction of azide leads to azido derivative 23 , which cyclizes on thermolysis to furazan‐oxide derivative 24 . Again, the thermolysis conditions were investigated by DSC. 5‐Chloro‐5‐nitro‐pyrido[3,2,1‐jk]carbazole‐4,6‐dione, obtained from 4 by subsequent nitration and chlorination, forms by exchange of both 5‐substituents 5,5‐dihydroxy‐pyridocarbazoledione 17 , which acylates phenol to give 5‐hydroxy‐5‐(p‐hydroxyphenyl)‐pyridocarbazoledione 20 . Acid‐catalyzed cyclodehydration of 20 forms a highly fused benzofuro‐pyridocarbazole 21 . Another C–C coupling at position 5 starts from 4‐chloro‐5‐nitro‐pyridocarbazolone 22 and diethyl malonate 2a , which forms the diethyl (nitrocarbazolyl)malonate 25 . With dimethyl malonate 2c , the intermediate dimethyl (nitrocarbazolyl)malonate gives on thermolysis the (nitrocarbazolyl)acetate 27 by loss of one ester group.