54 research outputs found

    An Asymmetric Synthesis of l-Pyrrolysine

    Full text link
    An efficient asymmetric synthesis of the 22nd amino acid l-pyrrolysine has been accomplished. The key stereogenic centers were installed by an asymmetric conjugate addition reaction. A Staudinger/aza-Wittig cyclization was used to form the acid-sensitive pyrroline ring. Pyrrolysine was synthesized in 13 steps in 20% overall yield

    Aminodiols via Stereocontrolled Oxidation of Methyleneaziridines

    Full text link
    A highly diastereoselective Ru-catalyzed oxidation/reduction sequence of bicyclic methyleneaziridines provides a facile route to complex 1-amino-2,3-diol motifs. The relative <i>anti</i> stereochemistry between the amine and the vicinal alcohol are proposed to result from 1,3-bischelation in the transition state by the C1 and C3 heteroatoms

    Aminodiols via Stereocontrolled Oxidation of Methyleneaziridines

    Full text link
    A highly diastereoselective Ru-catalyzed oxidation/reduction sequence of bicyclic methyleneaziridines provides a facile route to complex 1-amino-2,3-diol motifs. The relative <i>anti</i> stereochemistry between the amine and the vicinal alcohol are proposed to result from 1,3-bischelation in the transition state by the C1 and C3 heteroatoms

    Structural, Spectroscopic, and Computational Characterization of the Azide Adduct of Fe<sup>III</sup>(2,6-diacetylpyridinebis(semioxamazide)), a Functional Analogue of Iron Superoxide Dismutase

    Full text link
    We have prepared and thoroughly characterized, using X-ray crystallographic, spectroscopic, and computational methods, the diazide adduct of [Fe<sup>III</sup>(dapsox)­(H<sub>2</sub>O)<sub>2</sub>]<sup>+</sup> [dapsox = 2,6-diacetylpyridinebis­(semioxamazide)], (<b>1</b>), a low-molecular weight, functional analogue of iron superoxide dismutase (FeSOD). The X-ray crystal structure of the dimeric form of <b>1</b>, (Na­[Fe<sup>III</sup>(dapsox)­(N<sub>3</sub>)<sub>2</sub>]·DMF)<sub>2</sub> (<b>2</b>) shows two axially coordinated, symmetry inequivalent azides with differing Fe–N<sub>3</sub> bond lengths and Fe–N–N<sub>2</sub> bond angles. This inequivalence of the azide ligands likely reflects the presence of an interdimer hydrogen bonding interaction between a dapsox NH group and the coordinated nitrogen of one of the two azide ligands. Resonance Raman (rR) data obtained for frozen aqueous solution and solid-state samples of <b>2</b> indicate that the azides remain inequivalent in solution, suggesting that one of the azide ligands of <b>1</b> engages in an intermolecular hydrogen bonding interaction with a water molecule. Density functional theory (DFT) and time-dependent DFT calculations have been used to study two different computational models of <b>1</b>, one using coordinates taken from the X-ray crystal structure of <b>2</b>, and the other generated via DFT geometry optimization. An evaluation of these models on the basis of electronic absorption, magnetic circular dichroism, and rR data indicates that the crystal structure based model yields a more accurate electronic structure description of <b>1</b>, providing further support for the proposed intermolecular hydrogen bonding of <b>1</b> in the solid state and in solution. An analysis of the experimentally validated DFT results for this model reveals that the azides have both σ- and π-bonding interactions with the Fe<sup>III</sup> center and that more negative charge is located on the Fe-bound, rather than on the terminal, nitrogen atom of each azide. These observations are reminiscent of the results previously reported for the azide adduct of FeSOD and provide clues regarding the origin of the high catalytic activity of Fe-dapsox for superoxide disproportionation

    Aminodiols via Stereocontrolled Oxidation of Methyleneaziridines

    Full text link
    A highly diastereoselective Ru-catalyzed oxidation/reduction sequence of bicyclic methyleneaziridines provides a facile route to complex 1-amino-2,3-diol motifs. The relative <i>anti</i> stereochemistry between the amine and the vicinal alcohol are proposed to result from 1,3-bischelation in the transition state by the C1 and C3 heteroatoms

    An Asymmetric Synthesis of l-Pyrrolysine

    Full text link
    An efficient asymmetric synthesis of the 22nd amino acid l-pyrrolysine has been accomplished. The key stereogenic centers were installed by an asymmetric conjugate addition reaction. A Staudinger/aza-Wittig cyclization was used to form the acid-sensitive pyrroline ring. Pyrrolysine was synthesized in 13 steps in 20% overall yield

    Structural, Spectroscopic, and Computational Characterization of the Azide Adduct of Fe<sup>III</sup>(2,6-diacetylpyridinebis(semioxamazide)), a Functional Analogue of Iron Superoxide Dismutase

    Full text link
    We have prepared and thoroughly characterized, using X-ray crystallographic, spectroscopic, and computational methods, the diazide adduct of [Fe<sup>III</sup>(dapsox)­(H<sub>2</sub>O)<sub>2</sub>]<sup>+</sup> [dapsox = 2,6-diacetylpyridinebis­(semioxamazide)], (<b>1</b>), a low-molecular weight, functional analogue of iron superoxide dismutase (FeSOD). The X-ray crystal structure of the dimeric form of <b>1</b>, (Na­[Fe<sup>III</sup>(dapsox)­(N<sub>3</sub>)<sub>2</sub>]·DMF)<sub>2</sub> (<b>2</b>) shows two axially coordinated, symmetry inequivalent azides with differing Fe–N<sub>3</sub> bond lengths and Fe–N–N<sub>2</sub> bond angles. This inequivalence of the azide ligands likely reflects the presence of an interdimer hydrogen bonding interaction between a dapsox NH group and the coordinated nitrogen of one of the two azide ligands. Resonance Raman (rR) data obtained for frozen aqueous solution and solid-state samples of <b>2</b> indicate that the azides remain inequivalent in solution, suggesting that one of the azide ligands of <b>1</b> engages in an intermolecular hydrogen bonding interaction with a water molecule. Density functional theory (DFT) and time-dependent DFT calculations have been used to study two different computational models of <b>1</b>, one using coordinates taken from the X-ray crystal structure of <b>2</b>, and the other generated via DFT geometry optimization. An evaluation of these models on the basis of electronic absorption, magnetic circular dichroism, and rR data indicates that the crystal structure based model yields a more accurate electronic structure description of <b>1</b>, providing further support for the proposed intermolecular hydrogen bonding of <b>1</b> in the solid state and in solution. An analysis of the experimentally validated DFT results for this model reveals that the azides have both σ- and π-bonding interactions with the Fe<sup>III</sup> center and that more negative charge is located on the Fe-bound, rather than on the terminal, nitrogen atom of each azide. These observations are reminiscent of the results previously reported for the azide adduct of FeSOD and provide clues regarding the origin of the high catalytic activity of Fe-dapsox for superoxide disproportionation

    Polymorphism of Nifedipine: Crystal Structure and Reversible Transition of the Metastable β Polymorph

    Full text link
    We report the first structural determination of the metastable β polymorph of nifedipine (NIF) by single-crystal X-ray diffraction. Stable, high-quality crystals were grown from the melt in the presence of a polymer dopant. Our β NIF structure is characterized by a unit cell similar to that of the structure recently proposed from powder diffraction, but significantly different molecular conformations. Unlike the stable α polymorph, β NIF undergoes a reversible solid-state transformation near 60 °C. The now available β NIF structure clarifies some confusion concerning NIF polymorphs and enables inquiries into the structural basis for the selective crystallization of β NIF from glasses. We report that another polymorph crystallizes concomitantly with β NIF from the supercooled melt and transforms to β NIF at room temperature; this polymorph also undergoes reversible solid-state transformation

    Oxidation Products of Doubly Trimethylene-Bridged Tetrabenzyl <i>p</i>‑Phenylenediamine Paracyclophane

    Full text link
    We report synthesis and investigation of doubly trimethylene-bridged tetrabenzyl-<i>p</i>-phenylenediamine <b>1­(Bz)</b> in its singly and doubly charged redox states. The singly oxidized monoradical cation, which is a mixed-valence (MV) system with directly interacting charge-bearing units, shows broad and solvent-sensitive intervalence bands consistent with class II compounds according to the Robin–Day classification. The doubly oxidized diradical dication of <b>1­(Bz)</b> exists in the spin-paired singlet state with thermally accessible triplet state. It has similar conformations as the other dimeric <i>p</i>-phenylenediamines, such as derivatives <b>1­(Me)</b> and <b>1­(Et)</b>, in both the solid-state and solution phases. The successful isolation of the single-crystalline <b>1­(Bz)</b><sup><b>2+</b></sup> diradical dications with two different in nature counteranions, relatively highly coordinating SbF<sub>6</sub><sup>–</sup> and weakly coordinating carborane [undecamethylcarborane HCB<sub>11</sub>Me<sub>11</sub><sup>–</sup> (CB<sup>–</sup>)], reveals the distinct effect of the nature of counterions on the structural features of diradical dication. Cyclic voltammetry measurements of <b>1­(Bz)</b> in dichloromethane reveal separation of the first and second oxidation potential by 0.12 V (2.8 kcal/mol), indicating relatively stable mixed-valence state in the dichloromethane, whereas in the acetonitrile both the first and second oxidation potentials overlap into one unresolved redox peak with minimal separation

    Modular Functionalization of Allenes to Aminated Stereotriads

    Full text link
    Nitrogen-containing stereotriads, compounds with three adjacent stereodefined carbons, are commonly found in biologically important molecules. However, the preparation of molecules bearing these motifs can be challenging. Herein, we describe a modular oxidation protocol which converts a substituted allene to a triply functionalized amine of the form C–X/C–N/C–Y. The key step employs a Rh-catalyzed intramolecular conversion of the allene to a strained bicyclic methylene aziridine. This reactive intermediate is further elaborated to the target products, often in one reaction vessel and with effective transfer of the axial chirality of the allene to point chirality in the stereotriad
    • …
    corecore