Net Intermolecular Silyloxypyrone-Based (5+2) Cycloadditions Utilizing Amides as Enabling and Cleavable Tethers

Silyloxypyrone-based (5+2) cycloadditions were facilitated by amides that allowed for increased reactivity and a pathway for cleaving the tether to afford net intermolecular cycloadducts. Various amides underwent facile cycloaddition, and several experiments revealed steric and electronic factors that accelerate the reaction. tert-Butyl amides reacted faster than less hindered variants in multiple cases. In the case of dearomative oxidopyrylium-indole (5+2) cycloadditions, an amine-based tether was ineffective, whereas amides enabled this powerful transformation. Theoretical calculations evidenced a concerted asynchronous reaction in which the amide facilitates a conformational driving force enabling cycloaddition. Finally, a one-pot acylation/(5+2) cycloaddition/nucleophilic lactam opening and other examples of tosyl lactam opening of a modified cycloadduct were demonstrated.

Experimental and Theoretical Investigation of the Synchronicity of Ambident Silyloxypyrone-Based (5 + 2) Cycloadditions

The reaction pathway of silyloxypyrone-based (5 + 2) cycloadditions was determined to be extremely dependent on the nature of the dipolarophile. Neutral alkenes were the least reactive, whereas both electron-deficient and electron-rich dipolarophiles were more reactive, thus providing evidence for ambident oxidopyrylium intermediates. Qualitative rate studies, Hammett linear free energy relationships, and theoretical calculations combined to provide evidence for a spectrum of reactivity that passes through the borderlands of concerted and stepwise.

Investigation of Transfer Group, Tether Proximity, and Alkene Substitution for Intramolecular Silyloxypyrone-Based [5 + 2] Cycloadditions

Systematic investigation of intramolecular silyloxypyrone-based [5 + 2] cycloadditions revealed three significant factors impacting conversion to cycloadduct: (1) the silyl transfer group has a substantial influence on the rate of reaction, and the robust t-butyldiphenylsilyl group was found to be more effective overall than the conventional t-butyldimethylsilyl group; (2) α,β-unsaturated esters were generally more reactive than terminal olefins and afforded appreciable quantity of cycloadduct even at room temperature; and (3) the proximity of the tether to the silyl transfer group revealed a critical alignment trend between the pyrone and the alkene. Taken together, these investigations provided insight regarding the steric and electronic parameters that impact the scope and limitation of these reactions.

Investigation of oxidopyrylium-alkene [5+2] cycloaddition conjugate addition cascade (C3) sequences

Novel oxidopyrylium-alkene [5+2] cycloaddition conjugate addition cascade (C(3)) sequences are described. Various acetoxypyranone-alkenes with pendant nucleophiles undergo [5+2] cycloaddition followed by conjugate addition from the concave face of the intermediate pyranone toward bridged, tetracyclic ethers. In several cases, 3 new rings, 4 new bonds, and 6 new contiguous stereocenters are constructed with excellent diastereoselectivity. Finally, an asynchronous concerted reaction pathway is proposed to explain the high diastereoselectivity of the oxidopyrylium-alkene [5+2] C(3).

Synthesis and evaluation of N-alkyl-9-aminoacridines with antibacterial activity

A series of 9-alkylaminoacridines were synthesized and evaluated for activity against two strains of methicillin-resistant and one strain of methicillin-sensitive Staphylococcus aureus. Results are presented that show a clear structure activity relationship between the N-alkyl chain length and antibacterial activity with peak MIC99 values of 2-3 μM for alkyl chains ranging from 10 to 14 carbons in length. Although prior work has linked the function of acridine-based compounds to intercalation and topoisomerase inhibition, the present results show that 9-alkylaminoacridines likely function as amphiphilic membrane-active disruptors potentially in a similar manner as quaternary ammonium antimicrobials.

Multidimensional reaction screening for photochemical transformations as a tool for discovering new chemotypes

We have developed an automated photochemical microfluidics platform that integrates a 1 kW high-pressure Hg vapor lamp and allows for analytical pulse flow or preparative continuous flow reactions. Herein, we will discuss the use of this platform toward the discovery of new chemotypes through multidimensional reaction screening. We will highlight the ability to discretely control wavelengths with optical filters, allowing for control of reaction outcomes.

Synthesis and in silico screening of a library of β-carboline-containing compounds

The synthesis of a library of tetrahydro-β-carboline-containing compounds in milligram quantities is described. Among the unique heterocyclic frameworks are twelve tetrahydroindolizinoindoles, six tetrahydrocyclobutanindoloquinolizinones and three tetrahydrocyclopentenoneindolizinoindolones. These compounds were selected from a virtual combinatorial library of 11,478 compounds. Physical chemical properties were calculated and most of them are in accordance with Lipinski's rules. Virtual docking and ligand-based target evaluations were performed for the β-carboline library compounds and selected synthetic intermediates to assess the therapeutic potential of these small organic molecules. These compounds have been deposited into the NIH Molecular Repository (MLSMR) and may target proteins such as histone deacetylase 4, endothelial nitric oxide synthase, 5-hydroxytryptamine receptor 6 and mitogen-activated protein kinase 1. These in silico screening results aim to add value to the β-carboline library of compounds for those interested in probes of these targets.

Synthesis and reactivity of bicyclo[3.2.1]octanoid-derived cyclopropanes

Photochemical oxa-di-π-methane rearrangement of bicyclo[3.2.1]octanoid scaffolds affords multifunctional, donor-acceptor cyclopropanes. A related photochemical reaction of an iminium ether substrate uncovered an unprecedented aza-di-π-methane rearrangement of a β,γ-unsaturated iminium. Donor-acceptor cyclopropanes have been evaluated as substrates for reactions generating several new chemotypes.

Multidimensional Screening and Methodology Development for Condensations Involving Complex 1,2-Diketones

Multidimensional reaction screening employing complex 1,2-cycloheptanediones is described. The studies have enabled the discovery of regioselective, Lewis acid-mediated condensations with substituted ureas and a diastereoselective hydrogenation process which proceeds via an interesting allylpalladium hydride isomerization.

Reaction discovery using microfluidic-based multidimensional screening of polycyclic iminium ethers

Polycyclic iminium ethers are ambident electrophilic intermediates that react with a range of nucleophiles in a highly condition-dependent manner to afford densely functionalized lactams. In an effort to expand the scope of reactivity and assist in the generation of new chemotypes from these intermediates, several iminium ethers were subjected to reaction screening using an automated microfluidics reaction platform. Application of this approach led to the discovery of several interesting reaction pathways involving the iminium ether intermediates that will be described.

Development of an automated microfluidic reaction platform for multidimensional screening: reaction discovery employing bicyclo[3.2.1]octanoid scaffolds

An automated, silicon-based microreactor system has been developed for rapid, low-volume, multidimensional reaction screening. Use of the microfluidic platform to identify transformations of densely functionalized bicyclo[3.2.1]octanoid scaffolds will be described.

Novel acridine-based compounds that exhibit an anti-pancreatic cancer activity are catalytic inhibitors of human topoisomerase II

We have identified a small library of novel substituted 9-aminoacridine derivatives that inhibit cell proliferation of pancreatic cancer cell lines by inducing apoptosis [Goodell, J.R. et al., 2008. J. Med. Chem. 51, 179-182.]. To further investigate their antiproliferative activities, we have assessed the antiproliferative activity of these acridine-based compounds against several pancreatic cancer cell lines. All four compounds used in this study inhibited the proliferation of pancreatic cancer cell lines in vitro. In addition, we have employed a xenograft tumor model and found that these compounds also inhibit the proliferation of pancreatic cancer in vivo. In light of the potential importance of the anticancer activity of these acridine-based compounds, we have conducted a series of biochemical assays to determine the effect of these compounds on human topoisomerase II. Unlike amsacrine, these compounds do not poison topoisomerase II. Similar to amsacrine, however, these compounds intercalate into DNA in a way that they would alter the apparent topology of the DNA substrate. Thus, inhibition of the relaxation activity of topoisomerase II by these compounds has been reexamined using a DNA strand passage assay. We have found that these compounds, indeed, inhibit the catalytic activity of topoisomerase II. Thus, these novel acridine-based compounds with anti-pancreatic cancer activity are catalytic inhibitors, not poisons, of human topoisomerase II.

Acridine-based agents with topoisomerase II activity inhibit pancreatic cancer cell proliferation and induce apoptosis

A series of substituted 9-aminoacridines is evaluated for antiproliferative activity toward pancreatic cancer cells. The results indicate that the compounds inhibit cell proliferation by inducing a G1-S phase arrest. A model is also developed that explains the molecular basis to inhibition through a DNA "threading" mechanism. We conclude that the drug-DNA complex formed blocks topoisomerase II binding and activity leading to catalytic inhibition of the enzyme and the induction of apoptosis and programmed cell death.

Spectrophotometric determination and computational evaluation of the rates of hydrolysis of 9-amino-substituted acridines

Aminoacridines have a long history in the drug and dye industries and display a wide range of biological and physical properties. Despite the historical relevance of 9-aminoacridines, there have been few studies investigating their stability. 9-Aminoacridines are known to hydrolyze at the C9-N15 bond, yielding acridones. In this study, the pH-dependent hydrolysis rates of a series of 9-substituted aminoacridines are investigated. In addition, ground-state physical properties of the compounds are determined using ab initio quantum mechanics calculations to gain insight into the forces that drive hydrolysis. An analysis of the bond orders, bond dissociation energies, and conformational energies show that the rate of hydrolysis depends on two main factors: delocalization across the C9-N15 bond and steric effects. The computational results are applied to explain the change in experimental rates of hydrolysis going from primary to secondary and to tertiary substituted 9-aminoacridines. In the case of tertiary substituted amines, the calculations indicate the C9-N15 bond is forced into a more gauche-like conformation, greatly diminishing delocalization (as shown by reductions in bond orders and bond energy), which leads to rapid hydrolysis. A model of intramolecular hydrogen bonding is also presented, which explains the increased rate of hydrolysis observed for highly substituted compounds under acidic conditions.

Synthesis and evaluation of acridine- and acridone-based anti-herpes agents with topoisomerase activity

The discovery of new non-nucleoside antiviral compounds is of significant and growing interest for treating herpes virus infections due to the emergence of nucleoside-resistant strains. Using a whole cell virus-induced cytopathogenic assay, we tested a series of substituted triaryl heterocyclic compounds including acridones, xanthones, and acridines. The compounds which showed activity against Herpes Simplex-1 and/or Herpes Simplex-2 were further assayed for inhibition of topoisomerase activity to gain insight into the mechanism of action. The results indicate that the acridine analogs bearing substituted carboxamides and bulky 9-amino functionalities are able to inhibit herpes infections as well as inhibit topoisomerase II relaxation of supercoiled DNA. Given the mechanism of action of amsacrine (a closely related, well-studied 9-amino substituted acridine), the compounds were further tested in a DNA topoisomerase II cleavage assay to determine if the compounds function as poisons. The results show that the acridines synthesized in this study function through a different mechanism to that of amsacrine, most likely by blocking topoisomerase binding to DNA (akin to that of aclarubicin). This not only suggests a unique mechanism of action in treating herpes virus infections, but also may be of great interest in the development of anticancer agents that target topoisomerase II activity.

Identification of compounds with anti-West Nile Virus activity

The lack of antiviral compounds targeting flaviviruses represents a significant problem in the development of strategies for treating West Nile Virus (WNV), Dengue, and Yellow Fever infections. Using WNV high-throughput screening techniques developed in our laboratories, we report the identification of several small molecule anti-WNV compounds belonging to four different structural classes including pyrazolines, xanthanes, acridines, and quinolines. The initial set of "hits" was further refined using cell viability-cytotoxicity assays to two 1,3,5-triaryl pyrazoline compounds: 1-(4-chlorophenylacetyl)-5-(4-nitrophenyl)-3-(thiophen-2-yl)-4,5-dihydro-1H-pyrazole and 1-benzoyl-5-(4-chlorophenyl)-3-(thiophen-2-yl)-4,5-dihydro-1H-pyrazole. On the basis of their activity and favorable therapeutic indexes, these compounds were identified as viable leads and subjected to additional evaluation using an authentic viral titer reduction assay employing an epidemic strain of WNV. The compounds were further evaluated in a transient replicon reporting system to gain insight into the mechanism of action by identifying the step at which inhibition takes place during viral replication. The results indicate the pyrazolines inhibit RNA synthesis, pointing to viral RNA polymerase, RNA helicase, or other viral replication enzymes as potential targets. Progress was also made in understanding the structural requirements for activity by synthesizing a focused chemical library of substituted pyrazolines. Preliminary SAR data are presented that show the aryl-rings are required for activity against WNV. More importantly, the results indicate WNV activity is tolerant to aryl-substitutions paving the way for the design and development of much larger combinatorial libraries with varied physicochemical properties.

Serine-cis-proline and serine-trans-proline isosteres: stereoselective synthesis of (Z)- and (E)-alkene mimics by Still-Wittig and Ireland-Claisen rearrangements

Two new amide isosteres of Ser-cis-Pro and Ser-trans-Pro dipeptides were designed and stereoselectively synthesized to be incorporated into potential inhibitors of the phosphorylation-dependent peptidylprolyl isomerase Pin1, an essential regulator of the cell cycle. The cis mimic, the (Z)-alkene isomer, was formed through the use of a Still-Wittig [2,3]-sigmatropic rearrangement, while the trans mimic, the (E)-alkene, was synthesized through the use of an Ireland-Claisen [3,3]-sigmatropic rearrangement. Starting from N-Boc-Ser(OBn)-N(OMe)Me, both mimics were synthesized in Boc-protected form suitable for peptide synthesis with an overall yield of 20% in 10 steps for the cis mimic and 13% in eight steps for the trans mimic.