A sequential Ugi multicomponent/Cu-catalyzed azide-alkyne cycloaddition approach for the continuous flow generation of cyclic peptoids

Visible Light Excitation of Poly-(para-Phenylene Ethynylene) Enables Heterogeneous Photocatalytic Oxidations of Amines in Flow

Heterogeneous visible light photocatalysis is a compelling approach to address sustainability in synthetic photochemistry. However, the use of solid-state photocatalysts remains very unpopular in organic synthesis because of their limited accessibility and the black-box effect associated to the lack of rational between their molecular structure and their photochemical properties. Herein, we disclose the synthesis, characterization, photocatalytic properties and synthetic applications of a simple and readily available solid-state conjugated organic polymer, poly-(para-phenylene ethynylene) 1, which exhibits a strong oxidative power upon irradiation with visible light (E(1*/1⋅-)=+1.67 V vs SCE). Comparisons with structural analogues highlighted the superior photocatalytic activity of this linear semiconductor, on account of its fully conjugated architecture. The associated excited-state reactivity enabled the transformation of various amines into imines in batch and continuous flow reactors together with straightforward photocatalyst recycling. Mechanistic investigations revealed concomitant photoredox and energy transfer pathways, that led to the formation of the desired products. Ultimately, the inline generation of imines was exploited in telescoped three-component Ugi reactions (3CR) in batch and flow toward biologically relevant α-acylaminoamides.

Intramolecular Click Cycloaddition Reactions: Synthesis of 1,2,3-Triazoles

Click Chemistry, as a powerful tool, has been used for the synthesis of a variety of 1,2,3-triazoles. Among click cycloaddition reactions, intramolecular click reactions carried out in azido-alkyne precursors has not been thoroughly reviewed. Hence, in this review, we have summarized and categorised the recent literature (from 2012 on) based on the azidoalkynyl precursor's type and a brief and concise description of the involved mechanisms is presented. Accordingly, we have classified the relevant literature into three categories: (1) substitution precursors (2) addition and (3) multi-component reaction (MCR) products.

Growing Impact of Intramolecular Click Chemistry in Organic Synthesis

Click Chemistry, a modular, rapid, and one of the most reliable tool for the regioselective 1,2,3-triazole forming [3+2] reaction of organic azide and terimal alkyne is widely explored in various emerging domains of research ranging from chemical biology to catalysis and medicinal chemistry to material science. This regioselective reaction from a diverse range of azido-alkyne scaffolds has been well performed in both intermolecular as well as intramolecular fashions. In comparison to the intermolecular metal (Cu/Ru/Ni) variant of 'Click Chemistry', the intramolecular click tool is little addressed. The intramolecular click chemistry is exemplified as a mordern tool of cyclization which involves metal-catalyzed (CuAAC/RuAAC) cyclization, organo-catalyzed cyclization, and thermal-induced topochemical reaction. Thus, we report herein the recent approaches on intramolecular azide-alkyne cycloaddition 'Click Chemistry' with their wide-spread emerging applications in the developement of a diverse range of molecules including fused-heterocycles, well-defined peptidomemics, and macrocyclic architectures of various notable features.

Advances in the Synthesis of Fused 1,2,3-Triazoles via a MCR-Intramolecular Azide-Alkyne Cycloaddition Approach

The present review narrates several reports which deal with the synthesis of fused 1,2,3-triazole containing scaffolds following a sequential multicomponent reaction (MCR)-intramolecular azide-alkyne cycloaddition (IAAC) approach. The reviewed reactions were cleverly designed so as to incorporate azide and alkyne functionalities in the MCR product which was then subjected to IAAC. The review is divided into two sections based on the number of components in the multicomponent reaction. We have aimed at a critical discussion and also have highlighted either advantages or disadvantages of each methodology.

Amide Bonds Meet Flow Chemistry: A Journey into Methodologies and Sustainable Evolution

Formation of amide bonds is of immanent importance in organic and synthetic medicinal chemistry. Its presence in "traditional" small-molecule active pharmaceutical ingredients, in linear or cyclic oligo- and polypeptidic actives, including pseudopeptides, has led to the development of dedicated synthetic approaches for the formation of amide bonds starting from, if necessary, suitably protected amino acids. While the use of solid supported reagents is common in traditional peptide synthesis, similar approaches targeting amide bond formation in continuous-flow mode took off more significantly, after a first publication in 2006, only a couple of years ago. Most efforts rely upon the transition of traditional approaches in flow mode, or the combination of solid-phase peptide synthesis principles with flow chemistry, and advantages are mainly seen in improving space-time yields. This Review summarizes and compares the various approaches in terms of basic amide formation, peptide synthesis, and pseudopeptide generation, describing the technological approaches and the advantages that were generated by the specific flow approaches. A final discussion highlights potential future needs and perspectives in terms of greener and more sustainable syntheses.

Impact of Microwaves on Organic Synthesis and Strategies toward Flow Processes and Scaling Up

Microwave-assisted organic synthesis has been widely studied and deliberated, opening up some controversial issues as well. Nowadays, microwave chemistry is a mature technology that has been well demonstrated in many cases with numerous advantages in terms of the reaction rate and yield. The strategies toward scaling up find an ally in continuous-flow reactor technology comparing dielectric and conductive heating.

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

Due to their intrinsic physical properties, which includes being able to perform as volatile liquids at room and biological temperatures, fragrance ingredients/intermediates make ideal candidates for continuous-flow manufacturing. This review highlights the potential crossover between a multibillion dollar industry and the flourishing sub-field of flow chemistry evolving within the discipline of organic synthesis. This is illustrated through selected examples of industrially important transformations specific to the fragrances and flavours industry and by highlighting the advantages of conducting these transformations by using a flow approach. This review is designed to be a compendium of techniques and apparatus already published in the chemical and engineering literature which would constitute a known solution or inspiration for commonly encountered procedures in the manufacture of fragrance and flavour chemicals.

1,2,3-Triazoles as Biomimetics in Peptide Science

Natural peptides are an important class of chemical mediators, essential for most vital processes. What limits the potential of the use of peptides as drugs is their low bioavailability and enzymatic degradation in vivo. To overcome this limitation, the development of new molecules mimicking peptides is of great importance for the development of new biologically active molecules. Therefore, replacing the amide bond in a peptide with a heterocyclic bioisostere, such as the 1,2,3-triazole ring, can be considered an effective solution for the synthesis of biologically relevant peptidomimetics. These 1,2,3-triazoles may have an interesting biological activity, because they behave as rigid link units, which can mimic the electronic properties of amide bonds and show bioisosteric effects. Additionally, triazole can be used as a linker moiety to link peptides to other functional groups.

Multicomponent synthesis and anti-proliferative screening of biaryl triazole-containing cyclophanes

We report a practical two-step approach involving a Ugi 4-CR/ azide-alkyne cycloaddition for the synthesis of biaryl-containing cyclophanes. The series represents an extension of our previously reported macrocycles as an effort to enhance the anti-proliferative activity of this scaffold. In this variant, we incorporate a biphenyl moiety in the framework, thus enhancing the macrocycle size, lipophilicity, and structural diversity. Macrocycles were tested against different cell lines, being more cytotoxic against prostate (PC-3 and DU-145) and breast (MCF-7) tumor cells. Gratifyingly, the most active compound showed a significative enhancement of PC-3 growth inhibition with respect to our previous series, reaffirming the potential anti-proliferative activity of this kind of cyclophanes.

Synthesis of Nβ-Protected Amino Sulfenyl Methyl Formamides and Sulfonyl Methyl Formamides: A Simple Protocol

Chiral amino acid-derived formamides represent one of the most versatile components in multicomponent reactions. Herein, we describe a facile synthesis of N^β-protected amino sulfenyl methyl formamides and sulfonyl methyl formamides via the Mannich reaction of N^α-protected amino alkyl thiols followed by oxidation using 3-chloroperbenzoic acid (m-CPBA). This protocol is applicable to a wide range of Fmoc- and Cbz-protected amino acids. Notably, the reaction provides high yield and retains the stereochemistry of the chiral center of the starting component.

1,3-Dipolar Cycloaddition Reactions of Nitrile Oxides under "Non-Conventional" Conditions: Green Solvents, Irradiation, and Continuous Flow

The 1,3-dipolar cycloaddition reactions (DCs) of nitrile oxides (NOs) to alkenes and alkynes are useful methods for the synthesis of 2-isoxazolines and isoxazoles respectively, which are important classes of heterocyclic compounds in organic and medicinal chemistry. Most of these reactions are carried out in organic solvents and under thermal activation. Nevertheless the use of supercritical carbon dioxide (scCO₂ ) and ionic liquids (Ils) as alternative solvents and the application of microwave (MW) and ultrasound (US) as alternative activation procedures have evident advantages from the "Green Chemistry" point of view. The critical discussion on the applications of these "unconventional" activation methods and reaction conditions in the 1,3-DCs of NOs is the objective of the present Review.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

As a versatile therapeutic modality, peptides attract much attention because of their great binding affinity, low toxicity, and the capability of targeting traditionally "undruggable" protein surfaces. However, the deficiency of cell permeability and metabolic stability always limits the success of in vitro bioactive peptides as drug candidates. Peptide macrocyclization is one of the most established strategies to overcome these limitations. Over the past decades, more than 40 cyclic peptide drugs have been clinically approved, the vast majority of which are derived from natural products. The de novo discovered cyclic peptides on the basis of rational design and in vitro evolution, have also enabled the binding with targets for which nature provides no solutions. The current review summarizes different classes of cyclic peptides with diverse biological activities, and presents an overview of various approaches to develop cyclic peptide-based drug candidates, drawing upon series of examples to illustrate each strategy.

Tri-fluid mixing in a microchannel for nanoparticle synthesis

It is becoming more difficult to use bulk mixing and bi-fluid micromixing in multi-step continuous-flow reactions, multicomponent reactions, and nanoparticle synthesis because they typically involve multiple reactants. To date, most micromixing studies, both passive and active, have focused on how to efficiently mix two fluids, while micromixing of three or more fluids together (multi-fluid mixing) has been rarely explored. This study is the first on tri-fluid mixing in microchannels. We investigated tri-fluid mixing in three microchannel models: a straight channel, a classical staggered herringbone mixing (SHM) channel, and a three-dimensional (3D) X-crossing microchannel. Numerical simulations and experiments were jointly conducted. A two-step experimental process was performed to determine the tri-fluid mixing efficiencies of these microchannels. We found that the SHM cannot significantly enhance mixing of three streams especially for a Reynolds number (Re) higher than 10. However, the 3D X-crossing channel based on splitting-and-recombination (SAR) showed effective tri-mixing performance over a wide Re range up to 275 (with a corresponding flow rate of 1972.5 μL min^-1), thereby enabling high microchannel throughput. Furthermore, this tri-fluid micromixing process was used to synthesize a kind of Si-based nanoparticle. This achieved a narrower particle size distribution than traditional bulk mixing. Therefore, SAR-based tri-fluid mixing is an alternative for chemical and biochemical reactions where three reactants need to be mixed.

A Mild, Fast, and Scalable Synthesis of Substituted α-Acyloxy Ketones via Multicomponent Reaction Using a Continuous Flow Approach

A continuous flow approach for the synthesis of α-acyloxy ketone derivatives from the corresponding arylglyoxals, isocyanides, and carboxylic acids is described. The target products were obtained in excellent yields in short residence times and with high purities via the first transcription of the microwave-to-flow paradigm to the isocyanide-based Passerini reaction. Furthermore, this methodology allowed a 10-fold scale-up using the same experimental conditions initially established.

Heteroaryl Rings in Peptide Macrocycles

This Review is devoted to the chemistry of macrocyclic peptides having heterocyclic fragments in their structure. These motifs are present in many natural products and synthetic macrocycles designed against a particular biochemical target. Thiazole and oxazole are particularly common constituents of naturally occurring macrocyclic peptide molecules. This frequency of occurrence is because the thiazole and oxazole rings originate from cysteine, serine, and threonine residues. Whereas other heteroaryl groups are found less frequently, they offer many insightful lessons that range from conformational control to receptor/ligand interactions. Many options to develop new and improved technologies to prepare natural products have appeared in recent years, and the synthetic community has been pursuing synthetic macrocycles that have no precedent in nature. This Review attempts to summarize progress in this area.

Protein-Catalyzed Capture Agents

Protein-catalyzed capture agents (PCCs) are synthetic and modular peptide-based affinity agents that are developed through the use of single-generation in situ click chemistry screens against large peptide libraries. In such screens, the target protein, or a synthetic epitope fragment of that protein, provides a template for selectively promoting the noncopper catalyzed azide-alkyne dipolar cycloaddition click reaction between either a library peptide and a known ligand or a library peptide and the synthetic epitope. The development of epitope-targeted PCCs was motivated by the desire to fully generalize pioneering work from the Sharpless and Finn groups in which in situ click screens were used to develop potent, divalent enzymatic inhibitors. In fact, a large degree of generality has now been achieved. Various PCCs have demonstrated utility for selective protein detection, as allosteric or direct inhibitors, as modulators of protein folding, and as tools for in vivo tumor imaging. We provide a historical context for PCCs and place them within the broader scope of biological and synthetic aptamers. The development of PCCs is presented as (i) Generation I PCCs, which are branched ligands engineered through an iterative, nonepitope-targeted process, and (ii) Generation II PCCs, which are typically developed from macrocyclic peptide libraries and are precisely epitope-targeted. We provide statistical comparisons of Generation II PCCs relative to monoclonal antibodies in which the protein target is the same. Finally, we discuss current challenges and future opportunities of PCCs.

Synthesis of macrocyclic peptidomimetics via the Ugi-click-strategy

The Ugi-click-strategy was employed for the synthesis of 12–28 membered 1,2,3-triazole derived macrocyclic peptidomimetics.

A methanol and protic ionic liquid Ugi multicomponent reaction path to cytotoxic α-phenylacetamido amides

The Ugi four component reaction of an aldehyde, amine, isocyanide and an ethanoic acid was effected smoothly in protic ionic liquids ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) to afford analogues of α-phenylacetamido amides in good to excellent isolated yields. The corresponding reactions in [BMIM][PF₆] and the protic ionic liquid ethanolammonium nitrate (ETAN) failed. Microwave irradiation in EAN facilitated rapid access to three focused libraries, based on the parent isocyanide: cyclohexyl isocyanide, benzyl isocyanide and ethyl isocyanoacetate. Analysis of the structure activity relationship data suggested the presence of a bulky moiety originating from the isocyanide (cyclohexyl and benzyl) enhanced cytotoxicity. Removal of the acetylenic H-atom from the ethanoic acid moiety was detrimental to cytotoxicity. The most active analogues produced, N-(2-cyclohexylamino)-1-(4-methoxyphenyl)-2-oxoethyl-N-(3,5-dimethoxyphenyl)propiolamide, returned average GI₅₀ values of ≤1 μM across the cancer cell lines evaluated. Combined, these data suggest that analogues of this nature are interesting potential anti-cancer development leads.

The Ugi reaction (aldehyde, amine, isocyanide and an ethanoic acid) in the protic ionic liquids ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) gave excellent yields of α-phenylacetamido amides.

In situ epoxide generation by dimethyldioxirane oxidation and the use of epichlorohydrin in the flow synthesis of a library of β-amino alcohols

The flow coupling of epichlorohydrin with substituted phenols, while efficient, limits the nature of the epoxide available for the development of focused libraries of β-amino alcohols. This limitation was encountered in the production of analogues of 1-(4-nitrophenoxy)-3-((2-((4-(trifluoromethyl)pyrimidin-2-yl)amino)ethyl)amino)propan-2-ol 1, a potential antibiotic lead. The in situ (flow) generation of dimethyldoxirane (DMDO) and subsequent flow olefin epoxidation abrogates this limitation and afforded facile access to structurally diverse β-amino alcohols. Analogues of 1 were readily accessed either via (i) a flow/microwave hybrid approach, or (ii) a sequential flow approach. Key steps were the in situ generation of DMDO, with olefin epoxidation in typically good yields and a flow-mediated ring opening aminolysis to form an expanded library of β-amino alcohols 1 and 10a-18g, resulting in modest (11a, 21%) to excellent (12g, 80%) yields. Alternatively flow coupling of epichlorohydrin with phenols 4a-4m (22%-89%) and a Bi(OTf)₃ catalysed microwave ring opening with amines afforded a select range of β-amino alcohols, but with lower levels of aminolysis regiocontrol than the sequential flow approach.

Sequential Ugi reaction/base-induced ring closing/IAAC protocol toward triazolobenzodiazepine-fused diketopiperazines and hydantoins

A practical three-step protocol for the assembly of triazolobenzodiazepine-fused diketopiperazines and hydantoins has been developed. The synthesis of these tetracyclic ring systems was initiated by an Ugi reaction, which brought together all necessary functionalities for further transformations. The Ugi adducts were then subjected to a base-induced ring closing and an intramolecular azide–alkyne cycloaddition reaction in succession to obtain highly fused benzodiazepine frameworks.

Multi-step continuous-flow synthesis

Organic chemistry is continually evolving to improve the syntheses of value added and bioactive compounds. Through this progression, a concomitant advancement in laboratory technology has occurred. Many researchers now choose to mediate transformations in continuous-flow systems given the many benefits over round bottom flasks. Furthermore, reaction scale up is often less problematic as this is addressed at the inception of the science. Although single-step transformations in continuous-flow systems are common, multi-step transformations are more valuable. In these systems, molecular complexity is accrued through sequential transformations to a mobile scaffold, much like an in vitro version of Nature's polyketide synthases. Utilizing this methodology, multi-step continuous-flow systems have improved the syntheses of active pharmaceutical ingredients (APIs), natural products, and commodity chemicals. This Review details these advancements while highlighting the rapid progress, benefits, and diversification of this expanding field.

Synthesis of α-amino-1,3-dicarbonyl compounds via Ugi flow chemistry reaction: access to functionalized 1,2,3-triazoles

The Ugi multicomponent reaction has been used as an important synthetic route to obtain compounds with potential biological activity. We present the rapid and efficient synthesis of [Formula: see text]-amino-1,3-dicarbonyl compounds in moderate to good yields via Ugi flow chemistry reactions performed with a continuous flow reactor. Such [Formula: see text]-amino-1,3-dicarbonyl compounds can act as precursors for the production of [Formula: see text]-amino acids via hydrolysis of the ethyl ester group as well as building blocks for the synthesis of novel compounds with the 1,2,3-triazole ring. The [Formula: see text]-amino acid derivatives of the Ugi flow chemistry reaction products were then used for dipeptide synthesis.

The Hitchhiker's Guide to Flow Chemistry ∥

Flow chemistry involves the use of channels or tubing to conduct a reaction in a continuous stream rather than in a flask. Flow equipment provides chemists with unique control over reaction parameters enhancing reactivity or in some cases enabling new reactions. This relatively young technology has received a remarkable amount of attention in the past decade with many reports on what can be done in flow. Until recently, however, the question, "Should we do this in flow?" has merely been an afterthought. This review introduces readers to the basic principles and fundamentals of flow chemistry and critically discusses recent flow chemistry accounts.

A facile hybrid 'flow and batch' access to substituted 3,4-dihydro-2H-benzo[b][1,4]oxazinones

We describe a simple flow chemistry approach to libraries of ethyl 3-oxo-2-(substituted-phenylamino)-3,4-dihydro-2H-benzo[b][1,4]oxazine-6-carboxylates (12a-l) and N-ethyl-3-oxo-2-(substituted-phenylamino)-3,4-dihydro-2H-benzo[b][1,4]oxazine-6-carboxamides (13a-l) in 38-87% yields. This scaffold is poorly described in the chemical literature. Screening against a panel of 11 cancer and one normal cell line showed that the amide linked library 13a-l was devoid of toxicity. Whereas the ester linked analogues 12b, 12c, 12g, 12j and 12l were highly cytotoxic with growth inhibition (GI₅₀) values from 0.34 to >50 μM across all cell lines, with the 2-OH-Ph substituted 12l analogue presenting with sub-micromolar potency against the A2780 (ovarian; 0.34 ± 0.04 μM), BEC-2 (glioblastoma; 0.35 ± 0.06 μM), MIA (pancreas; 0.91 ± 0.054 μM) and SMA (murine glioblastoma; 0.77 ± 0.029 μM) carcinoma cell lines. Interestingly, the U87 glioblastoma cell line showed inherent resistance to growth inhibition by all analogues (GI₅₀ 32 to >50 μM) while the A2780 cells were highly sensitive (GI₅₀ 3.8-0.34 μM), suggesting that the analogues developed herein may be valuable lead compounds for the development of ovarian carcinoma specific cytotoxic agents. The differences in amide versus ester cytotoxicity was consitent with esterase cleaveage to release the cytotoxic warhead.

Safe generation and use of bromine azide under continuous flow conditions--selective 1,2-bromoazidation of olefins

Bromine azide (BrN3), a useful but extremely toxic and explosive reagent for the preparation of vicinal 1,2-bromine azide compounds, was safely generated and reacted in situ with alkenes in a continuous flow photoreactor. BrN3 was generated by a novel procedure from NaBr and NaN3 in water, and efficiently extracted into an organic phase containing the alkene thus avoiding decomposition. The resulting addition products have been used for the preparation of several useful building blocks.

Flow chemistry as a versatile tool for the synthesis of triazoles

This review surveys the continuous-flow strategies for the synthesis of triazoles by means of copper-catalyzed and catalyst-free cycloadditions between azides and various dipolarophiles.