Ring Framework Research Articles

Studies on the Synthesis of Schisandraceae Natural Products: Exploring a Cyclopropylcarbinol Ring Expansion Strategy

AbstractAcid mediated cyclopropylcarbinol ring expansion has been shown to be a viable method for the construction of the AB ring framework of lancifodilactone F and related terpenoids of the Schisandraceae family of natural products. We found that this rearrangement proceeds with good stereochemical control based on inversion of the C10 cyclopropyl center. Our studies indicate that the cis‐decalin motif of 31 could be used as a key synthetic precursor of certain Schisandraceae metabolites.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Stereoselective synthesis of the ABCD ring framework of azaspiracids

A stereoselective synthesis of the ABCD ring framework of azaspiracid-1 and azaspiracid-3 has been achieved using a tandem bis-spiroketalization protocol in the presence of a mild proton source from 1,4-diketone precursor. A tetrahydrofuran intermediate with the correct stereochemistry for the D ring of azaspiracids-1 and 3 was then taken through a linear sequence of reactions to afford the desired diketone precursor. The D-ring of azaspiracid-1 was then constructed by employing a Sharpless asymmetric dihydroxylation followed by etherification using a homoallyl derivative. The structure of the ABCD ring framework with four contiguous rings was established by extensive NMR analysis.

Novel spin-dimer system of a microporous polyborate NaCuB7O12.nH2O

AbstractA new microporous polyborate, NaCuB7O12.nH2O, is synthesized using boric acid as a flux. It contains a 14-membered ring framework built up by CuO6 octahedra, BO4 tetrahedra and BO3 triangles. The spin-1/2 dimers of Cu2+ ions are connected by BO3 to form an infinite ladder along the b-axis. Magnetic susceptibility data between 2 and 330 K are presented, and the data are accurately fitted by theoretical predictions of Bleaney-Bowers for the spin-1/2 dimer, with J/kB = -153.8 K, and g = 1.98, where J is the intradimer exchange interaction constant and g is the Landé g factor.

Synthesis and Protonation Constants of an Amide-Based Chelating Cyclophane

Our research group have reported that, under high dilution conditions, condensation reactions between 4,4’-ethylenebis(2,6-morpholinedione), 1, and aromatic diamines gave a new series of tetraaza chelating cylophanes, which have amide and aromatic groups in the ring framework and pendant carboxymethyl groups [1].[...]

The crystal structures of polymorphic SUZ-4

Abstract SUZ-4 is an aluminosilicate zeolite originally synthesized at British Petroleum and known to be related somehow to ferrierite. Although a model had been suggested for the framework structure no quantitative proof of its existence has been given in previous work. It is now found to be polymorphic. The originally identified ambient, hydrated form crystallizes in space group Imma where a = 18.882(1), b = 14.872(1), c = 14.211(1) Å. A model constructed by trial and error refines by Rietveld methods to give Rwp = 0.1643, R(F 2) = 0.1665. Powder data collected at 100 °C, on the other hand, correspond to the Cmmm model proposed earlier by Lawton et al., where a = 18.790(1), b = 14.2305(8), c = 7.4511(4) Å. The Rietveld match gives Rwp = 0.1473, R(F 2) = 0.1508. Although the two forms crystallize in different space groups, the framework topology remains the same, as revealed by vertex symbols and coordination sequences. Evidence is seen for potassium ions that cannot be completely removed by ammonium exchange. Later, the origin of the polymorphic change was discovered to be hydration of microporous channels rather than a thermal effect. Solid-state NMR measurements of the hydrated and anhydrous forms show that a more symmetric environment around the Al species is observed without affecting the coordination of the Al sites. Synchrotron powder data collected at room temperature from anhydrous material conformed to the Cmmm structural model, where a = 18.8064(4), b = 14.2298(3), c = 7.4548(2) Å. Rietveld refinement, including three potassium sites gives R wp = 0.1127, R(F 2) = 0.1296. (Elemental analysis indicates a stoichiometric assembly: K4T36O72; the Rietveld refinement finds 3.86 of the 4.00 potassium atoms.) The material represents a one-dimensional 10-membered ring framework.

Inhibitory Effects of Momordin I Derivatives on the Formation of Fos-Jun-AP-1 DNA Complex

In our previous studies, we have observed that curcumin and momordin I isolated from Ampelopsis radix inhibit the formation of Fos-Jun-activation protein-1 (AP-1) DNA complex. We have screened more effective compounds which have a 5-membered ring framework like momordin I and have modified disaccharide or carboxylic acid portions in momordin I. We synthesized momordin I derivatives according to the published method with slight modification. Synthetic momordin I derivatives showed remarkable inhibitory activities on Fos-Jun-AP-1 DNA complex formation results in in vitro assays. The <TEX>$IC_{50}$</TEX> values of momordin I derivatives were about 4.0 <TEX>$\mu$</TEX>M in an electrophoretic mobility shift assay (EMSA). This value is about 125 times higher than that of curcumin and about 12 times higher than that for curcumin derivative C1, and moreover about 30 times higher than that for momordin I. We found momordin I derivatives (a) and (b) are the strongest inhibitory compound for Fos-Jun-AP-1 DNA complex formation.

An onion ring framework for developing and assessing mobile commerce security

A five-layer 'onion ring' framework for analysing mobile commerce security requirements and for improving system security performance is presented in this research. Two quantifiable approaches, based on weighted scores applied to either a spider diagram or a decision solution matrix, are used to demonstrate how the security level can actually be objectively measured and evaluated in addition to the technical discussions on the framework's architecture.

All-cis Cyclic Peptides

Amide bonds -NH-CO- preferentially exist in trans conformations, the cis conformation being thermodynamically unfavored with respect to the trans by about 2 kcal/mol. Yet, the main reason most proteins or peptides cannot be made from cis-peptide plaques only lies in that connecting them into open chains appears to be sterically impracticable. It is possible, however, to build all-cis cyclic peptides in which all cis-plaques are efficiently locked. The present work examines, through quantum calculations, the structural and energetic issues associated with these peculiar arrangements. Systematic exploration at DFT-B3LYP level of the potential-energy surfaces for all-cis cyclopolyglycines cG(n)(c) (n = 2-10,15), and to a lesser extent, for all-cis cyclopolyalanines and all-cis cyclopolyphenylalanines confirms that all these structures are true minima. Optimal ring size occurs around eight peptide units, resulting in planar cG7(c), cG8(c), and cG9(c). In smaller systems, the ring strain is relieved through nonplanar cup-like distortions, particularly in cG6(c). From 10 peptide units and beyond, the ring framework distorts into a saddle-edge shape. These molecules disclose some molecular flexibility, as combinatorial tilting of the plaques may give sets of minima close in energy. Indexes based on isodesmic reactions are used to estimate the energy for joining all-cis or all-trans plaques into cyclic peptides. One of them, the mean plaque-junction energy (MPJE) suggests that within sensible sizes from six peptide units and beyond, all-cis plaque association is almost equally favorable as all-trans one. The frame of radiating cis-amide bonds can be considered as defining a new kind of peptidic material, endowed with specific self-assembling properties.

Studies Toward the Synthesis of Azadirachtin. Part 1. Total Synthesis of a Fully Functionalized ABC Ring Framework and Coupling with a Norbornene Domain

AbstractFor Abstract see ChemInform Abstract in Full Text.

DFT—Quantum chemical study of the HZSM-5-cyclohexene interaction pathways

A theoretical analysis of the interaction between cyclohexene and a HZSM-5 zeolite model system is presented. Two different models were used to represent this catalyst: a ring structure model consisting of 10 TO 4 tetrahedral sites (where T = Si, Al) and a smaller model containing 3 TO 4 tetrahedral sites. Two different reaction pathways were studied: the hydrogen exchange between the HZSM-5 cluster model and the cyclohexene molecule, and the proton addition to double bond of the cyclohexene. The alkoxy species can be stabilized/destabilized depending of the arrangement restriction between the olefin and the local geometry of the active site. When cyclohexene molecule interacts with the 3T model the reaction involves an alkoxy species formation. Due to the allowed relaxation in the ring framework around the active site, the local arrangement restriction of the alkoxy species and therefore the covalent alkoxy bond is too weak giving rise to a more ionic intermediate species (carbenium ion-like), which is characterized as a minimum in the PES. By using the non-interacting system, as a reference, and based on the calculated changes in the total energy of the different complexes, one is able to determine that both carbenium ion-like and alkoxy species are destabilized with respect to physisorbed complexes.

Studies toward the Synthesis of Azadirachtin, Part 1: Total Synthesis of a Fully Functionalized ABC Ring Framework and Coupling with a Norbornene Domain

Das komplexe Decalin-Intermediat 1 wurde synthetisiert und in die potenzielle Azadirachtin-Vorstufe 2 überführt. Zu dieser herausfordernden Synthese gehörten mehrere Schutzgruppenmanipulationen und das Kuppeln von 1 mit einer geeigneten Norborneneinheit.

Studies toward the Synthesis of Azadirachtin, Part 1: Total Synthesis of a Fully Functionalized ABC Ring Framework and Coupling with a Norbornene Domain

Studies toward the Synthesis of Azadirachtin, Part 1: Total Synthesis of a Fully Functionalized ABC Ring Framework and Coupling with a Norbornene Domain

FIA/MS analysis of temporally ozonated benzo[ a]pyrene and pyrene and their reaction products: Inhibition of gap junction-mediated intercellular communication

In this study, the polycyclic aromatic hydrocarbons, benzo[ a]pyrene (BaP) and pyrene, were subjected to temporal ozonation. The products from ozonation of 5 mg l −1 BaP and 5 mg l −1 pyrene, for varying time intervals (0, 1, 2, 3, 4, 5, 6, 8, 10, 20, and 30 min) were tested for their effects on gap junction-mediated intercellular communication (GJIC) in Clone 9 rat liver cells. Additionally, the ozonation products were also analyzed by flow injection analysis/mass spectrometry (FIA/MS) and the results were compared with the toxicity observed in the GJIC assay. Treatment of the Clone 9 cells with 5 mg l −1 of ozonated BaP products resulted in a decrease in GJIC that was inversely proportional to the length of ozonation. The products from 1 min of ozonation resulted in a 92% decrease in the rate of GJIC, but with >5 min ozonation, the products did not suppress GJIC. In contrast, pyrene (0.5 mg l −1) required >10 min of ozonation to alleviate its effects on GJIC. FIA/MS, using atmospheric pressure chemical ionization (APCI), demonstrated products with higher molecular weights (MW) than their corresponding parent compounds, BaP (MW 252) and pyrene (MW 202). Ozonation of pyrene formed significantly fewer products than BaP. More importantly, pyrene ozonation products were constant from 1 to 10 min, while BaP ozonation products seemed to vary between time intervals. With the longer ozonation times (20 and 30 min), BaP and pyrene formed similar products ( m/z peaks 157, 111, and 96). The suppression of GJIC by ozonated products seemed to correlate with oxidation of the aromatic ring framework. Further oxidation (longer ozonation times) to lower MW products correlated with restoration of normal GJIC.

Structural revision and total synthesis of azaspiracid-1, part 2: definition of the ABCD domain and total synthesis.

Structural revision and total synthesis of azaspiracid-1, part 2: definition of the ABCD domain and total synthesis.

Determination of the excited state structure of 7-azaindole using a Franck-Condon analysis

The change in structure of 7-azaindole upon electronic excitation was determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of six different vibronic bands. A total of 107 emission band intensities were fit, together with the changes in the rotational constants of four 7-azaindole isotopomers. The geometry change of the ring framework upon electronic excitation from the electronic ground state to the 1Lb state (ππ*) can be described by an overall expansion of the pyridine ring of 7-azaindole, with minor changes of the pyrrole ring. The resulting geometry changes are interpreted on the basis of ab initio calculations.

Conformational and configurational analysis of 2-phenoxy-2-oxo-1,3,2-dioxaphosphorinanes. Conformational and configurational dependence upon conformation of the diol precursor

Diastereomeric 5- tert-butyl-4-methyl-2-phenoxy-2-oxo-1,3,2-dioxaphosphorinanes were synthesized, and studied by NMR and computational methods in order to determine their predominant conformations as well as their relative configurations. The study was performed assuming a novel criteria, in which, the conformation and configuration of the diastereomeric 5- tert-butyl-4-methyl-2-phenoxy-2-oxo-1,3,2-dioxaphosphorinanes depend upon the conformation of the corresponding diol precursors. In other words, the orientation or pseudo orientation of the groups into the ring framework of the heterocyclic is initially acquired by the direct phosphorylation reaction with the diol precursor in the most stable conformation, and then, once the heterocyclic is formed, the final conformation is dictated by stereoelectronic and steric effects.

Study of the Activation of C−H and H−H Chemical Bonds by the [ZnOZn]2+ Oxycation: Influence of the Zeolite Framework Geometry

The reactivity of the [ZnOZn]2+ oxycation, located inside the zeolite micropores, has been studied by the periodical density functional method. Two different types of rings, six- and eight-membered, have been used as hosts for this cation. In addition, within each of these ring structures the arrangement of the two aluminum atoms has been explored, generating different configurations. Two different probe molecules have been used in this investigation: methane and molecular hydrogen. Both molecules prefer to chemisorb on this active site. Their dissociation reactions are strongly influenced by the surrounding zeolite environment and by the Al distribution within the zeolite ring framework. The [ZnOZn]2+ cation is able to activate both C−H and H−H bonds, particularly the latter one. These results are in line with previous calculations and recent experimental results. They also confirm that this active site is present in high-silica zeolites. However, this species seems unlikely to be the catalytic site for the alkane dehydrogenation.

Construction of the benzylic quaternary carbon center of zoanthenol by intramolecular Mizoroki-Heck reaction of enone.

[reaction: see text]. Stereocontrolled synthesis of the ABC ring framework of zoanthenol has been achieved. Our studies show that a beta,beta-disubstituted enone can act as a good acceptor of arylpalladium intermediates in the formation of a congested benzylic quaternary carbon center through an intramoleculer Mizoroki-Heck reaction. The cis B/C ring system was stereoselectively converted to the trans-fused framework through a SmI2-promoted deoxygenation of the alpha-hydroxy ketone.

Syntheses of the AB and EFGH ring segments of gambierol

Stereocontrolled syntheses of the AB and EFGH ring systems of gambierol ( 1 ) are described. The two key intermediates 3 and 55 , representing the AB and EFGH ring frameworks, were prepared from 2-deoxy- d-ribose via linear sequences. Brown's asymmetric allylboration and the intramolecular hetero-Michael reaction were successfully applied to the construction of the A ring moiety. Synthesis of the EFGH ring segment 55 was achieved by the SmI 2 mediated reductive cyclization, constructing the EF ring bearing two 1,3-diaxial methyl groups, and the palladium catalyzed coupling of enol triflate and zinc bishomoenolate, making the GH ring moiety. Attempted convergent approaches toward the EFGH ring framework are also described.

Toward a Total Synthesis of Okilactomycin. 1. A Direct, Enantiocontrolled Route to the Western Sector

A synthesis of the western half of the macrocyclic ring framework of the antitumor antibiotic okilactomycin is described. The strategy employed rests on an efficient synthesis of meso-2,4-dimethylglutaric anhydride and ensuing resolution via reaction with (S)-(-)-α-methylbenzylamine, diborane reduction, and selective crystallization. Following acid-catalyzed cyclization to (2S,4R)-2,4-dimethyl-6-valerolactone, an acyclic stereocontrol strategy was adopted to achieve chain lengthening with appropriate incorporation of functionality. The sensitive aldehyde 2 was further homologated to β-keto ester 17 in a model reaction sequence performed to simulate its ultimate projected coupling to 3.