Structure simplification of the Securinine skeleton reveals the importance of BCD ring system for the cytotoxic activity on HCT116 and HL60 cell lines.

Function-oriented molecular editing of the polycyclic scaffold of securinine led to the preparation of a library of simplified analogs that have been evaluated for their cytotoxicity potential against HCT116 and HL60 human cell lines. Chemical diversity at the C14 position (securinine numbering) was generated through the site-selective γ-iodination followed by Pd-catalyzed Sonogashira and Suzuki-Miyaura reactions. To explain the selectivity in the iodination step, a reaction mechanism has been proposed. Surprisingly, the piperidine ring (ring A) of the securinine skeleton has been found to be irrelevant for the cytotoxic activity. Based on this finding, the pharmacophoric core of securinine could be simplified to the key BCD motif. The nature of the substituent at the nitrogen can vary from a methyl or an isobutyl group to a benzyl or a carbamate moiety. Interestingly, the N-benzyl substituted simplified analog exhibited the same cytotoxic activity as the parent compound securinine. This functional group tolerance paves the way for the installation of reactive handles for the synthesis of molecular probes for target identification.

Chuanxiongdiolides R4 and R5, phthalide dimers with a complex polycyclic skeleton from the aerial parts of Ligusticum chuanxiong and their vasodilator activity.

Chuanxiongdiolides R4-R6 (1-3), three novel phthalide dimers featuring two classes of unreported monomeric units (ligustilide/senkyunolide A and ligustilide/neocnidilide) with an unprecedented linkage style (3a,7'/7a,7'a), were isolated from the aerial parts of Ligusticum chuanxiong, together with three pairs of enantiomeric phthalide dimers [(-)/(+)-4a/4b, 5a/5b, and 6a/6b]. The bioassays revealed that compounds 1, 3, 4, 5, and 6 showed significant vasodilation effects, and the mechanism may be attributed to Cav1.2 activation blockade. Based on the established compounds library, the structure activity relationship of the phthalides was proposed. Our findings afford possible leads for developing new vasodilator against cardiovascular and cerebrovascular diseases such as hypertension and ischemic stroke.

Structure-Based Design of Highly Potent HIV-1 Protease Inhibitors Containing New Tricyclic Ring P2-Ligands: Design, Synthesis, Biological, and X-ray Structural Studies.

We describe here design, synthesis, and biological evaluation of a series of highly potent HIV-1 protease inhibitors containing stereochemically defined and unprecedented tricyclic furanofuran derivatives as P2 ligands in combination with a variety of sulfonamide derivatives as P2' ligands. These inhibitors were designed to enhance the ligand-backbone binding and van der Waals interactions in the protease active site. A number of inhibitors containing the new P2 ligand, an aminobenzothiazole as the P2' ligand and a difluorophenylmethyl as the P1 ligand, displayed very potent enzyme inhibitory potency and also showed excellent antiviral activity against a panel of highly multidrug-resistant HIV-1 variants. The tricyclic P2 ligand has been synthesized efficiently in an optically active form using enzymatic desymmetrization of meso-1,2-(dihydroxymethyl)cyclohex-4-ene as the key step. We determined high-resolution X-ray structures of inhibitor-bound HIV-1 protease. These structures revealed extensive interactions with the backbone atoms of HIV-1 protease and provided molecular insights into the binding properties of these new inhibitors.

Biosynthetically Inspired Syntheses of Secu'amamine A and Fluvirosaones A and B.

Presented here is a concise synthesis of secu'amamine A, and fluvirosaones A and B from readily available allosecurinine and viroallosecurinine. The key C2-enamine derivative of (viro)allosecurinine, the presumed biosynthetic precursors of these natural products, was accessed, for the first time, by a VO(acac)2 -mediated regioselective Polonovski reaction. Formal hydration and 1,2-amine shift of this pluripotent enamine compound afforded secu'amamine A. Formal oxidative [3+2] cycloaddition reaction between this enamine and TMS-substituted methallyl iodide reagent paved the way to the precursors of fluvirosaones A and B. The relative stereochemistry at the C2 position of these advanced intermediates governs the fate of 1,2-amine shift leading to fluvirosaones A and B. The syntheses of potential biosynthetic precursors and investigations of their chemical reactivities have provided insights regarding the biogenesis of these natural products.

Synthesis of the Tetracyclic Cores of the Integrastatins, Epicoccolide A and Epicocconigrone A.

The integrastatins, epicoccolide A and epicoccongirone A, are natural products containing a unique [6.6.6.6]-tetracyclic core framework that exhibit a broad spectrum of biological activities. A synthesis of the common core of epicoccolide A and epicocconigrone A has been achieved using an umpolung alkylation-lactonization to assemble an isochromanone from which the bridged 1,3-dioxane was readily assembled. A different strategy was required to access the core on the integrastatins; an initial aryllithium addition to an aldehyde, followed by oxidation and treatment of the masked dihydroxyketone with acid gave the desired core structure.

Pyridine-Fused 2-Pyridones via Povarov and A3 Reactions: Rapid Generation of Highly Functionalized Tricyclic Heterocycles Capable of Amyloid Fibril Binding.

We here describe the use of three-component reactions to synthesize tricyclic pyridine ring-fused 2-pyridones. The developed protocols have a wide substrate scope and allow for the installation of diverse chemical functionalities on the tricyclic central fragment. Several of these pyridine-fused rigid polyheterocycles are shown to bind to Aß and α-synuclein fibrils, which are associated with neurodegenerative diseases.

Synthesis, Structure, and Cytotoxicity of a New Sulphanyl-Bridged Thiadiazolyl-Saccharinate Conjugate: The Relevance of S⋠⋠⋠N Interaction.

Reports showing that the copper concentration is considerably higher in neoplasms than in normal tissues prompted the need to develop selective copper chelators. We disclosed recently that some N-linked tetrazole-saccharinates bind selectively to copper, forming complexes that are highly cytotoxic towards cancer cells. Because tetrazole-saccharinates are photolabile, due to the photoreactivity of tetrazoles, we proposed thiadiazolyl-saccharinates as an alternative. Herein we describe the synthesis, structure, and monomeric photochemistry of a sulphanyl-bridged thiadiazolyl-saccharinate, 3-[(5-methyl-1,3,4-thiadiazol-2-yl)sulphanyl]-1,2-benzothiazole 1,1-dioxide (MTSB). The monomeric structure, charge density analysis, and characteristic infrared spectrum of MTSB were investigated theoretically, using quantum chemical calculations, and also experimentally, using matrix-isolation infrared spectroscopy. The crystal structure was investigated by combining X-ray crystallography with infrared and Raman spectroscopies. Results show that the structure of isolated MTSB is similar to that found in the crystal, with an S⋅⋅⋅N interaction clearly contributing to the structure of the molecule and of the crystal. Matrix irradiation revealed a high photostability of MTSB, compared to parent tetrazole-saccharinates and to the 5-methyl-1,3,4-thiadiazole building block, emphasizing the photostabilizing effect of the saccharyl system. Finally, in vitro toxicity assays of MTSB showed a copper concentration-dependent toxicity against cancer cells, without affecting normal cells. In particular, MTSB was most effective towards the hepatic (HepG2), neuroblastoma (SH-SY5), and lymphoma cell lines (U937). Thus, MTSB represents a promising lead for cancer chemotherapy based on chelating agents.

Novel delta opioid receptor agonists with oxazatricyclodecane structure showing potent agonistic activities.

We recently reported oxazatricyclodecane derivatives 1 as δ opioid receptor (DOR) agonists having a novel chemotype, but their DOR agonistic activities were relatively low. Based on the working hypothesis that the dioxamethylene moiety in 1 may be an accessory site and that it may interfere with the sufficient conformational change of the receptor required for exerting the full agonistic responses, we designed and synthesized new oxazatricyclodecane derivatives 2-4 lacking the dioxamethylene moiety. As we expected, the designed compounds 2-4 showed pronouncedly improved agonistic activities for the DOR. Compound 2a with the 17-cyclopropylmethyl substituent was a potent agonist with the highest selectivity for the DOR and was expected to be a lead compound for novel and selective DOR agonists.

Structure-Based Design of Tricyclic NF-κB Inducing Kinase (NIK) Inhibitors That Have High Selectivity over Phosphoinositide-3-kinase (PI3K).

We report here structure-guided optimization of a novel series of NF-κB inducing kinase (NIK) inhibitors. Starting from a modestly potent, low molecular weight lead, activity was improved by designing a type 11/2 binding mode that accessed a back pocket past the methionine-471 gatekeeper. Divergent binding modes in NIK and PI3K were exploited to dampen PI3K inhibition while maintaining NIK inhibition within these series. Potent compounds were discovered that selectively inhibit the nuclear translocation of NF-κB2 (p52/REL-B) but not canonical NF-κB1 (REL-A/p50).

Organocatalytic Synthesis of Methylene-Bridged N-Heterobiaryls.

A one-step synthesis of 1,1'- and 2,2'-methylene-bridged N-heterobiaryls directly from the corresponding N-heterocycles in a reaction with methylmagnesium chloride in the presence of catalytic amounts of N,N,N',N'-tetramethylethylenediamine under thermal and microwave conditions is reported. The split-and-merge methylenation of 2,2'-N-heterobiaryls and the direct ortho-alkylation of quinoline and isoquinoline with Grignard reagents have also been developed. Mechanistic studies identified several intermediates and provided insight into the formation and roles of magnesium hydride species in the process.

Biosynthesis-Inspired Total Synthesis of Bioactive Styryllactones (+)-Goniodiol, (6S,7S,8S)-Goniodiol, (-)-Parvistone D, and (+)-Parvistone E.

A protecting-group-free total synthesis of (+)-goniodiol (1), (6S,7S,8S)-goniodiol (2), (-)-parvistone D (4), and (+)-parvistone E (6) was efficiently achieved in five steps from commercially available trans-cinnamaldehyde with high overall yields (72-75%). The synthesis strategy was inspired from the proposed biosynthesis pathway of styryllactones. Key transformations of the strategy include a one-pot conversion of goniothalamin oxide to goniodiol or 9-deoxygoniopypyrone in aqueous media, stereoselective epoxidation, ring-closing metathesis, and stereoselective Maruoka allylation. The route is amenable to synthesis of various analogues for biological evaluation.

Design and Synthesis of Dimeric Securinine Analogues with Neuritogenic Activities.

Neurite outgrowth is crucial during neuronal development and regeneration, and strategies that aim at promoting neuritogenesis are beneficial for reconstructing synaptic connections after neuronal degeneration and injury. Using a bivalent analogue strategy as a successful approach, the current study identifies a series of novel dimeric securinine analogues as potent neurite outgrowth enhancers. Compounds 13, 14, 17-19, and 21-23, with different lengths of carbon chain of N,N-dialkyl substituting diacid amide linker between two securinine molecules at C-15 position, exhibited notable positive effects on both neuronal differentiation and neurite extension of neuronal cells. Compound 14, one of the most active compounds, was used as a representative compound for mechanistic studies. Its action on neurite outgrowth was through phosphorylation/activation of multiple signaling molecules including Ca2+/calmodulin-dependent protein kinase II (CaMKII), extracellular signal-regulated kinase (ERK) and Akt. These findings collectively identify a new group of beneficial compounds for neuritogenesis, and may provide insights on drug discovery of neural repair and regeneration.

Caged xanthones: Potent inhibitors of global predominant MRSA USA300.

Total of 22 caged xanthones were subjected to susceptibility testing of global epidemic MRSA USA300. Natural morellic acid showed the strongest potency (MIC of 12.5µM). However, its potent toxicity diminishes MRSA therapeutic potential. We synthetically modified natural morellic acid to yield 13 derivatives (3a-3m). Synthetically modified 3b retained strong potency in MRSA growth inhibition, yet the toxicity was 20-fold less than natural morellic acid, permitting the possibility of using caged xanthones for MRSA therapeutic.

Bidentate Lewis Acid Catalyzed Domino Diels-Alder Reaction of Phthalazine for the Synthesis of Bridged Oligocyclic Tetrahydronaphthalenes.

A domino process consisting of an inverse and a normal electron-demand Diels-Alder reaction is presented for the formation of bridged tri- and tetracyclic 1,2,3,4-tetrahydronaphthalenes catalyzed by a bidentate Lewis acid. The products were synthesized in a one-pot reaction from commercially available starting materials and contain up to six stereogenic centers. The tetrahydronaphthalenes were isolated as single diastereomers and are derivatives of phenylethylamine, which is well-known as a scaffold of amphetamine or dopamine.

Total Synthesis of Integrastatin B Enabled by a Benzofuran Oxidative Dearomatization Cascade.

The first total synthesis of integrastatin B, a potent HIV-1 integrase inhibitor, has been accomplished in seven steps with a 17.9% overall yield employing easily accessible starting compounds. The Oxone-mediated oxidative benzofuran dearomatization cascade has been employed as the key skeletal construct to forge the central tetracyclic nucleus.

Synthesis and biological evaluation of new securinine analogues as potential anticancer agents.

A series of new securinine analogues was prepared by Heck reaction from readily accessible securinine and commercially available iodoarenes. The in vitro cytotoxicity of the prepared compounds was assayed against a panel of four cancer cell lines: A375, A549, HCT-116 and HL-60 showing promising growth inhibition with excellent IC50 values in the nanomolar range. The plasmatic stability of the most potent analogue was also investigated demonstrating that they might serve as valuable leads for the development of anticancer drugs.

Tandem Chloropalladation/Cyclization and Dearomative Cyclization toward Functionalized Tricyclic Bridged [3.2.1] Skeleton Compounds.

A palladium-catalyzed tandem reaction is reported that involves chloropalladation/cyclization and dearomative cyclization to construct a tricyclic bridged [3.2.1] carbocyclic-skeleton and oxa- and aza-skeletons. In this domino process, a level of ring strain and other competitive reactions, i.e., protonolysis, ß-hydride elimination, and chlorination of the C-Pd bond, were suppressed to the lowest level under mild reaction conditions.

Total Synthesis of (-)-Secu'amamine A Exploiting Type II Anion Relay Chemistry.

A total synthesis of (-)-secu'amamine A has been achieved exploiting Type II Anion Relay Chemistry (ARC) to provide the full linear carbon and nitrogen skeleton in a single flask with the requisite stereochemistry and functionality. A mechanistic rationale is also proposed to account for the stereochemical outcome of the key aldol reaction leading to the advanced aza tricyclic core.

Racemic total synthesis of dactyloidin and demethyldactyloidin through the DL-proline-catalyzed Knoevenagel condensation/[4 + 2] cycloaddition cascade.

An efficient approach towards the first racemic total synthesis of dactyloidin (2) and demethyldactyloidin (3) is described. Their oxygen-bridged tricyclic ketal systems were rapidly constructed by using a remarkable biomimetic Knoevenagel condensation/[4 + 2] cycloaddition cascade as the critical strategy and the 1,5-dicarbonyl segment was assembled by Grignard addition.

Aza-Quaternary Scaffolds from Selective Bond Cleavage of Bridgehead-Substituted 7-Azabicyclo[2.2.1]heptane: Total Synthesis of (+)-Cylindricines†C-E and (-)-Lepadiformine†A.

A novel bridgehead-substituted aza-bicyclic framework has been designed and developed in both enantiomeric forms through an asymmetric desymmetrization reaction. Strategic exploitation of the ring strain in the aza-bicyclic framework has been utilized for the construction of the chiral aza-quaterenary scaffolds by selective bond fragmentation processes. Furthermore, a strategically designed precursor is employed for selective bond cleavage to initiate a cascade rearrangement for the total synthesis of the 1-azaspirotricyclic marine alkaloids (+)-cylindricines C, D, and E, as well as (-)-lepadiformine A. An oxidation/retro-aldol/aza-Michael sequence generated three new chiral centers with the required configuration in one pot.