Cyclization by Intramolecular Suzuki-Miyaura Cross-Coupling-A Review

Ring systems of all sizes are frequent core or substructures in natural products and they are important elements of many drug molecules, as they often confer high binding affinity to and selectivity for disease-relevant biological targets. A uniform key transformation in the synthesis of such structures is the cyclization step. Among the various approaches that have been developed for ring closure, the intramolecular Suzuki-Miyaura reaction has emerged as a powerful option for the construction of normal- and medium-sized rings as well as macrocycles, due to its stereospecificity, the mild reaction conditions, and the non-toxic nature of the boron by-products. In this review, we summarize the state-of-the-art of the application of intramolecular Suzuki-Miyaura cross-coupling reactions in the construction of (macro)cyclic frameworks of natural products and bioactive molecules of synthetic origin, covering (mostly) examples that have been reported since 2015. Target molecules prepared via intramolecular Suzuki-Miyaura cross-coupling as a key step range from natural products/natural product analogs to synthetic drug candidates, featuring ring sizes from 4 to ≫12. We highlight the utility, scope, and limitations of the reaction for different ring sizes and arrays of functional groups. Where possible, comparisons with other methods of cyclization are provided.

Rapid, Homogenous, B-Alkyl Suzuki-Miyaura Cross-Coupling of Boronic Esters

A rapid, anhydrous Suzuki-Miyaura cross-coupling of alkylboronic esters with aryl halides is described. Parallel experimentation revealed that the combination of AntPhos, an oxaphosphole ligand, neopentyldiol alkylboronic esters, and potassium trimethylsilanolate (TMSOK) enables successful cross-coupling. In general, reactions proceed in under 1 h with good yields and high linear/branched (l/b) selectivities. Crucially, two literature examples which previously took >20 h to reach completion were accomplished in a fraction of the time with the method described herein. Mechanistic studies revealed that the reaction proceeds through a stereoretentive pathway and identified the boronic ester skeleton as a predominant pathway for deleterious protodehalogenation.

Coordination of Pnictogenylboranes Towards Tl(I) Salts and a Tl- Mediated P-P Coupling

The coordination chemistry of only Lewis-base (LB)-stabilized pnictogenylboranes EH₂ BH₂ ⋅NMe₃ (E=P, As) towards Tl(I) salts has been studied. The reaction of Tl[BAr^Cl ] (BAr^Cl =[B(3,5-C₆ H₃ Cl₂ )₄ ]^- ) with the corresponding pnictogenylborane results in the formation of [Tl(EH₂ BH₂ ⋅NMe₃ )][BAr^Cl ] (1 a: E=P; 1 b: E=As). Whereas the Tl ion in 1 a/b is monocoordinated, the exchange of the weakly coordinating anion (WCA) in the Tl(I) salt leads to the formation of a trigonal pyramidal coordination mode at the Tl atom by coordination of three equivalents of EH₂ BH₂  ⋅ NMe₃ in [Tl(EH₂ BH₂  ⋅ NMe₃ )₃ ][WCA] (2 a: E=P, WCA=TEF^Cl ; 2 b: E=As, WCA=TEF) (TEF=[Al{OC(CF₃ )₃ }₄ ]^- , TEF^Cl =[Al{(OC(CF₃ )₂ (CCl₃ )}₄ ]^- ). Furthermore, by using two equivalents of PH₂ BH₂ ⋅NMe₃ , a Tl(I)-mediated P-P coupling takes place in CH₂ Cl₂ as solvent resulting in [Me₃ N⋅BH₂ PH₂ PHBH₂ ⋅NMe₃ ][WCA] (WCA=TEF, 3 a; BAr^Cl , 3 b; TEF^Cl , 3 c). In contrast, for the arsenic derivatives 1 b and 2 b, no coupling reaction is observed. The underlying chemical processes are elucidated by quantum chemical computations.

Synthesis and Bioactivity of Thiocarboxylic A and Derivatives

The Penicillium metabolite thiocarboxylic A (1a) and three close analogues were synthesized in 14 steps. The stereogenic elements were installed via stereoselective Sharpless epoxidation, (E)-selective reduction of a dibromide, and a Suzuki cross coupling. Thiocarboxylic A (1a) was obtained in 6% overall yield. The synthetic product and the natural isolate differed markedly in their specific rotations and antibiotic activities against Escherichia coli and Staphylococcus aureus. This modular synthetic route should be flexible enough to allow the synthesis of other natural and non-natural 3-methoxycarbonyldihydrofuran-4-ones for biological studies.

Direct access to various C3-substituted sialyl glycal derivatives from 3-iodo-sialyl glycals

A new and efficient method was developed for the synthesis of C3-substituted sialyl glycals that are useful for novel sialidase inhibitor discovery. This method was based on the cross-coupling reactions of 3-iodo-sialyl glycal methyl ester with boronic acids, alkenes and alkynes to directly introduce various functional groups to the sialyl glycal C3-position. A series of C3-aryl, alkyl, alkenyl, and alkynyl derivatives of sialyl glycal were efficiently and conveniently synthesized for the first time by this method, which has demonstrated its wide application scope.

The Stereochemical Course of Pd-Catalyzed Suzuki Reactions Using Primary Alkyltrifluoroborate Nucleophiles

Using deuterium-labeled stereochemical probes, we show that primary alkyltrifluoroborate nucleophiles undergo transmetalation to palladium exclusively via a stereoretentive pathway and that the resulting stereospecificity is broadly independent of electronic and steric effects. This stands in stark contrast to the stereochemical course of transmetalation for secondary alkyltrifluoroborates, which varies between net stereoretention and net stereoinversion depending upon the electronic properties of the supporting phosphine ligand, the electronic properties of the aryl electrophile, and the steric properties of the alkylboron nucleophile. In this study, we additionally show that the stereochemical course of transmetalation for secondary alkylboron reagents can be under reagent steric control, while no such steric control exists for analogous primary alkylboron nucleophiles. The combined study reveals fundamental mechanistic differences between transmetalations of primary and secondary alkylboron reagents in Pd-catalyzed Suzuki reactions.

Samarium(iii) catalyzed synthesis of alkenylboron compounds via hydroboration of alkynes

The homoleptic lanthanide complex Sm[N(TMS)₂]₃ is an efficient rare-earth catalyst for the hydroboration of alkynes to the corresponding alkenylboron compounds.

Enantioselective Preparation of Arenes with β-Stereogenic Centers: Confronting the 1,1-Disubstituted Olefin Problem Using CuH/Pd Cooperative Catalysis

Arenes with β-stereogenic centers are important substructures in pharmaceuticals and natural products. We have developed an asymmetric anti-Markovnikov hydroarylation of 1,1-disubstituted olefins by dual palladium and copper hydride catalysis as a convenient and general approach to access these substructures. This efficient one-step process addresses several limitations of the traditional stepwise approaches. The use of cesium benzoate as a base and a common phosphine ligand for both the Cu- and Pd-catalyzed processes were important discoveries that allow these challenging olefin substrates to be efficiently transformed. A variety of aryl bromide coupling partners, including numerous heterocycles, were coupled with 1,1-disubstituted alkenes to generate arenes with β-stereogenic centers.

Recent Advances in Metal-Catalyzed Alkyl–Boron (C(sp3)–C(sp2)) Suzuki-Miyaura Cross-Couplings

Boron chemistry has evolved to become one of the most diverse and applied fields in organic synthesis and catalysis. Various valuable reactions such as hydroborylations and Suzuki–Miyaura cross-couplings (SMCs) are now considered as indispensable methods in the synthetic toolbox of researchers in academia and industry. The development of novel sterically- and electronically-demanding C(sp3)–Boron reagents and their subsequent metal-catalyzed cross-couplings attracts strong attention and serves in turn to expedite the wheel of innovative applications of otherwise challenging organic adducts in different fields. This review describes the significant progress in the utilization of classical and novel C(sp3)–B reagents (9-BBN and 9-MeO-9-BBN, trifluoroboronates, alkylboranes, alkylboronic acids, MIDA, etc.) as coupling partners in challenging metal-catalyzed C(sp3)–C(sp2) cross-coupling reactions, such as B-alkyl SMCs after 2001.

Recent Advances in the Hofmann Rearrangement and Its Application to Natural Product Synthesis

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C-N bond formation reactions are the most important transformations in (bio)organic chemistry because of the widespread occurrence of amines in pharmaceuticals, natural products, and biologically active compounds. The Hofmann rearrangement is a well-known method used for the preparation of primary amines from amides. But, the traditional version of the Hofmann rearrangement often gave relatively poor yields due to over-oxidation or due to the poor solubility of some amides in aqueous base, and created an enormous amount of waste products. Developments over the last two decades, in particular, have focused on refining both of these factors affecting the reaction. This review covers both the description of recent advances (2000-2019) in the Hofmann rearrangements and its applications in the synthesis of heterocycles, natural products and complex molecules of biological interest. It is revealed that organo-catalytic systems especially hypervalent iodine-based catalysts have been developed for the green and environmentally friendly conversion of carboxamides to primary amines and carbamates.

Studies toward the Total Synthesis of the Marine Macrolide Salarin C

A convergent strategy for the synthesis of dideoxysalarin C (3) as a potential intermediate for the total synthesis of the marine macrolide salarin C (1) is described. The macrolactone core of 3 was assembled by Suzuki coupling between alkyl iodide 9 and vinyl iodide 8 and Shiina macrolactonization as key transformations. All macrocyclic intermediates were found to be of low stability.

Total Syntheses of Thailanstatins A-C, Spliceostatin D, and Analogues Thereof. Stereodivergent Synthesis of Tetrasubstituted Dihydro- and Tetrahydropyrans and Design, Synthesis, Biological Evaluation, and Discovery of Potent Antitumor Agents

Efficient and selective total syntheses of spliceosome modulating natural products thailanstatins A-C and spliceostatin D are reported. A number of stereoselective methods for the construction of various tetrasubstituted dihydro- and tetrahydropyrans were developed as a prerequisite for the syntheses of these naturally occurring molecules and variations thereof. The pyran-forming reactions utilize a Heck/Saegusa-Ito cascade sequence to generate hydroxy α,β,γ,δ-unsaturated aldehyde precursors followed by a catalyst-controlled oxa-Michael cyclization to furnish tetrasubstituted dihydropyrans with high stereocontrol. Subsequent optimized homogeneous or heterogeneous hydrogenations of these dihydropyran systems afford their tetrahydropyran counterparts, also in a highly stereoselective manner. The synthesized thailanstatins and related analogues were biologically evaluated for their cytotoxic properties, leading to the identification of a number of compounds with exceptionally potent antitumor activities suitable for further development as potential antibody-drug conjugate payloads, single drugs, or drug combinations for cancer therapies. Important structure-activity relationships within the thailanstatin family and structurally related compounds are discussed and are expected to be path-pointing for future studies.

4-Cyanopyridine-catalyzed anti-Markovnikov selective hydroboration of alkenes

4-Cyanopyridine catalyzed anti-Markovnikov selective hydroboration of alkenes with B₂pin₂ affords the corresponding alkylboronates in good yields via a radical pathway.

Copper-catalyzed borylative coupling of vinylazaarenes and N-Boc imines

Diastereoselective Cu-catalyzed three-component couplings of vinylazaarenes, B₂(pin)₂, and N-Boc imines are described.

Pd-catalyzed cross-coupling of 1,1-diborylalkanes with aryl triflates

The Pd-catalyzed synthesis of benzylboronic esters through coupling of aryl triflates with 1,1-diborylalkane under ambient conditions is described.

FeCl2-catalyzed hydroboration of aryl alkenes with bis(pinacolato)diboron

Alkylboronates were synthesized by a ligand-free ferrous chloride catalyzed anti-Markovnikov hydroboration of un-activated aryl alkenes with bis(pinacolato)diboron.

Catalytic, enantioselective, intramolecular carbosulfenylation of olefins. Preparative and stereochemical aspects

The first catalytic, enantioselective, intramolecular carbosulfenylation of isolated alkenes with aromatic nucleophiles is described. The combination of N-phenylsulfenylphthalimide, a chiral selenophosphoramide derived from BINAM, and ethanesulfonic acid as a cocatalytic Brønsted acid induced an efficient and selective cyclofunctionalization of various alkenes (aliphatic and aromatic) tethered to a 3,4-methylenedioxyphenyl ring. Under these conditions, 6-phenylthio-5,6,7,8-tetrahydronaphthalenes are formed diastereospecifically in good yields (50-92%) and high enantioselectivities (71:29-97:3 er). E-Alkenes reacted much more rapidly and with much higher selectivity than Z-alkenes, whereas electron-rich alkenes reacted more rapidly but with comparable selectivity to electron-neutral alkenes and electron-deficient alkenes. The Brønsted acid played a critical role in effecting reproducible enantioselectivity. A model for the origin of enantioselectivity and the dependence of rate and selectivity on alkene structure is proposed along with a rationale for the site selectivity in reactions with monoactivated arene nucleophiles.

Asymmetric synthesis from terminal alkenes by cascades of diboration and cross-coupling

Terminal, monosubstituted alkenes are ideal prospective starting materials for organic synthesis because they are manufactured on very large scales and can be functionalized via a broad range of chemical transformations. Alkenes also have the attractive feature of being stable in the presence of many acids, bases, oxidants and reductants. In spite of these attributes, relatively few catalytic enantioselective transformations have been developed that transform aliphatic α-olefins into chiral products with an enantiomeric excess greater then 90 per cent. With the exception of site-controlled isotactic polymerization of α-olefins, none of these catalytic enantioselective processes results in chain-extending carbon-carbon bond formation to the terminal carbon. Here we describe a strategy that directly addresses this gap in synthetic methodology, and present a single-flask, catalytic enantioselective conversion of terminal alkenes into a number of chiral products. These reactions are facilitated by a neighbouring functional group that accelerates palladium-catalysed cross-coupling of 1,2-bis(boronates) relative to non-functionalized alkyl boronate analogues. In tandem with enantioselective diboration, this reactivity feature transforms alkene starting materials into a diverse array of chiral products. We note that the tandem diboration/cross-coupling reaction generally provides products in high yield and high selectivity (>95:5 enantiomer ratio), uses low loadings (1-2 mol per cent) of commercially available catalysts and reagents, offers an expansive substrate scope, and can address a broad range of alcohol and amine synthesis targets, many of which cannot be easily addressed with current technology.

Chemo-, regio-, and stereoselective iron-catalysed hydroboration of alkenes and alkynes

The highly chemo-, regio-, and stereoselective synthesis of alkyl- and vinyl boronic esters with good functional group tolerance has been developed using in situ activation of a bench-stable iron(II) pre-catalyst and pinacolborane (16 examples, 45-95% yield, TOF up to 30,000 mol h(-1)). The first iron-catalysed alkene hydrogermylation is also reported.

Chemoselective and regiospecific Suzuki coupling on a multisubstituted sp(3)-carbon in 1,1-diborylalkanes at room temperature

The palladium-catalyzed Suzuki-Miyaura cross-coupling on a multisubstituted sp(3)-carbon in 1,1-diborylalkanes was achieved at room temperature. The generation of a monoborate intermediate by virtue of the adjacent B atom could result in the chemoselective coupling reaction under ambient conditions.

31P NMR study of the stoichiometry, stability and thermodynamics of complex formation between palladium(II) acetate and bis(diphenylphosphino)ferrocene

The complexation reaction between palladium (II) acetate, and 1,1'-bis(diphenylphosphino)ferrocene, DPPF, was investigated in two different deuterated solvents CDCl(3) and DMSO at various temperatures using (31)P NMR spectroscopy. The exchange between free and complexed DPPF is slow on the NMR time scale and consequently, two (31)P NMR signals were observed. At metal ion-to-ligand mole ratio larger than 1, only one (31)P NMR signal was observed, indicating the formation of a 1:1 Pd(2+)-DPPF complex in solution. The formation constant of the resulting 1:1 complexes was determined from the integration of two (31)P signals. The values of the thermodynamic parameters (DeltaH, DeltaS and DeltaG(298)) for complexation were determined from the temperature dependence of stability constants. It was found that, in both solvents, the resulting complex is mainly entirely enthalpy stabilized and the DeltaH compensates the TDeltaS contribution.

Suzuki-Miyaura cross-coupling reactions of primary alkyltrifluoroborates with aryl chlorides

Parallel microscale experimentation was used to develop general conditions for the Suzuki-Miyaura cross-coupling of diversely functionalized primary alkyltrifluoroborates with a variety of aryl chlorides. These conditions were found to be amenable to coupling with aryl bromides, iodides, and triflates as well. The conditions that were previously identified through similar techniques to promote the cross-coupling of secondary alkyltrifluoroborates with aryl chlorides were not optimal for the primary alkyltrifluoroborates, thus demonstrating the value of parallel experimentation to develop novel, substrate specific results.