54 research outputs found
An Asymmetric Synthesis of l-Pyrrolysine
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
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
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
We have prepared and thoroughly characterized,
using X-ray crystallographic, spectroscopic, and computational methods,
the diazide adduct of [Fe<sup>III</sup>(dapsox)Â(H<sub>2</sub>O)<sub>2</sub>]<sup>+</sup> [dapsox = 2,6-diacetylpyridinebisÂ(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>, (NaÂ[Fe<sup>III</sup>(dapsox)Â(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
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
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
We have prepared and thoroughly characterized,
using X-ray crystallographic, spectroscopic, and computational methods,
the diazide adduct of [Fe<sup>III</sup>(dapsox)Â(H<sub>2</sub>O)<sub>2</sub>]<sup>+</sup> [dapsox = 2,6-diacetylpyridinebisÂ(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>, (NaÂ[Fe<sup>III</sup>(dapsox)Â(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
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
We report synthesis and investigation
of doubly trimethylene-bridged
tetrabenzyl-<i>p</i>-phenylenediamine <b>1Â(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>1Â(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>1Â(Me)</b> and <b>1Â(Et)</b>, in both the
solid-state and solution phases. The successful isolation of the single-crystalline <b>1Â(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>1Â(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
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
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