AB Ring Research Articles

Approaches to the synthesis of arisugacin A

Approaches to the synthesis of the important acetylcholinesterase inhibitor, arisugacin A, are described. Two different routes to the key AB ring system are described: the first utilizes an intramolecular Diels–Alder reaction on a furan substrate and the second a 6π-electrocyclization of a substituted triene followed by cycloaddition with singlet oxygen. The successful synthesis of a fully functionalized AB ring system of arisugacin A, the tetraol 52 from hydroxy-β-ionone 22 in 16 steps and 9.3% overall yield is described. Several useful synthetic transformations to this molecule and its analogues are reported, e.g., the formation of the furan Diels–Alder cycloadduct 14 and its conversion into the oxa-bridged structures 17 and 21, the preparation of the dienes 25 and 26 and the conversion of the later into the endoperoxide 30 and its diol 36, the preparation of the endoperoxide 40 and the oxa-bridged system 42, and finally the use of the enelactone 43 and its ultimately successful conversion into 52. In addition, several novel rearrangements are described, producing the unusual compounds 62, 65, and 67. Finally, the successful coupling of the pyrone unit to the AB ring system is described to give compounds 70 and 71. The novel reduction of these compounds to the cyclic ether 74 is described.

Synthetic Study Towards Azadirachtin: An Efficient and Stereoselective Construction of the AB Rings with Full Functionality

Synthetic Study Towards Azadirachtin: An Efficient and Stereoselective Construction of the AB Rings with Full Functionality

Synthetic Study Towards Azadirachtin: An Efficient and Stereoselective Construction of the AB Rings with Full Functionality

Synthetic Study Towards Azadirachtin: An Efficient and Stereoselective Construction of the AB Rings with Full Functionality

Spin transport through QD with spin–orbital interaction and magnetic field

By employing non-equilibrium Green's function method, we calculated the spin transport through quantum dot (QD) embedded on AB ring with Rashba spin–orbit (SO) interaction and magnetic field. The results show that the spin polarization can be controlled by adjusting either SO interaction or the magnetic field applied on the QD. Furthermore, a new parameter ( λ) is defined to describe the effect of magnetic field on SO coupling interaction. And it is found that the effect of SO coupling is suppressed with increasing the magnetic field.

QUANTUM TRANSPORT THROUGH A MESOSCOPIC AB RING WITH THREE LEADS

We study the quantum transport through a mesoscopic AB ring with three leads. An exact solution for the quantum transport is obtained and it is shown that the transmitted currents j1 and j2 in the outgoing leads are modulated by the AB phase respectively and oscillate with the magnetic flux. We demonstrate that the corresponding probability currents I1, I2 and I3 in the three arms l1, l2 and l3 also oscillate with the magnetic flux ΦAB for the cases l1=l2=l3=L and l1=L, l2=l3=2L.

Enantioselective Total Synthesis of (−)‐Acylfulvene and (−)‐Irofulven

Antitumor agents (−)-acylfulvene and (−)-irofulven are prepared in an approach that employs the powerful enyne ring-closing metathesis reaction to secure the spiro-bicyclic AB rings. Other key features of this synthesis include an efficient aldol-based introduction of the stereocenter at C2, a diazene-mediated reductive allylic transposition, and a ring-closing metathesis/oxidation sequence.

Oscillations of quantum transport through double-AB rings with magnetic impurity

We have studied the effect of impurity scattering on the quantum transport through double AB rings in the presence of spin-flipper in the middle lead in terms of one-dimensional quantum waveguide theory. The electron interacts with the impurity through the exchange interaction leading to spin-flip scattering. Transmissions in the spin-flipped and non-spin-flipped channels are calculated explicitly. It is found that the overall transmission and the conductance are distorted due to the impurity scattering. The extent of distortion not only depends on the strength of the impurity potential but also on the impurity position. Moreover, the transmission probability and the conductance are modulated by the magnetic flux, the size of the ring and the impurity potential strength as well.

Synthesis and Biological Evaluation of 10,11-Methylenedioxy-14-azacamptothecin

[reaction: see text] 10,11-Methylenedioxy-14-azacamptothecin, a potent analogue of the antitumor agent camptothecin (CPT), has been prepared via a key condensation between AB and DE ring precursors. The biological testing of this compound validated a strategy for modulation of the off-rate of camptothecin analogues from the topoisomerase-DNA-CPT ternary complex via structural modification.

Synthesis and antibacterial activity of substituted flavones, 4-thioflavones and 4-iminoflavones

Synthesis of flavones, 4-thioflavones and 4-iminoflavones was carried out with the substitution of variable halogens, methyl, methoxy and nitro groups in the A, B and AB rings of the respective compounds and we also report here their antibacterial activity. Most of the synthesized compounds were found to be active against Escherichia coli, Bacillus subtilis, Shigella flexnari, Salmonella aureus, Salmonella typhi and Pseudomonas aeruginosa. Activity of 4-thioflavones and 4-iminoflavones was found to be higher than that of their corresponding flavone analogues. Investigated compounds having substituents like F, OMe and NO 2 at 4′-position in ring-B exhibited enhanced activity and the presence of electronegative groups in the studied compounds showed a direct relationship to the antibacterial activity.

Total synthesis and evaluation of [Psi[CH2NH]Tpg4]vancomycin aglycon: reengineering vancomycin for dual D-Ala-D-Ala and D-Ala-D-Lac binding.

An effective synthesis of [Psi[CH(2)NH]Tpg(4)]vancomycin aglycon (5) is detailed in which the residue 4 amide carbonyl of vancomycin aglycon has been replaced with a methylene. This removal of a single atom was conducted to enhance binding to D-Ala-D-Lac, countering resistance endowed to bacteria that remodel their D-Ala-D-Ala peptidoglycan cell wall precursor by a similar single atom change (ester O for amide NH). Key elements of the approach include a synthesis of the modified vancomycin ABCD ring system featuring a reductive amination coupling of residues 4 and 5 for installation of the deep-seated amide modification, the first of two diaryl ether closures for formation of the modified CD ring system (76%, 2.5-3:1 kinetic atropodiastereoselectivity), a Suzuki coupling for installation of the hindered AB biaryl bond (90%) on which the atropisomer stereochemistry could be thermally adjusted, and a macrolactamization closure of the AB ring system (70%). Subsequent DE ring system introduction enlisted a room-temperature aromatic nucleophilic substitution reaction for formation of the remaining diaryl ether (86%, 6-7:1 kinetic atropodiastereoselectivity), completing the carbon skeleton of 5. Consistent with expectations and relative to the vancomycin aglycon, 5 exhibited a 40-fold increase in affinity for D-Ala-D-Lac (K(a) = 5.2 x 10(3) M(-1)) and a 35-fold reduction in affinity for D-Ala-D-Ala (K(a) = 4.8 x 10(3) M(-1)), providing a glycopeptide analogue with balanced, dual binding characteristics. Beautifully, 5 exhibited antimicrobial activity (MIC = 31 microg/mL) against a VanA-resistant organism that remodels its D-Ala-D-Ala cell wall precursor to d-Ala-d-Lac upon glycopeptide antibiotic challenge, displaying a potency that reflects these binding characteristics.

Enantioselective Total Synthesis of (‐)‐Candelalide A, a Novel Blocker of the Voltage‐Gated Potassium Channel Kv1.3 for an Immunosuppressive Agent.

A convergent route to (−)-candelalide A involved the union of a trans-decalin portion (AB ring) and a γ-pyrone moiety through the C16−C3‘ bond to assemble the whole carbon framework and subsequent formation of the dihydropyran ring (C ring) as the crucial steps. A strategic [2,3]-Wittig rearrangement was employed for establishing the stereogenic center at C9 and an exo-methylene function at C8 present in the decalin portion.

Synthetic studies toward merrilactone A: a short synthesis of AB ring motif

An efficient route to the AB ring motif of merrilactone A has been established by remarkable regioselective reduction of cyclic anhydrides 3 and 8 to the γ-lactone moiety, followed by the successive Stille and Heck reactions of 1,1-dibromo-1-alkene.

Enantioselective Total Synthesis of (−)-Candelalide A, a Novel Blocker of the Voltage-Gated Potassium Channel Kv1.3 for an Immunosuppressive Agent

A convergent route to (-)-candelalide A involved the union of a trans-decalin portion (AB ring) and a gamma-pyrone moiety through the C16-C3' bond to assemble the whole carbon framework and subsequent formation of the dihydropyran ring (C ring) as the crucial steps. A strategic [2,3]-Wittig rearrangement was employed for establishing the stereogenic center at C9 and an exo-methylene function at C8 present in the decalin portion. [reaction: see text]

Syntheses of Oxygenated Spongiane Diterpenes from Carvone. Synthesis of Dorisenone C.

Abstract The synthesis of dorisenone C, a representative member of the spongiane-type diterpene family, is described. The synthesis follows a B→AB→ABC→ABCD approach and is based on the initial preparation of the previously known hydroxy-aldehyde 14 (AB rings) from R -(−)-carvone, followed by an intramolecular Diels–Alder reaction between an oxygenated diene moiety and an acetylenic dienophile for the construction of the C ring (compound 22 ), and adequate manipulation of the Diels–Alder adduct functionality for completion of the spongiane framework.

Dephasing by electron–phonon interaction in an AB ring with an embedded quantum dot

AbstractWe study the dephasing effect on the transport properties in an Aharonov–Bohm ring with an embedded quantum dot, due to the electron–phonon interaction. The differential conductance is calculated under finite bias using Keldysh Green function. The electron–phonon interaction is treated (i) by the canonical transformation, or (ii) by the second‐order perturbation. By the latter method, we find that the Fano resonance is suppressed by the electron–phonon interaction and that the suppression increases with increasing bias voltage. The calculations by the former method do not yield the bias‐voltage dependence of the dephasing effect. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fano Resonance Through Quantum Dots in Tunable Aharonov-Bohm Rings

The main goal of the present work is to investigate novel resonant phenomena which may arise in the AB rings with specially-designed dots. First, we investigate the total transmission probability through the coupled double quantum dots (QDs) embedded in one of the arms of the AB ring with a magnetic flux passing through its center. We calculate the electron transmission through this AB ring with three short-range potential barriers using a scattering matrix at each junction and a transfer matrix in each arm. The Fano resonance splitting and disappearing in the transmission can be seen by adjusting the interaction between coupled QDs, whose coupling is controlled by the modulation of the center potential barrier. We also show the Fano resonance shifting to the higher energy by changing the interaction parameter, and the swing from Breit-Wigner to Fano resonance (or vice-versa) by tuning the magnetic AB flux threading through the AB ring. Secondly, we study the effects of an asymmetry in the arm by inserting an attractive potential well (dot) in one arm. The combined transmission resonance effects (Young's interference, T-stub and Fano resonances) as a function of parameters of the ring structure and the potential well will be presented. For large amplitude of the potential dot, the transmission zeroes are seen to match the theory of resonance due to reflections from a closed "T-stub". Fano-interference in an asymmetric AB ring with an embedded attractive potential appears when quasi-bound states in the dot coherently interact and interfere with propagating waves in the arms. Our theoretical AB-ring model is compared to experimental results from measurements of an asymmetric ring fabricated in a GaAs/AlGaAs-heterostructure, where Fano resonances have been observed (Keyser et al., 2002).

Synthesis of 14-Azacamptothecin, a Water-Soluble Topoisomerase I Poison

14-Azacamptothecin, a potent, water-soluble analogue of the antitumor agent camptothecin, has been prepared by a convergent synthesis. The key condensation of the AB and DE rings with concomitant formation of ring C of 14-aza CPT was carried out in two stages, the latter of which involved a radical cyclization strategy. [structure: see text]

Syntheses of oxygenated spongiane diterpenes from carvone. Synthesis of dorisenone C

The synthesis of dorisenone C, a representative member of the spongiane-type diterpene family, is described. The synthesis follows a B→AB→ABC→ABCD approach and is based on the initial preparation of the previously known hydroxy-aldehyde 14 (AB rings) from R-(−)-carvone, followed by an intramolecular Diels–Alder reaction between an oxygenated diene moiety and an acetylenic dienophile for the construction of the C ring (compound 22 ), and adequate manipulation of the Diels–Alder adduct functionality for completion of the spongiane framework.

Asymmetric Synthesis of the AB Ring System of Lactonamycin

[reaction: see text] An enantiospecific synthesis of the AB fragment of lactonamycin (5) is achieved in eight steps from dimethyl D-tartrate. Ester enolate chemistry features prominently in the sequence.

Selective 1,5‐Alkylidenecarbene Insertion Reactions on [3.2.1] Oxabicyclic Ethers: A New Approach Toward the AB Ring System (I) of Ingenol (II).

Selective 1,5‐Alkylidenecarbene Insertion Reactions on [3.2.1] Oxabicyclic Ethers: A New Approach Toward the AB Ring System (I) of Ingenol (II).