Synthesis of marine polycyclic polyethers via endo-selective epoxide-opening cascades

The proposed biosynthetic pathways to ladder polyethers of polyketide origin and oxasqualenoids of terpenoid origin share a dramatic epoxide-opening cascade as a key step. Polycyclic structures generated in these biosynthetic pathways display biological effects ranging from potentially therapeutic properties to extreme lethality. Much of the structural complexity of ladder polyether and oxasqualenoid natural products can be traced to these hypothesized cascades. In this review we summarize how such epoxide-opening cascade reactions have been used in the synthesis of ladder polyethers and oxasqualenoid natural products.

Synthesis and antibacterial activity of carvacryl ethers

Structural modifications of phenolic monoterpenoid obtained by reacting carvacrol with various substituted alpha-chloro acetanilides, to improve biological activities which give the product with better yield and higher purity under mild reaction conditions with the help of microwave irradiation techniques.

Uptake and metabolism of new synthetic lipophilic derivatives, hydroxytyrosyl ethers, by human hepatoma HepG2 cells

As a response to the increasing demand by the food industry for new synthetic lipophilic antioxidants, hydroxytyrosyl methyl, ethyl, propyl and butyl ethers have been synthesized from hydroxytyrosol, with similar or even higher antioxidant activity than free hydroxytyrosol. The uptake and metabolism of hydroxytyrosyl ethers with different alkyl side chain lengths (methyl, ethyl, propyl, and butyl) was studied after incubation for 2 and 18 h with HepG2 cells as a model of the human liver. LC-DAD and LC-MS were used for the identification of metabolites in culture media, cell lysates and samples hydrolyzed with beta-glucuronidase and sulfatase. In vitro conjugation reactions of pure phenols were also performed. The results show an extensive uptake and metabolism by HepG2 cells after 18 h of incubation. A direct relationship between the lipophilic nature of the compound and the biotransformation yield was observed. Similar ratio of methyl and glucuronide forms were detected after 2 h of incubation while at 18 h high amounts of methylglucuronides and glucuronide metabolites were identified together with low amounts of methyl conjugates. In conclusion, alkyl hydroxytyrosyl ethers could be metabolized by the liver, their metabolic rate being higher for the more lipophilic compounds.

[Screening of potential antibiotics, inhibitors of the nonmevalonate pathway of isoprenoid biosynthesis--2-C-methyl-D-erythritol-2,4-cyclodiphosphate derivatives]

The recently discovered nonmevalonate pathway of isoprenoid biosynthesis is a prospective target in screening of new antibiotics. Because of the absence of the pathway in the animal cells, the specific inhibitors of the pathway will be a new class of antibiotics against many pathogens (which cause, e.g., malaria, tuberculosis, etc), combining high efficiency and low toxicity. Several derivatives of 2-C-methyl-D-erythritol-2,4-cyclodiphosphate (MEC) were synthesized. 4-Phospho-methyl-D-erythritol-1,2-cyclophosphate, benzyl ether and benzyliden derivative of MEC inhibited the 14C-MEC incorporation into isoprenoids of chromoplasts from red pepper with IC50 of 1.7-5 MM. Some inhibition (about 10%) was also observed with the use of dimethyl ether and isopropyliden derivative of MEC.

Enantioselective total synthesis of brevetoxin A: unified strategy for the B, E, G, and J subunits

Brevetoxin A is a decacyclic ladder toxin that possesses 5-, 6-, 7-, 8-, and 9-membered oxacycles, as well as 22 tetrahedral stereocenters. Herein, we describe a unified approach to the B, E, G, and J rings based upon a ring-closing metathesis strategy from the corresponding dienes. The enolate technologies developed in our laboratory allowed access to the precursor acyclic dienes for the B, E, and G medium-ring ethers. The strategies developed for the syntheses of these four monocycles ultimately provided multigram quantities of each of the rings, supporting our efforts toward the completion of a convergent synthesis of brevetoxin A.

Distance dependent interhelical couplings of organic rods incorporated in DNA 4-helix bundles

The synthesis of a conjugated linear organic module containing terminal salicylaldehyde groups and a central activated ester, designed for conjugation to amino-modified oligonucleotides, is presented. The organic module has a phenylene-ethynylene backbone and is highly fluorescent. It is conjugated to oligonucleotide sequences and incorporated into specific locations in a well-defined DNA 4-helix bundle (4-HB). The DNA-nanostructure offers precise location control of the organic modules which allows for selective interhelical coupling reactions. In this study, metal-salen formation as well as dihydrazone formation are used to covalently interlink the organic modules. Both coupling reactions are highly dependent on the distances between the organic modules in the 4-HB. Neighboring modules dimerize easier, whereas more distanced modules are less prone to react, even when the linkers are extended. The dimeric products are characterized by denaturing polyacrylamide gel electrophoresis (PAGE), high performance liquid chromatography (HPLC), and matrix assisted laser desorption/absorption ionization time-of-flight (MALDI TOF) mass spectrometry.

Preparation and characterization of catechin sulfates, glucuronides, and methylethers with metabolic interest

Catechins are major polyphenols in many plant foods that have been related to health promotion. In the human organism they are largely metabolized to different conjugates (sulfates, glucuronides, and methylethers), which are further found in plasma and would contribute to the biological effects associated with the intake of the parent compounds. Circulating metabolites are likely to possess biological properties different from those of the original compounds, and therefore, it is important to evaluate their activity, for which sufficient amounts of them are required that cannot be obtained by isolation from biological fluids. This paper describes the preparation of the methyl, sulfate, and glucuronide derivatives of catechins using different chemical syntheses and their characterization by HPLC-DAD-ESI/MS. MS2 fragmentation of the compounds was also described that allowed the determination of the location of the different substituents on the catechin aglycones. The procedures optimized allowed the preparation of (epi)catechin sulfates, glucuronides, and methylethers conjugated at positions 3' and 4', as well as the sulfates at positions 5 and 7 with satisfactory yields for their further isolation by semipreparative-HPLC in view of their use in in vitro/ex vivo assays.

Epoxide-opening cascades in the synthesis of polycyclic polyether natural products

The structural features of polycyclic polyether natural products can, in some cases, be traced to their biosynthetic origin. However in case that are less well understood, only biosynthetic pathways that feature dramatic, yet speculative, epoxide-opening cascades are proposed. We summarize how such epoxide-opening cascade reactions have been used in the synthesis of polycyclic polyethers (see scheme) and related natural products.The group of polycyclic polyether natural products is of special interest owing to the fascinating structure and biological effects displayed by its members. The latter includes potentially therapeutic antibiotic, antifungal, and anticancer properties, and extreme lethality. The polycyclic structural features of this class of compounds can, in some cases, be traced to their biosynthetic origin, but in others that are less well understood, only to proposed biosynthetic pathways that feature dramatic, yet speculative, epoxide-opening cascades. In this review we summarize how such epoxide-opening cascade reactions have been used in the synthesis of polycyclic polyethers and related natural products.

Synthesis of alkynyl ethers and low-temperature sigmatropic rearrangement of allyl and benzyl alkynyl ethers

Alpha-alkoxy ketones 3 can be transformed into 1-alkynyl ethers 5 by a two-step procedure involving formation of the enol triflate or phosphate and base-induced elimination. Performing the same reaction sequence with allylic alcohols (R2OH, R2 = allyl) furnishes instead gamma,delta-unsaturated carboxylic acid derivatives 6, derived from [3,3]-sigmatropic rearrangement of the intermediate allyl alkynyl ethers at -78 degrees C and trapping of the subsequently formed ketene with nucleophiles (Nu-H). Benzyl alkynyl ether 5 (R2 = benzyl) rearranges to indanone 7 upon heating to 60 degrees C.

Synthesis and structure-activity relationship studies of cytotoxic ester and ether anhydrovinblastine derivatives

Two new series of 3-demethoxycarbonyl-3-ester and 3-demethoxycarbonyl-3-ether methyl anhydrovinblastine derivatives were designed, synthesized, and evaluated for their inhibitory activities against human non-small-cell lung cancer (A549) and cervical epithelial adenocarcinoma (HeLa) cell lines. Ester anhydrovinblastine derivatives exhibited potent cytotoxicity, whereas the ether analogues were much less active. The size of the introduced substituents was the foremost factor in determining the resultant cytotoxic activity. Compound 12b showed a similar cytotoxic potency to the positive control, anhydrovinblastine (1a).

Ligand controlled highly regio- and enantioselective synthesis of alpha-acyloxyketones by palladium-catalyzed allylic alkylation of 1,2-enediol carbonates

The palladium catalyzed decarboxylative asymmetric allylic alkylation of allyl 1,2-enediol carbonates 1 can decompose to either alpha-hydroxyketones 3 or alpha-hydroxyaldehydes 4. The product distribution is largely controlled by the ligand. Using Lnaph in DME we exclusively obtained the ketone product in good to excellent yields and high enantiomeric excesses. The reaction proceeds under extremely mild conditions, so we can have a broad range of choices of OR. Besides the commonly used protection groups such as OAc and OPiv, a more functionalized group such as methyl but-2-enoyl group can also be used, downstream process of which can afford other synthetically interesting structures.

C-bis-pivot lariat ethers: synthesis and spectral investigations on new 15- and 17-membered coronands containing dimethoxyphosphoryl groups

The reactions of dibenzo-diaza crown ethers (coronands) (1 and 2) with dimethylphosphite led to the formation of the mixture of meso and racemic C-bis-pivot lariat ethers (3 and 4) containing dimethoxyphosphoryl groups. We have failed to make the resolution of the mixture, nevertheless, the detailed characterization and spectral investigations of compounds 3 and 4 have been made by elemental analyses, FTIR, (1)H NMR, (13)C NMR, (31)P NMR, COSY, DEPT, HETCOR and HMBC spectral data. The salient features of the spectral data of these compounds have been presented.

Design and synthesis of aryl ether inhibitors of the Bacillus anthracis enoyl-ACP reductase

The problem of increasing bacterial resistance to the current generation of antibiotics is well documented. Known resistant pathogens such as methicillin-resistant Staphylococcus aureus are becoming more prevalent, while the potential exists for developing drug-resistant pathogens for use as bioweapons, such as Bacillus anthracis. The biphenyl ether antibacterial agent, triclosan, exhibits broad-spectrum activity by targeting the fatty acid biosynthetic pathway through inhibition of enoyl-acyl carrier protein reductase (ENR) and provides a potential scaffold for the development of new, broad-spectrum antibiotics. We used a structure-based approach to develop novel aryl ether analogues of triclosan that target ENR, the product of the fabI gene, from B. anthracis (BaENR). Structure-based design methods were used for the expansion of the compound series including X-ray crystal structure determination, molecular docking, and QSAR methods. Structural modifications were made to both phenyl rings of the 2-phenoxyphenyl core. A number of compounds exhibited improved potency against BaENR and increased efficacy against both the Sterne strain of B. anthracis and the methicillin-resistant strain of S. aureus. X-ray crystal structures of BaENR in complex with triclosan and two other compounds help explain the improved efficacy of the new compounds and suggest future rounds of optimization that might be used to improve their potency.

Biphenylalkylacetylhydroquinone ethers suppress the proliferation of murine B16 melanoma cells

Hydroquinone, an activator of caspase-9 activity via reactive oxygen species, and farnesol, a post-translational down-regulator of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity suppress the growth of murine 816 melanoma cells. Our previous studies have shown that farnesyl-O-acetylhydroquinone has a markedly greater growth-suppressive activity than that predicted by the responses to the parent compounds. Perillyl alcohol, a modulator of small G-protein activity, and biphenyl compounds, activators of Fas-mediated death pathways, suppress B16 growth. A similar synergistic increase in the potency of each compound when ether-linked to acetylhydroquinone is reported. Perillyl-O-acetylhydroquinone, biphenylethyl-O-acetylhydroquinone and biphenylpropyl-O-acetylhydroquinone had dose-dependent impacts on the proliferation of B16 cells with 50% inhibitory concentration (IC50) values of 8.0, 4.2 and 1.4 micromol/L, respectively. The growth-suppression effected by biphenylpropyl-O-acetylhydroquinone was accompanied by a dose-dependent arrest at the G1/S interface of the cell cycle, an impact greater than that previously reported for farnesyl-O-acetylhydroquinone (IC50 = 2.5 micromol/L). These new hydroquinone derivatives may have potential in cancer chemoprevention and/or therapy.

The continuing saga of the marine polyether biotoxins

The unprecedented structure of the marine natural product brevetoxin B was elucidated by the research group of Nakanishi and Clardy in 1981. The ladderlike molecular architecture of this fused polyether molecule, its potent toxicity, and fascinating voltage-sensitive sodium channel based mechanism of action immediately captured the imagination of synthetic chemists. Synthetic endeavors resulted in numerous new methods and strategies for the construction of cyclic ethers, and culminated in several impressive total syntheses of this molecule and some of its equally challenging siblings. Of the marine polyethers, maitotoxin is not only the most complex and most toxic of the class, but is also the largest nonpolymeric natural product known to date. This Review begins with a brief history of the isolation of these biotoxins and highlights their biological properties and mechanism of action. Chemical syntheses are then described, with particular emphasis on new methods developed and applied to the total syntheses. The Review ends with a discussion of the, as yet unfinished, story of maitotoxin, and projects into the future of this area of research.

Peptidomimetic inhibitors of farnesyltransferase with high in vitro activity and significant cellular potency

2-o-Tolyl or 2-o-anisyl substituted 4-hydroxy- and 4-carboxybenzamides of methionine, etherified and amidified with 2-hydroxymethyl- and 2-aminomethylpyridodioxane, respectively, are described as inhibitors of Ras protein farnesyltransferase (FTase). Of the sixteen compounds, resulting from the substitution pattern of benzamide and the configuration of the two stereocenters, seven inhibited FTase activity with potencies in the nanomolar range. They were all 2-oxymethylpyridodioxane ethers and, among them, the four o-tolyl substituted stereoisomers also showed micromolar antiproliferative effect on human aortic smooth muscle cells interfering with Ras farnesylation. The docking analysis enlightened significant differences in enzyme interaction between oxymethylpyridodioxane and aminomethylpyridodioxane derivatives.

Catalytic enantioselective O-H insertion reactions

The first effective method for catalytic enantioselective insertions into O-H bonds has been developed. Specifically, a copper/bisazaferrocene catalyst couples alcohols such as 2-trimethylsilylethanol with alpha-aryl-alpha-diazo esters in high yield and good ee.

Alkynyl polysaccharides: synthesis of propargyl potato starch followed by subsequent derivatizations

Potato starch propargyl ethers (PgS) with degrees of substitution (DS) from 0.1 to 2.2 have been prepared by etherification of starch with sodium hydroxide or Li dimsyl in Me(2)SO and propargyl bromide. DS values and substituent distribution were determined after hydrolysis and acetylation by GC-MS. The order of reactivity was 2>6>>3, with O-3 substitution being preferably observed in the trisubstituted units. Repeated analysis of the starch derivatives revealed that propargyl residues were lost during storage, a phenomenon that was not fully understood until now. Selected PgS were further functionalized: (a) O- and C-methylated to O-(2-butynyl)-O-methyl starch (BMS), (b) in a Mannich type reaction with diethylamine and formaldehyde to yield O-(4-diethylamino)-2-butinyl starch (DEABiS), (c) in a 1,3-dipolar cycloaddition with benzyl azide ('click-chemistry') to a N-benzyltriazole derivatized starch (BTrS), and (d) with carbon dioxide to O-(3-carboxy)-2-butinyl starch (CBiS). While the yield of carboxylation was only poor, conversion was high or nearly quantitative for reactions a-c. Thus, it is demonstrated that starch propargyl ethers are valuable intermediates for the preparation of functional polysaccharides.

Synthetic antimicrobial oligomers induce a composition-dependent topological transition in membranes

Antimicrobial peptides (AMPs) are cationic amphiphiles that comprise a key component of innate immunity. Synthetic analogues of AMPs, such as the family of phenylene ethynylene antimicrobial oligomers (AMOs), recently demonstrated broad-spectrum antimicrobial activity, but the underlying molecular mechanism is unknown. Homologues in this family can be inactive, specifically active against bacteria, or nonspecifically active against bacteria and eukaryotic cells. Using synchrotron small-angle X-ray scattering (SAXS), we show that observed antibacterial activity correlates with an AMO-induced topological transition of small unilamellar vesicles into an inverted hexagonal phase, in which hexagonal arrays of 3.4-nm water channels defined by lipid tubes are formed. Polarized and fluorescence microscopy show that AMO-treated giant unilamellar vesicles remain intact, instead of reconstructing into a bulk 3D phase, but are selectively permeable to encapsulated macromolecules that are smaller than 3.4 nm. Moreover, AMOs with different activity profiles require different minimum threshold concentrations of phosphoethanolamine (PE) lipids to reconstruct the membrane. Using ternary membrane vesicles composed of DOPG:DOPE:DOPC with a charge density fixed at typical bacterial values, we find that the inactive AMO cannot generate the inverted hexagonal phase even when DOPE completely replaces DOPC. The specifically active AMO requires a threshold ratio of DOPE:DOPC = 4:1, and the nonspecifically active AMO requires a drastically lower threshold ratio of DOPE:DOPC = 1.5:1. Since most gram-negative bacterial membranes have more PE lipids than do eukaryotic membranes, our results imply that there is a relationship between negative-curvature lipids such as PE and antimicrobial hydrophobicity that contributes to selective antimicrobial activity.

Synthesis of new polyether glycodendrons as oligosaccharide mimetics

Divalent and tetravalent glycomimetics based on polyether glycodendrons have been prepared. The branched scaffolds were decorated with galactose moieties on one hand and were elaborated into new glycodendrons of a 'mixed' type on the other, carrying both galactose and mannose moieties as biologically important sugar epitopes. All synthesized glycodendrons possess a focal point that can be employed for further derivatization and functionalization.

Epoxide-opening cascades promoted by water

Selectivity rules in organic chemistry have been inferred largely from nonaqueous environments. In contrast, enzymes operate in water, and the chemical effect of the medium change remains only partially understood. Structural characterization of the "ladder" polyether marine natural products raised a puzzle that persisted for 20 years: Although the stereochemistry of adjacent tetrahydropyran (THP) cycles would seem to arise from a biosynthetic cascade of epoxide-opening reactions, experience in organic solvents argued consistently that such a pathway would be kinetically disfavored. We report that neutral water acts as an optimal promoter for the requisite ring-opening selectivity, once a single templating THP is appended to a chain of epoxides. This strategy offers a high-yielding route to the naturally occurring ladder core and highlights the likely importance of aqueous-medium effects in underpinning certain noteworthy enzymatic selectivities.

Interplay between core and peripheral chirality in polyethers

A chiral-base mediated reaction has been used to synthesise the polyethers illustrated and their enantiomers; CD spectroscopy revealed that the homochiral isomers A may be viewed as a C3-symmetric arrangement of fixed paddles while the heterochiral isomers B form a C3-symmetric arrangement of rotating paddles.

Cisplatinum and Transplatinum Complexes with Benzyliminoether Ligands; Synthesis, Characterization, Structure−Activity Relationships, and In Vitro and In Vivo Antitumor Efficacy

New benzyliminoether derivatives [PtCl2{N(H)=C(OMe)CH2Ph}2] of cis (1a, 1b) and trans (2a, 2b) geometry were prepared and characterized by means of elemental analysis, multinuclear NMR and FT-IR techniques, and X-ray crystallography; this latter was carried out for 1b. The cytotoxic properties of these new platinum(II) complexes were evaluated in terms of cell growth inhibition against a panel of different types of human cancer cell lines. cis-[PtCl2{E-N(H)=C(OMe)CH2Ph}2] (1a) was significantly more potent than cisplatin against all tumor cell lines tested, showing IC50 values from about 2- to 17-fold lower than the reference compound. Chemosensitivity tests performed on cisplatin-sensitive and -resistant cell lines have demonstrated that complex 1a is able to overcome cisplatin resistance. Analyzing the mechanism by which complex 1a led to cell death, we have found that it induced apoptosis in a dose-dependent manner, accompanied by the activation of caspase-3. The in vivo studies carried out using two transplantable tumor models (L1210 leukemia and Lewis lung carcinoma) showed that derivative 1a induced a remarkable antitumor activity in both tumor models, as measured by prolonged survival and reduced tumor mass compared to control groups.

Synthesis of indenyl ethers by gold(I)-catalyzed intramolecular carboalkoxylation of alkynes

The gold(I)-catalyzed carboalkoxylation of alkynes to form indanone derivatives from readily available ortho-acetylenic benzylic ethers is described. Importantly, the gold(I)-catalyzed rearrangement of enantioenriched benzylic ethers proceeds with chirality transfer, thus providing a practical method for the enantioselective synthesis of indenyl ethers.

Structure-activity relationship studies of carboxamido-biaryl ethers as opioid receptor antagonists (OpRAs). Part 1

A structurally unique and new class of opioid receptor antagonists (OpRAs) that bear no structural resemblance with morphine or endogenous opioid peptides has been discovered. A series of carboxamido-biaryl ethers were identified as potent receptor antagonists against mu, kappa and delta opioid receptors. The structure-activity relationship indicated para-substituted aryloxyaryl primary carboxamide bearing an amine tether on the distal phenyl ring was optimal for potent in vitro functional antagonism against three opioid receptor subtypes.

Trisubstituted aziridine ring-opening by phenol derivatives: stereo- and regioselective formation of chiral tertiary alkyl-aryl ethers

An unprecedented stereo- and regioselective trisubstituted aziridine ring-opening by phenol derivatives was discovered. The reaction features very mild reaction conditions and broad functional group compatibility, which provides a good method for the stereoselective formation of tertiary alkyl-aryl ethers in highly functionalized systems. [reaction: see text]

Stereoselective synthesis of (E)- and (Z)-enol ethers from β-amino aldehydes

Stereocontrolled methods for the direct and divergent synthesis of the silylenol ethers possessing amino group from beta-amino aldehydes have been achieved. These enol ethers with the defined olefin geometry could be key building blocks for the synthesis of the medicinally impor tant compounds.

Synthesis and Biological Studies of Novel 2-Aminoalkylethers as Potential Antiarrhythmic Agents for the Conversion of Atrial Fibrillation

A series of 2-aminoalkylethers prepared as potential antiarrhythmic agents is described. The present compounds are mixed sodium and potassium ion channel blockers and exhibit antiarrhythmic activity in a rat model of ischemia-induced arrhythmias. Structure-activity studies led to the identification of three compounds 5, 18, and 26, which were selected based on their particular in vivo electrophysiological properties, for studies in two canine atrial fibrillation (AF) models. The three compounds converted AF in both models, but only compound 26 was shown to be orally bioavailable. Resolution of the racemate 26 into its corresponding enantiomers 40 and 41 and subsequent biological testing of these enantiomers led to the selection of (1S,2S)-1-(1-naphthalenethoxy)-2-(3-ketopyrrolidinyl)cyclohexane monohydrochloride (41) as a potential atrial selective antiarrhythmic candidate for further development.

Amphiphilic polyether branched molecules to increase the circulation time of cationic particles

The preparation, physicochemical and biological properties of amphiphilic polyether branched molecules is described. These 'bunch shaped' molecules when inserted into cationic liposomes/DNA complexes have shown efficient surface charge shielding. As a consequence they efficiently inhibited the non specific interactions with blood components and significantly enhanced circulation time of the particles in the blood track. Formulations containing these molecules compared positively with those containing PEG lipids, providing a 5-fold increase in circulation time.

Structure-based design, synthesis, and A-site rRNA cocrystal complexes of functionally novel aminoglycoside antibiotics: C2" ether analogues of paromomycin

A series of 2"-O-substituted ether analogues of paromomycin were prepared based on new site-selective functionalizations. X-ray cocrystal complexes of several such analogues revealed a new mode of binding in the A-site rRNA, whereby rings I and II adopted the familiar orientation and position previously observed with paromomycin, but rings III and IV were oriented differently. With few exceptions, all of the new analogues showed potent inhibitory activity equal or better than paromomycin against a sensitive strain of S. aureus. Single digit microM MIC values were obtained against E. coli, with some of the ether appendages containing polar or basic end groups. Two analogues showed excellent survival rate in a mouse septicemia protection assay. Preliminary histopathological analysis of the kidney showed no overt signs of toxicity, while controls with neomycin and kanamycin were toxic at lower doses.

Synthetic diether-linked cationic lipids for gene delivery

Quaternary ammonium lipids 2a-p, with diether linkages between hydrocarbon chains and their ammonium headgroups, were synthesized as potential vectors for cationic liposome-mediated gene delivery. Varying the length of carbon chains and quaternary ammonium heads as well as different anionic complexes will enable the study of the structure-function relationships of these cationic lipids in terms of gene delivery properties.

Synthesis of novel diaryl ethers and their evaluation as antimitotic agents

A series of novel diaryl ethers possessing various functional groups were synthesized and evaluated for antiproliferative activity in human myeloid leukemia HL-60 cells. Among the compounds examined, compounds 10, 17, 20, 24, and 33 showed moderate to potent antiproliferative activity. These derivatives were further examined in terms of their abilities to inhibit tubulin polymerization; however, all of the tested compounds were relatively ineffective. The reference compound E7010 with an IC(50) of 0.34 microM exhibited potent antiproliferative activity and significantly inhibited tubulin polymerization in a dose-dependent manner.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2005 is provided. In a similar vein to earlier installments in this series, the emphasis is on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments.

Synthesis of densely functionalised arenes using [2 + 2 + 2] cycloaddition reactions

Rh(I)-catalysed [2 + 2 + 2] cycloaddition allows the synthesis of aryl ethers and diaryl methanes containing a high degree of steric hindrance from relatively simple diyne and alkyne precursors. The diarylmethanes made in this way show no evidence in their NMR spectra, however, of rotational restriction.

Intermolecular Electrophilic O-Amination of Alcohols

We report the first examples of an intermolecular electrophilic O-amination of aliphatic alcohols. Thus, the new reagents, fluorenone oxime tosylate, 5a, or mesylate, 5b, permit O-amination of diverse alcohols in the presence of NaH under mild conditions. By following the formation of the resulting oxime ethers, 6, the reaction was shown to be sensitive to steric effects in the alcohol. Furthermore, the presence of an aromatic ring or of a double bond in the alcohol molecule (benzyl, allyl) was found to increase the reaction rate.

Alcohols and Di-tert-butyl Dicarbonate: How the Nature of the Lewis Acid Catalyst May Address the Reaction to the Synthesis of tert-Butyl Ethers

The reaction between alcohols and Boc2O leads to the formation of tert-butyl ethers and/or Boc-alcohols, depending on the nature of the Lewis acid catalyst. Product distribution is mainly tuned by the anionic part of the salt. Perchlorates and triflates, anions with highly delocalized negative charge, give prevalent or exclusive ether formation. On the other hand, Boc alcohols are the main or exclusive products with un-delocalized isopropoxide or low-delocalized acetate ions. The metal ion influences only the reaction rate, roughly following standard parameters for calculating Lewis acidity. A reaction mechanism is supposed, and a series of experimental evidences is reported to support it. These studies allowed us to conclude that, to synthesize tert-butyl ethers, in reactions involving aliphatic alcohols, Mg(ClO4)2 or Al(ClO4)3 represents the best compromise between costs and efficiency of the reaction, while, in reactions involving phenols, Sc(OTf)3 is the best choice, since aromatic tert-butyl ethers are not stable in the presence of perchlorates.

Convergent Assembly of Polycyclic Ethers via Acyl Radical Addition to Unactivated Enol Ether

A new convergent strategy for assembling 6/6- and 6/7-fused ether ring systems was developed. The key features in our method include Ag+-promoted facile formation of chemically labile enol ether from O,S-acetal and addition of an acyl radical to unactivated enol ether to cyclize a six- or seven-membered ether ring. [reaction: see text]

Total Synthesis of (S)-Equol

The first enantioselective total synthesis of (S)-equol is reported. The described route relies on an Evans alkylation to form the stereocenter and an intramolecular Buchwald etherification to generate the chroman ring. Key features of this method include its brevity, its scalability, and the low cost of starting materials. [reaction: see text].

Automation of fluorous solid-phase extraction for parallel synthesis

An automatic fluorous solid-phase extraction (F-SPE) technique is developed by using FluoroFlash SPE cartridges on the RapidTrace workstation. A 10-module workstation has the capability to complete a maximum of 100 SPEs each round in 1-2 h. Another important feature of the RapidTrace system is that it has the capability to load slurry samples onto the F-SPE cartridges. The F-SPE cartridge charged with 2 g of fluorous silica gel is used to purify up to 200 mg of crude sample. Sample loading, elution solvent, cartridge reuse, and SPE reproducibility are evaluated. The automatic SPE system is used for purification of a small urea library generated from amine-scavenging reactions using fluorous dichlorotriazine, a 96-membered amide library generated using 2-chloro-4,6-bis[(perfluorohexyl)propyloxy]-1,3,5-triazine as the coupling agent, and another 96-membered library generated from fluorous Mitsunobu reactions. Approximately 90% of the products have > 90% purity after F-SPE.

Discovery of adamantane ethers as inhibitors of 11beta-HSD-1: Synthesis and biological evaluation

A novel class of adamantane ethers 11beta-hydroxysteroid hydrogenase type I inhibitors has been discovered. These compounds have excellent HSD-1 potency and selectivity against HSD-2. The structure-activity relationships, selectivity, metabolism, PK, ex vivo pharmacodynamic data, and an X-ray crystal structure of one of these inhibitors bound to h-HSD-1 are discussed.

Stereochemically General Approach to Adjacent Bis(tetrahydrofuran) Cores of Annonaceous Acetogenins

A series of six 2,5-disubstituted adjacent bis(tetrahydrofuran) stereoisomers with trans/erythro/cis, trans/threo/trans, or cis/threo/cis relative stereochemistry have been synthesized from known dihydroxycyclooctenes via ring opening/cross metathesis and Pd(0)-mediated asymmetric double cycloetherification. The stereochemistry of four of these isomers has been found in the biologically active annonaceous acetogenin natural products. [reaction: see text].

Synthesis of gerfelin and related analogous compounds

Gerfelin, an inhibitor of human geranylgeranyl diphosphate (GGPP) synthase that has been isolated from a culture broth of Beauveria felina QN22047, was synthesized in 4 and 3 steps starting from 2,4-dihydroxy-6-methylbenzoic acid and 3,4,5-trihydroxytoluene, respectively. An effective ligand, 2-(di-tert-butylphosphino)biphenyl, was used in the palladium-catalyzed diaryl ether-forming reaction. Five analogous compounds of gerfelin were also synthesized for a study of the structure-activity relationship.

Organolithium-Induced Alkylative Ring Opening of Aziridines: Synthesis of Unsaturated Amino Alcohols and Ethers

Organolithium-induced alkylative ring opening of N-sulfonyl-protected aziridinyl ethers is described. The reactions were efficiently carried out with a variety of organolithiums, providing a promising new strategy to unsaturated amino alcohols and ethers. Cis- and trans-1,4-dimethoxybut-2-ene-derived aziridines were prepared, and their propensity to undergo organolithium- induced alkylative desymmetrization is detailed. Use of a single enantiomer of the latter aziridine provides a route to enantiopure unsaturated amino ethers.

Solid-phase synthesis and structure–activity relationships of novel biarylethers as melanin-concentrating hormone receptor-1 antagonists

Melanin-concentrating hormone (MCH) is a cyclic 19 amino acid orexigenic neuropeptide. The action of MCH on feeding is thought to involve the activation of its respective G protein-coupled receptor MCH-R1. Consequently, antagonists that block MCH regulated MCH-R1 activity may provide a viable approach to the treatment of diet-induced obesity. This communication reports the discovery of a novel MCH-R1 receptor antagonist, the biarylether 7, identified through high throughput screening. The solid-phase synthesis and structure-activity relationship of related analogs is described.