One-Pot Reactions Accelerate the Synthesis of Active Pharmaceutical Ingredients

Multifunctional Metal-Organic Frameworks as Catalysts for Tandem Reactions

Owing to well-defined structure as well as easy synthesis and modification, metal-organic frameworks (MOFs) have emerged as promising catalysts for tandem reactions. In this article, we aim to summarize the development of multifunctional MOFs, including mixed metal MOFs, MOFs that are synergistically catalyzed by metal nodes and organic linkers, MOFs loaded with metal nanoparticles, etc, as heterogenous catalysts for tandem reactions over the past five years. This concept briefly discusses on present challenges, future trends, and prospects of multifunctional MOFs catalysts in tandem reactions.

Multistep Cascade Catalytic Reactions Employing Bifunctional Framework Compounds

Multistep cascade reactions are important to achieve atom as well as step economy over conventional synthesis. This approach, however, is limited due to the incompatibility of the available reactive centers in a catalyst. In the present study, new MOF compounds, [Zn₂(SDBA)(3-ATZ)₂]·solvent, I and II, with tetrahedral Zn centers as good Lewis acidic sites and the free amino group of the 3-amino triazole ligand as a strong Lewis base center were shown to perform 4-step cascade/tandem reaction in a facile manner. Effective conversion of benzaldehyde dimethyl acetal in the presence of excess nitromethane at 100 °C in water to 1-(1,3-dinitropropan-2-yl) benzene was achieved in 10 h with yields of ∼95% (I) and ∼94% (II). This 4-step cascade reaction proceeds via deacetalization (Lewis acid), Henry (Lewis base), and Michael (Lewis base) reactions. The present work highlights the importance of spatially separated functional groups in multistep tandem catalysis─the examples of which are not common.

One-Pot Synthesis of Unprotected 2-Acylpyrroles from 1,2,3-Triazoles and 2-Hydroxymethylallyl Carbonates

An efficient, tandem one-pot approach to synthesize multisubstituted 2-acylpyrroles from readily prepared N-tosyl triazoles and 2-hydroxymethylallyl carbonates is reported. The reaction proceeds via Rh(II)-catalyzed O-H insertion, [3,3]-sigmatropic rearrangement, Pd(0)-catalyzed oxidative addition, intramolecular cyclization, DBU-promoted E1cB elimination, double bond isomerization, and aromatization, enabling the disconnection and formation of multiple bonds in one reactor. The approach represents a highly regioselective way to access di-, tri-, and tetra-substituted NH pyrroles with high efficiency.

Epoxides: Small Rings to Play with under Asymmetric Organocatalysis

Optically pure epoxides are recognized as highly valuable products and key intermediates, useful in different areas from pharmaceutical and agrochemical industries to natural product synthesis and materials science. The predictable fate of the ring-opening process, in terms of stereoselectivity and often of regioselectivity, enables useful functional groups to be installed at vicinal carbon atoms in a desired manner. In this way, products of widespread utility either for synthetic applications or as final products can be obtained. The advent of asymmetric organocatalysis provided a new convenient tool, not only for their preparation but also for the elaboration of this class of heterocycles. In this review, we focus on recent developments of stereoselective organocatalytic ring-opening reactions of meso-epoxides, kinetic resolution of racemic epoxides, and Meinwald-type rearrangement. Examples of asymmetric organocatalytic processes toward specific synthetic targets, which include ring opening of an epoxide intermediate, are also illustrated.

Synthesis of 3-epi-Hypatulin B Featuring a Late-Stage Photo-Oxidation in Flow

A synthesis of 3-epi-hypatulin B, a highly oxygenated and densely functionalized bicyclic scaffold, is reported. The carbon skeleton was prepared by functionalization of a cyclopentanone and an intramolecular Mukaiyama aldol reaction. Highlights include a late-stage photo-oxidation of a methoxyallene to provide an ester group. The problems encountered in the batch process were solved by translation into a flow protocol. Our synthesis highlights the value of flow chemistry to enable challenging late-stage transformations in natural product synthesis.

In-Situ Bromination Enables Formal Cross-Electrophile Coupling of Alcohols with Aryl and Alkenyl Halides

Although alcohols are one of the largest pools of alkyl substrates, approaches to utilize them in cross-coupling and cross-electrophile coupling are limited. We report the use of 1° and 2° alcohols in cross-electrophile coupling with aryl and vinyl halides to form C(sp³)-C(sp²) bonds in a one-pot strategy utilizing a very fast (<1 min) bromination. The reaction's simple benchtop setup and broad scope (42 examples, 56% ± 15% ave yield) facilitates use at all scales. The potential in parallel synthesis applications was demonstrated by successfully coupling all combinations of 8 alcohols with 12 aryl cores in a 96-well plate.

Combining flow synthesis and heterogeneous catalysis for the preparation of conjugated polymers

We report the first successful synthesis of a conjugated polymer using a heterogeneous palladium catalyst in a flow system. The resulting polymer with an Mn of 13.6 kDa and high reproducibility shows the great potential of this system.

An Iterative Phosphate Tether Mediated Approach for the Synthesis of Complex Polyol Subunits

A pot-economical approach to advanced polyol subunits is reported. The key reactions involved are iterative use of a phosphate tether-mediated one-pot sequential RCM/CM/H₂ with subsequent utilization of either a regio-/diasteroselective cuprate addition or a Pd-catalyzed reductive allylic transposition. This method highlights the asymmetric synthesis of 12 complex polyol subunits in 4-6 one-pot sequential operations with a total of 12-14 reactions, of which 4-5 are catalytic, with minimal workup and purification procedures.

Catalytic Enantioselective Access to Dihydroquinoxalinones via Formal α-Halo Acyl Halide Synthon in One Pot

An enantioselective one-pot catalytic strategy to dihydroquinoxalinones, featuring novel 1-phenylsulfonyl-1-cyano enantioenriched epoxides as masked α-halo acyl halide synthons, followed by a domino ring-opening cyclization (DROC), is documented. A popular quinine-derived urea served as the catalyst in two out of the three steps performed in the same solvent using commercially available aldehydes, (phenylsulfonyl)acetonitrile, cumyl hydroperoxide and 1,2-phenylendiamines. Medicinally relevant 3-aryl/alkyl-substituted heterocycles are isolated in generally good to high overall yield and high enantioselectivity (up to 99 % ee). A rare example of excellent reusability of an organocatalyst at higher scale, subjected to oxidative conditions, is demonstrated. Mechanistically, labile α-ketosulfone has been detected as the intermediate involved in the DROC process. Theoretical calculations on the key epoxidation step rationalize the observed stereocontrol, highlighting the important role played by the sulfone group.

Synthesis of 2-Methylquinoxaline Derivatives from Glycerol and Diamines Catalyzed by Iridium Complexes Bearing an N-Heterocyclic Carbene Ligand

2-Methylquinoxaline derivatives are widely used as intermediates in the synthesis of pharmaceuticals, natural products, and dyes; however, their syntheses usually require excess reagents, making them environmentally burdensome. Meanwhile, glycerol can be sustainably obtained in large quantities as a by-product in the production of biodiesel fuel using waste oil as a raw material. Thus, it is worthwhile to develop a new catalytic system that utilizes glycerol as a C3 source. In this study, an efficient catalytic system was developed to obtain 2-methylquinoxaline derivatives from glycerol and 1,2-phenylenediamines. This system is beneficial because it is environmentally friendly and has excellent atom efficiency.

Regio-controllable Cobalt-Catalyzed Sequential Hydrosilylation/Hydroboration of Arylacetylenes

Regiodivergent addition reactions provide straightforward and atom-economic approaches to access different regioisomers. However, the regio-chemistry control to access all the possible results is still challenging especially for the reaction involving multiple addition steps. Herein, we reported regio-controllable cobalt-catalyzed sequential hydrosilylation/hydroboration of arylacetylenes, delivering all the possible regio-outcomes with high regioselectivities (up to >20/1 rr for all the cases). Each regioisomer of value-added silylboronates could be efficiently and regioselectively obtained from the same materials. The adjustment of the ligands of cobalt catalysts combined with dual catalysis relay strategy is the key to achieve regio-chemistry control. This regio-controllable research might inspire the exploration of the diversity-oriented synthesis that involves multiple additions and provide full sets of regioisomers of other synthetic useful molecules.

Alcohol Dehydrogenases and N-Heterocyclic Carbene Gold(I) Catalysts: Design of a Chemoenzymatic Cascade towards Optically Active β,β-Disubstituted Allylic Alcohols

The combination of gold(I) and enzyme catalysis is used in a two-step approach, including Meyer-Schuster rearrangement of a series of readily available propargylic alcohols followed by stereoselective bioreduction of the corresponding allylic ketone intermediates, to provide optically pure β,β-disubstituted allylic alcohols. This cascade involves a gold N-heterocyclic carbene and an enzyme, demonstrating the compatibility of both catalyst types in aqueous medium under mild reaction conditions. The combination of [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene][bis(trifluoromethanesulfonyl)-imide]gold(I) (IPrAuNTf₂ ) and a selective alcohol dehydrogenase (ADH-A from Rhodococcus ruber, KRED-P1-A12 or KRED-P3-G09) led to the synthesis of a series of optically active (E)-4-arylpent-3-en-2-ols in good yields (65-86 %). The approach was also extended to various 2-hetarylpent-3-yn-2-ol, hexynol, and butynol derivatives. The use of alcohol dehydrogenases of opposite selectivity led to the production of both allyl alcohol enantiomers (93->99 % ee) for a broad panel of substrates.

Microwave-assisted Synthesis of Novel Mannich Base and Conazole Derivatives Containing Biologically Active Pharmacological Groups

Background:

The aim of this study was to synthesize new mannich bases and conazol derivatives with biological activity by the microwave-assisted method.

Introduction:

1,2,4-Triazole-3-one (3) acquired from tryptamine was transformed to the corresponding carbox(thio)amides (6a-c) via several steps. Compounds 6a-c were refluxed with sodium hydroxide to yield 1,2,4-triazole derivatives (7a-c). Compounds 3 and 7a-c on treatment with different heterocyclic secondary amines in an ambiance with formaldehyde afforded the mannich bases 8-15 having diverse pharmacophore units with biologically active sites. The reaction of compound 3 and 2-bromo-1-(4-chlorophenyl) ethanone in the presence of sodium ethoxide gave the corresponding product 2-substituted-1,2,4-triazole-3-one, 16, which was reduced to 1,2,4-triazoles (17). Synthesis of compounds 18, 19, and 20 was carried out starting from compounds 17 with 4-chlorobenzyl chloride (for 18), 2,4-dichlorobenzyl chloride (for 19), and 2,6-dichlorobenzyl chloride (for 20).

Methods:

he conventional technique was utilized for the synthesis of compounds, 3-7, and microwave- assisted technique for the compounds, 8-20. That is, green chemistry techniques were applied during these reactions. The structures of molecules were elucidated on the foundation of 1H NMR, 13C NMR, FT-IR, EI-MS methods, and elemental analysis. Novel synthesized molecules were investigated for their antimicrobial activity using MIC (minimum inhibitory concentration) method.

Results:

Aminoalkylation of triazole derivatives 3 and 7a-c with fluoroquinolones such as ciprofloxacin and norfloxacin provided an enhancement to the bioactivity of mannich bases 8-11 against the tested microorganisms. The MIC values ranged between <0.24 and 3.9 μg/mL. Moreover, molecules 10 and 11 exhibited more effects on M. smegmatis than the other compounds by the MIC values of <1 μg/mL. They have shown very good antituberculosis activity.

Conclusion:

Most of the synthesized structures were observed to have excellent antimicrobial activity against most microorganisms taken into account. These molecules have better activity than the standard drug ampicillin and streptomycin.

In Pursuit of Synthetic Efficiency: Convergent Approaches

The field of total synthesis has reached a stage in which emphasis has been increasingly focused on synthetic efficiency rather than merely achieving the synthesis of a target molecule. The pursuit of synthetic efficiency, typically represented by step count and overall yield, is a rich source of inspiration and motivation for synthetic chemists to invent innovative strategies and methods. Among them, convergent strategy has been well recognized as an effective approach to improve efficiency. This strategy generally involves coupling of fragments with similar complexity to furnish the target molecule via subsequent cyclization or late-stage functionalization. Thus, methodologies that enable effective connection of fragments are critical to devising a convergent plan. In our laboratory, convergent strategy has served as a long-standing principle for pursuing efficient synthesis during the course of planning and implementing synthetic projects. In this Account, we summarize our endeavors in the convergent synthesis of natural products over the last ten years. We show how we identify reasonable bond disconnections and employ enabling synthetic methodologies to maximize convergency, leading to the efficient syntheses of over two-dozen highly complex molecules from eight disparate families.In detail, we categorize our work into three parts based on the diverse reaction types for fragment assembly. First, we demonstrate the application of a powerful single-electron reducing agent, SmI₂, in a late-stage cyclization step, forging the polycyclic skeletons of structurally fascinating Galbulimima alkaloids and Leucosceptrum sesterterpenoids. Next, we showcase how three different types of cycloaddition reactions can simultaneously construct two challenging C-C bonds in a single step, providing concise entries to three distinct families, namely, spiroquinazoline alkaloids, gracilamine, and kaurane diterpenoids. In the third part, we describe convergent assembly of ent-kaurane diterpenoids, gelsedine-type alkaloids, and several drug molecules via employing some bifunctional synthons. To access highly oxidized ent-kaurane diterpenoids, we introduce the hallmark bicyclo[3.2.1]octane ring system at an early stage, and then execute coupling and cyclization by means of a Hoppe's homoaldol reaction and a Mukaiyama-Michael-type addition, respectively. Furthermore, we showcase how the orchestrated combination of an asymmetric Michael addition, a tandem oxidation-aldol reaction and a pinacol rearrangement can dramatically improve the efficiency in synthesizing gelsedine-type alkaloids, with nary a protecting group. Finally, to address the supply issue of several drugs, including anti-influenza drug zanamivir and antitumor agent Et-743, we exploit scalable and practical approaches to provide advantages over current routes in terms of cost, ease of execution, and efficiency.

Pot-Economical Total Synthesis of Clinprost

The pot-economical synthesis of clinprost is reported, in which the core bicyclo[3.3.0]octenone structure was synthesized by two key steps: an asymmetric domino Michael/Michael reaction catalyzed by diphenylprolinol silyl ether and an intramolecular Horner-Wadsworth-Emmons reaction. The trisubstituted endocyclic alkene was selectively introduced by 1,4-reduction followed by trapping of the generated enolate with Tf₂NPh and subsequent utilization of the Suzuki-Miyaura coupling reaction. Chiral, nonracemic clinprost was synthesized in seven pots with a 17% total yield and excellent enantioselectivity.

One-Pot Cross-Coupling/C-H Functionalization Reactions: Quinoline as a Substrate and Ligand through N-Pd Interaction

Herein, we report a one-pot process that marries mechanistically distinct, traditional cross-coupling reactions with C-H functionalization using the same precatalyst. The reactions proceed in yields of up to 95%, in air, and require no extraneous ligand. The reactions are thought to be facilitated by harnessing the substrate quinoline as an N-ligand, and evidence of the palladium-quinoline interaction is provided by ¹H-¹⁵N HMBC NMR spectroscopy and X-ray crystallographic structures. Application of the methodology is demonstrated by the quick formation of fluorescent, π-extended frameworks.

Five-Step Synthesis of Yaequinolones J1 and J2

A concise synthesis of yaequinolones J1 and J2 is reported. The route is based on the aryne insertion into the σ-C-N bond of an unsymmetric imide followed by a diastereoselective aldol cyclization of the resulting N-acylated aminobenzophenone. The chromene motif is generated in the first step by an organocatalytic tandem Knoevenagel electrocyclization of citral and 2-bromoresorcinol. The approach adheres to the ideality principle, using almost exclusively strategic bond-forming reactions.

Recent advances in metal-catalysed asymmetric sequential double hydrofunctionalization of alkynes

Recent advances in various metal-catalysed asymmetric sequential double hydrofunctionalizations of alkynes have been highlighted in this feature article.

Multicomponent synthesis of diphenyl-1,3-thiazole-barbituric acid hybrids and their fluorescence property studies

A series of novel diphenyl-1,3-thiazole linked barbituric acid hybrids (4) were prepared by two catalyst-free methods from readily available starting materials.

Metal–organic frameworks as acid- and/or base-functionalized catalysts for tandem reactions

In this article, we have reviewed the development of MOFs anchored with acidic and/or basic sites as heterogeneous catalysts for tandem/cascade (domino) reactions over the past five years.

Multicatalysis Combining 3D-Printed Devices and Magnetic Nanoparticles in One-Pot Reactions: Steps Forward in Compartmentation and Recyclability of Catalysts

A tricatalytic compartmentalized system that immobilizes metallic species to perform one-pot sequential functionalization is described: a three-dimensional (3D)-printed palladium monolith, ferritic copper(I) magnetic nanoparticles, and a 3D-printed polypropylene capsule-containing copper(II) loaded onto polystyrene-supported 1,5,7-triazabicyclo[4.4.0]dec-5-ene (PS-TBD) allowed the rapid synthesis of diverse substituted 1-([1,1'-biphenyl]-4-yl)-1H-1,2,3-triazoles. The procedure is based on the Chan-Lam azidation/copper alkyne-azide cycloaddition/Suzuki reaction strategy in the solution phase. This catalytic system enabled the efficient assembly of the final compounds in high yields without the need for special additives or intermediate isolation. The monolithic catalyst-containing immobilized palladium species was synthesized by surface chemical modification of a 3D-printed silica monolith using a soluble polyimide resin as a key reagent, thus creating an extremely robust composite. All three immobilized catalysts described here were easily recovered and reused in numerous cycles. This work exemplifies the role of 3D printing in the design and manufacture of devices for compartmented multicatalytic systems to carry out complex one-pot transformations.

Rapid Assembly of Protoilludane Skeleton through Tandem Catalysis: Total Synthesis of Paesslerin A and Its Structural Revision

A multicomponent domino reaction involving three mechanistically distinct Tf₂NH-catalyzed reactions was developed. The reaction cascade enables the assembly of a skewed 5/6/4 tricyclic motif with migration of the reactive site with the assistance of a catalyst. The tricyclic product was used to achieve the first total synthesis of cytotoxic paesslerin A by regioselective C-H insertion of the sulfonyl carbenoid and base-promoted olefin isomerization. Our results led to the revision of the originally proposed tricyclic structure of paesslerin A.

Biomimetic Enantioselective Total Synthesis of (-)-Robustanoids A and B and Analogues

We report a step-economical, enantioselective total synthesis of (-)-robustanoid B and (-)-robustanoid A and four novel natural product-like compounds. Our strategy relied on our biosynthetic hypothesis and on a novel complexity generation methodology, namely, the one-pot hydroxylative double cyclization reaction. The latter consists of a modified 3,3-dimethyldioxirane-triggered epoxidation-epoxide-ring-opening cyclization reaction cascade and Trost's regioselectivity umpolung methodology ("anti-Michael addition").

A new efficient domino approach for the synthesis of coumarin-pyrazolines as antimicrobial agents targeting bacteriald-alanine-d-alanine ligase

The inhibition ofd-alanine-d-alanine ligase (Ddl) prevents bacterial growth, which makes this enzyme an attractive and viable target in the urgent search for novel effective antimicrobial drugs.

Synthesis of Quinolinone Alkaloids via Aryne Insertions into Unsymmetric Imides in Flow

A general strategy for the synthesis of 3,4-dioxygenated quinolin-2-one natural products is reported. The key step is a regioselective insertion of arynes into unsymmetric imides. When performed in continuous flow, the reaction proceeds within minutes, while lower yields and longer reaction times are observed in batch. The resulting N-acylated 2-aminobenzophenones were transformed to (±)-peniprequinolone, (±)-aflaquinolones E and F, (±)-6-deoxyaflaquinolone E, (±)-quinolinones A and B, and (±)-aniduquinolone C in 1-3 steps.

Enabling strategies for step efficient syntheses

The field of natural product total synthesis has reached the point where synthetic efficiency has become more important than merely defining a viable (yet less ideal) route to the target molecule. Synthetic efficiency is best represented by the number of steps it takes to finish the target molecule from readily available starting materials, as by reducing the number of steps, all other factors of synthetic efficiency are influenced positively. By comparing several total syntheses from the recent years, the most successful strategies for step efficient syntheses will be highlighted. Each synthesis will be presented using a color-coded synthetic flowchart, in which each step is categorized by a colored box. Five categories of transformations are defined and rated according to their synthetic value. Each class will be signified by different colors so that the reader can quickly see which parts of the synthesis are productive and those that are not.