The novel reaction of ketones with o-oxazoline-substituted anilines

Synthesis and Biological Evaluation of Aurachin D Analogues as Inhibitors of Mycobacterium tuberculosis Cytochrome bd Oxidase

A revised total synthesis of aurachin D (1a), an isoprenoid quinolone alkaloid that targets Mycobacterium tuberculosis (Mtb) cytochrome bd (cyt-bd) oxidase, was accomplished using an oxazoline ring-opening reaction. The ring opening enabled access to a range of electron-poor analogues, while electron-rich analogues could be prepared using the Conrad-Limpach reaction. The aryl-substituted and side-chain-modified aurachin D analogues were screened for inhibition of Mtb cyt-bd oxidase and growth inhibition of Mtb. Nanomolar inhibition of Mtb cyt-bd oxidase was observed for the shorter-chain analogue 1d (citronellyl side chain) and the aryl-substituted analogues 1g/1k (fluoro substituent at C6/C7), 1t/1v (hydroxy substituent at C5/C6) and 1u/1w/1x (methoxy substituent at C5/C6/C7). Aurachin D and the analogues did not inhibit growth of nonpathogenic Mycobacterium smegmatis, but the citronellyl (1d) and 6-fluoro-substituted (1g) inhibitors from the Mtb cyt-bd oxidase assay displayed moderate growth inhibition against pathogenic Mtb (MIC = 4-8 μM).

Synthesis and Antimicrobial Activity of Burkholderia-Related 4-Hydroxy-3-methyl-2-alkenylquinolines (HMAQs) and Their N-Oxide Counterparts

The Burkholderia genus offers a promising potential in medicine because of the diversity of biologically active natural products encoded in its genome. Some pathogenic Burkholderia spp. biosynthesize a specific class of antimicrobial 2-alkyl-4(1H)-quinolones, i.e., 4-hydroxy-3-methyl-2-alkenylquinolines (HMAQs) and their N-oxide derivatives (HMAQNOs). Herein, we report the synthesis of a series of six HMAQs and HMAQNOs featuring a trans-Δ² double bond at the C2-alkyl chain. The quinolone scaffold was obtained via the Conrad-Limpach approach, while the (E)-2-alkenyl chain was inserted through Suzuki-Miyaura cross-coupling under microwave radiation without noticeable isomerization according to the optimized conditions. Subsequent oxidation of enolate-protected HMAQs cleanly led to the formation of HMAQNOs following cleavage of the ethyl carbonate group. Synthetic HMAQs and HMAQNOs were evaluated in vitro for their antimicrobial activity against different Gram-negative and Gram-positive bacteria as well as against molds and yeasts. The biological results support and extend the potential of HMAQs and HMAQNOs as antimicrobials, especially against Gram-positive bacteria. We also confirm the involvement of HMAQs in the autoregulation of the Hmq system in Burkholderia ambifaria.

Microwave-Assisted Synthesis of Novel Pyrazolo[3,4-g][1,8]naphthyridin-5-amine with Potential Antifungal and Antitumor Activity

The microwave assisted reaction between heterocyclic o-aminonitriles 1 and cyclic ketones 2 catalyzed by zinc chloride led to new series of pyrazolo[3,4-b][1,8]naphthyridin-5-amines 3 in good yields. This procedure provides several advantages such as being environmentally friendly, high yields, simple work-up procedure, broad scope of applicability and the protocol provides an alternative for the synthesis of pyrazolonaphthyridines. The whole series showed antifungal activities against Candida albicans and Cryptococcus neoformans standardized strains, being compounds with a 4-p-tolyl substituent of the naphthyridin scheleton (3a, 3d and 3g), the most active ones mainly against C. albicans, which appear to be related to their comparative hydrophobicity. Among them, 3d, containing a cyclohexyl fused ring, showed the best activity. The anti-Candida activity was corroborated by testing the three most active compounds against clinical isolates of albicans and non-albicans Candida strains. These compounds were also screened by the US National Cancer Institute (NCI) for their ability to inhibit 60 different human tumor cell lines. Compounds 3a and 3e showed remarkable antitumor activity against cancer cell lines, with the most important GI₅₀ values ranging from 0.62 to 2.18 μM.

A new route for the synthesis of highly substituted 4-aminoquinoline drug like molecules via aza hetero–Diels–Alder reaction

A new route has been developed for the synthesis of 4-aminoquinoline drug like moleculesviaaza hetero–Diels–Alder reaction starting from 2H-indazole as a diene for the first time.

2-Pyridylquinolone antimalarials with improved antimalarial activity and physicochemical properties

2-Pyridylquinolones with improved solubility, an improved metabolic stability profile, reduced off-target toxicity and 12 nM Plasmodium falciparum antimalarial activity are described.

Heterocycle-heterocycle strategies: (2-nitrophenyl)isoxazole precursors to 4-aminoquinolines, 1H-indoles, and quinolin-4(1H)-ones

Reductive heterocycle-heterocycle (heterocycle → heterocycle; H-H) transformations that give 4-aminoquinolines, 3-acylindoles, and quinolin-4(1H)-ones from 2-nitrophenyl substituted isoxazoles are reported. When this methodology is applied to 3,5-, 4,5-, and 3,4-bis(2-nitrophenyl)isoxazoles, chemoselective heterocyclization gives quinolin-4(1H)-ones, and 4-aminoquinolines, exclusively.

Identification, design and biological evaluation of bisaryl quinolones targeting Plasmodium falciparum type II NADH:quinone oxidoreductase (PfNDH2)

A program was undertaken to identify hit compounds against NADH:ubiquinone oxidoreductase (PfNDH2), a dehydrogenase of the mitochondrial electron transport chain of the malaria parasite Plasmodium falciparum. PfNDH2 has only one known inhibitor, hydroxy-2-dodecyl-4-(1H)-quinolone (HDQ), and this was used along with a range of chemoinformatics methods in the rational selection of 17 000 compounds for high-throughput screening. Twelve distinct chemotypes were identified and briefly examined leading to the selection of the quinolone core as the key target for structure–activity relationship (SAR) development. Extensive structural exploration led to the selection of 2-bisaryl 3-methyl quinolones as a series for further biological evaluation. The lead compound within this series 7-chloro-3-methyl-2-(4-(4-(trifluoromethoxy)benzyl)phenyl)quinolin-4(1H)-one (CK-2-68) has antimalarial activity against the 3D7 strain of P. falciparum of 36 nM, is selective for PfNDH2 over other respiratory enzymes (inhibitory IC₅₀ against PfNDH2 of 16 nM), and demonstrates low cytotoxicity and high metabolic stability in the presence of human liver microsomes. This lead compound and its phosphate pro-drug have potent in vivo antimalarial activity after oral administration, consistent with the target product profile of a drug for the treatment of uncomplicated malaria. Other quinolones presented (e.g., 6d, 6f, 14e) have the capacity to inhibit both PfNDH2 and P. falciparum cytochrome bc₁, and studies to determine the potential advantage of this dual-targeting effect are in progress.

Identification, design and biological evaluation of heterocyclic quinolones targeting Plasmodium falciparum type II NADH:quinone oxidoreductase (PfNDH2)

Following a program undertaken to identify hit compounds against NADH:ubiquinone oxidoreductase (PfNDH2), a novel enzyme target within the malaria parasite Plasmodium falciparum, hit to lead optimization led to identification of CK-2-68, a molecule suitable for further development. In order to reduce ClogP and improve solubility of CK-2-68 incorporation of a variety of heterocycles, within the side chain of the quinolone core, was carried out, and this approach led to a lead compound SL-2-25 (8b). 8b has IC₅₀s in the nanomolar range versus both the enzyme and whole cell P. falciparum (IC₅₀ = 15 nM PfNDH2; IC₅₀ = 54 nM (3D7 strain of P. falciparum) with notable oral activity of ED₅₀/ED₉₀ of 1.87/4.72 mg/kg versus Plasmodium berghei (NS Strain) in a murine model of malaria when formulated as a phosphate salt. Analogues in this series also demonstrate nanomolar activity against the bc₁ complex of P. falciparum providing the potential added benefit of a dual mechanism of action. The potent oral activity of 2-pyridyl quinolones underlines the potential of this template for further lead optimization studies.