The Biomimetic Total Syntheses of the Antiplasmodial Tomentosones A and B

The Kornblum DeLaMare rearrangement in natural product synthesis: 25 years of innovation

Covering: 1998 up to the end of 2023Since its initial disclosure in 1951, the Kornblum DeLaMare rearrangement has proved an important synthetic transformation and has been widely adopted as a biomimetic step in natural product synthesis. Utilising the base catalysed decomposition of alkyl peroxides to yield a ketone and alcohol has found use in many syntheses as well as a key strategic step, including the unmasking of furans, as a biomimetic synthetic tool, and the use of the rearrangement to install oxygen enantioselectively. Since ca. 1998, its impact as a synthetic transformation has grown significantly, especially given the frequency of use in natural product syntheses, therefore this 25 year time period will be the focus of the review.

Oligomeric phloroglucinols with hAChE inhibitory and antibacterial activities from tropic Rhodomyrtus tomentosa

Alzheimer's diseases (AD) and other infectious diseases caused by drug-resistance bacteria have posed a serious threat to human lives and global health. With the aim to search for human acetylcholinesterase (hAChE) inhibitors and antibacterial agents from medicinal plants, 16 phloroglucinol oligomers, including two new phloroglucinol monomers (1a and 1b), four new phloroglucinol dimers (3a, 3b, 4b, and 5a), six new phloroglucinol trimers (6a, 6b, 7a, 7b, 8a, and 8b), and two naturally occurring phloroglucinol monomers (2a and 2b), along with two known congeners (4a and 5b), were purified from the leaves of tropic Rhodomyrtus tomentosa. The structures and absolute configurations of these new isolates were unequivocally established by comprehensive analyses of their spectroscopic data (NMR and HRESIMS), ECD calculation, and single crystal X-ray diffraction. Structurally, 3a/3b shared a rare C-5' formyl group, whereas 6a/6b possessed a unique C-7' aromatic ring. In addition, 7a/7b and 8a/8b were rare phloroglucinol trimers with a bis-furan and a C-6' hemiketal group. Pharmacologically, the mixture of 3a and 3b showed the most potent human acetylcholinesterase (hAChE) inhibitory activity with an IC50 value of 1.21 ± 0.16 μM. The molecular docking studies of 3a and 3b in the hAChE binding sites were performed, displaying good agreement with the in vitro inhibitory effects. In addition, the mixture of 3a and 3b displayed the most significant anti-MRSA (methicillin-resistant Staphylococcus aureus) with MIC and MBC values of both 0.50 μg/mL, and scanning electron microscope (SEM) studies revealed that they could destroy the biofilm structures of MRSA. The findings provide potential candidates for the further development of anti-AD and anti-bacterial agents.

Direct Organocatalytic Reductive Alkylation of Syncarpic Acid: Scope and Applications

Biologically important 4-alkylsyncarpic acids, which resemble the core structure of many natural products, were synthesized in one-pot through the organocatalytic three-component reductive alkylation with excellent yields and C-selectivity. Synthetic applications of 4-alkylsyncarpic acids were demonstrated by converting into the functionally rich molecules through different reactions like Michael, retro-Michael, reduction, and oxidation reactions. In a continuation, formal total synthesis of (±)-triumphalone, (±)-isotriumphalone, and monomeric phloroglucinol derivatives was reported in a few steps starting from 4-alkylsyncarpic acids in overall very good yields. Further showcasing the importance of C-alkylated products, 4-benzylsyncarpic acid and its Michael adduct with methyl vinyl ketone were synthesized in a gram scale without compromising rate/yields.

Recent advances in the synthesis of natural products containing the phloroglucinol motif

Covering: June 2009 to 2021Natural products containing a phloroglucinol motif include simple and oligomeric phloroglucinols, polycyclic polyprenylated acylphloroglucinols, phloroglucinol-terpenes, xanthones, flavonoids, and coumarins. These compounds represent a major class of secondary metabolites which exhibit a wide range of biological activities such as antimicrobial, anti-inflammatory, antioxidant and hypoglycaemic properties. A number of these compounds have been authorized for therapeutic use or are currently being studied in clinical trials. Their structural diversity and utility in both traditional and conventional medicine have made them popular synthetic targets over the years. In this review, we compile and summarise the recent synthetic approaches to the natural products bearing a phloroglucinol motif. Focus has been given on ingenious strategies to functionalize the phloroglucinol moiety at multiple positions. The isolation and bioactivities of the compounds are also provided.

Discovery and Biomimetic Synthesis of a Polycyclic Polymethylated Phloroglucinol Collection from Rhodomyrtus tomentosa

Inspired by a previously reported biomimetic synthesis study, four new naturally occurring phloroglucinol trimers 1-4 with unusual 6/5/5/6/6/6-fused hexacyclic ring systems, along with two known analogues (5 and 6) and two known biogenetically related dimers (10 and 11), were isolated from Rhodomyrtus tomentosa. Their structures and absolute configurations were unambiguously elucidated by spectroscopic analysis, X-ray diffraction, and electronic circular dichroism calculation. By mimicking two potentially alternative biosynthetic pathways, the first asymmetric syntheses of 1-4 and the racemic syntheses of 5 and 6 were achieved in only five to six steps without the need for protecting groups. Furthermore, phloroglucinol dimers 10 and 11 exhibited significant in vitro antiviral activity against the respiratory syncytial virus.

Bioinspired syntheses of cryptoflavanones C and D, oboflavanones A and B, and cryptoyunnanones G and H enabled by an acid-triggered cascade sequence

Collective total syntheses of oboflavanones A-B, cryptoflavanones C-D, and cryptoyunnanones G-H via a bioinspired acid-triggered olefin isomerization/hemiacetalization/dehydration/formal [3 + 3]-type cycloaddition cascade process are presented.

Total syntheses of ericifolione and its analogues via a biomimetic inverse-electron-demand Diels-Alder reaction

Driven by bioinspiration and appreciation of the structure of ericifolione, a biomimetic tautomerization/intermolecular inverse-electron-demand hetero Diels-Alder reaction cascade sequence promoted by sodium acetate to rapidly construct sterically hindered dihydropyran scaffolds was established, which allowed the first straightforward biomimetic total syntheses of ericifolione and its analogues with high simplicity. Moreover, this methodology set the stage for the preparation of relevant natural products or derivatives.

Acylphloroglucinol trimers from Callistemon salignus seeds: Isolation, configurational assignment, hAChE inhibitory effects, and molecular docking studies

Alzheimer's disease (AD) diagnoses are greatly increasing in frequency as the global population ages, highlighting an urgent need for new anti-AD strategies. With the aim to search for human acetylcholinesterase (hAChE) inhibitors from the species of Myrtaceae family, ten acylphloroglucinol trimers (APTs), including eight new APTs, callistemontrimers A-H (1a, 1b, 2a, 2b, 3a, 3b, 4b, and 5b), and two naturally occurring ones (4a and 5a), along with one reported triketone-acylphloroglucinol-monoterpene adduct (6), were obtained and structurally characterized from the hAChE inhibitory acetone extract of Callistemon salignus seeds. The structures and their absolute configurations for new APTs were unequivocally established via the detailed interpretation of extensive spectroscopic data (HRESIMS and NMR), ECD calculations, and single crystal X-ray diffraction, whereas the absolute configurations of known APTs were determined by further chiral separation, and calculated ECD calculations. The results of hAChE inhibitory assay revealed that an enantiomeric mixture of 2a/2b, 2a, and 2b are good hAChE inhibitors with IC₅₀ values of 1.22 ± 0.23, 2.28 ± 0.19, and 4.96 ± 0.39 μM, respectively. Molecular docking was used to uncover the modes of interactions for bioactive compounds with the active site of hAChE. In addition, 2 and 6 displayed moderate neurite outgrowth-promoting effects with differentiation rates of 6.16% and 6.19% at a concentration of 1.0 μM, respectively.

Total syntheses of melodienones by redox isomerization of propargylic alcohols

A remarkable base-promoted methodology for the rapid construction of the (E)- and (Z)-γ-oxo-α,β-alkenoic ester skeletons from readily accessible vinyl propargylic alcohols through modified redox isomerization was uncovered. This approach manifested its high simplicity and efficiency with excellent tolerance of functional substituents, which led to the straightforward structural modifications of various natural products and efficient total syntheses of melodienone, homomelodienone, isomelodienone, and homoisomelodienone within 4 linear steps.

Rhodomentosones A and B: Two Pairs of Enantiomeric Phloroglucinol Trimers from Rhodomyrtus tomentosa and Their Asymmetric Biomimetic Synthesis

Rhodomentosones A and B (1 and 2), two pairs of novel enantiomeric phloroglucinol trimers featuring a unique 6/5/5/6/5/5/6-fused ring system were isolated from Rhodomyrtus tomentosa. Their structures with absolute configurations were elucidated by NMR spectroscopy, X-ray crystallography, and ECD calculation. The bioinspired syntheses of 1 and 2 were achieved in six steps featuring an organocatalytic asymmetric dehydroxylation/Michael addition/Kornblum-DeLaMare rearrangement/ketalization cascade reaction. Compounds 1 and 2 exhibited promising antiviral activities against respiratory syncytial virus (RSV).