Synthesis and bio-inspired optimization of drimenal: Discovery of chiral drimane fused oxazinones as promising antifungal and antibacterial candidates

Design, synthesis, and exploration of antibacterial activity of 6H-1,2-oxazin-6-ones

This study reports the in silico design of 30 6H-1,2-oxazin-6-ones against DHFR and PTC antimicrobial targets. Docking compounds 1, 3, 4, 6, and 8 with both enzymes was favorable, outperforming Trimethoprim with DHFR. Therefore, 12 6H-1,2-oxazin-6-ones, including the most promising compounds, were synthesized through an aminolysis reaction of β-cyanoketones with hydroxylamine hydrochloride, obtaining moderate to high yields (55-88%). Subsequently, antibacterial studies were conducted against five bacteria: four Gram-positive MRSA (ATCC 43300 and three clinical isolates) and one Gram-negative (E. coli ATCC 25922). Compounds 1, 2, 3, 4, 6, and 8 inhibited bacterial growth with MIC values ranging from 3.125 to 200 μg mL-1. Compound 1 showed better activity against Gram-positive bacteria than Linezolid. Toxicity assays indicated no adverse effects of the active oxazinones in silico and in vitro. This study demonstrated the antibacterial potential of the selected 6H-1,2-oxazin-6-ones against resistant human pathogenic bacteria.

Unlocking flavin photoacid catalysis through electrophotochemistry

Molecular flavins are one of the most versatile photocatalysts. They can coordinate single and multiple electron transfer processes, gift hydrogen atoms, form reversible covalent linkages that support group transfer mechanisms, and impart photonic energy to ground state molecules, priming them for downstream reactions. But one mechanism that has not featured extensively is the ability of flavins to act as photoacids. Herein, we disclose our proof-of-concept studies showing that electrophotochemistry can transform fully oxidized flavin quinones to super-oxidized flavinium photoacids that successfully guide proton-transfer and deliver acid-catalyzed products. We also show that these species can adopt a second mechanism wherein they react with water to release hydroxyl radicals that facilitate hydrogen-atom abstraction and sp3C-H functionalization protocols. Together, this unprecedented bimodal reactivity enables electro-generated flavinium salts to affect synthetic chemistries previously unknown to flavins, greatly expanding their versatility as catalysts.

Divergent and Stereoselective Synthesis of Ustusal A, (-)-Albrassitriol, and Elegansin D

The first synthesis of ustusal A as well as expeditious access to (-)-albrassitriol is described as featuring a singlet oxygen [4 + 2] cycloaddition, achieving the desired stereoselectivity for the 1,4-cis-hydroxyl groups. Transformation of (+)-sclareolide to III followed by a key Horner-Wadsworth-Emmons (HWE) reaction and stereospecific allylic oxidation facilitated the first synthesis of elegansin D. The biological evaluation of these natural products together with seven elegansin D analogues was performed, among which several elegansin D analogues exhibited potential anticancer activity against liver cancer HepG2 cells (IC50 = 11.99-25.58 μM) with low cytotoxicity on normal liver HL7702 cells (IC50 > 100 μM).

Bioactivity-Guided Subtraction of MIQOX for Easily Available Isoquinoline Hydrazides as Novel Antifungal Candidates

The discovery of novel and easily available leads provides a convincing solution to agrochemical innovation. A bioassay-guided scaffold subtraction of the previous "Chem-Bio Model" isoquinoline-3-oxazoline MIQOX was conducted for identifying the easily available isoquinoline-3-hydrazide as a novel antifungal scaffold. The special and practical potential of this model was demonstrated by a phenotypic antifungal bioassay, molecular docking, and cross-resistance evaluation. A panel of antifungal leads (LW2, LW3, and LW11) was acquired, showing much better antifungal performance than the positive controls. Specifically, compound LW3 exhibited a broad antifungal spectrum holding EC₅₀ values as low as 0.54, 0.09, 1.52, and 2.65 mg/L against B. cinerea, R. solani, S. sclerotiorum , and F. graminearum, respectively. It demonstrated a curative efficacy better than that of boscalid in controlling the plant disease caused by B. cinerea. The candidate LW3 did not show cross-resistance to the extensively used succinate dehydrogenase inhibitor (SDHI) fungicides and can efficiently inhibit resistant B. cinerea strains. The molecular docking of compound LW3 is quite different from that of the positive controls boscalid and fluopyram. This progress highlights the practicality of isoquinoline hydrazide as a novel model in fungicide innovation.

Isolation, identification, and antibacterial evaluation of endophytic fungi from Gannan navel orange

Gannan navel orange is a famous brand in China but the isolation of its endophytic fungi was rarely reported. In this study, a total of 54 strains of endophytic fungi were successfully isolated from the pulp, peel, twig, and leaf of Gannan navel orange; they were successfully identified to belong to 17 species of 12 genera. All these strains were fermented using potato-dextrose agar (PDA) medium, and their secondary metabolites were then extracted with ethyl acetate (EtOAc). The antibacterial assays of Escherichia coli (E. coli), methicillin-resistant Staphylococcus aureus (MRSA), and Xanthomonas citri subsp. citri (Xcc) were also performed for the EtOAc extracts of these strains. As a result, the extracts of both Geotrichum sp. (gc-1-127-30) and Diaporthe biconispora (gc-1-128-79) demonstrated significant antibacterial activities against Xcc, and the MIC value for the extract of Colletotrichum gloeosporioides against MRSA was low to 62.5 μg/mL. Moreover, the chemical components of the extracts of Colletotrichum sp., Diaporthe biconispora, and Annulohypoxylon atroroseum were primarily investigated, and they successfully led to the isolation of 24 compounds involving a new botryane sesquiterpene. Among the isolated products, compound 2 showed significant inhibitory activities toward SA, MRSA, E. coli, and Xcc with MIC values of 12.5, 3.1, 125, and 12.5 μg/mL, respectively. This study revealed that the endophytic fungi of Gannan navel orange showed high potency to produce secondary metabolites with significant antibacterial effects.

Synthesis of Aminoalkyl Sclareolide Derivatives and Antifungal Activity Studies

Sclareolide was developed as an efficient C-nucleophilic reagent for an asymmetric Mannich addition reaction with a series of N-tert-butylsulfinyl aldimines. The Mannich reaction was carried out under mild conditions, affording the corresponding aminoalkyl sclareolide derivatives with up to 98% yield and 98:2:0:0 diastereoselectivity. Furthermore, the reaction could be performed on a gram scale without any reduction in yield and diastereoselectivity. Additionally, deprotection of the obtained Mannich addition products to give the target sclareolide derivatives bearing a free N-H group was demonstrated. In addition, target compounds 4-6 were subjected to an antifungal assay in vitro, which showed considerable antifungal activity against forest pathogenic fungi.

Cytotoxic Activity, Topoisomerase I Inhibition and In Silico Studies of New Sesquiterpene-aryl Ester Derivatives of (-) Drimenol

In this study, we aimed to evaluate two sets of sesquiterpene-aryl derivatives linked by an ester bond, their cytotoxic activities, and their capacity to activate caspases 3/7 and inhibit human topoisomerase I (TOP1). A total of 13 compounds were synthesized from the natural sesquiterpene (-)-drimenol and their cytotoxic activity was evaluated in vitro against three cancer cell lines: PC-3 (prostate cancer), HT-29 (colon cancer), MCF-7 (breast cancer), and an immortalized non-tumoral cell line (MCF-10). From the results, it was observed that 6a was the most promising compound due to its cytotoxic effect on three cancer cell lines and its selectivity, 6a was 100-fold more selective than 5-FU in MCF-7 and 20-fold in PC-3. It was observed that 6a also induced apoptosis by caspases 3/7 activity using a Capsase-Glo-3/7 assay kit and inhibited TOP1. A possible binding mode of 6a in a complex with TOP1-DNA was proposed by docking and molecular dynamics studies. In addition, 6a was predicted to have a good pharmacokinetic profile for oral administration. Therefore, through this study, it was demonstrated that the drimane scaffold should be considered in the search of new antitumoral agents.

Synthesis and Biological Evaluation of Sclareolide-Indole Conjugates and Their Derivatives

Sclareolide is a sesquiterpene lactone isolated from various plant sources in tons every year and is commercially used as a flavor ingredient in the cosmetic and food industries. Antitumor and antiviral activities of sclareolide have been previously reported. However, biological studies of sclareolide synthetic analogous are few. In view of these, we developed a robust synthetic method that allows the assembly of 36 novel sclareolide-indole conjugates and their derivatives. The synthetic method was based on TiCl₄-promoted nucleophilic substitution of sclareolide-derived hemiacetal 4, while electron-rich aryles including indoles, polyphenol ethers, and pyrazolo [1,5-a]pyridine were good substrates. The stereochemistry of the final products was confirmed by single-crystal X-ray diffraction analysis, while the antiproliferative activities of selected final products were tested in K562 and MV4-11 cancer cell lines. Cytometric flow analysis shows that lead compounds 8k- and 10-induced robust apoptosis in MV4-11 cancer cells, while they exhibited weak impact on cell cycle progression. Taken together, our study suggests that sclareolide could be a good template and substrate for the synthesis of novel antiproliferative compounds.

Asymmetric Synthesis of Sakuranetin-Relevant Flavanones for the Identification of New Chiral Antifungal Leads

Discovery and efficient synthesis of new promising leads have a central role in agrochemical science. Reported herein is the sakuranetin-directed synergistic exploration of an asymmetric synthesis and an antifungal evaluation of chiral flavanones. A new palladium catalytic system with CarOx-type ligands was successfully identified for the highly enantioselective addition of arylboronic acids to chromones. This enabled the facile and programmable construction of a constellation of chiral flavanones (up to 98% yield and 97% ee), in which (R)-pinostrobin was efficiently constructed without laborious protecting/deprotecting operations. Its good performance in asymmetric induction and functional tolerance expanded the chemical space of pharmaceutically important flavanones. The chiral differentiation of flavanones based on antifungal activity and a concise structure-activity relationship model was disclosed and summarized. This synergistic project culminated with acquisition of the naturally unprecedented flavanones with better antifungal potentials than sakuranetin, in which the R-enantiomer of flavanone 54 (EC₅₀ = 0.8 μM) demonstrated better performance than boscalid against Rhizoctonia solani. The novel scaffold and predicted new target compared with the commercial fungicides in the FRAC reinforce the value of further exploration.

Facile and divergent optimization of chromazonarol enabled the identification of simplified drimane meroterpenoids as novel pharmaceutical leads

The diversity of drimane hydroquinones was significantly expanded by the facile construction of (+)-chromazonarol relevant natural products, isomers, and analogues for the discovery of new pharmaceutical leads. The structure-activity relationship of (+)-chromazonarol relevant (non)-natural products was delineated via the synergistic interaction of the programmable synthesis and bioactivity-guided screening. The first divergent derivatization of (+)-chromazonarol demonstrated that the phenolic hydroxyl group is one inviolable requirement for antifungal effect. Pinpoint modification of (+)-yahazunol manifested the position of hydroxyl group was crucial for both antifungal and antitumor activities. (+)-Albaconol, (+)-neoalbaconol, and two (+)-yahazunol isomers (24 and 25) proved to be the novel pharmaceutical leads. The probable macromolecular targets were estimated to deliver new information about the biological potentials resident in (+)-yahazunol relevant products. This work also featured the first synthesis of (+)-albaconol and (+)-neoalbaconol, the first biological exploration of (+)-dictyvaric acid and improved preparation of (+)-8-epi-puupehedione and a promising pelorol analogue.

Organocatalytic β,γ-Selective Activation of Deconjugated Butenolides Access to Chiral Tricyclic Chroman-butyrolactones

A highly efficient method for the β,γ-selective activation of deconjugated butenolides has been developed through an organocatalytic asymmetric vinylogous cascade reaction. This protocol enables the construction of a broad range of substituted tricyclic chroman-butyrolactones by vinylogous Michael/oxa-Michael pathways in good yield (up to 89%) with good to high enantioselectivity (up to 97:3 er) and excellent diastereoselectivity. The ring-opening esterification of butyrolactones was also demonstrated.

Structure-antifungal relationships and preventive effects of 1-(2,4-dihydroxyphenyl)-2-methylpropan-1-one derivatives as potential inhibitors of class-II fructose-1,6-bisphosphate aldolase

Emerging fungal phytodiseases are a food security threat and novel fungicides are in an urgent need. Herein, a series of isobutyrophenone derivatives were designed and synthesized. The derivatives exhibited excellent fungicidal activities against seven fungi. The structure-activity relationship (SAR) study indicated that the introduction of a bromo group at the position 3 or 5 of the phenyl ring, as well as esterification of the 4-hydroxy with a chloroacetyl group, could substantially increase the antifungal activity and spectrum of the compounds. Among all 23 compounds, 2-bromo-3-hydroxy-4-isobutyryl-6-methylphenyl 2-chloroacetate (12b) showed the highest fungicidal activity against all seven tested fungal pathogens with EC₅₀ values ranging from 1.22 to 39.94 μg/mL and exhibited the most potent inhibition against class II fructose-1,6-bisphosphate aldolase with an IC₅₀ of 3.63 μM. The lead compounds were proven to be safe to NIH3T3/293 T cells and silkworm larvae, and relatively stable under different harsh conditions. Detached fruit tests showed the practical potential of lead compounds for fruit (or plant) protection. Taken together, our results indicated that the isobutyrophenone derivatives could be further optimized and developed as advanced leads for new fungicides.

Synthesis and In Vitro Antibacterial Activity of Quaternized 10-Methoxycanthin-6-one Derivatives

Background: Based on our previous work, we found that 10-methoxycanthin-6-one displayed potential antibacterial activity and quaternization was an available method for increasing the antibacterial activity. Here, we explored the antibacterial activity of quaternized 10-methoxy canthin-6-one derivatives. Methods and Results: Twenty-two new 3-N-benzylated 10-methoxy canthin-6-ones were designed and synthesized through quaternization reaction. The in vitro antibacterial activity against three bacteria was evaluated by the double dilution method. Moreover, the structure–activity relationships (SARs) were carefully summarized in order to guide the development of antibacterial canthin-6-one agents. Two highly active compounds (6p and 6t) displayed 8-fold superiority (MIC = 3.91 µg/mL) against agricultural pathogenic bacteria R. solanacearum and P. syringae compared to agrochemical streptomycin sulfate, and showed potential activity against B. cereus. Moreover, these two compounds exhibited good “drug-like” properties, low cytotoxicity, and no inhibition on seed germination. Conclusions: This work provides two new effective quaternized canthin-6-one derivatives as candidate bactericide, promoting the development of natural-sourced bactericides and preservatives.

Expediently Scalable Synthesis and Antifungal Exploration of (+)-Yahazunol and Related Meroterpenoids

The efficient synthesis and antifungal exploration of (+)-yahazunol and related natural products are described. Central to this strategy is the Barton decarboxylative coupling, comprising a one-pot radical decarboxylation and quinone addition cascade. The scalable synthesis of (+)-yahazunol was accomplished in five longest linear sequences (LLS) starting from commercially available and inexpensive (-)-sclareol. The divergent translational potential of (+)-yahazunol was demonstrated by the expedient preparation of (-)-zonarone, (-)-isozonarone, (-)-zonarol, (-)-isozonarol, (+)-chromazonarol, and (+)-yahazunone. This approach also enables the formal synthesis of puupehenol, puupehedione, and hongoquercin A. Antifungal evaluation was performed, and this represents the first biological profiles for (+)-yahazunone, (+)-8- O-acetylyahazunone, and (+)-8- O-acetylyahazunol. (+)-Chromazonarol and (+)-yahazunone are promising candidates against Sclerotinia scleotiorum, with EC₅₀ values of 24.1 and 28.7 μM, respectively, demonstrating advantages over the original model (DM) and synthesized heterocyclic mimic (3a) of meroterpenoids. This will favor the establishment of a chemical repertoire in the management of different plant diseases.

Design and Discovery of Novel Chiral Antifungal Amides with 2-(2-Oxazolinyl)aniline as a Promising Pharmacophore

Inspired by established succinate dehydrogenase inhibitors (SDHIs), our continuing efforts toward the discovery of chiral antifungal amides turned to the optimization of their polar regions with 2-(2-oxazolinyl)aniline as a known pharmacophore. Scaffold hopping and bioactivity-guided convergent synthesis enabled the identification of promising antifungal categories. Fine tuning of the substituents and chirality furnished seven amides (1s, 1t, 2d, 2h, 2j, 3k, and 2l) as antifungal candidates, with EC₅₀ values lower than 5 mg/L. The first investigation of chiral amides of acyclic acids as SDHIs was conducted, and compound 2d was selected as a promising candidate against Botrytis cinerea, with a preventative efficacy of up to 93.9% at 50 mg/L, which is better than that of boscalid. The different binding models between compounds with different configurations were simulated for compound 2d and its diastereoisomers. The benefits of synthetic accessibility and cost-effectiveness highlight the practical potential for compound 2d as a good alternative to known SDHI fungicides.