A Concise and Highly Enantioselective Total Synthesis of (+)-anti- and (-)-syn-Mefloquine Hydrochloride: Definitive Absolute Stereochemical Assignment of the Mefloquines

Chiral separation and spectroscopic characterization of mefloquine analogues

Mefloquine, a widely used antimalarial agent, has spurred ongoing research into the development of derivatives with enhanced efficacy and reduced side effects. In this investigation, we synthesized two compounds containing N-allyl or N-tert-butylacetamid groups. A chiral liquid chromatography with polysaccharide chiral stationary phase was utilized to separate the enantiomers of both derivatives. We employed spectroscopic chiroptical and non-polarizable methods such as electronic and vibrational circular dichroism, infrared absorption and ultraviolet spectroscopies. Combined with density functional theory calculations, the stable conformers were found in solution and their spectra were subsequently simulated. We elucidated the three-dimensional structure of the enantiomerically pure compounds and assigned the absolute configuration of all prepared derivatives using both experimental and simulated spectra.

Bimetallic Ru/Ru-Catalyzed Asymmetric One-Pot Sequential Hydrogenations for the Stereodivergent Synthesis of Chiral Lactones

Asymmetric sequential hydrogenations of α-methylene γ- or δ-keto carboxylic acids are established in one-pot using a bimetallic Ru/Ru catalyst system, achieving the stereodivergent synthesis of all four stereoisomers of both chiral γ- and δ-lactones with two non-vicinal carbon stereocenters in high yields (up to 99%) and with excellent stereoselectivities (up to >99% ee and >20:1 dr). The compatibility of the two chiral Ru catalyst systems is investigated in detail, and it is found that the basicity of the reaction system plays a key role in the sequential hydrogenation processes. The protocol can be performed on a gram-scale with a low catalyst loading (up to 11000 S/C) and the resulting products allow for many transformations, particularly for the synthesis of several key intermediates useful for the preparation of chiral drugs and natural products.

Access to 2-thio/selenoquinolines via domino reaction of isocyanides with sulfur and selenium in water

A domino reaction of o-alkenylaryl isocyanides with elemental sulfur and selenium in pure water was developed for the efficient and green synthesis of quinoline-2-thione and diquinolyl diselenide derivatives. Mechanistical investigation reveals that intramolecular nucleophilic addition of an alkenyl group to the in situ generated isothio/isoselenocyanate accounts for the formation of a quinoline-ring. Moreover, this transformation is also amendable for the convenient preparation of 2-fluoromethylthio-/seleno-quinolines by a one-pot three-component reaction.

A Review of Modifications of Quinoline Antimalarials: Mefloquine and (hydroxy)Chloroquine

Late-stage modification of drug molecules is a fast method to introduce diversity into the already biologically active scaffold. A notable number of analogs of mefloquine, chloroquine, and hydroxychloroquine have been synthesized, starting from the readily available active pharmaceutical ingredient (API). In the current review, all the modifications sites and reactivity types are summarized and provide insight into the chemistry of these molecules. The approaches include the introduction of simple groups and functionalities. Coupling to other drugs, polymers, or carriers afforded hybrid compounds or conjugates with either easily hydrolyzable or more chemically inert bonds. The utility of some of the compounds was tested in antiprotozoal, antibacterial, and antiproliferative assays, as well as in enantiodifferentiation experiments.

Total Synthesis and Biological Evaluation of Paenilamicins from the Honey Bee Pathogen Paenibacillus larvae

Paenilamicins are a group of complex polycationic peptide secondary metabolites with antibacterial and antifungal activities produced by the devastating honey bee brood pathogen Paenibacillus larvae causing the lethal brood disease American Foulbrood (AFB). Here, we report the convergent total synthesis and structural revision of paenilamicin B2. Specific stereoisomers of paenilamicin B2 were synthesized for unambiguous confirmation of the natural product structure and for evaluation of biological activities. These studies revealed the N-terminal fragment of paenilamicin as an important pharmacophore. Infection assays using bee larvae and the insect pathogen Bacillus thuringiensis demonstrated that paenilamicins outcompete bacterial competitors in the ecological niche of P. larvae. Finally, we show first data that classifies paenilamicins as potential ribosome inhibitors. Hence, our synthesis route is a further step for understanding the pathogenicity of P. larvae and for thorough structure-activity-relationship as well as mode-of-action studies in the near future.

Chiral Vicinal Diamines Derived from Mefloquine

Novel 1,2-diamines based on the mefloquine scaffold prepared in enantiomerically pure forms resemble 9-amino-Cinchona alkaloids. Most effectively, 11-aminomefloquine with an erythro configuration was obtained by conversion of 11-alcohol into azide and hydrogenation. Alkylation of a secondary amine unit was needed to arrive at diastereomeric threo-11-aminomefloquine and to introduce diversity. Most of the substitution reactions of the hydroxyl group to azido group proceeded with net retention of the configuration and involved actual aziridine or plausible aziridinium ion intermediates. Enantiomerically pure products were obtained by the resolution of either the initial mefloquine or one of the final products. The evaluation of the efficacy of the obtained vicinal diamines in enantioselective transformations proved that erythro-11-aminomefloquine is an effective catalyst in the asymmetric Michael addition of nitromethane to cyclohexanone (up to 96.5:3.5 er) surpassing epi-aminoquinine in terms of selectivity.

Catalytic Enantioselective Pyridine N-Oxidation

The catalytic, enantioselective N-oxidation of substituted pyridines is described. The approach is predicated on a biomolecule-inspired catalytic cycle wherein high levels of asymmetric induction are provided by aspartic-acid-containing peptides as the aspartyl side chain shuttles between free acid and peracid forms. Desymmetrizations of bis(pyridine) substrates bearing a remote pro-stereogenic center substituted with a group capable of hydrogen bonding to the catalyst are demonstrated. Our approach presents a new entry into chiral pyridine frameworks in a heterocycle-rich molecular environment. Representative functionalizations of the enantioenriched pyridine N-oxides further document the utility of this approach. Demonstration of the asymmetric N-oxidation in two venerable drug-like scaffolds, Loratadine and Varenicline, show the likely generality of the method for highly variable and distinct chiral environments, while also revealing that the approach is applicable to both pyridines and 1,4-pyrazines.

The antimalarial drug mefloquine enhances TP53 premature termination codon readthrough by aminoglycoside G418

Nonsense mutations constitute ~10% of TP53 mutations in cancer. They introduce a premature termination codon that gives rise to truncated p53 protein with impaired function. The aminoglycoside G418 can induce TP53 premature termination codon readthrough and thus increase cellular levels of full-length protein. Small molecule phthalimide derivatives that can enhance the readthrough activity of G418 have also been described. To determine whether readthrough enhancers exist among drugs that are already approved for use in humans, we tested seven antimalarial drugs for readthrough of the common R213X TP53 nonsense mutation in HDQ-P1 breast cancer cells. Mefloquine induced no TP53 readthrough activity as a single agent but it strongly potentiated readthrough by G418. The two enantiomers composing pharmaceutical mefloquine potentiated readthrough to similar levels in HDQ-P1 cells and also in SW900, NCI-H1688 and HCC1937 cancer cells with different TP53 nonsense mutations. Exposure to G418 and mefloquine increased p53 phosphorylation at Ser15 and P21 transcript levels following DNA damage, indicating p53 produced via readthrough was functional. Mefloquine does not appear to enhance readthrough via lysosomotropic effects as it did not significantly affect lysosomal pH, the cellular levels of G418 or its distribution in organellar or cytosolic fractions. The availability of a readthrough enhancer that is already approved for use in humans should facilitate study of the therapeutic potential of TP53 readthrough in preclinical cancer models.

Asymmetric Synthesis of (+)-anti- and (-)-syn-Mefloquine Hydrochloride

The asymmetric (er > 99:1) total synthesis of (+)-anti- and (-)-syn-mefloquine hydrochloride from a common intermediate is described. The Sharpless asymmetric dihydroxylation is the key asymmetric transformation used in the synthesis of this intermediate. It is carried out on an olefin that is accessed in three steps from commercially available materials, making the overall synthetic sequence very concise. The common diol intermediate derived from the Sharpless asymmetric dihydroxylation is converted into either a trans- or cis-epoxide, and these are subsequently converted to (+)-anti- and (-)-syn-mefloquine, respectively. X-ray crystallographic analysis of derivatives of (+)-anti- and (-)-syn-mefloquine is used to lay to rest a 40 year argument regarding the absolute stereochemistry of the mefloquines. A formal asymmetric (er > 99:1) synthesis of (+)-anti-mefloquine hydrochloride is also presented that uses a Sharpless asymmetric epoxidation as a key step.

Absolute configuration of the antimalarial erythro-mefloquine – vibrational circular dichroism and X-ray diffraction studies of mefloquine and its thiourea derivative

The long-standing discussion of the absolute configuration of erythro-mefloquine is revisited, showcasing the strength of a combination of experimental and calculated vibrational circular dichroism spectroscopy.

A Robust, Recyclable Resin for Decagram Scale Resolution of (±)-Mefloquine and Other Chiral N-Heterocycles

Decagram quantities of enantiopure (+)-mefloquine have been produced via kinetic resolution of racemic mefloquine using a ROMP-gel supported chiral acyl hydroxamic acid resolving agent. The requisite monomer was prepared in a few synthetic steps without chromatography and polymerization was safely performed on a >30 gram scale under ambient conditions. The reagent was readily regenerated and reused multiple times for the resolution of 150 grams of (±)-mefloquine and other chiral N-heterocylces.