Characterization and heterologous reconstitution of Taxus biosynthetic enzymes leading to baccatin III

Paclitaxel is a well known anticancer compound. Its biosynthesis involves the formation of a highly functionalized diterpenoid core skeleton (baccatin III) and the subsequent assembly of a phenylisoserinoyl side chain. Despite intensive investigation for half a century, the complete biosynthetic pathway of baccatin III remains unknown. In this work, we identified a bifunctional cytochrome P450 enzyme [taxane oxetanase 1 (TOT1)] in Taxus mairei that catalyzes an oxidative rearrangement in paclitaxel oxetane formation, which represents a previously unknown enzyme mechanism for oxetane ring formation. We created a screening strategy based on the taxusin biosynthesis pathway and uncovered the enzyme responsible for the taxane oxidation of the C9 position (T9αH1). Finally, we artificially reconstituted a biosynthetic pathway for the production of baccatin III in tobacco.

Applications of oxetanes in drug discovery and medicinal chemistry

The compact and versatile oxetane motifs have gained significant attention in drug discovery and medicinal chemistry campaigns. This review presents an overview of the diverse applications of oxetanes in clinical and preclinical drug candidates targeting various human diseases, including cancer, viral infections, autoimmune disorders, neurodegenerative conditions, metabolic disorders, and others. Special attention is given to biologically active oxetane-containing compounds and their disease-related targets, such as kinases, epigenetic and non-epigenetic enzymes, and receptors. The review also details the effect of the oxetane motif on important properties, including aqueous solubility, lipophilicity, pKa, P-glycoprotein (P-gp) efflux, metabolic stability, conformational preferences, toxicity profiles (e.g., cytochrome P450 (CYP) suppression and human ether-a-go-go related gene (hERG) inhibition), pharmacokinetic (PK) properties, potency, and target selectivity. We anticipate that this work will provide valuable insights that can drive future discoveries of novel bioactive oxetane-containing small molecules, enabling their effective application in combating a wide range of human diseases.

Oxetanes in Drug Discovery Campaigns

The oxetane ring is an emergent, underexplored motif in drug discovery that shows attractive properties such as low molecular weight, high polarity, and marked three-dimensionality. Oxetanes have garnered further interest as isosteres of carbonyl groups and as molecular tools to fine-tune physicochemical properties of drug compounds such as pKa, LogD, aqueous solubility, and metabolic clearance. This perspective highlights recent applications of oxetane motifs in drug discovery campaigns (2017-2022), with emphasis on the effect of the oxetane on medicinally relevant properties and on the building blocks used to incorporate the oxetane ring. Based on this analysis, we provide an overview of the potential benefits of appending an oxetane to a drug compound, as well as potential pitfalls, challenges, and future directions.

Hexahydroazulene-2(1H)-one Sesquiterpenoids with Bridged Cyclobutane, Oxetane, and Tetrahydrofuran Rings from the Stems of Daphne papyracea with α-Glycosidase Inhibitory Activity

Chemical investigation of an alcoholic extract from the stem of Daphne papyracea ("Xuehuagou") led to the isolation of the tetracyclic sesquiterpenoid daphnepapytone A (1), containing a unique caged skeleton with a cyclobutane ring having three tetrasubstituted chirality centers. Also isolated were new guaiane sesquiterpenoids, namely, daphnepapytones B-H (2-8), and one 1,5-diphenylpentanone 2-hydroxy-5-oxo-daphneone (9), together with 26 known compounds. The cyclic metabolites share a 5-isoprenyl-hexahydroazulene-2(1H)-one skeleton with different substitution patterns and a bridged cyclobutane, oxetane, or tetrahydrofuran ring. The planar structures and relative configuration of the new compounds were elucidated on the basis of spectroscopic analysis aided by DFT ¹³C NMR calculations. The absolute configurations of 1-7 were determined by X-ray single-crystal diffraction or TDDFT-ECD calculations. Daphnepapytones A and C (1 and 3), 2-hydroxy-5-oxodaphneone (9), daphnenone (10), daphneone (11), and 3-methyldaphneolone (12) showed α-glycosidase inhibitory activity, with IC₅₀ values of 159.0, 102.3, 139.3, 43.3, 145.0, and 126.1 μM, respectively.

Discovery of Novel Cyclic Ethers with Synergistic Antiplasmodial Activity in Combination with Valinomycin

With drug resistance threatening our first line antimalarial treatments, novel chemotherapeutics need to be developed. Ionophores have garnered interest as novel antimalarials due to their theorized ability to target unique systems found in the Plasmodium-infected erythrocyte. In this study, during the bioassay-guided fractionation of the crude extract of Streptomyces strain PR3, a group of cyclodepsipeptides, including valinomycin, and a novel class of cyclic ethers were identified and elucidated. Further study revealed that the ethers were cyclic polypropylene glycol (cPPG) oligomers that had leached into the bacterial culture from an extraction resin. Molecular dynamics analysis suggests that these ethers are able to bind cations such as K⁺, NH₄⁺ and Na⁺. Combination studies using the fixed ratio isobologram method revealed that the cPPGs synergistically improved the antiplasmodial activity of valinomycin and reduced its cytotoxicity in vitro. The IC₅₀ of valinomycin against P. falciparum NF54 improved by 4-5-fold when valinomycin was combined with the cPPGs. Precisely, it was improved from 3.75 ± 0.77 ng/mL to 0.90 ± 0.2 ng/mL and 0.75 ± 0.08 ng/mL when dosed in the fixed ratios of 3:2 and 2:3 of valinomycin to cPPGs, respectively. Each fixed ratio combination displayed cytotoxicity (IC₅₀) against the Chinese Hamster Ovary cell line of 57-65 µg/mL, which was lower than that of valinomycin (12.4 µg/mL). These results indicate that combinations with these novel ethers may be useful in repurposing valinomycin into a suitable and effective antimalarial.

FEMA GRAS assessment of natural flavor complexes: Eucalyptus oil and other cyclic ether-containing flavoring ingredients

In 2015, the Expert Panel of the Flavor and Extract Manufacturers Association (FEMA) initiated a program for the re-evaluation of the safety of over 250 natural flavor complexes (NFCs) used as flavor ingredients. This publication, the sixth in the series, will summarize the re-evaluation of eight NFCs whose constituent profiles are characterized by significant amounts of eucalyptol and/or other cyclic ethers. This re-evaluation was based on a procedure first published in 2005 and subsequently updated in 2018 that evaluates the safety of naturally occurring mixtures for their intended use as flavoring ingredients. The procedure relies on a complete chemical characterization of the NFC intended for commerce and the organization of its chemical constituents into well-defined congeneric groups. The safety of the NFC is evaluated using the well-established and conservative threshold of toxicological concern (TTC) concept in addition to data on absorption, metabolism and toxicology of the constituents of the congeneric groups and the NFC under evaluation. Eight NFCs derived from the Eucalyptus, Melaleuca, Origanum, Laurus, Rosmarinus and Salvia genera were affirmed as generally recognized as safe (GRAS) under their conditions of intended use as flavor ingredients based on an evaluation of each NFC and the constituents and congeneric groups therein.

Direct Synthesis of Polyaromatic Cyclophanes Containing Bis-Methylene-Interrupted Z-Double Bonds and Study of Their Antitumor Activity In Vitro

An original synthetic route was developed for the preparation of previously unknown unsaturated polyaromatic macrolactones containing a 1Z,5Z-diene moiety in 48–71% yields and with >98% stereoselectivity. The method is based on intermolecular cyclocondensation of aromatic dicarboxylic acids with α,ω-alka-nZ,(n+4)Z-dienediols (1,12-dodeca-4Z,8Z-dienediol, 1,14-tetradeca-5Z,9Z-dienediol, 1,18-octadeca-7Z,11Z-dienediol) mediated by N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC)/4-dimethylaminopyridine (DMAP). The unsaturated diols were prepared by successive homo-cyclomagnesiation of tetrahydropyran ethers of O-containing 1,2-dienes with EtMgBr in the presence of Mg metal and the Cp₂TiCl₂ catalyst (10 mol.%) and subsequent treatment with 0.1 equiv. of para-toluenesulfonic acid of pyran ethers formed after the acid hydrolysis of magnesacyclopentanes. The resulting cyclophanes exhibited high cytotoxic activity in vitro against Jurkat, K562, U937, and HL60 cancer lines. Additionally, the synthesized products were studied for their effect on mitochondria, ability to induce apoptosis, and influence on the cell cycle using modern flow cytometry methods.

A Case Study in Catalyst Generality: Simultaneous, Highly-Enantioselective Brønsted- and Lewis-Acid Mechanisms in Hydrogen-Bond-Donor Catalyzed Oxetane Openings

Generality in asymmetric catalysis can be manifested in dramatic and valuable ways, such as high enantioselectivity across a wide assortment of substrates in a given reaction (broad substrate scope) or as applicability of a given chiral framework across a variety of mechanistically distinct reactions (privileged catalysts). Reactions and catalysts that display such generality hold special utility, because they can be applied broadly and sometimes even predictably in new applications. Despite the great value of such systems, the factors that underlie generality are not well understood. Here, we report a detailed investigation of an asymmetric hydrogen-bond-donor catalyzed oxetane opening with TMSBr that is shown to possess unexpected mechanistic generality. Careful analysis of the role of adventitious protic impurities revealed the participation of competing pathways involving addition of either TMSBr or HBr in the enantiodetermining, ring-opening event. The optimal catalyst induces high enantioselectivity in both pathways, thereby achieving precise stereocontrol in fundamentally different mechanisms under the same conditions and with the same chiral framework. The basis for that generality is analyzed using a combination of experimental and computational methods, which indicate that proximally localized catalyst components cooperatively stabilize and precisely orient dipolar enantiodetermining transition states in both pathways. Generality across different mechanisms is rarely considered in catalyst discovery efforts, but we suggest that it may play a role in the identification of so-called privileged catalysts.

Disproof of the Proposed Structures of Bradyoxetin, a Putative Bradyrhizobium japonicum Signaling Molecule, and HMCP, a Putative Ralstonia solanacearum Quorum-Sensing Molecule

First, we revisited the reported NMR data of bradyoxetin, a putative cell density factor of Bradyrhizobium japonicum, and found some inconsistencies in the proposed structure. To elucidate the correct structure, we synthesized model oxetane compounds and confirmed that the NMR data of the synthetic compounds did not match those of the reported bradyoxetin. After reinterpreting the reported NMR data, we concluded that bradyoxetin must be chloramphenicol. Next, some derivatives of 2-hydroxy-4-((methylamino)(phenyl)methyl)cyclopentanone (HMCP), which is a putative quorum-sensing molecule of Ralstonia solanacearum, were synthesized. The NMR spectra of the synthesized compounds were completely different from those of the reported natural products. Based on theoretical studies, including the estimation of ¹H and ¹³C NMR chemical shifts using density functional theory calculations, we confirmed the correctness of the structure of the synthesized compound. These results strongly suggest that the proposed structure of HMCP could be incorrect.

Construction of Supramolecular Polymers with Different Topologies by Orthogonal Self-Assembly of Cryptand-Paraquat Recognition and Metal Coordination

Recently, metal-coordinated orthogonal self-assembly has been used as a feasible and efficient method in the construction of polymeric materials, which can also provide supramolecular self-assembly complexes with different topologies. Herein, a cryptand with a rigid pyridyl group on the third arm derived from BMP32C10 was synthesized. Through coordination-driven self-assembly with a bidentate organoplatinum(II) acceptor or tetradentate Pd(BF4)2•4CH3CN, a di-cryptand complex and tetra-cryptand complex were prepared, respectively. Subsequently, through the addition of a di-paraquat guest, linear and cross-linked supramolecular polymers were constructed through orthogonal self-assembly, respectively. By comparing their proton nuclear magnetic resonance (1H NMR) and diffusion-ordered spectroscopy (DOSY) spectra, it was found that the degrees of polymerization were dependent not only on the concentrations of the monomers but also on the topologies of the supramolecular polymers.

Generation of Mixed Anhydrides via Oxidative Fragmentation of Tertiary Cyclopropanols with Phenyliodine(III) Dicarboxylates

Oxidative fragmentation of tertiary cyclopropanols with phenyliodine(III) dicarboxylates in aprotic solvents (dichloromethane, chloroform, toluene) produces mixed anhydrides. The fragmentation reaction is especially facile with phenyliodine(III) reagents bearing electron-withdrawing carboxylate ligands (trifluoroacetyl, 2,4,6-trichlorobenzoyl, 3-nitrobenzoyl), and affords 95−98% yields of the corresponding mixed anhydride products. The latter can be straightforwardly applied for the acylation of various nitrogen, oxygen and sulfur-centered nucleophiles (primary and secondary amines, hydroxylamines, primary alcohols, phenols, thiols). Intramolecular acylation yielding macrocyclic lactones can also be performed. The developed transformation has bolstered the synthetic utility of cyclopropanols as pluripotent intermediates in diversity-oriented synthesis of bioactive natural products and their synthetic congeners. For example, it was successfully applied for the last-stage modification of a cyclic peptide to produce a precursor of a known histone deacetylase inhibitor.

Chemical Space Exploration of Oxetanes

This paper focuses on new derivatives bearing an oxetane group to extend accessible chemical space for further identification of kinase inhibitors. The ability to modulate kinase activity represents an important therapeutic strategy for the treatment of human illnesses. Known as a nonclassical isoster of the carbonyl group, due to its high polarity and great ability to function as an acceptor of hydrogen bond, oxetane seems to be an attractive and underexplored structural motif in medicinal chemistry.

Copper-Catalyzed Enantioselective Hydroalkoxylation of Alkenols for the Synthesis of Cyclic Ethers

The copper-catalyzed enantioselective intramolecular hydroalkoxylation of unactivated alkenes for the synthesis of tetrahydrofurans, phthalans, isochromans, and morpholines from 4- and 5-alkenols is reported. The substrate scope is complementary to existing enantioselective alkene hydroalkoxylations and is broad with respect to substrate backbone and alkene substitution. The asymmetric induction and isotopic labeling studies support a polar/radical mechanism involving enantioselective oxycupration followed by C-[Cu] homolysis and hydrogen atom transfer. Synthesis of the antifungal insecticide furametpyr was accomplished.

Catalytic Hydroxycyclopropanol Ring-Opening Carbonylative Lactonization to Fused Bicyclic Lactones

A novel palladium-catalyzed ring opening carbonylative lactonization of readily available hydroxycyclopropanols was developed to efficiently synthesize tetrahydrofuran (THF) or tetrahydropyran (THP)-fused bicyclic γ-lactones, two privileged scaffolds often found in natural products. The reaction features mild reaction conditions, good functional group tolerability, and scalability. Its application was demonstrated in a short total synthesis of (±)-paeonilide. The fused bicyclic γ-lactone products can be easily diversified to other medicinally important scaffolds, which further broadens the application of this new carbonylation method.

Alternating Cascade Metathesis Polymerization of Enynes and Cyclic Enol Ethers with Active Ruthenium Fischer Carbenes

Ruthenium alkoxymethylidene complexes have rarely been demonstrated as active species in metathesis reactions and are frequently regarded as inert. Herein, we highlight the ability of these Fischer-type carbenes to participate in cascade alternating ring-opening metathesis polymerization through their efficient alkyne addition reactions. When enyne monomers are combined with low-strain cyclic vinyl ethers, a controlled chain-growth copolymerization occurs that exhibits high degrees of alternation (>90% alternating diads) and produces degradable poly(vinyl ether) materials with low dispersities and targetable molecular weights. This new method is amenable to the synthesis of alternating diblock polymers that can be degraded to small-molecule fragments under aqueous acidic conditions. This work furthers the potential of Fischer-type ruthenium alkylidenes in polymerization strategies and presents new avenues for the generation of functional metathesis materials.

Weak Interaction of the Antimetabolite Drug Methotrexate with a Cavitand Derivative

Formation of inclusion complexes involving a cavitand derivative (as host) and an antimetabolite drug, methotrexate (as guest) was investigated by photoluminescence measurements in dimethyl sulfoxide solvent. Molecular modeling performed in gas phase reflects that, due to the structural reasons, the cavitand can include the methotrexate in two forms: either by its opened structure with free androsta-4-en-3-one-17α-ethinyl arms or by the closed form when all the androsta-4-en-3-one-17α-ethinyl arms play role in the complex formation. Experiments reflect enthalpy driven complex formation in higher temperature range while at lower temperature the complexes are stabilized by the entropy gain.

Highly Sensitive and Selective Fluorescent Probes for Cu(II) Detection Based on Calix[4]arene-Oxacyclophane Architectures

A new topological design of fluorescent probes for sensing copper ion is disclosed. The calix[⁴]arene-oxacyclophane (Calix-OCP) receptor, either wired-in-series in arylene-alt-ethynylene conjugated polymers or standing alone as a sole molecular probe, display a remarkable affinity and selectivity for Cu(II). The unique recognition properties of Calix-OCP system toward copper cation stem from its pre-organised cyclic array of O-ligands at the calixarene narrow rim, which is kept in a conformational rigid arrangement by a tethered oxacyclophane sub-unit. The magnitude of the binding constants (K_a = 5.30 - 8.52 × 10⁴ M^-1) and the free energy changes for the inclusion complexation (-ΔG = 27.0 - 28.1 kJmol^-1), retrieved from fluorimetric titration experiments, revealed a high sensitivity of Calix-OCP architectures for Cu(II) species. Formation of supramolecular inclusion complexes was evidenced from UV-Vis spectroscopy. The new Calix-OCP-conjugated polymers (polymers 4 and 5), synthesized in good yields by Sonogashira-Hagihara methodologies, exhibit high fluorescence quantum yields (Φ_F = 0.59 - 0.65). Density functional theory (DFT) calculations were used to support the experimental findings. The fluorescence on-off behaviour of the sensing systems is tentatively explained by a photoinduced electron transfer mechanism.

Highly Enantioselective, Hydrogen-Bond-Donor Catalyzed Additions to Oxetanes

A precisely designed chiral squaramide derivative is shown to promote the highly enantioselective addition of trimethylsilyl bromide (TMSBr) to a broad variety of 3-substituted and 3,3-disubstituted oxetanes. The reaction provides direct and general access to synthetically valuable 1,3-bromohydrin building blocks from easily accessed achiral precursors. The products are readily elaborated both by nucleophilic substitution and through transition-metal-catalyzed cross-coupling reactions. The enantioselective catalytic oxetane ring opening was employed as part of a three-step, gram-scale synthesis of pretomanid, a recently approved medication for the treatment of multidrug-resistant tuberculosis. Heavy-atom kinetic isotope effect (KIE) studies are consistent with enantiodetermining delivery of bromide from the H-bond-donor (HBD) catalyst to the activated oxetane. While the nucleophilicity of the bromide ion is expected to be attenuated by association to the HBD, overall rate acceleration is achieved by enhancement of Lewis acidity of the TMSBr reagent through anion abstraction.

Algal Toxin Goniodomin A Binds Potassium Ion Selectively to Yield a Conformationally Altered Complex with Potential Biological Consequences

The marine toxin goniodomin A (GDA) is a polycyclic macrolide containing a spiroacetal and three cyclic ethers as part of the macrocycle backbone. GDA is produced by three species of the Alexandrium genus of dinoflagellates, blooms of which are associated with "red tides", which are widely dispersed and can cause significant harm to marine life. The toxicity of GDA has been attributed to stabilization of the filamentous form of the actin group of structural proteins, but the structural basis for its binding is not known. Japanese workers, capitalizing on the assumed rigidity of the heavily substituted macrolide ring, assigned the relative configuration and conformation by relying on NMR coupling constants and NOEs; the absolute configuration was assigned by degradation to a fragment that was compared with synthetic material. We have confirmed the absolute structure and broad features of the conformation by X-ray crystallography but have found GDA to complex with alkali metal ions in spite of two of the heterocyclic rings facing outward. Such an arrangement would have been expected to impair the ability of GDA to form a crown-ether-type multidentate complex. GDA shows preference for K⁺, Rb⁺, and Cs⁺ over Li⁺ and Na⁺ in determinations of relative affinities by TLC on metal-ion-impregnated silica gel plates and by electrospray mass spectrometry. NMR studies employing the K⁺ complex of GDA, formed from potassium tetrakis[pentafluorophenyl]borate (KBArF₂₀), reveal a major alteration of the conformation of the macrolide ring. These observations argue against the prior assumption of rigidity of the ring. Alterations in chemical shifts, coupling constants, and NOEs indicate the involvement of most of the molecule other than ring F. Molecular mechanics simulations suggest K⁺ forms a heptacoordinate complex involving O_A, O_B, O_C, O_D, O_E, and the C-26 and C-27 hydroxy groups. We speculate that complexation of K⁺ with GDA electrostatically stabilizes the complex of GDA with filamentous actin in marine animals due to the protein being negatively charged at physiological pH. GDA may also cause potassium leakage through cell membranes. This study provides insight into the structural features and chemistry of GDA that may be responsible for significant ecological damage associated with the GDA-producing algal blooms.

Solvent Switched Weak Interaction of a 4-Quinazolinone with a Cavitand Derivative

Interaction of 4-quinazolinone with tetrakis (3,5-dicarboxylatophenoxy)-cavitand derivative has been studied in methanol and dimethylformamide media using fluorescence spectroscopy and molecular modeling methods. Results show temperature dependent complex formation: either the entropy gain or the high enthalpy changes are responsible for the formation of stable complexes in two separated temperature regions. However, different thermodynamic parameters are associated to different conformations of the complexes: while the high entropy gain associated to the formation of deeply included guest in methanol, the high entropy gain is associated with the formation of weakly included guest in dimethylformamide solvent. This finding highlights the importance of dynamic properties of the species interacted in different solvents.

Substituent effect on inter-ring interaction in paracyclophanes

The theoretical calculations, namely multipole-derived charge analysis, quantum theory of atom in molecules, and non-bonding interaction (NCI), were performed for [2.2]paracyclophanes, [2.2]paracyclophane-7,9-dienes, and [3.3]paracyclophanes optimized at B3LYP/6-311++G** level, including dispersion correction. The substituent effect of the electron donor N(Me)2 and electron acceptor NO2 group and the influence of the length of bridges joining the aromatic ring on aromatic ring interaction energy (AIE) and strain energy were discussed. The local and electrostatic character of the substituent effect in paracyclophanes was shown. The presence of the weak orbital through-space C···C interaction between the [3.3]paracyclophane ring and weak CH···O hydrogen bonds between the substituents in the different rings was shown.

An Overview of Saturated Cyclic Ethers: Biological Profiles and Synthetic Strategies

Saturated oxygen heterocycles are widely found in a broad array of natural products and other biologically active molecules. In medicinal chemistry, small and medium rings are also important synthetic intermediates since they can undergo ring-opening and -expansion reactions. These applications have driven numerous studies on the synthesis of oxygen-containing heterocycles and considerable effort has been devoted toward the development of methods for the construction of saturated oxygen heterocycles. This paper provides an overview of the biological roles and synthetic strategies of saturated cyclic ethers, covering some of the most studied and newly discovered related natural products in recent years. This paper also reports several promising and newly developed synthetic methods, emphasizing 3-7 membered rings.

Potent HIV-1 protease inhibitors incorporating squaramide-derived P2 ligands: Design, synthesis, and biological evaluation

We describe the design, synthesis, and biological evaluation of novel HIV-1 protease inhibitors containing a squaramide-derived scaffold as the P2 ligand in combination with a (R)-hydroxyethylamine sulfonamide isostere. Inhibitor 3h with an N-methyl-3-(R)-aminotetrahydrofuranyl squaramide P2-ligand displayed an HIV-1 protease inhibitory Ki value of 0.51 nM. An energy minimized model of 3h revealed the major molecular interactions between HIV-1 protease active site and the tetrahydrofuranyl squaramide scaffold that may be responsible for its potent activity.

Building upon Nature's Framework: Overview of Key Strategies Toward Increasing Drug-Like Properties of Natural Product Cyclopeptides and Macrocycles

The pharmaceutical industry has focused mainly in the development of small-molecule entities intended for oral administration for the past decades. As a result, the majority of existing drugs address only a narrow range of biological targets. In the era of post-genomics, transcriptomics, and proteomics, there is an increasing interest on larger modulators of proteins that can span larger surfaces, access new therapeutic mechanisms of action, and provide greater target specificity. Traditional drug-like molecules developed using "rule-of-five" (Ro5) guidelines have been proven ineffective against a variety of challenging targets, such as protein-protein interactions, nucleic acid complexes, and antibacterial modalities. However, natural products are known to be effective at modulating such targets, leading to a renewed focus by medicinal chemists on investigating underrepresented chemical scaffolds associated with natural products. Here we describe recent efforts toward identification of novel natural cyclopeptides and macrocycles as well as selected medicinal chemistry strategies to increase drug-like properties or further exploration of their activity.

Probing the Structural Determinants of Amino Acid Recognition: X-Ray Studies of Crystalline Ditopic Host-Guest Complexes of the Positively Charged Amino Acids, Arg, Lys, and His with a Cavitand Molecule

Crystallization of tetraphosphonate cavitand Tiiii[H, CH₃, CH₃] in the presence of positively charged amino acids, namely arginine, lysine, or histidine, afforded host-guest complex structures. The X-ray structure determination revealed that in all three structures, the fully protonated form of the amino acid is ditopically complexed by two tetraphosphonate cavitand molecules. Guanidinium, ammonium, and imidazolium cationic groups of the amino acid side chain are hosted in the cavity of a phosphonate receptor, and are held in place by specific hydrogen bonding interactions with the P=O groups of the cavitand molecule. In all three structures, the positively charged α-ammonium groups form H-bonds with the P=O groups, and with a water molecule hosted in the cavity of a second tetraphosphonate molecule. Furthermore, water-assisted dimerization was observed for the cavitand/histidine ditopic complex. In this 4:2 supramolecular complex, a bridged water molecule is held by two carboxylic acid groups of the dimerized amino acid. The structural information obtained on the geometrical constrains necessary for the possible encapsulation of the amino acids are important for the rational design of devices for analytical and medical applications.

Synthesis, microstructure and mechanical properties of partially biobased biodegradable poly(ethylene brassylate-co-ε-caprolactone) copolyesters

High-molecular-weight poly(ethylene brassylate-co-ε-caprolactone) copolyesters within a wide composition range were prepared via triphenyl bismuth catalyzed copolymerization of ethylene brassylate (EB) and ε-caprolactone (ε-CL) in bulk. Microstructural analysis of the resulting copolyesters demonstrated that the comonomer units were completely random distribution. DSC and WAXD recognized that the copolyesters cocrystallize within the lattices analogous to either of the parent homopolymers. It confirmed the isodimorphism behavior with a pseudo-eutectic point of melting temperatures as well as lattice spacings at 75 mol% ε-CL units. The crystal cell would be stretched in one dimension rather than expanding in both dimensions with the incorporation of comonomer units according to the result of WAXD. The mechanical properties of the copolyesters are well tunable by the composition, and its trend is consistent with the isodimorphism behavior, in particular, the maximum elongation at break over 2000% is located at the pseudo-eutectic point. The intralamellar shear occurred at the low tensile rate while both intralamellar shear and interlamellar shear occurred at high tensile rate. The copolymers exhibit excellent hydrolytic stability.

Crystalline Cyclophane-Protein Cage Frameworks

Cyclophanes are macrocyclic supramolecular hosts famous for their ability to bind atomic or molecular guests via noncovalent interactions within their well-defined cavities. In a similar way, porous crystalline networks, such as metal-organic frameworks, can create microenvironments that enable controlled guest binding in the solid state. Both types of materials often consist of synthetic components, and they have been developed within separate research fields. Moreover, the use of biomolecules as their structural units has remained elusive. Here, we have synthesized a library of organic cyclophanes and studied their electrostatic self-assembly with biological metal-binding protein cages (ferritins) into ordered structures. We show that cationic pillar[5]arenes and ferritin cages form biohybrid cocrystals with an open protein network structure. Our cyclophane-protein cage frameworks bridge the gap between molecular frameworks and colloidal nanoparticle crystals and combine the versatility of synthetic supramolecular hosts with the highly selective recognition properties of biomolecules. Such host-guest materials are interesting for porous material applications, including water remediation and heterogeneous catalysis.

Ribocyclophanes A-E, Glycosylated Cyclophanes with Antiproliferative Activity from Two Cultured Terrestrial Cyanobacteria

The cell extracts of two cultured freshwater Nostoc spp., UIC 10279 and UIC 10366, both from the suburbs of Chicago, showed antiproliferative activity against MDA-MB-231 and MDA-MB-435 cancer cell lines. Bioassay-guided fractionation led to the isolation of five glycosylated cylindrocyclophanes, named ribocyclophanes A-E (1-5) and cylindrocyclophane D (6). The structure determination was carried out by HRESIMS and 1D and 2D NMR analyses and confirmed by single-crystal X-ray crystallography. The structures of ribocyclophanes A-E (1-5) contain a β-d-ribopyranose glycone in the rare 1 C4 conformation. Among isolated compounds, ribocyclophane D (4) showed antiproliferative activity against MDA-MB-435 and MDA-MB-231 cancer cells with an IC50 value of less than 1 μM.

The Thermodynamics of Anion Complexation to Nonpolar Pockets

The interactions between nonpolar surfaces and polarizable anions lie in a gray area between the hydrophobic and Hofmeister effects. To assess the affinity of these interactions, NMR and ITC were used to probe the thermodynamics of eight anions binding to four different hosts whose pockets each consist primarily of hydrocarbon. Two classes of host were examined: cavitands and cyclodextrins. For all hosts, anion affinity was found to follow the Hofmeister series, with associations ranging from 1.6-5.7 kcal mol-1. Despite the fact that cavitand hosts 1 and 2 possess intrinsic negative electrostatic fields, it was determined that these more enveloping hosts generally bound anions more strongly. The observation that the four hosts each possess specific anion affinities that cannot be readily explained by their structures, points to the importance of counter cations and the solvation of the "empty" hosts, free guests, and host-guest complexes, in defining the affinity.

Pharmacological use of a novel scaffold, anomeric N,N-diarylamino tetrahydropyran: molecular similarity search, chemocentric target profiling, and experimental evidence

Rational drug design against a determined target (disease, pathway, or protein) is the main strategy in drug discovery. However, regardless of the main strategy, chemists really wonder how to maximize the utility of their new compounds by drug repositioning them as clinical drug candidates in drug discovery. In this study, we started our drug discovery "from curiosity in the chemical structure of a drug scaffold itself" rather than "for a specific target". As a new drug scaffold, anomeric diarylamino cyclic aminal scaffold 1, was designed by combining two known drug scaffolds (diphenylamine and the most popular cyclic ether, tetrahydropyran/tetrahydrofuran) and synthesized through conventional Brønsted acid catalysis and metal-free α-C(sp3)-H functionalized oxidative cyclization. To identify the utility of the new scaffold 1, it was investigated through 2D and 3D similarity screening and chemocentric target prediction. The predicted proteins were investigated by an experimental assay. The scaffold 1 was reported to have an antineuroinflammatory agent to reduce NO production, and compound 10 concentration-dependently regulated the expression level of IL-6, PGE-2, TNF-α, ER-β, VDR, CTSD, and iNOS, thus exhibiting neuroprotective activity.

Site-Selective Modification of Proteins with Oxetanes

Oxetanes are four-membered ring oxygen heterocycles that are advantageously used in medicinal chemistry as modulators of physicochemical properties of small molecules. Herein, we present a simple method for the incorporation of oxetanes into proteins through chemoselective alkylation of cysteine. We demonstrate a broad substrate scope by reacting proteins used as apoptotic markers and in drug formulation, and a therapeutic antibody with a series of 3-oxetane bromides, enabling the identification of novel handles (S-to-S/N rigid, non-aromatic, and soluble linker) and reactivity modes (temporary cysteine protecting group), while maintaining their intrinsic activity. The possibility to conjugate oxetane motifs into full-length proteins has potential to identify novel drug candidates as the next-generation of peptide/protein therapeutics with improved physicochemical and biological properties.

Organocatalytic Coupling of CO2 with Oxetane

The organocatalytic coupling of CO₂ with oxetanes is investigated under solvent-free conditions. The influence of the main reaction parameters (type of organocatalytic system, pressure, and temperature) on the yield, the product formed, and the selectivity of the reaction are discussed. An onium salt combined with a fluorinated alcohol promotes the efficient and selective organocatalytic synthesis of α,ω-hydroxyl oligocarbonates by coupling CO₂ with oxetanes at 130 °C and at a CO₂ pressure as low as 2 MPa. NMR characterizations were correlated with matrix-assisted laser desorption/ionization with time-of-flight mass spectrometer (MALDI-TOF) analyses for elucidating the structure of the oligomers. Online FTIR studies under pressure, NMR titrations, and DFT calculations allowed an in-depth understanding of the reaction mechanism. Finally, CO₂ -based poly(carbonate-co-urethane)s were synthesized by step-growth polymerization of hydroxyl telechelic oligocarbonates with 4,4'-methylene diphenyl diisocyanate (MDI). The organocatalytic system described herein constitutes an innovative sustainable route to the selective preparation of hydroxyl telechelic carbonates of high interest for many applications, notably for the polyurethane business (especially for coatings or foams).

Direct evidence of specific localization of sesquiterpenes and marchantin A in oil body cells of Marchantia polymorpha L

Liverworts are a rich source of a diverse array of specialized metabolites, such as terpenoids and benzenoids, which are potentially useful for pharmaceutical or agrochemical applications, and also provide clues to elucidate the strategy by which liverworts adapt to the terrestrial environment. Liverworts, belonging to orders Marchantiales and Jungermanniales, possess oil bodies. In Marchantia polymorpha L., oil bodies are confined to scattered idioblastic oil body cells. It has been assumed that the specialized metabolites in M. polymorpha specifically accumulate in the oil bodies in oil body cells; however, no direct evidence was previously available for this specific accumulation. In this study, direct evidence was obtained using micromanipulation techniques coupled with MS analysis that demonstrated the specific accumulation of sesquiterpenoids and marchantin A in the oil body cells of M. polymorpha thalli. It was also observed that the number of oil body cells increased in thalli grown in low-mineral conditions. The amounts of sesquiterpenoids and marchantin A detected in crude extract prepared from the whole thallus were roughly proportional to the number of oil body cells found in a given volume of thallus, suggesting that oil body cell differentiation and sesquiterpenoid and marchantin A biosynthetic pathways are coordinated with each other.

Bis-bibenzyls from the Cameroon Liverwort Marchantia debilis

Liverworts are rich sources of terpenoids and aromatic compounds among which bis-bibenzyls are well known for their wide spectrum of biological activities. This is the first report of chemical analysis of the African liverwort Marchantia debilis Goebel. From the methanol extract marchantinquinone-l'-methyl ether was newly isolated together with three known bis-bibenzyls, marchantin C, marchantinquinone and perrottetin E. The presence of bis-bibenzyls with a quinone moiety is noted for the first time in the Marchantia genus.

Nickel-Catalyzed Intramolecular C-O Bond Formation: Synthesis of Cyclic Enol Ethers

An efficient and exceptionally mild intramolecular nickel-catalyzed carbon-oxygen bond-forming reaction between vinyl halides and primary, secondary, and tertiary alcohols has been achieved. Zinc powder was found to be an essential additive for obtaining high catalyst turnover and yields. This operationally simple method allows direct access to cyclic vinyl ethers in high yields in a single step.

Additional Insights into the Obtusallene Family: Components of Laurencia marilzae

The obtusallenes are a significant subset of C15-halogenated acetogenins that incorporate 12-membered cyclic ethers. We have recently reported the isolation from Laurencia marilzae of 12-epoxyobtusallene IV (1) and its related α,β-unsaturated carboxylate ester (2), both of special biogenetic relevance. Here we describe the final step of our study, the isolation of three new analogues (3-5), among these, the first bromopropargylic derivative (3) of this class of macrocyclic C15-acetogenins. The structures were elucidated by analysis of NMR and X-ray data. 12-Epoxyobtusallene IV (1), its new isomer 4, and known obtusallene IV (6) were evaluated for their apoptosis-inducing activities in a human hepatocarcinoma cell line.

Cavitand-Based Polyphenols as Highly Reactive Organocatalysts for the Coupling of Carbon Dioxide and Oxiranes

A variety of cavitand-based polyphenols was prepared from cheap and accessible aldehyde and resorcinol/pyrogallol reagents to give the respective resorcin[4]- or pyrogallol[4]arenes. The preorganization of the phenolic units allows intra- and intermolecular hydrogen bond (HB) networks that affect both the reactivity and stability of these HB-donor catalysts. Unexpectedly, we found that the resorcin[4]arenes show cooperative catalysis behavior compared to the parent resorcinol in the catalytic coupling of epoxides and CO2 with a significantly higher turnover. At elevated reaction temperatures, the resorcin[4]arene-based catalyst 3 d displays the best catalytic performance with very high turnover numbers and frequencies, combining increased reactivity and stability compared to pyrogallol, and an ample substrate scope. This type of polyphenol structure thus illustrates the importance of a new, highly competitive organocatalyst design to devise sustainable CO2 conversion processes.

Supramolecular Surface Photochemistry: Cascade Energy Transfer between Encapsulated Dyes Aligned on a Clay Nanosheet Surface

Three coumarin derivatives (7-propoxy coumarin, coumarin-480, and coumarin-540a, 2, 3, and 4, respectively) having different absorption and emission spectra were encapsulated within a water-soluble organic capsule formed by the two positively charged ammonium-functionalized cavitand octaamine (OAm, 1). Guests 2, 3, and 4 absorb in ultraviolet, violet, and blue regions and emit in violet, blue, and green regions, respectively. Energy transfer between the above three coumarin@(OAm)2 complexes assembled on the surface of a saponite clay nanosheet was investigated by steady-state and time-resolved emission techniques. Judging from their emission and excitation spectra, we concluded that the singlet-singlet energy transfer proceeded from 2 to 3, from 2 to 4, and from 3 to 4 when OAm-encapsulated 2, 3, and 4 were aligned on a clay surface as two-component systems. Under such conditions, the energy transfer efficiencies for the paths 2* to 3, 2* to 4, and 3* to 4 were calculated to be 33, 36, and 50% in two-component systems. When all three coumarins were assembled on the surface and 2 was excited, the energy transfer efficiencies for the paths 2* to 3, 2* to 4, and 3* to 4 were estimated to be 32, 34, and 33%. A comparison of energy transfer efficiencies of the two-component and three-component systems revealed that excitation of 2 leads to emission from 4. Successful merging of supramolecular chemistry and surface chemistry by demonstrating novel multi-step energy transfer in a three-component dye encapsulated system on a clay surface opens up newer opportunities for exploring such systems in an artificial light-harvesting phenomenon.

Alkanes from Bioderived Furans by using Metal Triflates and Palladium-Catalyzed Hydrodeoxygenation of Cyclic Ethers

Using a metal triflate and Pd/C as catalysts, alkanes were prepared from bioderived furans in a one-pot hydrodeoxygenation (HDO) process. During the reaction, the metal triflate plays a crucial role in the ring-opening HDO of furan compounds. The entire reaction process has goes through two major phases: at low temperatures, saturation of the exocyclic double bond and furan ring are catalyzed by Pd/C; at high temperatures, the HDO of saturated furan compounds is catalyzed by the metal triflate. The reaction mechanism was verified by analyzing the changes of the intermediates during the reaction. In addition, different metal triflates, solvents, and catalyst recycling were also investigated.

Copper-Catalyzed Cyclopropanol Ring Opening Csp(3)-Csp(3) Cross-Couplings with (Fluoro)Alkyl Halides

Novel and general copper-catalyzed cyclopropanol ring opening cross-coupling reactions with difluoroalkyl bromides, perfluoroalkyl iodides, monofluoroalkyl bromides, and 2-bromo-2-alkylesters to synthesize various β-(fluoro)alkylated ketones are reported. The reactions feature mild conditions and excellent functional group compatibility and can be scaled up to gram scale. Preliminary mechanistic studies suggest the involvement of radical intermediates. The difluoroalkyl-alkyl cross-coupling products can also be readily converted to more valuable and diverse gem-difluoro-containing compounds by taking advantage of the carbonyl group resulting from cyclopropanol ring opening.

Synthesis of a Spirocyclic Oxetane-Fused Benzimidazole

A new synthesis of 2-oxa-7-azaspiro[3.5]nonane is described. Spirocyclic oxetanes, including 2-oxa-6-azaspiro[3.3]heptane were converted into o-cycloalkylaminoacetanilides for oxidative cyclizations using Oxone^® in formic acid. The expanded spirocyclic oxetane successfully gave the [1,2-a] ring-fused benzimidazole. X-ray crystal structure of the resultant new tetracyclic system, 1ʹ,2ʹ-dihydro-4ʹH-spiro[oxetane-3,3ʹ-pyrido[1,2-a]benzimidazole] and the azetidine ring-opened adduct, N-(2-acetamido-4-bromophenyl)-N-{[3-(chloromethyl)oxetan-3-yl]methyl}acetamide are disclosed.

Stability and in vivo behavior of Rh[16aneS4-diol]211 at complex: a potential precursor for astatine radiopharmaceuticals

INTRODUCTION

The heavy halogen (211)At is of great interest for targeted radiotherapy because it decays by the emission of short-range, high-energy α-particles. However, many astatine compounds that have been synthesized are unstable in vivo, providing motivation for seeking other (211)At labeling strategies. One relatively unexplored approach is to utilize prosthetic groups based on astatinated rhodium (III) complex stabilized with a tetrathioether macrocyclic ligand - Rh[16aneS(4)-diol](211)At. The purpose of the current study was to evaluate the in vitro and in vivo stability of this complex in comparison to its iodine analog - Rh[16aneS(4)-diol](131)I.

METHODS

Rh[16aneS(4)-diol](211)At and Rh[16aneS(4)-diol](131)I complexes were synthesized and purified by HPLC. The stability of both complexes was evaluated in vitro by incubation in phosphate-buffered saline (PBS) and human serum at different temperatures. The in vivo behavior of the two radiohalogenated complexes was assessed by a paired-label biodistribution study in normal Balb/c mice.

RESULTS

Both complexes were synthesized in high yield and purity. Almost no degradation was observed for Rh[16aneS(4)-diol](131)I in PBS over a 72 h incubation. The astatinated analog exhibited good stability in PBS over 14 h. A slow decline in the percentage of intact complex was observed for both tracers in human serum. In the biodistribution study, retention of (211)At in most tissues was higher than that of (131)I at all time points, especially in spleen and lungs. Renal clearance of Rh[16aneS(4)-diol](211)At and Rh[16aneS(4)-diol](131)I predominated, with 84.1 ± 2.3% and 94.6 ± 0.9% of injected dose excreted via the urine at 4 h.

CONCLUSIONS

The Rh[16aneS(4)-diol](211)At complex might be useful for constructing prosthetic groups for the astatination of biomolecules and further studies are planned to evaluate this possibility.

Eribulin mesylate: mechanism of action of a unique microtubule-targeting agent

Eribulin mesylate (eribulin), an analogue of the marine natural product halichondrin B, is a microtubule-depolymerizing drug that has utility in the treatment of patients with breast cancer. Clinical trial results have demonstrated that eribulin treatment provides a survival advantage to patients with metastatic or locally advanced breast cancer previously treated with an anthracycline and a taxane. Furthermore, a pooled analysis of two pivotal phase III trials has demonstrated that eribulin also improves overall survival in several patient subgroups, including in women with HER2-negative disease and triple-negative breast cancer. This review covers the preclinical research that led to the clinical testing and approval of eribulin, as well as subsequent research that was prompted by distinct and unexpected effects of eribulin in the clinic. Initial studies with halichondrin B demonstrated unique effects on tubulin binding that resulted in distinct microtubule-dependent events and antitumor actions. Consistent with the actions of the natural product, eribulin has potent microtubule-depolymerizing activities and properties that distinguish it from other microtubule-targeting agents. Here, we review new results that further differentiate the effects of eribulin from other agents on peripheral nerves, angiogenesis, vascular remodeling, and epithelial-to-mesenchymal transition. Together, these data highlight the distinct properties of eribulin and begin to delineate the mechanisms behind the increased survival benefit provided by eribulin for patients.

The effect of number and position of P=O/P=S bridging units on cavitand selectivity toward methyl ammonium salts

The present work reports the synthesis and complexation properties of five mixed bridge P=O/P=S cavitands toward N,N-methyl butyl ammonium chloride (1) as prototype guest. The influence of number and position of P=O and P=S groups on the affinity of phosphonate cavitands toward 1 is assessed via ITC titrations in DCE as solvent. Comparison of the resulting Kass values, the enthalpic and entropic contributions to the overall binding with those of the parent tetraphosphonate Tiiii and tetrathiophosphonate TSiiii cavitands allows one to single out the simultaneous dual H-bond between the cavitand and the salt as the major player in complexation.

Anionic deep cavitands enable the adhesion of unmodified proteins at a membrane bilayer

An anionic self-folding deep cavitand is capable of immobilizing unmodified proteins and enzymes at a supported lipid bilayer interface, providing a simple, soft bioreactive surface that allows enzymatic function under mild conditions. The adhesion is based on complementary charge interactions, and the hosts are capable of binding enzymes such as trypsin at the bilayer interface: the catalytic activity is retained upon adhesion, allowing selective reactions to be performed at the membrane surface.

Probing cavitand-organosilane hybrid bilayers via sum-frequency vibrational spectroscopy

Quinoxaline cavitands (QxCav) are transferred by Langmuir-Schaefer method on self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) and N,N-dimethyl-N-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP) on fused silica substrates. The molecular architectures of both the hydrophobic SAMs templates and the hybrid cavitand-organosilanes bilayers at the solid-air interface are investigated and correlated by sum-frequency vibrational spectroscopy. The results show that QxCav are always in the closed vase configuration and orient with their principal axis normal to the substrates. The role of the alkyl chains density in the SAM templates on the QxCav transfer ratio is pointed out.

Selected hybrid natural products as tubulin modulators

Modulators of microtubule dynamics have received increasing attention because of their potential to stop cancer growth. Although it belongs to the category of complex protein-protein interactions (PPIs), which are generally considered difficult to modulate through small molecules, the use of microtubule is considered a well-validated target. There are a number of bioactive natural products and related compounds that are currently in use as drugs or in clinical trials as next generation anti-cancer agents. The present review article is focused on two such bioactive natural products, epothilone and halichondrin B, and covers some of the key papers published after 2005 that outline various synthetic approaches to obtain next generation structural analogs as well as the synthesis of hybrid compounds.

Selective hydrogenation of furan-containing condensation products as a source of biomass-derived diesel additives

In this study, we demonstrate that while the energy density and lubricity of the C15 and C16 products of furan condensation of biomass-derived aldehydes with 2-methylfuran are consistent with requirements for diesel, these products do not meet specifications for cetane number and pour point due to their aromatic furan rings. However, a novel class of products that fully meet or exceed most specifications for diesel can be produced by converting the furan rings in these compounds to cyclic ether moieties. Full hydrodeoxygenation of furan condensation products to alkanes would require 55-60% higher hydrogen demand, starting from biomass, compared to the products of furan ring saturation, providing an additional incentive to support the saturated products. We also report here on a tunable class of catalysts that contain Pd nanoparticles supported on ionic liquid-modified SiO2 that can achieve complete saturation of the furan rings in yields of 95% without opening these rings.

A sensitive LC-MS/MS assay for brevisulcenal and brevisulcatic acid toxins produced by the dinoflagellate Karenia brevisulcata

A toxic dinoflagellate, Karenia brevisulcata, devastated almost all marine life in Wellington Harbour, New Zealand during the late summer of 1998. Brevisulcatic acids (BSXs) and brevisulcenals (KBTs), both polycyclic ether toxins, have been identified as the causative agents. A liquid chromatography tandem mass spectrometry (LC-MS/MS) method has been developed and validated for the sensitive and specific determination of BSXs and KBTs in culture medium, seawater and shellfish. Acidified algal culture, or seawater, was extracted using reverse phase solid phase extraction cartridges. Shellfish tissue homogenate was blended with methanol-water (9:1) and partitioned with hexane to remove non-polar lipids. This extraction protocol is similar to that used for analysis of lipophilic shellfish toxins. LC-MS/MS (triple quadrupole) was used for quantitative analysis with gradient elution (acidic buffer), positive electrospray ionization and multiple-reaction monitoring. Purified toxins were available for 4 KBTs (KBT-F, -G, -H and -I) and 4 BSXs (-1, -2, -4, and -5), and were used to calibrate the instrument responses. Relative response factors were used for semi-quantitative analysis of BSX-3 and BSX-6, using BSX-1 and BSX-4 respectively. Calibration curves for all toxins monitored were linear over the concentration range tested (5-200 ng mL(-1)) with r(2) values >0.99. The method limit of quantitation was determined to be 2 ng mL(-1) for BSXs and KBTs, except KBT-I, which was 5 ng mL(-1). Validation data was generated for culture medium and shellfish. Toxin recoveries were typically >70% with relative standard deviations <20% across all of the matrices tested. In addition, toxins specific to K. brevisulcata were able to be detected in seawater at a cell concentration of 10,000 cells L(-1), which represents the suggested trigger level for this harmful algal species. This method shows suitable performance characteristics to be regarded a useful tool to monitor toxin levels in a variety of sample matrices during future bloom events.

Rigid spiroethers targeting the decoding center of the bacterial ribosome

Continuing our efforts towards understanding the principles governing ribosomal recognition and function, we have synthesized and evaluated a series of diversely functionalized 5,6-, 6,6- and 7,6-spiroethers. These compounds successfully mimic natural aminoglycosides regarding their binding to the decoding center of the bacterial ribosome. Their potential to inhibit prokaryotic protein production in vitro along with their antibacterial potencies have also been examined.