Semisynthesis and Biological Evaluation of Platensimycin Analogues with Varying Aminobenzoic Acids

Alkoxyalkylation of Electron-Rich Aromatic Compounds

Alkoxyalkylation and hydroxyalkylation methods utilizing oxo-compound derivatives such as aldehydes, acetals or acetylenes and various alcohols or water are widely used tools in preparative organic chemistry to synthesize bioactive compounds, biosensors, supramolecular compounds and petrochemicals. The syntheses of such molecules of broad relevance are facilitated by acid, base or heterogenous catalysis. However, degradation of the N-analogous Mannich bases are reported to yield alkoxyalkyl derivatives via the retro-Mannich reaction. The mutual derivative of all mentioned species are quinone methides, which are reported to form under both alkoxy- and aminoalkylative conditions and via the degradation of the Mannich-products. The aim of this review is to summarize the alkoxyalkylation (most commonly alkoxymethylation) of electron-rich arenes sorted by the methods of alkoxyalkylation (direct or via retro-Mannich reaction) and the substrate arenes, such as phenolic and derived carbocycles, heterocycles and the widely examined indole derivatives.

Using a Bioactive Eremophila-Derived Serrulatane Scaffold to Generate a Unique Carbamate Library for Anti-infective Evaluations

The known Eremophila microtheca-derived diterpenoid 3,7,8-trihydroxyserrulat-14-en-19-oic acid (1) was targeted for large-scale purification, as this bioactive plant compound has proven to be an attractive scaffold for semisynthetic studies and subsequent library generation. Compound 1 was converted to a selectively protected trimethyl derivative, 3-hydroxy-7,8-dimethoxyserrulat-14-en-19-oic acid methyl ester (2), using simple and rapid methylation conditions. The resulting scaffold 2 was reacted with a diverse series of commercially available isocyanates to generate an 11-membered carbamate-based library. The chemical structures of the 11 new semisynthetic analogues were fully characterized by spectroscopic and spectrometric analysis. All natural products and semisynthetic compounds were evaluated for their anthelmintic, antimalarial, and anti-HIV activities. Compound 3 was shown to elicit the greatest antiplasmodial activity of all compounds tested, with IC₅₀ values of 4.6 and 11.6 μM against Plasmodium falciparum 3D7 and Dd2, respectively. Compound 11 showed the greatest inhibition of development to fourth-stage Haemonchus contortus larvae (L4) and induction of a skinny (Ski) phenotype (67.5% of nematodes) at 50 μM. Compound 7, which inhibited 59.0% of HIV production at 100 μg/mL, was the carbamate analogue that displayed the best antiviral activity.

Morphing Natural Product Platensimycin via Heck, Sonogashira, and One-Pot Sonogashira/Cycloaddition Reactions to Produce Antibiotics with In Vivo Activity

Type II fatty acid synthases are promising drug targets against major bacterial pathogens. Platensimycin (PTM) is a potent inhibitor against β-ketoacyl-[acyl carrier protein] synthase II (FabF) and β-ketoacyl-[acyl carrier protein] synthase I (FabB), while the poor pharmacokinetics has prevented its further development. In this work, thirty-two PTM derivatives were rapidly prepared via Heck, Sonogashira, and one-pot Sonogashira/cycloaddition cascade reactions based on the Gram-scale synthesis of 6-iodo PTM (4). About half of the synthesized compounds were approximately equipotent to PTM against the tested Staphylococcus aureus strains. Among them, the representative compounds 4, A4, and B8 exhibited different plasma protein binding affinity or stability in the human hepatic microsome assay and showed improved in vivo efficacy over PTM in a mouse peritonitis model. In addition, A4 was also effective in an S. aureus-infected skin mouse model. Our study not only significantly expands the known PTM derivatives with improved antibacterial activities in vivo, but showcased that C–C cross-coupling reactions are useful tools to functionalize natural product drug leads.

An Experimental Toolbox for Structure-Based Hit Discovery for P. aeruginosa FabF, a Promising Target for Antibiotics

FabF (3-oxoacyl-[acyl-carrier-protein] synthase 2), which catalyses the rate limiting condensation reaction in the fatty acid synthesis II pathway, is an attractive target for new antibiotics. Here, we focus on FabF from P. aeruginosa (PaFabF) as antibiotics against this pathogen are urgently needed. To facilitate exploration of this target we have set up an experimental toolbox consisting of binding assays using bio-layer interferometry (BLI) as well as saturation transfer difference (STD) and WaterLOGSY NMR in addition to robust conditions for structure determination. The suitability of the toolbox to support structure-based design of FabF inhibitors was demonstrated through the validation of hits obtained from virtual screening. Screening a library of almost 5 million compounds resulted in 6 compounds for which binding into the malonyl-binding site of FabF was shown. For one of the hits, the crystal structure in complex with PaFabF was determined. Based on the obtained binding mode, analogues were designed and synthesised, but affinity could not be improved. This work has laid the foundation for structure-based exploration of PaFabF.

Late-Stage Functionalization of Platensimycin Leading to Multiple Analogues with Improved Antibacterial Activity in Vitro and in Vivo

Bacterial fatty acid synthases are promising antibacterial targets against multidrug-resistant pathogens. Platensimycin (PTM) is a potent FabB/FabF inhibitor, while its poor pharmacokinetics hampers the clinical development. In this study, a focused library of PTM derivatives was prepared through thiolysis of PTM oxirane (1), followed by various C-C cross-coupling reactions in high yields. Antibacterial screening of these compounds in vitro yielded multiple hits with improved anti-Staphylococcus activities over PTM. Among them, compounds A1, A3, A17, and A28 exhibited improved antibacterial activities over PTM against methicillin-resistant Staphylococcus aureus (MRSA) in a mouse peritonitis model. Compound A28 was further shown to be effective against MRSA infection in a mouse wound model, in comparison to mupirocin. Therefore, the facile preparation and screening of these PTM derivatives, together with their potent antibacterial activities in vivo, suggest a promising strategy to improve the antibacterial activity and pharmacokinetic properties of PTM.

Evaluation of Platensimycin and Platensimycin-Inspired Thioether Analogues against Methicillin-Resistant Staphylococcus aureus in Topical and Systemic Infection Mouse Models

Staphylococcus aureus is one of the most common pathogens causing hospital-acquired and community-acquired infections. Methicillin-resistant S. aureus (MRSA)-formed biofilms in wounds are difficult to treat with conventional antibiotics. By targeting FabB/FabF of bacterial fatty acid synthases, platensimycin (PTM) was discovered to act as a promising natural antibiotic against MRSA infections. In this study, PTM and its previously synthesized sulfur-Michael derivative PTM-2t could reduce over 95% biofilm formation by S. aureus ATCC 29213 when used at 2 μg/mL in vitro. Topical application of ointments containing PTM or PTM-2t (2 × 4 mg/day/mouse) was successfully used to treat MRSA infections in a BABL/c mouse burn wound model. As a potential prodrug lead, PTM-2t showed improved in vivo efficacy in a mouse peritonitis model compared with PTM. Our study suggests that PTM and its analogue may be used topically or locally to treat bacterial infections. In addition, the use of prodrug strategies might be instrumental to improve the poor pharmacokinetic properties of PTM.

Stereoselective functionalization of platensimycin and platencin by sulfa-Michael/aldol reactions

Bioinspired sulfa-Michael/aldol cascade reactions have been developed for the semisynthesis of sulfur-containing heterocyclic derivatives of platensimycin and platencin, with three newly formed contiguous stereogenic centers. Density functional theory calculations revealed the mechanism for the stereochemistry control. This method was used in a synthesis of a platensimycin thiophene analogue with potent antibacterial activities against Staphylococcus aureus.