Browse
Submit Data
Databases
API
Help

Dataset Information

0 Views

0 Connections

0 Citations

0 Reanalyses

0 Downloads

Omics score: 0

Effects of alpha-Alkoxy Substitution and Conformational Constraints on 6-exo Radical Cyclizations of Hydrazones via Reversible Thiyl and Stannyl Additions.

ABSTRACT: Access to multifunctional hydrazones of relevance to dysiherbaine synthesis studies is described. Subsequent radical cyclizations of multifunctional hydrazones via a Si- and C-linked tethering strategy are shown to function effectively in 6-exo fashion. Conformational constraints are proposed to play a key role in suppressing unproductive premature reduction pathways. The stereochemical outcomes suggest that minimizing the dipole repulsion between neighboring C=N and C-O bonds favors a C(alpha)-C(=N) dihedral angle placing the C=N bond axial within a chairlike transition state, in contrast to the usual Beckwith-Houk model.

SUBMITTER: Friestad GK

PROVIDER: S-EPMC2034276 | biostudies-literature | 2007 Sep

REPOSITORIES: biostudies-literature

ACCESS DATA

Json Xml

Publications

Effects of alpha-Alkoxy Substitution and Conformational Constraints on 6-exo Radical Cyclizations of Hydrazones via Reversible Thiyl and Stannyl Additions.

Friestad Gregory K GK Mathies Alex K AK

Tetrahedron 20070901 38

Access to multifunctional hydrazones of relevance to dysiherbaine synthesis studies is described. Subsequent radical cyclizations of multifunctional hydrazones via a Si- and C-linked tethering strategy are shown to function effectively in 6-exo fashion. Conformational constraints are proposed to play a key role in suppressing unproductive premature reduction pathways. The stereochemical outcomes suggest that minimizing the dipole repulsion between neighboring C=N and C-O bonds favors a C(alpha)- ...[more]

PMID: 18797497

Similar Datasets

Stereoselective additions of thiyl radicals to terminal ynamides.

Project description:Two complementary sets of conditions for radical additions of thiols to terminal ynamides are described. The use of 1 equiv of thiol affords the cis-beta-thioenamide adducts in rapid fashion (10 min) and good dr, whereas employing excess thiol and longer reaction times favors the trans products.

| S-EPMC2878391 | biostudies-literature

Michael-type cyclizations in lantibiotic biosynthesis are reversible.

Project description:Lanthipeptides are members of the ribosomally synthesized and post-translationally modified peptides (RiPPs). They are generated in two biosynthetic steps: dehydration of Ser and Thr residues to the corresponding dehydroamino acids and subsequent conjugate addition by the thiol of Cys residues to generate the characteristic lanthionine and methyllanthionine thioether-bridged structures. Typically, a lanthipeptide contains multiple thioether cross-links. Recent studies have proposed that the final ring topology may be under thermodynamic control. If so, then the Michael-type cyclization reaction would need to be reversible, but such reversibility has never been demonstrated. We show here for the class I lanthipeptide cyclase NisC and class II lanthipeptide synthetase HalM2 that, indeed, the conjugate addition reactions are reversible and that the enzymes can open up all thioether rings in their products. We also propose that a His residue that is conserved among the lanthipeptide cyclases acts as the acid or base that protonates or generates the enolate intermediate during thioether ring formation and opening, respectively.

| S-EPMC4433588 | biostudies-literature

Chalcogen Bonding due to the Exo-Substitution of Icosahedral Dicarbaborane.

Project description:Chalcogen atoms are a class of substituents capable of generating inner and outer derivatives of boron clusters. It is well known that chalcogenated boron clusters can form strong σ-hole interactions when a chalcogen atom is a part of an icosahedron. This paper studies σ-hole interactions of dicarbaboranes with two exopolyhedral chalcogen atoms bonded to carbon vertices. Specifically, a computational investigation has been carried out on the co-crystal of (1,2-C2B10H10)2Se4•toluene and a single crystal of (1,2-C2B10H10)2Te4.

| S-EPMC6680755 | biostudies-literature

Exploration of cascade cyclizations terminated by tandem aromatic substitution: total synthesis of (+)-schweinfurthin A.

Project description:The termination of epoxide-initiated cascade cyclizations with a range of "protected" phenols is described. When the protecting group can be lost as a stabilized electrophile, the cascade process continues beyond ring closure to afford products which have undergone a tandem electrophilic aromatic substitution. A number of groups have proven viable in this process and the regiochemistry of their substitution reactions has been studied. Application of this methodology in the first total synthesis of (+)-schweinfurthin A, a potent antiproliferative agent, has been achieved.

| S-EPMC3069933 | biostudies-literature

Impact of Substitution Pattern and Chain Length on the Thermotropic Properties of Alkoxy-Substituted Triphenyl-Tristriazolotriazines.

Project description:Tristriazolotriazines (TTTs) with a threefold alkoxyphenyl substitution were prepared and studied by DSC, polarized optical microscopy (POM) and X-ray scattering. Six pentyloxy chains are sufficient to induce liquid-crystalline behavior in these star-shaped compounds. Thermotropic properties of TTTs with varying substitution patterns and a periphery of linear chains of different lengths, branching in the chain and swallow-tails, are compared. Generally, these disks display broad and stable thermotropic mesophases, with the tangential TTT being superior to the radial isomer. The structure-property relationships of the number of alkyl chains, their position, length and structure were studied.

| S-EPMC7729490 | biostudies-literature

Enantioselective Nickel-Catalyzed anti-Carbometallative Cyclizations of Alkynyl Electrophiles Enabled by Reversible Alkenylnickel E/Z Isomerization.

Project description:Nickel-catalyzed additions of arylboronic acids to alkynes, followed by enantioselective cyclizations of the alkenylnickel species onto tethered ketones or enones, are reported. These reactions are reliant upon the formal anti-carbonickelation of the alkyne, which is postulated to occur by the reversible E/Z isomerization of an alkenylnickel species.

| S-EPMC4957849 | biostudies-literature

Synthesis of highly enantioenriched 3,4-dihydroquinolin-2-ones by 6-exo-trig radical cyclizations of axially chiral alpha-halo-ortho-alkenyl anilides.

Project description:Radical cyclizations (Bu(3)SnH, Et(3)B/air, rt) of racemic alpha-halo-ortho-alkenyl anilides provide 3,4-dihydroquinolin-2-ones in high yield. Cyclizations of enantioenriched precursors occur in similarly high yields and with transfer of axial chirality to the new stereocenter of the products with exceptionally high fidelity (often >95%). Single and tandem cyclizations of alpha-halo-ortho-alkenyl anilides bearing an additional substituent on the alpha-carbon occur with high chirality transfer and high diastereoselectivity. Straightforward models are proposed to interpret both the chirality transfer and diastereoselectivity aspects. These first examples of an approach for axial chiral transfer from a reactive species in the amide to an acceptor suggest broad potential for extension both within and beyond radical reactions.

| S-EPMC2784126 | biostudies-literature

Bronsted acid catalyzed phosphoramidic acid additions to alkenes: diastereo- and enantioselective halogenative cyclizations for the synthesis of C- and P-chiral phosphoramidates.

Project description:The first highly diastereo- and enantioselective additions of a halogen and phosphoramidic acid to unactivated alkenes have been developed, catalyzed by a chiral Brønsted acid. A unique feature of these additions is the opportunity for stereocontrol at two noncontiguous chiral centers, carbon and phosphorus, leading to cyclic P-chiral phosphoramidates. In addition to their inherent value, the phosphoramidates are precursors to enantioenriched epoxy allylamines.

| S-EPMC4210055 | biostudies-literature

Reversible hydrogen transfer reactions of cysteine thiyl radicals in peptides: the conversion of cysteine into dehydroalanine and alanine, and of alanine into dehydroalanine.

Project description:The photodissociation of disulfide bonds in model peptides containing Ala and Ala-d(3) generates a series of photoproducts following the generation of a CysS(•) thiyl radical pair. These photoproducts include transformations of Cys to dehydroalanine (Dha) and Ala, as well as Ala to Dha. Intramolecular Michael addition of an intact Cys with a photolytically generated Dha results in the formation of cyclic thioethers. The conversion of Cys into Dha likely involves a 1,3-H-shift from the Cys (?)C-H bond to the thiyl radical, followed by elimination of HS(•). The conversion of Dha into Ala most likely involves hydrated electrons, which are generated through the photolysis of Cys, the photoproduct of disulfide photolysis. Prior to stable product formation, CysS(•) radicals engage in reversible hydrogen transfer reactions with (?)C-H and (?)C-H bonds of the surrounding amino acids. Especially for the (?)C-H bonds of Ala, such hydrogen transfer reactions are unexpected on the basis of thermodynamic grounds; however, the replacement of deuterons in Ala-d(3) by hydrogens in H(2)O provides strong experimental evidence for such reactions.

| S-EPMC3736842 | biostudies-literature

Triggered reversible substitution of adaptive constitutional dynamic networks dictates programmed catalytic functions.

Project description:The triggered substitution of networks and their resulting functions play an important mechanism in biological transformations, such as intracellular metabolic pathways and cell differentiation. We describe the triggered, cyclic, reversible intersubstitution of three nucleic acid-based constitutional dynamic networks (CDNs) and the programmed catalytic functions guided by the interconverting CDNs. The transitions between the CDNs are activated by nucleic acid strand displacement processes acting as triggers and counter triggers, leading to the adaptive substitution of the constituents and to emerging catalytic functions dictated by the compositions of the different networks. The quantitative evaluation of the compositions of the different CDNs is achieved by DNAzyme reporters and complementary electrophoresis experiments. By coupling a library of six hairpins to the interconverting CDNs, the CDN-guided, emerging, programmed activities of three different biocatalysts are demonstrated. The study has important future applications in the development of sensor systems, finite-state logic devices, and selective switchable catalytic assemblies.

| S-EPMC6510552 | biostudies-literature

OmicsDI is part of the ELIXIR infrastructure

OmicsDI is an Elixir interoperability service. Learn more ›

OmicsDI Databases

PRIDE
PeptideAtlas
MassIVE
JPOST Repository
Physiome Model Repository

EGA
EVA
ENA
LINCS
PAXDB
Cell Collective

MetaboLights
Metabolomics Workbench
MetabolomeExpress
GNPS
BioModels
FAIRDOMHub

ArrayExpress
dbGaP
ExpressionAtlas
GEO
NODE

Information

Databases
Help
API
Contact us
Code on GitHub
Terms of use
Submit Data