Amidative Cyclization of Alkynyl Esters to Access Pyrazin-1(2H)-ones: Application to the Synthesis of Peramine and Dibromophakellin.

Herein, we report an efficient 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD)-catalyzed tandem intermolecular amidation and regioselective intramolecular 6-exo-dig cyclization of alkynyl esters to efficiently access pyrazine-1(2H)-one scaffolds. This organo-catalyzed [5 + 1] annulation features a broad substrate scope concerning both annulating partners. Total syntheses of peramine and formal syntheses of dibromophakellin natural products were achieved to show the application potential of the method.

Design, Synthesis, Molecular Docking Study and Biological Evaluation of Novel γ-Carboline Derivatives of Latrepirdine (Dimebon) as Potent Anticancer Agents.

A series of novel γ-Carboline derivatives were designed and synthesized using the Suzuki coupling reaction to identify the leads for the activity against cancer. Interestingly, these compounds were tested for their anticancer activity against the cell lines, particularly human cancer cell lines MCF7 (breast), A549 (lung), SiHa (cervix), and Colo-205 (colon). Most of the γ-Carboline derivatives showed potent inhibitory activity in four cancer cell lines, according to in vitro anticancer activity screening. Two compounds, specifically LP-14 and LP-15, showed superior activity in cancer cell lines among the γ-Carboline derivatives from LP-1 to LP-16. Additionally, the compound LP-14, LP-15 and Etoposide carried out molecular docking studies on human topoisomerase II beta in complex with DNA and Etoposide (PDB ID: 3QX3). The docking studies' results showed that the derivative LP-15 was strongly bound with the receptor amino acid residues, including Glu477 and DC8 compared with the marked drug Etoposide.

Design and Synthesis of Tetrazole- and Pyridine-Containing Itraconazole Analogs as Potent Angiogenesis Inhibitors.

Itraconazole, a widely used antifungal drug, was found to possess antiangiogenic activity and is currently undergoing multiple clinical trials for the treatment of different types of cancer. However, it suffers from extremely low solubility and strong interactions with many drugs through inhibition of CYP3A4, limiting its potential as a new antiangiogenic and anticancer drug. To address these issues, a series of analogs in which the phenyl group is replaced with pyridine or fluorine-substituted benzene was synthesized. Among them the pyridine- and tetrazole-containing compound 24 has significantly improved solubility and reduced CYP3A4 inhibition compared to itraconazole. Similar to itraconazole, compound 24 inhibited the AMPK/mTOR signaling axis and the glycosylation of VEGFR2. It also induced cholesterol accumulation in the endolysosome and demonstrated binding to the sterol-sensing domain of NPC1 in a simulation study. These results suggested that compound 24 may serve as an attractive candidate for the development of a new generation of antiangiogenic drug.

Thienopyrimidinone Derivatives That Inhibit Bacterial tRNA (Guanine37-N1)-Methyltransferase (TrmD) by Restructuring the Active Site with a Tyrosine-Flipping Mechanism.

Among the >120 modified ribonucleosides in the prokaryotic epitranscriptome, many tRNA modifications are critical to bacterial survival, which makes their synthetic enzymes ideal targets for antibiotic development. Here we performed a structure-based design of inhibitors of tRNA-(N1G37) methyltransferase, TrmD, which is an essential enzyme in many bacterial pathogens. On the basis of crystal structures of TrmDs from Pseudomonas aeruginosa and Mycobacterium tuberculosis, we synthesized a series of thienopyrimidinone derivatives with nanomolar potency against TrmD in vitro and discovered a novel active site conformational change triggered by inhibitor binding. This tyrosine-flipping mechanism is uniquely found in P. aeruginosa TrmD and renders the enzyme inaccessible to the cofactor S-adenosyl-l-methionine (SAM) and probably to the substrate tRNA. Biophysical and biochemical structure-activity relationship studies provided insights into the mechanisms underlying the potency of thienopyrimidinones as TrmD inhibitors, with several derivatives found to be active against Gram-positive and mycobacterial pathogens. These results lay a foundation for further development of TrmD inhibitors as antimicrobial agents.

Novel Tetrazole-Containing Analogues of Itraconazole as Potent Antiangiogenic Agents with Reduced Cytochrome P450 3A4 Inhibition.

Itraconazole has been found to possess potent antiangiogenic activity, exhibiting promising antitumor activity in several human clinical studies. The wider use of itraconazole in the treatment of cancer, however, has been limited by its potent inhibition of the drug metabolizing enzyme cytochrome P450 3A4 (CYP3A4). In an effort to eliminate the CYP3A4 inhibition while retaining its antiangiogenic activity, we designed and synthesized a series of derivatives in which the 1,2,4-triazole ring is replaced with various azoles and nonazoles. Among these analogues, 15n with tetrazole in place of 1,2,4-triazole exhibited optimal inhibition of human umbilical vein endothelial cell proliferation with an IC50 of 73 nM without a significant effect on CYP3A4 (EC50 > 20 µM). Similar to itraconazole, 15n induced Niemann-Pick C phenotype (NPC phenotype) and blocked AMPK/mechanistic target of rapamycin signaling. These results suggest that 15n is a promising angiogenesis inhibitor that can be used in combination with most other known anticancer drugs.

Thiazolidine-Masked α-Oxo Aldehyde Functionality for Peptide and Protein Modification.

α-Oxo aldehyde-based bioconjugation chemistry has been widely explored in peptide and protein modifications for various applications in biomedical research during the past decades. The generation of α-oxo aldehyde via sodium periodate oxidation is usually limited to the N-terminus of a target protein. Internal-site functionalization of proteins with the α-oxo aldehyde handle has not been achieved yet. Herein we report a novel method for site-specific peptide and protein modification using synthetically or genetically incorporated thiazolidine-protected α-oxo aldehyde. Efficient unmasking of the aldehyde was achieved by silver ion-mediated hydrolysis of thiazolidine under mild conditions for the first time. A model peptide and a recombinant protein were used to demonstrate the utility of this new method, which were site-specifically modified by oxime ligation with an oxyamine-functionalized peptide labeling reagent. Therefore, our current method has enriched the α-oxo aldehyde synthetic tool box in peptide and protein bioconjugation chemistry and holds great potential to be explored in novel applications in the future.

Genetic incorporation of 1,2-aminothiol functionality for site-specific protein modification via thiazolidine formation.

Here we report a new site-specific conjugation strategy to modify proteins via thiazolidine ligation. Proteins harbouring a 1,2-aminothiol moiety introduced by amber codon suppression technology could be modified chemoselectively with aldehyde-functionalized reagents, such as a biotin-labeled peptide or ubiquitin, under mild conditions to yield homogeneous biotinylated or ubiquitinated products.

5-Methylisoxazole-3-carboxamide-Directed Palladium-Catalyzed γ-C(sp(3))-H Acetoxylation and Application to the Synthesis of γ-Mercapto Amino Acids for Native Chemical Ligation.

Palladium-catalyzed acetoxylation of the primary γ-C(sp(3))-H bonds in the amino acids Val, Thr, and Ile was achieved using a newly discovered 5-methylisoxazole-3-carboxamide directing group. The γ-acetoxylated α-amino acid derivatives could be easily converted to γ-mercapto amino acids, which are useful for native chemical ligation (NCL). The first application of NCL at isoleucine in the semisynthesis of a Xenopus histone H3 protein was also demonstrated.

Auxiliary-Directed Pd-Catalyzed γ-C(sp(3))-H Bond Activation of α-Aminobutanoic Acid Derivatives.

New bidentate auxiliaries derived from the isoxazole-3-carboxamide and oxazole-4-carboxamide moieties were used for Pd-catalyzed C(sp(3))-H bond activation. The results show that, when placed on a primary amine compound, 5-methylisoxazole-3-carboxamide (MICA) directs Pd-catalyzed activation of inert γ-C(sp(3))-H bonds for C-C bond formation. Selective and efficient arylation and alkylation of several α-aminobutanoic acid derivatives led to various γ-substituted non-natural amino acids. The MICA directing group can be conveniently removed and recovered under very mild conditions.

Benzofuran-based estrogen receptor α modulators as anti-cancer therapeutics: in silico and experimental studies.

In the search for new estrogen receptor alpha (ERα) modulators, a trial molecular screening was conducted and 5,6-dihydroxybenzofuran was identified as a possible drug target for ERα. The target molecular modelling molecule 1 and a series of 5,6-dihydroxybenzofurans have been synthesized and evaluated for their anti-proliferation activities against MCF-7 and MDA-MB-231 cells. From the SAR studies, potential functional groups have been identified, the two hydroxyl groups at C-5 and C-6 and the phenyl ring at C-2, which showed considerable cytotoxicity in MCF-7 breast cancer cells. In addition, the apoptotic abilities of the compounds have been measured in both MCF-7 ER(+) and MDA-MB-231 ER(-) breast cancer cells. The results demonstrated that our compounds inhibit MCF-7 breast cancer cells via ER(+). These preliminary results provide valuable information towards the identification of important functional groups present on 5,6-dihydroxybenzofuran, which could be a promising scaffold for designing novel ER ligands.

Adamantyl derivative as a potent inhibitor of Plasmodium FK506 binding protein 35.

FKBP35, FK506 binding protein family member, in Plasmodium species displays a canonical peptidyl-prolyl isomerase (PPIase) activity and is intricately involved in the protein folding process. Inhibition of PfFKBP35 by FK506 or its analogues were shown to interfere with the in vitro growth of Plasmodium falciparum. In this study, we have synthesized adamantyl derivatives, Supradamal (SRA/4a) and its analogues SRA1/4b and SRA2/4c, which demonstrate submicromolar inhibition of Plasmodium falciparum FK506 binding domain 35 (FKBD35) PPIase activity. SRA and its analogues not only inhibit the in vitro growth of Plasmodium falciparum 3D7 strain but also show stage specific activity by inhibiting the trophozoite stage of the parasite. SRA/4a also inhibits the Plasmodium vivax FKBD35 PPIase activity and our crystal structure of PvFKBD35 in complex with the SRA provides structural insights in achieving selective inhibition against Plasmodium FKBPs.

Synthesis of K48-linked diubiquitin using dual native chemical ligation at lysine.

The dual native chemical ligation at lysine strategy was revised by replacing the acid-labile Cbz protecting group with photolabile NVOC at the 4-mercaptolysine side chain. The optimized strategy was subsequently applied to the synthesis of K48-linked diubiquitin.

Microwave-assisted copper(II)-catalyzed one-pot four-component synthesis of multifunctionalized dihydropyridines.

A fast and highly efficient copper-catalyzed multicomponent synthesis of 1,4-dihydropyridines under microwave irradiation is described. The protocol utilizes mild reaction conditions with low catalytic loading, leading to high yields. This methodology provides us with biologically active 1,4-dihydropyridine library for medicinal chemistry applications.

Synthesis of 4-mercapto-L-lysine derivatives: potential building blocks for sequential native chemical ligation.

A general and diastereoselective synthesis of (2S, 4S)-4-mercapto-L-lysine derivative was described. The key features of this synthesis include Zn-mediated diastereoselective Reformatsky reaction and selective reduction of methyl ester with sodium borohydride. Introduction of thiol functional group on lysine side chain proved to be appropriate for dual native chemical ligation. This methodology allows to develop various 4-substituted L-lysine derivatives.

Dual native chemical ligation at lysine.

A thiol group introduced on the gamma-carbon of lysine mediates robust native chemical ligation at both the alpha- and epsilon-amines in two consecutive steps. Desulfurization then affords the final product, in which the lysine residue at the ligation site has an isopeptide bond on its side chain. The method is useful for the synthesis of proteins containing special post-translational modifications on lysine.