Achmatowicz Rearrangement-Inspired Development of Green Chemistry, Organic Methodology, and Total Synthesis of Natural Products

Recent Advances in the Synthesis of Diarylheptanoids

In the quest for synthesizing biologically important natural products, medicinal chemists embark on an endless journey. This review focuses on the reports published towards the syntheses of diarylheptanoids, classifying them into linear, tetrahydropyran, diarylether, and biphenyl categories. The synthesis methods for each class from 2013 to 2023 are discussed, providing a comprehensive overview of the advancements in the field. Representative natural product examples are highlighted for each category. The review emphasizes the importance of diarylheptanoids in the realms of chemistry and medicine, showcasing their potential as valuable compounds for medicinal and synthetic chemists.

Preparative scale Achmatowicz and aza-Achmatowicz rearrangements catalyzed by Agrocybe aegerita unspecific peroxygenase

The unspecific peroxygenase (UPO) from Agrocybe aegerita (rAaeUPO-PaDa-I-H) is an effective and practical biocatalyst for the oxidative expansion of furfuryl alcohols/amines on a preparative scale, using the Achmatowicz and aza-Achmatowicz reaction. The high activity and stability of the enzyme, which can be produced on a large scale as an air-stable lyophilised powder, renders it a versatile and scalable biocatalyst for the preparation of synthetically valuable 6-hydroxypyranones and dihydropiperidinones. In several cases, the biotransformation out-performed the analogous chemo-catalysed process, and operates under milder and greener reaction conditions.

Ecofriendly Protocol for ipso-Bromination of Arylboronic Acids

We report a novel and environmentally friendly method for the ipso-bromination of arylboronic acids by exploiting the oxone/KBr system. We discovered that CuBr can catalyze the reaction and increase the yield from 63 to 97%. We believe that CuBr might catalyze the in situ generation of HOBr from oxone/KBr. The mild reaction condition permits tolerance of a diverse array of functional groups with exclusive regio- and chemoselectivity and allows low-cost large-scale reaction without explosion risk.

Controllable multi-halogenation of a non-native substrate by SyrB2 iron halogenase

Geminal, multi-halogenated functional groups are widespread in natural products and pharmaceuticals, yet no synthetic methodologies exist that enable selective multi-halogenation of unactivated C-H bonds. Biocatalysts are powerful tools for late-stage C-H functionalization, as they operate with high degrees of regio-, chemo-, and stereoselectivity. 2-oxoglutarate (2OG)-dependent non-heme iron halogenases chlorinate and brominate aliphatic C-H bonds offering a solution for achieving these challenging transformations. Here, we describe the ability of a non-heme iron halogenase, SyrB2, to controllably halogenate non-native substrate alpha-aminobutyric acid (Aba) to yield mono-chlorinated, di-chlorinated, and tri-chlorinated products. These chemoselective outcomes are achieved by controlling the loading of 2OG cofactor and SyrB2 biocatalyst. By using a ferredoxin-based biological reductant for electron transfer to the catalytic center of SyrB2, we demonstrate order-of-magnitude enhancement in the yield of tri-chlorinated product that were previously inaccessible using any single halogenase enzyme. We also apply these strategies to broaden SyrB2's reactivity scope to include multi-bromination and demonstrate chemoenzymatic conversion of the ethyl side chain in Aba to an ethylyne functional group. We show how steric hindrance induced by the successive addition of halogen atoms on Aba's C4 carbon dictates the degree of multi-halogenation by hampering C3-C4 bond rotation within SyrB2's catalytic pocket. Overall, our work showcases the synthetic potential of iron halogenases to facilitate multi-C-H functionalization chemistry.

Oxidative Transformation of 2-Furylanilines into Indolin-3-ones

Oxidation of 2-furylaninlies with m-CPBA followed by treatment with a base provides access to functionalized indolin-3-ones. The designed oxidative transformation utilizes an underassessed chemical behavior of furyl-containing amines to form a C-N bond via engaging a β-carbon atom of the furan core upon a ring-forming step, thereby providing an alternative disconnection toward nitrogen-containing heterocycles.

Aza-Achmatowicz rearrangement coupled with intermolecular aza-Friedel-Crafts enables total syntheses of uleine and aspidosperma alkaloids

Aspidosperma and uleine alkaloids belong to the large family of monoterpene indole alkaloids with diverse biological activities and thus have attracted extensive synthetic interest. Reported is the development of a new synthetic strategy that allows direct C3-C2' linkage of indoles with functionalized 2-hydroxypiperidines to construct the core common to all aspidoserma and uleine alkaloids. Such indole-piperidine linkage is enabled by coupling aza-Achmatowicz rearrangement (AAR) with indoles via an intermolecular aza-Friedel-Crafts (iAFC) reaction. This AAR-iAFC reaction proceeds under mild acidic conditions with wide tolerance of functional groups (33 examples). The synthetic application of the AAR-iAFC method was demonstrated with collective total syntheses of 3 uleine-type and 6 aspidosperma alkaloids: (+)-3-epi-N-nor-dasycarpidone, (+)-3-epi-dasycarpidone, (+)-3-epi-uleine, 1,2-didehydropseudoaspidospermidine, 1,2-dehydroaspidospermidine, vincadifformine, winchinine B, aspidospermidine, and N-acetylaspidospermidine. We expect that this AAR-iAFC strategy is applicable to other monoterpene indole alkaloids with the C3-C2' linkage of indoles and piperidines.

BiBr3 -Mediated Intramolecular Aza-Prins Cyclization of Aza-Achmatowicz Rearrangement Products: Asymmetric Total Synthesis of Suaveoline and Sarpagine Alkaloids

An intramolecular aza-Prins cyclization of aza-Achmatowicz rearrangement products was developed in which bismuth tribromide (BiBr3 ) plays a dual role as an efficient Lewis acid and source of the bromide nucleophile. This approach enables the facile construction of highly functionalized 9-azabicyclo[3.3.1]nonanes (9-ABNs), which are valuable synthetic building blocks and a powerful platform for the synthesis of a variety of alkaloid natural products and drug molecules. Suitable substrates for the aza-Prins cyclization include 1,1-disubstituted alkenes, 1,2-disubstituted alkenes, alkynes, and allenes, with good to excellent yields observed. Finally, we showcase the application of this new approach to the enantioselective total synthesis of six indole alkaloids: (-)-suaveoline (1), (-)-norsuaveoline (2), (-)-macrophylline (3), (+)-normacusine B (4), (+)-Na -methyl-16-epipericyclivine (5) and (+)-affinisine (6) in a total of 9-14 steps. This study significantly expands the synthetic utility of the aza-Achmatowicz rearrangement, and the strategy (aza-Achmatowicz/aza-Prins) is expected to be applicable to the total synthesis of other members of the big family of macroline and sarpagine indole alkaloids.

Total Syntheses of Polycyclic Diterpenes Phomopsene, Methyl Phomopsenonate, and iso-Phomopsene via Reorganization of C-C Single Bonds

The first total syntheses of polycyclic diterpenes phomopsene (1), methyl phomopsenonate (2), and iso-phomopsene (3) have been accomplished through the unusual cascade reorganization of C-C single bonds. This approach features: (i) a synergistic Nazarov cyclization/double ring expansions in one-step, developed by authors, to rapid and stereospecific construction of the 5/5/5/5 tetraquinane scaffold bearing contiguous quaternary centers and (ii) a one-pot strategic ring expansion through Beckmann fragmentation/recombination to efficiently assemble the requisite 5/5/6/5 tetracyclic skeleton of the target molecules 1-3. This work enables us to determine that the correct structure of iso-phomopsene is, in fact, the C7 epimer of the originally assigned structure. Finally, the absolute configurations of three target molecules were confirmed through enantioselective synthesis.

Chemo-divergent Cyano Group Migration: Involving Elimination and Substitution of the Key α-Thianthrenium Cyano Species

Herein, we report a light-driven, radical-type cyano migration in the absence of a photocatalyst, enabling a chemo-divergent synthesis of (Z)-alkenyl nitriles and ketones. Trifluoromethyl thianthrenium salt (TT-CF3+OTf-) plays multiple roles: (a) absorbing light to generate trifluoromethyl radicals to initiate the reaction and (b) forming α-thianthrenium cyano species by in situ capture of TT• +. (Z)-Alkenyl nitriles were formed through the elimination of thianthrenium salts, and aryl ketones were obtained via the nucleophilic substitution of thianthrenium salts.

Deconstructive and Divergent Synthesis of Bioactive Natural Products

Natural products play a key role in innovative drug discovery. To explore the potential application of natural products and their analogues in pharmacology, total synthesis is a key tool that provides natural product candidates and synthetic analogues for drug development and potential clinical trials. Deconstructive synthesis, namely building new, challenging structures through bond cleavage of easily accessible moieties, has emerged as a useful design principle in synthesizing bioactive natural products. Divergent synthesis, namely synthesizing many natural products from a common intermediate, can improve the efficiency of chemical synthesis and generate libraries of molecules with unprecedented structural diversity. In this review, we will firstly introduce five recent and excellent examples of deconstructive and divergent syntheses of natural products (2021-2023). Then, we will summarize our previous work on the deconstructive and divergent synthesis of natural products to demonstrate the high efficiency and simplicity of these two strategies in the field of total synthesis.

Green Halogenation of Indoles with Oxone-Halide

Oxidative functionalization of indoles is one of the most widely used approaches to exploit the synthetic utility of indoles. In continuation of our research interest in the green oxidation of indoles, we further explore the oxidation of indoles with oxone-halide and discover that the protecting group on the nitrogen of indoles plays a decisive role in controlling the pathways of indole oxidation with oxone-halide. An electron-withdrawing group on the nitrogen of indoles (N-EWG) enables C2 halogenation with stoichiometric halide, while C3 halogenation could be selectively achieved by using stoichiometric halide without dependence on the electronic property of the protecting group on the indole nitrogen. Different from our previous results obtained by using catalytic halide, these findings lead to the development of an environmentally friendly, efficient, and mild protocol for access to 2- or 3-haloindoles (chloro and bromo). As compared to the previous synthetic methods for 2-/3-haloindoles, our method exploits the in situ-generated reactive halogenating species from oxone-halide for halogenation of indoles and thus eliminates the use of stoichiometric halogenating agents and the production of toxic and hazardous organic byproducts derived from oxidants.

A Comprehensive Review on Metal Catalysts for the Production of Cyclopentanone Derivatives from Furfural and HMF

The catalytic transformation of biomass-based furan compounds (furfural and HMF) for the synthesis of organic chemicals is one of the important ways to utilize renewable biomass resources. Among the numerous high-value products, cyclopentanone derivatives are a kind of valuable compound obtained by the hydrogenation rearrangement of furfural and HMF in the aqueous phase of metal-hydrogen catalysis. Following the vast application of cyclopentanone derivatives, this reaction has attracted wide attention since its discovery, and a large number of catalytic systems have been reported to be effective in this transformation. Among them, the design and synthesis of metal catalysts are at the core of the reaction. This review briefly introduces the application of cyclopentanone derivatives, the transformation mechanism, and the pathway of biomass-based furan compounds for the synthesis of cyclopentanone derivatives. The important progress of metal catalysts in the reaction since the first report in 2012 up to now is emphasized, the characteristics and catalytic performance of different metal catalysts are introduced, and the critical role of metal catalysts in the reaction is discussed. Finally, the future development of this transformation process was prospected.

Facile preparation of N-tert-butyl amides under heat-, metal- and acid-free conditions by using tert-butyl nitrite (TBN) as a practical carbon source

tert-Butyl nitrite (TBN) is a nontoxic substance that has frequently been used as a source of nitrogen, oxygen, or nitric oxide (NO), but not as a carbon source in chemical transformations. Here, for the first time to the best of our knowledge, we introduced TBN as a source of carbon (tert-butyl group) for the synthesis of highly valuable N-tert-butyl amides from nitriles and water under very mild conditions.

Total Synthesis of Vinorine

The asymmetric total synthesis of vinorine, a polycyclic and cage-like alkaloid, has been realized in a flexible approach. Key features of the current synthesis include an aza-Achmatowicz rearrangement/Mannich-type cyclization to install the highly functional 9-azabicyclo-[3.3.1]nonane scaffold, a high yield Fischer indole annulation to synthesize the common intermediate for sarpagine-ajamaline type alkaloids, and an Ireland-Claisen rearrangement to construct the C15-C20 bond.

Diastereoselective Synthesis of 2,6-Disubstituted Tetrahydropyranones via Prins Cyclization of 3-Bromobut-3-en-1-ols and Aldehydes

Tetrahydropyranones are synthesized from 3-bromobut-3-en-1-ols and aldehydes in good yields with excellent diastereoselectivity at -35 °C. The reaction involves an initial formation of a most stable six-membered chairlike tetrahydropyranyl carbocation followed by nucleophilic attack of the hydroxyl group and subsequent elimination of HBr to give tetrahydropyranone. The carbonyl moiety of the tetrahydropyranone is converted to enol ether and esters using Wittig reaction. It is also transformed into 4-hydroxy-2,6-disubstituted tetrahydropyran with 2,4- and 4,6-cis configuration by lithium aluminum hydride in up to 96% diastereoselectivity. Furthermore, the methodology is extended toward the synthesis of novel anticancer aminoguanidine compounds.

Ru-Catalyzed Isomerization of Achmatowicz Derivatives: A Sustainable Route to Biorenewables and Bioactive Lactones

A Ru-catalyzed isomerization of Achmatowicz derivatives that opens unexplored routes to diversify the biogenic furanic platform is reported. The mechanistic insights of this formally redox-neutral intramolecular process were studied computationally and by deuterium labeling. The transformation proved to be a robust synthetic tool to achieve the synthesis of bioderived-monomers and a series of 4-keto-δ-valerolactones that further enabled the development of a flexible strategy for the synthesis of acetogenins. A concise and protective group-free asymmetric total synthesis of two natural products, namely, (S,S)-muricatacin and the (S,S)-L-factor, is also described.

Two Green Protocols for Halogenative Semipinacol Rearrangement

Semipinacol rearrangement is a special type of Wagner-Meerwein rearrangement that involves carbocation 1,2-rearrangement to provide carbonyl compounds with an α-quaternary carbon center. It has been strategically used for natural product synthesis and construction of highly congested quaternary carbons. Herein, we report a safe and green protocol that uses oxone/halide and Fenton bromide to achieve halogenative semipinacol rearrangement. The key feature of this method is the green in situ generation of reactive halogenating species from oxidation of halide with oxone or H₂O₂, which produces a nontoxic byproduct (potassium sulfate or water). Easy operation (insensitive to air and moisture) at room temperature without using special equipment adds additional advantage over previous methods.

De novo asymmetric Achmatowicz approach to oligosaccharide natural products

The development and application of the asymmetric synthesis of oligosaccharides from achiral starting materials is reviewed. This de novo asymmetric approach centers around the use of asymmetric catalysis for the synthesis of optically pure furan alcohols in conjunction with Achmatowicz oxidative rearrangement for the synthesis of various pyranones. In addition, the use of a diastereoselective palladium-catalyzed glycosylation and subsequent diastereoselective post-glycosylation transformation was used for the synthesis of oligosaccharides. The application of this approach to oligosaccharide synthesis is discussed.

Natural product total synthesis using rearrangement reactions

The synthetic utility of rearrangement reactions in total synthesis for the rapid construction of core skeletons, the precise control of stereochemistry, and the identification of suitable synthons has been discussed.