Unified Syntheses of Cavicularin and Riccardin C: Addressing the Synthesis of an Arene Adopting a Boat Configuration

Exploring the plasticity of the InhA substrate-binding site using new diaryl ether inhibitors

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), remains a worldwide scourge with more than 10 million people affected yearly. Among the proteins essential for the survival of Mtb, InhA has been and is still clinically validated as a therapeutic target. A new family of direct diaryl ether inhibitors, not requiring prior activation by the catalase peroxidase enzyme KatG, has been designed with the ambition of fully occupying the InhA substrate-binding site. Thus, eleven compounds, featuring three pharmacophores within the same molecule, were synthesized. One of them, 5-(((4-(2-hydroxyphenoxy)benzyl)(octyl)amino)methyl)-2-phenoxyphenol (compound 21), showed good inhibitory activity against InhA with IC50 of 0.70 µM. The crystal structure of compound 21 in complex with InhA/NAD+ showed how the molecule fills the substrate-binding site as well as the minor portal of InhA. This study represents a further step towards the design of new inhibitors of InhA.

Discovery of new diaryl ether inhibitors against Mycobacterium tuberculosis targeting the minor portal of InhA

Tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) affects 10 million people each year and the emergence of resistant TB augurs for a growing incidence. In the last 60 years, only three new drugs were approved for TB treatment, for which resistances are already emerging. Therefore, there is a crucial need for new chemotherapeutic agents capable of eradicating TB. Enzymes belonging to the type II fatty acid synthase system (FAS-II) are involved in the biosynthesis of mycolic acids, cell envelope components essential for mycobacterial survival. Among them, InhA is the primary target of isoniazid (INH), one of the most effective compounds to treat TB. INH acts as a prodrug requiring activation by the catalase-peroxidase KatG, whose mutations are the major cause for INH resistance. Herein, a new series of direct InhA inhibitors were designed based on a molecular hybridization approach. They exhibit potent inhibitory activities of InhA and, for some of them, good antitubercular activities. Moreover, they display a low toxicity on human cells. A study of the mechanism of action of the most effective molecules shows that they inhibit the biosynthesis of mycolic acids. The X-ray structures of two InhA/NAD+/inhibitor complexes have been obtained showing a binding mode of a part of the molecule in the minor portal, rarely seen in the InhA structures reported so far.

Bisbibenzyls from Serbian Primula veris subsp. Columnae (Ten.) Lȕdi and P. acaulis (L.) L

Bisbibenzyls are specialized metabolites found exclusively in liverworts, until recently; they represent chemical markers of liverworts. Their occurrence in vascular plants was noticed in 2007, when they were found in Primula veris subsp. Macrocalyx from Russia. This report prompted us to chemically analyze the two most common Serbian Primula species, P. veris subsp. Columnae and P. acaulis, in order to determine the presence of bisbibenzyls in them. Our study revealed nine structurally distinct bisbibenzyls (1-9), identified based on 1D and 2D NMR, IR, UV and HRESIMS data. Among them were five previously undescribed compounds (2-6). The remaining compounds found and previously described in the literature were: the bisbibenzyls riccardin C (1), isoperrottetin A (7), isoplagiochin E (8) and 11-O-demethylmarchantin I (9), as well as 4-hydroxyphenylmethylketone (10) and 4-hydroxy-3-methoxyphenylmethylketone (11). Riccardin C was the most dominant bisbibenzyl in both species studied. Previously, it was the first bisbibenzyl found in vascular plants (P. veris subsp. macrocalyx). An assessment of the cytotoxic activity of the isolated compounds against A549 lung cancer and healthy MRC5 cell lines was also the subject of our study. Compounds 6 and 9 exhibited significant cytotoxic activity expressed by IC₅₀ values of 12 μM, but the selectivity was not satisfactory.

One-Pot Tandem Coupling Method for the Short-Step Formal Synthesis of Riccardin C

One-pot reactions reduce reagent amounts and circumvent process treatments, such as work-up and purifications in multi-step reactions. In this study, we achieved the formal total synthesis of riccardin C through a one-pot reaction by simultaneously linking four units through two Sonogashira coupling reactions and one Suzuki coupling reaction, followed by reduction and deprotection. Thus, this one-pot method comprised five steps and did not require the purification of intermediate reaction mixtures, which saves resources, such as reagents and solvents, and expedites the work process.

Bis-bibenzyls, Bibenzyls, and Terpenoids in 33 Genera of the Marchantiophyta (Liverworts): Structures, Synthesis, and Bioactivity

The Marchantiophyta (liverworts) are rich sources of phenolic substances, especially cyclic and acyclic bis-bibenzyls, which are rare natural products in the plant kingdom, together with bibenzyls and characteristic terpenoids. At present, more than 125 bis-bibenzyls have been found in liverworts. They are biosynthesized from the dimerization of lunularic acid via dihydrocoumaric acid and prelunularin. The structurally unusual cyclic and acyclic bis-bibenzyls show various biological activities such as antimicrobial, antifungal, cytotoxic, muscle relaxation, antioxidant, tubulin polymerization inhibitory, and antitrypanosomal activities, among others. The present review article deals with the distribution and structure of bis-bibenzyls, bibenzyls, and several characteristic ent-sesqui- and diterpenoids in liverworts. Furthermore, the biosynthesis and total syntheses and biological activities of bis-bibenzyls are also surveyed.

Direct Synthesis of Polyaromatic Cyclophanes Containing Bis-Methylene-Interrupted Z-Double Bonds and Study of Their Antitumor Activity In Vitro

An original synthetic route was developed for the preparation of previously unknown unsaturated polyaromatic macrolactones containing a 1Z,5Z-diene moiety in 48–71% yields and with >98% stereoselectivity. The method is based on intermolecular cyclocondensation of aromatic dicarboxylic acids with α,ω-alka-nZ,(n+4)Z-dienediols (1,12-dodeca-4Z,8Z-dienediol, 1,14-tetradeca-5Z,9Z-dienediol, 1,18-octadeca-7Z,11Z-dienediol) mediated by N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC)/4-dimethylaminopyridine (DMAP). The unsaturated diols were prepared by successive homo-cyclomagnesiation of tetrahydropyran ethers of O-containing 1,2-dienes with EtMgBr in the presence of Mg metal and the Cp₂TiCl₂ catalyst (10 mol.%) and subsequent treatment with 0.1 equiv. of para-toluenesulfonic acid of pyran ethers formed after the acid hydrolysis of magnesacyclopentanes. The resulting cyclophanes exhibited high cytotoxic activity in vitro against Jurkat, K562, U937, and HL60 cancer lines. Additionally, the synthesized products were studied for their effect on mitochondria, ability to induce apoptosis, and influence on the cell cycle using modern flow cytometry methods.

One-Step Synthesis of Triphenylphosphonium Salts from (Het)arylmethyl Alcohols

Two approaches for the synthesis of substituted phosphonium salts from easily available benzyl alcohols and their heterocyclic analogs have been developed. The developed protocols are complementary: the direct mixing of alcohol, trimethylsilyl bromide, and triphenylphosphine in 1,4-dioxane followed by heating at 80 °C was found to be more efficient for acid-sensitive substrates, such as salicyl or furfuryl alcohols as well as secondary benzyl alcohols, while a one-pot procedure including sequential addition of trimethylsilyl bromide and triphenylphosphine gave higher yields for benzyl alcohols bearing electroneutral or electron-withdrawing substituents.

A synthetic approach to chrysophaentin F

A synthetic approach to chrysophaentin F is described featuring an array of metal catalysed coupling reactions (Cu, Ni, Pd, W, Mo).

A new era for homolytic aromatic substitution: replacing Bu3SnH with efficient light-induced chain reactions

Herein is a pertinent review of recent photochemical homolytic aromatic substitution (HAS) literature. Issues with using the reductant Bu3SnH in an oxidative process where the net loss of a hydrogen atom occurs is discussed. Nowadays more efficient light-induced chain reactions are used resulting in HAS becoming a synthetic mechanism of choice rivaling organometallic, transition-metal and electrophilic aromatic substitution protocols. The review includes aromatic substitution as part of a tandem or cascade reaction, Pschorr reaction, as well as HAS facilitated by ipso-substitution, and Smiles rearrangement. Recently visible-light photoredox catalysis, which is carried out at room temperature has become one of the most important means of aromatic substitution. The main photoredox catalysts used are polypyridine complexes of Ru(ii) and Ir(iii), although eosin Y is an alternative allowing metal-free HAS. Other radical initiator-free aromatic substitutions have used 9-mesityl-10-methylacridinium ion and N,N-bis(2,6-diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide) as the photoredox catalyst, UV-light, photoinduced electron-transfer, zwitterionic semiquinone radical anions, and Barton ester intermediates.

Selected synthetic strategies to cyclophanes

In this review we cover various approaches to meta- and paracyclophanes involving popular reactions. Generally, we have included a strategy where the reaction was used for assembling the cyclophane skeleton for further functionalization. In several instances, after the cyclophane is made several popular reactions are used and these are not covered here. We included various natural products related to cyclophanes. To keep the length of the review at a manageable level the literature related to orthocyclophanes was not included.

Anti-MRSA activity of isoplagiochin-type macrocyclic bis(bibenzyl)s is mediated through cell membrane damage

We synthesized three geometrical isomers of a macrocyclic bis(bibenzyl) based on isoplagiochin, a natural product isolated from bryophytes, and evaluated their antibacterial activity towards methicillin-resistant Staphylococcus aureus (anti-MRSA activity). The isomer containing a 1,4-linked ring (5) showed only weak activity, whereas the isomers containing a 1,3-linked (6) or 1,2-linked (7) C ring showed potent anti-MRSA activity. Molecular dynamics calculations indicated that these differences are probably due to differences in the conformational flexibility of the macrocyclic ring; the active compounds 6 and 7 were more rigid than 5. In order to understand the action mechanism of anti-MRSA activity, we investigated the cellular flux of a fluorescent DNA-binder, ethidium bromide (EtBr), in the presence and absence of these macrocycles. The active compound 6 increased the levels of EtBr inflow and outflow in S. aureus cells, as did our potent anti-MRSA riccardin derivative (4), indicating that these compounds increased the permeability of the cytoplasmic membrane. Inactive 5 had no effect on EtBr inflow or outflow. Furthermore, compound 6 abrogated the normal intracellular concentration gradients of Na(+) and K(+) in S. aureus cells, increasing the intracellular Na(+) concentration and decreasing the K(+) concentration, while 5 had no such effect. These results indicate that anti-MRSA-active macrocyclic bis(bibenzyl) derivatives directly damage the gram-positive bacterial membrane, resulting in increased permeability.

6-Azabicyclo[3.2.1]octanes Via Copper-Catalyzed Enantioselective Alkene Carboamination

Bridged bicyclic rings containing nitrogen heterocycles are important motifs in bioactive small organic molecules. An enantioselective copper-catalyzed alkene carboamination reaction that creates bridged heterocycles is reported herein. Two new rings are formed in this alkene carboamination reaction where N-sulfonyl-2-aryl-4-pentenamines are converted to 6-azabicyclo[3.2.1]octanes using [Ph-Box-Cu](OTf)₂ or related catalysts in the presence of MnO₂ as stoichiometric oxidant in moderate to good yields and generally excellent enantioselectivities. Two new stereocenters are formed in the reaction, and the C-C bond-forming arene addition is a net C-H functionalization.

Barton esters for initiator-free radical cyclisation with heteroaromatic substitution

S-(1-Oxido-2-pyridinyl)-1,1,3,3-tetramethylthiouronium hexafluorophosphate (HOTT) facilitates the first examples of efficient radical cyclisation with (hetero)aromatic substitution via Barton ester intermediates. Cyclopropyl and alkyl radicals allow access to five, six and seven-membered alicyclic-ring fused heterocycles with and without an additional fused cyclopropane, including the skeleton of the anti-cancer agent, cyclopropamitosene, expanded, and diazole analogues. Radical initiators are not required for cyclisation from carboxylic acid precursors.