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.

Chemo-, Regio- and Stereoselective Preparation of (Z)-2-Butene-1,4-Diol Monoesters via Pd-Catalyzed Decarboxylative Acyloxylation

(Z)-alkenes are useful synthons but thermodynamically less stable than their (E)-isomers and typically more difficult to prepare. The synthesis of 1,4-hetero-bifunctionalized (Z)-alkenes is particularly challenging due to the inherent regio- and stereoselectivity issues. Herein we demonstrate a general, chemoselective and direct synthesis of (Z)-2-butene-1,4-diol monoesters. The protocol operates within a Pd-catalyzed decarboxylative acyloxylation regime involving vinyl ethylene carbonates (VECs) and various carboxylic acids as the reaction partners under mild and operationally attractive conditions. The newly developed process allows access to a structurally diverse pool of (Z)-2-butene-1,4-diol monoesters in good yields and with excellent regio- and stereoselectivity. Various synthetic transformations of the obtained (Z)-2-butene-1,4-diol monoesters demonstrate how these synthons are of great use to rapidly diversify the portfolio of these formal desymmetrized (Z)-alkenes.

Macrolactonization of methyl 15-hydroxypentadecanoate to cyclopentadecanolide using KF-La/γ-Al2O3 catalyst

It has been a challenge to synthesize macrolide musk in excellent yields with high purity. KF-La/γ-Al₂O₃ catalyst was prepared from a highly basic mesoporous framework using a mild method. The prepared KF-La/γ-Al₂O₃ catalyst was employed for the synthesis of cyclopentadecanolide from methyl 15-hydroxypentadecanoate. The morphology and structure of prepared catalysts were characterized using XRD, TG-DTG, SEM, EDX, TEM, BET and CO₂-TPD. The results revealed that the K₃AlF₆ and LaOF are produced on the surface of KF-La/γ-Al₂O₃, and LaO can promote the dispersion of KF on the surface of Al₂O₃. Catalysts pore size main distribution ranges between 10 and 30 nm, the maximum CO₂ desorption temperature is 715°C when the La loading is 25%. Because F^- ion has a higher electronegativity than O^2- ion, the KF-promoted metal oxide (Al₂O₃ or/and La₂O₃) contained more strong basic sites, compared with that of the corresponding metal oxide. The yield of cyclopentadecanolide obtained at 0.5 g KF-25La/γ-Al₂O₃ catalyst and a reaction temperature of 190°C for 7 h were 58.50%, and the content after reactive distillation is 98.8%. The KF-La/γ-Al₂O₃ catalyst has a larger pore size and basic strength, which is more conducive to the macrolactonization of long-chain hydroxy ester.

Asymmetric Synthesis of Chiral Aza-macrodiolides via Iridium-Catalyzed Cascade Allylation/Macrolactonization

Iridium-catalyzed cascade allylation/macrolactonization between vinylethylene carbonate (VEC) and isatoic anhydride derivatives was successfully developed, readily generating a wide range of C₂-symmetric chiral macrodiolides bearing 14-membered rings in moderate to good yields with excellent diastereoselectivities and enantioselectivities (generally 99% ee). Control experiments revealed that racemic VEC as the precursor of electrophilic iridium-π-allyl species underwent kinetic resolution process. This expedient protocol features easily available substrates, excellent stereoselective control, and high step economy.

Regio- and enantioselective formation of tetrazole-bearing quaternary stereocenters via palladium-catalyzed allylic amination

A general and efficient method via catalysis by readily available Pd(0)/DACH-naphthyl catalyst under mild conditions, unlocks a new platform that permits the synthesis of elusive quaternary N2-allylic tetrazoles, even in the context of late-stage functionalization.

Selective approach to N-substituted tertiary 2-pyridones

Commercially available 2-hydroxypyridines are converted into enantiomerically enriched allylic 2-pyridones with elusive N-substituted tertiary carbon by means of Pd-catalyzed allylic amination of vinyl cyclic carbonates.

Synthesis of 2-trifluoromethylquinolines through rhodium-catalysed redox-neutral [3 + 3] annulation between anilines and CF3-ynones using traceless directing groups

Rhodium-catalysed [3 + 3]-cycloaddition of acetanilides with CF3-ynones was achieved through a traceless directing-group strategy, which delivered 2-trifluromethylquinolines exhibiting favorable blue emissions.

Five-Membered Cyclic Carbonates: Versatility for Applications in Organic Synthesis, Pharmaceutical, and Materials Sciences

This review presents the recent advances involving several applications of five-membered cyclic carbonates and derivatives. With more than 150 references, it covers the period from 2012 to 2020, with special emphasis on the use of five-membered cyclic carbonates as building blocks for organic synthesis and material elaboration. We demonstrate the application of cyclic carbonates in several important chemical transformations, such as decarboxylation, hydrogenation, and transesterification reactions, among others. The presence of cyclic carbonates in molecules with high biological potential is also displayed, together with the importance of these compounds in the preparation of materials such as urethanes, polyurethanes, and flame retardants.

Regioselective Synthesis of 2,4-Diaryl-6-trifluoromethylated Pyridines through Copper-Catalyzed Cyclization of CF3-Ynones and Vinyl Azides

A novel copper-catalyzed cyclization of readily available vinyl azides with CF₃-ynones is steadily achieved under mild conditions to furnish the versatile 2,4-diaryl-6-trifluoromethylated pyridine products, which are of great interest in medicinal chemistry. The generation of the vinyl iminophosphorane intermediates from vinyl azides through the Staudinger-Meyer reaction ensures the subsequent 1,4-addition process with CF₃-ynones in this transformation.

Palladium-Catalyzed Formal (5 + 6) Cycloaddition of Vinylethylene Carbonates with Isatoic Anhydrides for the Synthesis of Medium-Sized N,O-Containing Heterocycles

Under the reaction conditions of Pd(PPh₃)₄ (2.5 mol %) and PPh₃ (10 mol %) in EtOAc at 60 °C, the formal (5 + 6) cycloaddition of vinylethylene carbonates with isatoic anhydrides proceeded smoothly and furnished medium-sized N,O-containing heterocycles in reasonable chemical yields. The chemical structures of the title products were clearly identified by X-ray diffraction analysis.

Palladium-catalyzed stereoselective (3 + 2) cycloaddition of vinylethylene carbonates with cyclic N-sulfonyl ketimines

A diastereoselective (3 + 2) cycloaddition of N-sulfonyl ketimines with vinylethylene carbonates (VECs) in the presence of Pd2dba3·CHCl3 and PPh3 has been developed. The reaction of various substituted VECs and diverse cyclic N-sulfonyl ketimines proceeded smoothly under mild conditions, giving highly functionalized oxazolidine frameworks in good to excellent yields with moderate to good diastereoselectivities. With the use of spiroketal-based diphosphine SKP as a chiral ligand, an asymmetric version of the current (3 + 2) cycloaddition was achieved, and chiral products were obtained in >99% ee in most cases.

Catalytic enantioselective synthesis of macrodiolides and their application in chiral recognition

New types of C₂-symmetric chiral macrodiolides are readily obtained via chiral N,N′-dioxide-scandium(iii) complex-promoted asymmetric tandem Friedel–Crafts alkylation/intermolecular macrolactonization of ortho-quinone methides with C3-substituted indoles. This protocol provides an array of enantioenriched macrodiolides with 16, 18 or 20-membered rings in moderate to good yields with high diastereoselectivities and excellent enantioselectivities through adjusting the length of the tether at the C3 position of indoles. Density functional theory calculations indicate that the formation of macrocycles is more favorable than that of 9-membered-ring lactones in terms of kinetics and thermodynamics. The potential utility of these intriguing chiral macrodiolide molecules is demonstrated in the enantiomeric recognition of aminols and chemical recognition of metal ions.

An asymmetric tandem Friedel–Crafts alkylation/intermolecular macrolactonization of ortho-quinone methides with C3-substituted indoles was achieved by using a chiral N,N′-dioxide-scandium(iii) complex.

Palladium-catalysed cyclisation of vinylethylene carbonates and anhydrides: a new approach to diverse medium-sized bislactones

Efficient construction of medium-sized lactones has attracted considerable interest over several decades, but remains a formidable challenge in synthetic chemistry. Here, we describe an unprecedented palladium-catalysed regioselective [5 + n] cyclisation (n = 5, 6, and 7) between vinylethylene carbonates and various anhydrides. Catalytic transformation occurs under mild, room-temperature conditions and offers an exceptional substrate scope. A broad spectrum of medium-sized bislactones with skeletal diversity can be obtained easily.