Advances in the synthesis of rearranged homoisoflavonoids

Rearranged homoisoflavonoids constitute a unique group of natural products, renowned for their structural diversity and complexity. These compounds, derived from modifications in the 3-benzylchroman skeleton, are categorized into four subclasses: brazilin, caesalpin, protosappanin, and scillascillin homoisoflavonoids. This review examines the advancements in the total synthesis of these complex structures, aiming to highlight the challenges and opportunities encountered. A comparative analysis of the strategies employed thus far to synthesize these compounds provides a comprehensive understanding of the progress in this field.

Transition-metal catalyzed C-H activation as a means of synthesizing complex natural products

Over the past few decades, the advent of C-H activation has led to a rethink among chemists about the synthetic strategies employed for multi-step transformations. Indeed, deploying innovative and masterful tricks against the numerous classical organic transformations has been the need of the hour. Despite this, the immense importance of C-H activation remains unfulfilled unless the methodology can be deployed for large-scale industrial processes and towards the concise, step-economic synthesis of prodigious natural products and pharmaceutical drugs. Lately, the growing potential of C-H activation methodology has indeed driven the pioneers of synthetic organic chemists into finding more efficient methods to accelerate the synthesis of such complex molecular scaffolds. This review aims to draw a general overview of the various C-H activation procedures that have been adopted for synthesizing these vast majority of structurally complicated natural products. Our objective lies in drawing a complete picture and taking the readers through the synthesis of a series of such complex organic compounds by simplified techniques, making it step-economic on a larger scale and thus instigating the readers to trigger the use of such methodology and uncover new, unique patterns for future synthesis of such natural products.

Base-Promoted Decarboxylative Annulation of Methyl 2-(2-Bromophenyl)acetates and Ynones to Access Benzoxepines

A simple and efficient base-mediated decarboxylative annulation of ynones with methyl 2-(2-bromophenyl)acetates has been developed. A broad range of benzoxepines were prepared with a broad substrate scope and high regioselectivity in moderate to excellent yields under transition-metal-free conditions. This method proceeds through a tandem [2 + 4] annulation, ring-opening decarboxylative reaction, and the intramolecular nucleophilic aromatic substitution reaction. Additionally, the key intermediates were successfully obtained and characterized unambiguously by single-crystal X-ray crystallography, which could favorably support a decarboxylative annulation mechanism. Furthermore, gram-scale reaction and synthetic applications for the further functionalization are also studied.

Catalytic asymmetric synthesis of medium-sized bridged biaryls

Despite the persistent presence of medium-sized (seven- to nine-membered) scaffolds in natural products and biologically active molecules, their asymmetric syntheses have always been considered a formidable task; therefore, they have remained underdeveloped when compared to the enantioselective synthesis of five- and six-membered ring scaffolds. One important class of such medium-sized ring frameworks includes seven- to nine-membered biaryl bridged carbo- and heterocycles. These medium-ring-sized biaryl frameworks possess more configurational stability than the related smaller ring structures and are common features of valuable natural products, bioactive compounds, chiral catalysts, and molecular motors. Due to these exciting properties and broad applications, over the last few years, the catalytic enantioselective synthesis of medium-sized bridged biaryls has seen an upsurge. This highlight article describes the development of organocatalysed and transition-metal catalysed transformations for procuring seven-, eight-, and nine-membered bridged biaryls bearing a chiral axis/one or more asymmetric carbon centres.

Fluorine Labeling of ortho-Phenylenes to Facilitate Conformational Analysis

¹H NMR spectroscopy is a powerful tool for the conformational analysis of ortho-phenylene foldamers in solution. However, as o-phenylenes are integrated into ever more complex systems, we are reaching the limits of what can be analyzed by ¹H- and ¹³C-based NMR techniques. Here, we explore fluorine labeling of o-phenylene oligomers for analysis by ¹⁹F NMR spectroscopy. Two series of fluorinated oligomers have been synthesized. Optimization of monomers for Suzuki coupling enables an efficient stepwise oligomer synthesis. The oligomers all adopt well-folded geometries in solution, as determined by ¹H NMR spectroscopy and X-ray crystallography. ¹⁹F NMR experiments complement these methods well. The resolved singlets of one-dimensional ¹⁹F{¹H} spectra are very useful for determining relative conformer populations. The additional information from two-dimensional ¹⁹F NMR spectra is also clearly valuable when making ¹H assignments. The comparison of ¹⁹F isotropic shielding predictions to experimental chemical shifts is not, however, currently sufficient by itself to establish o-phenylene geometries.

2-Fluoro-5-nitrophenyldiazonium: A Novel Sanger-Type Reagent for the Versatile Functionalization of Alcohols

As a novel Sanger-type reagent, 2-fluoro-5-nitrophenyldiazonium tetrafluoroborate enabled the versatile functionalization of primary and secondary aliphatic alcohols. Based on a mild nucleophilic aromatic substitution of the fluorine atom under unprecedented, base-free conditions, the diazonium unit on the aromatic core of the resulting aryl-alkyl ether could be employed for such diverse transformations as radical C-H activation and cyclization, as well as palladium catalyzed cross-coupling reactions.

A visible-light-induced cascade reaction of etherification/C-C cyclization: efficient synthesis of dibenzo[b,d]oxepin-7(6H)-ones

A visible-light-induced palladium-catalyzed cascade reaction was developed by etherification/C-C coupling cyclization of α-bromoacetophenones with phenols. A series of dibenzo[b,d]oxepin-7(6H)-one derivatives were efficiently synthesized by using this method in good yields. Furthermore, this method was applied to the synthesis of protosappanin A. The protocol has advantages such as simple reaction conditions, wide range of substrates and high reaction efficiency.

One-Pot Synthesis of Dibenzo[ b, d]oxepines via Olefinic C-F Bond Functionalization and Intramolecular Pd-Catalyzed C-H Arylation

We report base-mediated reactions of 1-bromo-2-(2,2-difluorovinyl)benzenes with phenols followed by Pd-catalyzed intramolecular C-H arylation, which offers a novel route to a pharmaceutically relevant class of compounds, oxepines. Construction of these medium-sized ring structures proceeds as a one-pot two-step reaction through diaryl ketene acetals as intermediates. It allows to obtain various dibenz[ b, d]oxepine derivatives in moderate to high yields.

Visible-Light-Induced External Radical-Triggered Annulation To Access CF2-Containing Benzoxepine Derivatives

A facile and diversified synthesis of functionalized CF₂-containing benzoxepine derivatives via photoredox catalysis was achieved in this work. This novel protocol features broad substrate scope, mild reaction conditions, operational simplicity, easy scale-up, and versatile derivatization, which would facilitate its practical and broad applications in the construction of valuable and synthetically challenging heterocycles.

Molecular modeling study, synthesis and biological evaluation of combretastatin A-4 analogues as anticancer agents and tubulin inhibitors

As the major structural component of microtubules, tubulin is an interesting target for the development of anticancer agents. In this study, 64 tubulin polymerization inhibitors of five-membered heterocycle-based combretastatin A-4 analogues were studied by a combination of molecular modeling techniques including 3D-QSAR, molecular docking, and molecular dynamics (MD) simulation. The CoMFA (comparative molecular field analysis) and CoMSIA (comparative molecular similarity indices analysis) models were established with desirable statistical parameters and excellent predictive ability. 20 ns MD simulations were successfully performed to confirm the detailed binding mode and validate the rationality of docking results. Combining the binding free energy calculations and 3D-QSAR results, some new heterocycle-based combretastatin A-4 analogues were designed. Three of them were synthesized and biologically evaluated. Compound 13a displayed potent antiproliferative activity (IC₅₀ value of 1.31 μM against HepG2 cells, IC₅₀ value of 1.37 μM against A549 cells) and inhibition of tubulin polymerization activity (IC₅₀ value of 0.86 μM). Compound 13b also presented good activity against HepG2 cells (IC₅₀ value of 4.75 μM). The experimental results demonstrated that the built models were effective for the development of novel anticancer agents and tubulin inhibitors.

A conceptually novel construction of the 6a-hydroxypterocarpan skeleton - Synthesis of (±)-variabilin

A new access to the 6a-hydroxypterocarpan variabilin was established. Key step of this concise total synthesis is a challenging cyclization of a haloketone via halogen-metal exchange and subsequent intramolecular addition to the carbonyl function.

Synthesis of 8-azaprotosappanin A derivatives via intramolecular palladium-catalyzed ortho C–H activation/C–C cyclization and their antibacterial activity

A novel synthetic protocol for the construction of eight-membered heterocycles by intramolecular palladium-catalyzed ortho C–H activation/C–C cyclization was proposed.