Intramolecular formal oxa-[3 + 3] cycloaddition approach to the ABD system of phomactin A

The ABD-ring of phomactin A was synthesized using an intramolecular formal oxa-[3 + 3] cycloaddion of an alpha,beta-unsaturated iminium salt tethered to a 1,3-diketone. This represents the first time that the 12-membered ring has been formed simultaneously with the 1-oxadecalin and should afford a facile route to the challenging structure of phomactin A. [reaction: see text]     

A total synthesis of phomactin A

A total synthesis of phomactin A (1) based on a Cr(II)/Ni(II) macrocyclisation from the aldehyde vinyl iodide 11, leading to 12, followed by elaboration of the epoxyketone 16, which then undergoes spontaneous pyran-hemiacetal formation on deprotection, is described.     

Convergent enantioselective synthesis of the tricyclic core of phomactin A

[reaction: see text] The tricyclic core of phomactin A was synthesized from 6,6-dimethyl-2-cyclohexen-1-one. Key reactions include the addition of a cyclohexenyllithium reagent to an epoxyaldehyde and a regioselective intramolecular epoxide opening to install the oxadecalin core.     

Unique structural trends in the lanthanoid oxocarbonyl complexes

Reactions of laser-ablated lanthanoid atoms (except for radioactive Pm) with carbon dioxide molecules in solid argon have been investigated using matrix-isolation infrared spectroscopy. On the basis of isotopic shifts, mixed isotopic splitting patterns, and CCl4-doping experiments, the lanthanoid oxocarbonyl complexes have been identified. Density functional theory calculations have been performed on these products, which support the experimental assignments of the infrared spectra. Infrared spectroscopic studies of these lanthanoid complexes combined with theoretical calculations reveal that the early lanthanoid (La-Sm) oxocarbonyl complexes adopt trans configurations, the europium and ytterbium ones adopt side-on-bonded modes (Eu-(eta2-OC)O and Yb-(eta2-OC)O), and the late lanthanoid (Gd-Lu) ones adopt cis configurations. Natural bond orbital analysis indicates that the formation of the lanthanoid oxocarbonyl complexes involves the promotion of 6s and 4f electrons into the metal valence shell.     

Studies toward the synthesis of phomactin A. An approach to the macrocyclic core

An approach to the macrocyclic core of phomactin A is described. Central to this strategy is the use of a cis-fused oxadecalin intermediate, prepared using the dihydropyrone Diels-Alder reaction. The conformational bias inherent to this system is then used to facilitate macrocycle formation via an intramolecular B-alkyl Suzuki coupling.     

Synthetic studies towards congeners of phomactin A. Re-examination of the structure of Sch 49028

The total syntheses of the epoxy cyclic hemiacetal structures 8 and 9, which are isomeric with the structure 6 proposed for the phomactin known as Sch 49028 isolated from the marine fungus Phoma sp. are described. Neither of these structures showed spectroscopic data consistent with those reported for the purported natural product, adding credibility to the proposal that the structure Sch 49028 does not exist in nature and that its NMR spectroscopic data should have been assigned as phomactin A (1).     

Studies toward the tricyclic core of phomactin A. Synthesis of the reduced furanochroman subunit

An approach to the tricyclic core of phomactin A is described via the synthesis of a reduced furanochroman model. Key elements of this study include the use of a highly functionalized 1-oxadecalone derivative as a template for the stereoselective introduction of functionality and a tandem retro aldol-epoxide opening-cyclization sequence for elaboration of the dihydrofuran ring.     

Enantioselective synthesis of the oxadecalin core of phomactin A via a highly stereoselective Diels-Alder reaction

Phomactin A (1) is a selective antagonist of platelet activating factor (PAF). Herein, we report our progress toward the construction of the oxadecalin core of 1. This route is based on the Diels-Alder cycloaddition of an appropriately functionalized vinyl pyran and a complementary dienophile. A model of this reaction involving 2 and maleic anhydride was conducted. Adduct 3 contains the correct stereochemical arrangements between functional groups necessary for gaining access to phomactin A. Reaction: see text.     

Formation of polyurethane structural materials based on mixtures of oligoethers with different reactivities

Kinetics of the reaction of urethane formation from oligopolyols with primary and secondary hydroxy groups and the influence exerted on their interaction by products formed in the reaction of N,N,N′,N′-tetrakis-(-2-hydroxypropyl)-ethylenediamine with an isocyanate were studied. Samples of high-strength polyurethane composites based on mixtures of oligoetherpolyols and the oligomeric diamine were obtained. These samples approximately twice surpass in strength materials based on oligoepoxides. The working capacity of the samples under severe climatic conditions was demonstrated.     

Construction of the tricyclic furanochroman skeleton of phomactin A via the Prins/Conia-ene cascade cyclization approach

A substrate-controlled asymmetric Prins/Conia-ene cascade cyclization has been developed with In(OTf)(3) in CH(3)CN from 0 to 70 °C. These conditions afforded very good yields of the 1-oxadecalin product in one pot and effectively suppressed the racemization of the 1-oxadecalin product with almost no enantiomeric excess (ee) loss. This cascade cyclization has been successfully employed for the construction of the highly functionalized 1-oxadecalin unit of phomactin A with an acyclic β-keto ester and an alkynal as the substrates via a one-pot operation (66% yield, single diastereomer). The 1-oxadecalin moiety has been readily converted to the tricyclic furanochroman skeleton of phomactin A via the epoxidation/dealkoxycarbonylation protocol under very mild conditions with 52% yield in three steps.     

Synthesis of metal-complexes and their reactivities: -The difference of reactivities among the manganese complexes-

Manganese chelates were prepared and the autoxidation of isopropyl benzene (IP) was carried out. (Mn-Chelate-1) showed a long induction period, but the other complexes (Mn-Chelate-2 and −3) which contain two or more magnesium ions in each molecule had shorter induction periods. The difference comes from the conformation around the manganese ion. Mn(III) acetate prepared from the oxidation of manganese (II) acetate with potassium permanganate in acetic acid was discussed.     

Synthesis of the ring system of phomactin D using a Suzuki macrocyclization

[reaction: see text]The ring system of phomactin D was synthesized in racemic form in an efficient manner from 2,3-dimethylcyclohexanone. Notable transformations include (1) an alkylation of the enolate of a vinylogous thiolester to install a quaternary stereocenter, (2) a conjugate addition of cyanide to an alpha,beta-unsaturated aldehyde, (3) the formation of a Weinreb amide directly from a cyanohydrin, and (4) an intramolecular Pd-mediated Suzuki coupling of a B-alkyl-9-BBN derivative and a vinyl iodide to form the macrocyclic ring.     

Theoretical investigation of different reactivities of Fe(IV)O and Ru(IV)O complexes with the same ligand topology

The structures and mechanisms for hydrogen abstraction from isopropylbenzene for four high-valence complexes, cis-β-[Fe^IV(O)(BQCN)]²⁺ (Fe-2b and Fe-2b-2) and cis-β-[Ru^IV(O)(BQCN)]²⁺ (Ru-2b and Ru-2b-2) (BQCN = N,N′-dimethyl-N,N′-bis(8-quinolyl)-cyclohexanediamine), were investigated using density functional theory. Of the two iron complexes, Fe-2b-2 has more exposed FeO units than Fe-2b, with iron being further out of the equatorial plane because of the steric interaction of the same ligand topologies with the iron-oxo group trans to a quinolyl or amine nitrogen. The contribution of BQCN to Fe-2b is higher than the contribution to Fe-2b-2 as shown by the density-of-states spectra. The iron isomers can abstract hydrogen from isopropylbenzene via two-state reactivity patterns, whereas the ruthenium isomers react with isopropylbenzene via a single-state mechanism. In the gas phase, the relative reactivity exhibits the trend Fe-2b > Fe-2b-2, whereas with the addition of the ZPE correction and the SMD model, the relative reactivity follows Fe-2b-2 > Fe-2b. For the ruthenium complexes, the relative reactivity follows the trend Ru-2b-2 > Ru-2b in both the gas phase and solvent. Thus, the same ligand topologies with the metal-oxo group trans to a different nitrogen affect the reactivities of the iron and ruthenium complexes.     

Synthesis, structural, and spectroscopic characterization and reactivities of mononuclear cobalt(III)-peroxo complexes

Metal-dioxygen adducts are key intermediates detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. In this study, mononuclear cobalt(III)-peroxo complexes bearing tetraazamacrocyclic ligands, [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+), were synthesized by reacting [Co(12-TMC)(CH(3)CN)](2+) and [Co(13-TMC)(CH(3)CN)](2+), respectively, with H(2)O(2) in the presence of triethylamine. The mononuclear cobalt(III)-peroxo intermediates were isolated and characterized by various spectroscopic techniques and X-ray crystallography, and the structural and spectroscopic characterization demonstrated unambiguously that the peroxo ligand is bound in a side-on η(2) fashion. The O-O bond stretching frequency of [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+) was determined to be 902 cm(-1) by resonance Raman spectroscopy. The structural properties of the CoO(2) core in both complexes are nearly identical; the O-O bond distances of [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+) were 1.4389(17) ? and 1.438(6) ?, respectively. The cobalt(III)-peroxo complexes showed reactivities in the oxidation of aldehydes and O(2)-transfer reactions. In the aldehyde oxidation reactions, the nucleophilic reactivity of the cobalt-peroxo complexes was significantly dependent on the ring size of the macrocyclic ligands, with the reactivity of [Co(13-TMC)(O(2))](+) > [Co(12-TMC)(O(2))](+). In the O(2)-transfer reactions, the cobalt(III)-peroxo complexes transferred the bound peroxo group to a manganese(II) complex, affording the corresponding cobalt(II) and manganese(III)-peroxo complexes. The reactivity of the cobalt-peroxo complexes in O(2)-transfer was also significantly dependent on the ring size of tetraazamacrocycles, and the reactivity order in the O(2)-transfer reactions was the same as that observed in the aldehyde oxidation reactions.     

Enantiospecific synthesis of the ABC and ABD ring systems of the marine diterpenes aberraranes

Enantiospecific synthesis of the ABC and ABD ring systems present in the marine diterpenes aberraranes, starting from the readily available (S)-campholenaldehyde, has been accomplished.     

Comparing the structural topology of integral and peripheral membrane proteins utilizing electron paramagnetic resonance spectroscopy

The alignment of membrane proteins provides pertinent structural and dynamic information. Structural topology data gleaned from such studies can be used to determine the functional mechanisms associated with a wide variety of integral membrane proteins. In this communication, we successfully demonstrate, for the first time, the determination of the structural topology and helical tilt of an antimicrobial peptide magainin 2 using aligned X-band spin-label EPR spectroscopic techniques. This novel comparison unlocks many possibilities utilizing EPR spectroscopy to probe antimicrobial peptide topologies with increased sensitivity and may also give further clues to elucidate their corresponding mechanisms.     

A computational study of the reactivities of four-membered heavy carbene systems

The potential energy surfaces for the chemical reactions of four-membered N-heterocyclic group 14 heavy carbene species have been studied using density functional theory (B3LYP/LANL2DZ). Five four-membered group 14 heavy carbene species, (i-Pr)(2) NP(NR)(2) E:, in which E = C, Si, Ge, Sn, and Pb, were chosen as the model reactants in this work. Also, four kinds of chemical reactions, C-H bond insertion, water addition, alkene cycloaddition, and dimerization, have been used to study the chemical reactivities of these group 14 four-membered N-heterocyclic carbene species. Basically, our present theoretical work predicts that the larger the ∠NEN bond angle of the four-membered group 14 heavy carbene species, the smaller the singlet-triplet splitting, the lower the activation barrier, and, in turn, the more rapid, its chemical reactions to various chemical species. Moreover, our theoretical investigations suggest that the relative carbenic reactivity decreases in the order: C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is its four-membered carbene toward chemical reactions. As a result, our results predict that the four-membered group 14 heavy carbene species (E = Si, Ge, Sn, and Pb) should be more kinetically stable than the observed carbene species and, thus, can be also readily synthesized and isolated at room temperature. Furthermore, the singlet-triplet energy splitting of the four-membered group 14 carbene species, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.     

Synthetic studies towards the phomactins. Concise syntheses of the tricyclic furanochroman and the oxygenated bicyclo[9.3.1]pentadecane ring systems in phomactin A

A concise synthesis of the tricyclic furanochroman unit 3 found in the PAF antagonist phomactin A (1) isolated from the marine fungus Phoma sp., is described. In complementary studies, a variety of synthetic routes towards the bicyclo[9.3.1]pentadecane ring system 4 in phomactin A were explored. These studies culminated in a synthesis of the substituted ring system 79 containing all the carbon atoms and all the necessary oxygen centres for elaboration to phomactin A itself.