Search results for "metathesis"
showing 10 items of 150 documents
Stereoselective Synthesis and Structural Correction of the Naturally Occurring Lactone Stagonolide G
2010
A convergent synthesis of the structure proposed for the naturally occurring lactone stagonolide G is described. All three stereocenters were created with the aid of asymmetric Brown allylations. The lactone ring was built by means of a ring-closing metathesis (RCM). The synthetic and the natural compound differed in their spectral properties. A new structure is now proposed for stagonolide G and demonstrated by means of a chemical transformation.
ChemInform Abstract: Solution-, Solid-Phase, and Fluorous Synthesis of β,β-Difluorinated Cyclic Quaternary α-Amino Acid Derivatives: A Comparative St…
2008
The diastereoselective synthesis of cyclic β,β-difluorinated α-amino acid derivatives bearing a quaternary stereocenter is described. The process relies on the chemo- and diastereoselective addition of allylic organometallic reagents to fluorinated α-imino esters and a subsequent ring-closing metathesis reaction (RCM). Complete selectivity in the nucleophilic addition was achieved with (R)-phenylglycinol methyl ether as a chiral auxiliary. The resulting amino acids were introduced into peptide chains, which could facilitate the preparation of potentially bioactive dipeptide derivatives. In addition, the solution synthesis of these cyclic fluorinated α-amino acids was successfully adapted to…
Synthesis of Precision Poly(1,3-adamantylene alkylene)s via Acyclic Diene Metathesis Polycondensation
2019
[Image: see text] Fully saturated, aliphatic polymers containing adamantane moieties evenly distributed along the polymer backbone are of great interest due to their exceptional thermal stability, yet more synthetic strategies toward these polymers would be desirable. Herein, we report for the first time the synthesis of poly(1,3-adamantylene alkylene)s based on α,ω-dienes containing bulky 1,3-adamantylene defects precisely located on every 11th, 17th, 19th, and 21st chain carbon via acyclic diene metathesis polycondensation. All saturated polymers revealed excellent thermal stabilities (452–456 °C) that were significantly higher compared to those of structurally similar polyolefins with al…
Vitamin C Loaded Polyethylene: Synthesis and Properties of Precise Polyethylene with Vitamin C Defects via Acyclic Diene Metathesis Polycondensation
2020
A polyethylene-like polymer with an in-chain vitamin C group was synthesized by olefin metathesis polymerization. Here, we describe both the synthesis and a comprehensive physical characterization. Because of the olefin metathesis synthesis, the vitamin C groups are equidistantly arranged in the polyethylene (PE) main chain. Their separation was adjusted to 20 CH2 units. After hydrogenation, a semicrystalline polymer is obtained that is soluble in polar solvents. Because of its size and steric effect, the vitamin C acts as a chain defect, which is expelled from the crystal lattice, yielding a lamellar crystal with a homogeneous thickness corresponding to the interdefect distance. The physic…
Chemistry of transition-metal clusters with mixed Sb/S ligands: evidence for a terminal Sb=S double bond in Cp*3Rh3Sb2S5 (Cp* = C5Me5).
2005
The reaction of [Cp 2 *Rh 2 Cl 4 ] (Cp* = C 5 Me 5 ) with a slight excess of K 3 SbS 3 in boiling THF gave the neutral clusters [Cp* 4 Rh 4 S 5 ] (1), [Cp* 3 Rh 3 Sb 2 S 5 ] (2), and after salt metathesis [Cp* 3 Rh 3 SbS n ]PF 6 (3; n = 5 and 6). The structures of 1-3 are heterocubane clusters with Cp*Rh, S, and Sb vertices but with sulfur inserted into one (1 and 2) or two (3) edges. X-ray diffraction analysis of 2 additionally reveals a very short Sb-S distance of 2.297(1) A within the novel μ 3 -Sb 2 S 4 ligand. Density functional theory calculation of the model compounds [SSbS] 3 - , [HSSbS] 2 - , and [HSSbH 2 S] 0 provided strong evidence for the existence of a stable terminal Sb=S dou…
Synthetic Approaches to Anti-Inflammatory Macrolactones of the Oxacyclododecindione Type
2015
Various synthetic approaches to the oxacyclododecindione-type macrolactones, known for their potent anti-inflammatory activity, are presented. These include an attempted carbonylative ring closure, a hydroacylation route, and an approach by ring-closing metathesis and double bond isomerization, as well as a strategy including ring-closing metathesis/unsaturation. The last route allowed the preparation of a bioactive analogue of the recently described 14-deoxyoxacyclododecindione.
ChemInform Abstract: Synthetic Approaches to Antiinflammatory Macrolactones of the Oxacyclododecindione Type.
2015
A variety of attempts to synthesize oxacyclododecindione macrolactones through carbonylative ring-closure, intramolecular alkyne hydroacyalation, and Ru-catalyzed double bond isomerization fails.
Rare earth metal-based catalysts for the polymerization of nonpolar and polar monomers
2001
Abstract The synthesis of rare earth metal half-sandwich hydrido complexes [Ln (h5:h1-C5Me 4SiMe2NCMe3) (THF) (µ-H) ] 2 (Ln = Y, Lu) through s-bond metathesis of the easily accessible alkyl complexes [Ln (h5:h1-C5Me 4SiMe2NCMe3) (CH2 SiMe3) (THF) ] was developed. The dimeric yttrium hydrido complexes are highly fluxional, and a monomer-dimer equilibrium is present. They were tested as single-site, single-component catalysts for the polymerization of ethylene and styrene, as well as alkyl acrylate and acrylonitrile. The hydrido complexes polymerize ethylene slowly and form isolable mono (insertion) products with styrene. The yttrium n-alkyl complexes [Y (h5:h1-C5Me 4SiMe2NCMe3) (R) (THF) ] […
Stereoselective Synthesis of (+)-Boronolide
2002
The delta-lactone boronolide (+)-1, a pharmacologically active, naturally occurring product, has been synthesized in enantiopure form with L-erythrulose as the chiral starting material. The key steps of the synthesis were a highly stereoselective aldol-reduction one-pot sequence, an indium-mediated diastereoselective aldehyde allylation, and a ring-closing metathesis.
Single-Component Polymerization Catalysts for Ethylene and Styrene: Synthesis, Characterization, and Reactivity of Alkyl and Hydrido Yttrium Complex…
1999
Yttrium alkyl complexes Y(η5:η1-C5Me4SiMe2NCMe2R)(CH2SiMe3)(THF) (R = Me, Et) and Y(η5:η1-C9H6SiMe2NCMe3)(CH2SiMe3)(THF) can be prepared in high yields by a σ-bond metathesis reaction between Y(CH2SiMe3)3(THF)2 and amino-functionalized cyclopentadienes or indene. The structure of Y(η5:η1-C5Me4SiMe2NCMe2Et)(CH2SiMe3)(THF) was shown by single-crystal X-ray diffraction to be that of a three-legged piano stool. Reaction of Y(CH2SiMe3)3(THF)2 with the tridentate linked amido−cyclopentadienyl ligands (C5Me4H)SiMe2NHR (R = CH2CH2OMe, CH2CH2NMe2, CH2CH2CH2OMe, CMe2CH2OMe), which contain an additional donor site, results in the cleavage of the silicon−cyclopentadienyl bond and the formation of the t…