Flow Chemistry for Cycloaddition Reactions

Cyclic natural product oligomers: diversity and (bio)synthesis of macrocycles

Cyclic compounds are generally preferred over linear compounds for functional studies due to their enhanced bioavailability, stability towards metabolic degradation, and selective receptor binding. This has led to a need for effective cyclization strategies for compound synthesis and hence increased interest in macrocyclization mediated by thioesterase (TE) domains, which naturally boost the chemical diversity and bioactivities of cyclic natural products. Many non-ribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) derived natural products are assembled to form cyclodimeric compounds, with these molecules possessing diverse structures and biological activities. There is significant interest in identifying the biosynthetic pathways that produce such molecules given the challenge that cyclodimerization represents from a biosynthetic perspective. In the last decade, many groups have pursued the characterization of TE domains and have provided new insights into this biocatalytic machinery: however, the enzymes involved in formation of cyclodimeric compounds have proven far more elusive. In this review we focus on natural products that involve macrocyclization in their biosynthesis and chemical synthesis, with an emphasis on the function and biosynthetic investigation on the special family of TE domains responsible for forming cyclodimeric natural products. We also introduce additional macrocyclization catalysts, including butelase and the CT-mediated cyclization of peptides, alongside the formation of cyclodipeptides mediated by cyclodipeptide synthases (CDPS) and single-module NRPSs. Due to the interdisciplinary nature of biosynthetic research, we anticipate that this review will prove valuable to synthetic chemists, drug discovery groups, enzymologists, and the biosynthetic community in general, and inspire further efforts to identify and exploit these biocatalysts for the formation of novel bioactive molecules.

Diastereoselective Construction of Spirocyclic Isobenzofurans via a Tandem Michael Addition/5-Exo-dig Cyclization Reaction

A practical approach for rapid and efficient access to spirocyclic isobenzofurans is described. The reaction proceeds through the cycloaddition of 1,6-ynenone derivatives and 4-nitro-1,3-diarylbutan-1-ones, promoted by Cs2CO3 in the presence of l-proline as a catalyst. The advantages of this reaction include the formation of two C-C bonds and one C-O bond as well as mild reaction conditions. Extended spirocyclic isobenzofurans are obtained with good efficiency and diastereoselectivity under these mild conditions, and this new protocol avoids the use of any transition-metal reagents.

Advances in the Continuous Flow Synthesis of 3- and 4-Membered Ring Systems

Small carbo- and heterocyclic ring systems have experienced a significant increase in importance in recent years due to their relevance in modern pharmaceuticals, as building blocks for designer materials or as synthetic intermediates. This necessitated the development of new synthetic methods for the preparation of these strained ring systems focusing on effectiveness and scalability. The high ring strain of these entities as well as the use of high-energy reagents and intermediates has often challenged their synthesis. Continuous flow approaches have thus emerged as highly effective means to safely and reliably access these strained scaffolds. In this short review, key developments in this field are summarised showcasing the power of continuous flow approaches for accessing 3- and 4-membered ring systems via thermal, photo- and electrochemical processes.

Advances in Continuous Flow Fluorination Reactions

Fluorination reactions are important in constructing organofluorine motifs, which contribute to favorable biological properties in pharmaceuticals and agrochemicals. However, fluorination reagents and reactions are associated with various problems, such as their hazardous nature, high exothermicity, and poor selectivity and scalability. Continuous flow has emerged as a transformative technology to provide many advantages relative to batch syntheses. This review article summarizes recent continuous flow techniques that address the limitations and challenges of fluorination reactions. Approaches based on different flow techniques are discussed, including gas-liquid reactions, packed-bed reactors, in-line purifications, streamlined multistep synthesis, large-scale reactions well as flow photoredox- and electrocatalysis.

Inline purification in continuous flow synthesis – opportunities and challenges

Continuous flow technology has become the method of choice for many academic and industrial researchers when developing new routes to chemical compounds of interest. With this technology maturing over the last decades, robust and oftentimes automated processes are now commonly exploited to generate fine chemical building blocks. The integration of effective inline analysis and purification tools is thereby frequently exploited to achieve effective and reliable flow processes. This perspective article summarizes recent applications of different inline purification techniques such as chromatography, extractions, and crystallization from academic and industrial laboratories. A discussion of the advantages and drawbacks of these tools is provided as a guide to aid researchers in selecting the most appropriate approach for future applications. It is hoped that this perspective contributes to new developments in this field in the context of process and cost efficiency, sustainability and industrial uptake of new flow chemistry tools developed in academia.

Understanding flow chemistry for the production of active pharmaceutical ingredients

Multi-step organic syntheses of various drugs, active pharmaceutical ingredients, and other pharmaceutically and agriculturally important compounds have already been reported using flow synthesis. Compared to batch, hazardous and reactive reagents can be handled safely in flow. This review discusses the pros and cons of flow chemistry in today’s scenario and recent developments in flow devices. The review majorly emphasizes on the recent developments in the flow synthesis of pharmaceutically important products in last five years including flibanserin, imatinib, buclizine, cinnarizine, cyclizine, meclizine, ribociclib, celecoxib, SC-560 and mavacoxib, efavirenz, fluconazole, melitracen HCl, rasagiline, tamsulosin, valsartan, and hydroxychloroquine. Critical steps and new development in the flow synthesis of selected compounds are also discussed.

Study of the Pauson-Khand reaction in flow over alkynylphenyl vinyl ethers: towards the synthesis of tricyclic multisubstituted benzofurans

The use of flow methodology allows the use of alkynylphenyl vinyl ethers (benzo-fused 1,7 enynes) as substrates for the intramolecular Pauson-Khand reaction (PKr). Forced temperature and pressure conditions during a short reaction time minimize the substrate decomposition allowing the formation of the PK adduct. Substrates substituted at the internal position of the double bond and with internal triple bonds give better yields. The resulting products are cyclopentabenzofuranones present in diverse natural products and drugs that can be further functionalised.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

Cyclic Diaryl λ3-Bromanes: A Rapid Access to Molecular Complexity via Cycloaddition Reactions

Biaryls have widespread applications in organic synthesis. However, sequentially polysubstituted biaryls are underdeveloped due to their challenging preparation. Herein, we report the synthesis of dissymetric 2,3,2',3',4-substituted biaryls via pericyclic reactions of cyclic diaryl λ³-bromanes. The functional groups tolerance and atom economy allow access to molecular complexity in a single reaction step. Continuous flow protocol has been designed for the scale-up of the reaction, while postfunctionalizations have been developed taking advantage of the residual Br-atom.

Direct Metal-Free Transformation of Alkynes to Nitriles: Computational Evidence for the Precise Reaction Mechanism

Density functional theory calculations elucidated the precise reaction mechanism for the conversion of diphenylacetylenes into benzonitriles involving the cleavage of the triple C≡C bond, with N-iodosuccinimide (NIS) as an oxidant and trimethylsilyl azide (TMSN3) as a nitrogen donor. The reaction requires six steps with the activation barrier ΔG‡ = 33.5 kcal mol-1 and a highly exergonic reaction free-energy ΔGR = -191.9 kcal mol-1 in MeCN. Reaction profiles agree with several experimental observations, offering evidence for the formation of molecular I2, interpreting the necessity to increase the temperature to finalize the reaction, and revealing thermodynamic aspects allowing higher yields for alkynes with para-electron-donating groups. In addition, the proposed mechanism indicates usefulness of this concept for both internal and terminal alkynes, eliminates the option to replace NIS by its Cl- or Br-analogues, and strongly promotes NaN3 as an alternative to TMSN3. Lastly, our results advise increasing the solvent polarity as another route to advance this metal-free strategy towards more efficient processes.