Noncovalent dimerization of ubiquitin

Another kind of dynamics: Ubiquitin noncovalently dimerizes with a dissociation constant of approximately 5?mM. The two subunits adopt an array of relative orientations, utilizing an interface also for binding to other proteins (see picture). Quaternary fluctuation among members of the dimer ensemble constitutes a different kind of dynamics that complements the tertiary dynamics of each ubiquitin subunit.     

6,7,14,15-Tetrahydro[1,5]diazocino[1,2-a:6,5-a′]diindole. Synthesis of a novel pentacyclic ring system

In search of new lead structures for potent allosteric enhancers of antagonist binding to muscarinic M₂ receptors, the first representative of a novel heterocyclic ring system, 6,7,14,15-tetrahydro[1,5]diazocino[1,2-a:6,5-a′]diindole, has been synthesized. The new pentacyclic ring skeleton is obtained from [3-(2-dibenzylaminoethyl)indol-2-yl]-acetic acid methyl ester in three steps.     

Sequential Baylis-Hillman/RCM protocol for the stereoselective synthesis of (+)-MK7607 and (+)-streptol

Sequential Baylis-Hillman/ring-closing metathesis (RCM) approach toward the total synthesis of (+)-MK7607 and (+)-streptol starting from (R,R)-tartaric acid is reported.     

Addition of γ-silyloxyallyltins on ethyl glyoxylate: evaluation of the influence of the experimental conditions on the stereochemical course of the reaction

Ethyl glyoxylate was reacted with α-substituted γ-(t-butyldimethylsilyloxy)-allyltributyltin in order to obtain selectively each diastereomer of ethyl 3-(t-butyldimethylsilyloxy)-2-hydroxyhex-4-enoate and subsequently the corresponding diols. Diastereomers syn-E, anti-E and anti-Z were obtained in good yields with good to high selectivities and the obtained results were rationalized by consideration of cyclic or open transition states in agreement with the experimental conditions and with the structure of the starting reagents.     

The Effects of Microenvironment Polarity and Dendritic Branching of Aliphatic Hydrocarbon Dendrons on the Self‐Assembly of 2‐Ureido‐4‐pyrimidinones

Two series of aliphatic hydrocarbon‐based G1–G3 dendritic 2‐ureido‐4‐pyrimidinones (UPy) ( S‐Gn )₂ and ( L‐Gn )₂, differing from one another by the distance between the branching juncture to the urea end, were prepared and characterized. These hydrocarbon dendrons were also appended to a p‐aminonitrobenzene solvatochromic chromophore in order to probe their microenvironment polarity. While positive solvatochromism was observed which indicated the chromophore was solvent accessible, there was no significant difference between the microenvironment polarities on going from the G1 to the G3 dendrons. The self‐assembling behavior and tautomeric preference of the dendritic UPy derviatives were examined by ¹H NMR spectroscopy. The dimerization constants (K_dim*) of the DDAA tautomers were unchanged at 10⁷ M ^?1 in CDCl₃ at both 25 and 50 °C, which were comparable to those of UPy compounds bearing other nonpolar substitutents. Furthermore, the lower limits on the K′_dim* of the DADA tautomeric forms of the ( S‐Gn )₂ and ( L‐Gn )₂ series were determined to be 10⁶ and 10⁵ M ^?1 in CDCl₃, respectively. It was found that a closer proximity of the dendron branching juncture to the UPy unit could lead to a destabilization effect on the dimeric states. Hence, the ( L‐Gn )₂ dimers are more stable than those of ( S‐Gn )₂ in the DDAA form, but the latter are more stable than the former in the tautomeric DADA state. This study showed that both the highly nonpolar microenvironment and the proximity of the dendritic branching juncture to the UPy motif could alter the strength and profile of the hydrogen bond‐mediated self‐assembling process.     

Direct Synthesis of a Photoactive Inorganic–Organic Mesostructured Hybrid Material and its Application as a Photocatalyst

A direct route : Silylated triphenylmethanol is incorporated into mesoporous material MCM‐41 through a direct synthesis method. Under acidic conditions, this inorganic–organic hybrid generates trityl cations to give the photoactive material Tyl‐MCM41. Tyl‐MCM41 promotes the photosensitized dimerization of 1,3‐cyclohexadiene with an unprecedented selectivity towards the formation of the exo product (see scheme).

     

Structure of a C26H28 alkene formed via titanium-promoted reductive dimerization of 4-(spirocyclopropyl)-pentacyclo[5.4.0.02,6.03,10.05,9]undecan-8-one

A spiro-fused C₂₆H₂₈ cage≓ dimer hydrocarbon contains two three-membered rings, two four-membered rings, eight five-membered rings, and two six-membered rings. The molecule is calculated by molecular mechanics to have 902 kJ/mol of strain energy distributed primarily between angle strain (457 kJ/mol) and torsional interactions (368 kJ/mol). Molecular mechanics calculations and a geometry-optimized ab initio calculation are used to analyze the bond lengths and bond angles in the molecule. There is one major discrepancy between observed and calculated distances.     

丙烯腈催化二聚反应的研究进展

丙烯腈是廉价易得的化工原料.丙烯腈的线性二聚可以构建己(烯)二腈分子骨架,经过进一步的加氢还原可制备己二胺.己二胺是重要的工业中间体,有着广泛的用途和广阔的市场,主要用于合成尼龙66.因此,丙烯腈的线性二聚是非常重要的有机化学反应.与已经实现工业化的电解法相比,催化二聚有着能耗低、装置要求低等优点.除此之外,丙烯腈催化二聚还可能生成非线性聚合产物2-亚甲基戊二腈.它是生产广谱抗菌剂溴菌腈的重要中间体.溴菌腈的市场需求量虽然不如尼龙66巨大,但也是重要的化工产品之一.然而,在丙烯腈的催化二聚领域,至今未有系统性的综述报导.对丙烯腈催化二聚反应的研究进展进行整理与系统性地阐述,按照催化剂种类,主要分为钌催化、其它金属催化以及膦催化二聚等三部分,供国内相关领域技术人员参考.     

Insights into Functional‐Group‐Tolerant Polymerization Catalysis with Phosphine–Sulfonamide Palladium(II) Complexes

Two series of cationic palladium(II) methyl complexes {[(2‐MeOC₆H₄)₂PC₆H₄SO₂NHC₆H₃(2,6‐R¹,R²)]PdMe}₂[A]₂ ( ^X 1⁺‐A : R¹=R²=H: ^H 1⁺‐A ; R¹=R²=CH(CH₃)₂: ^DIPP 1⁺‐A ; R¹=H, R²=CF₃: ^CF3 1⁺‐A ; A =BF₄ or SbF₆) and neutral palladium(II) methyl complexes {[(2‐MeOC₆H₄)₂PC₆H₄SO₂NC₆H₃(2,6‐R¹,R²)]PdMe(L)} ( ^X 1‐acetone : L=acetone; ^X 1‐dmso : L=dimethyl sulfoxide; ^X 1‐pyr : L=pyridine) chelated by a phosphine–sulfonamide were synthesized and fully characterized. Stoichiometric insertion of methyl acrylate (MA) into all complexes revealed that a 2,1 regiochemistry dominates in the first insertion of MA. Subsequently, for the cationic complexes ^X 1⁺‐A , β‐H elimination from the 2,1‐insertion product ^X 2⁺‐A_MA‐2,1 is overwhelmingly favored over a second MA insertion to yield two major products ^X 4⁺‐A_MA‐1,2 and ^X 5⁺‐A_MA . By contrast, for the weakly coordinated neutral complexes ^X 1‐acetone and ^X 1‐dmso , a second MA insertion of the 2,1‐insertion product ^X 2_MA‐2,1 is faster than β‐H elimination and gives ^X 3_MA as major products. For the strongly coordinated neutral complexes ^X 1‐pyr , no second MA insertion and no β‐H elimination (except for ^DIPP 2‐pyr_MA‐2,1 ) were observed for the 2,1‐insertion product ^X 2‐pyr_MA‐2,1 . The cationic complexes ^X 1⁺‐A exhibited high catalytic activities for ethylene dimerization, affording butenes (C₄) with a high selectivity of up to 97.7 % (1‐butene: 99.3 %). Differences in activities and selectivities suggest that the phosphine–sulfonamide ligands remain coordinated to the metal center in a bidentate fashion in the catalytically active species. By comparison, the neutral complexes ^X 1‐acetone , ^X 1‐dmso , and ^X 1‐pyr showed very low activity towards ethylene to give traces of oligomers. DFT analyses taking into account the two possible coordination modes (O or N) of the sulfonamide ligand for the cationic system ^CF3 1⁺ suggested that the experimentally observed high activity in ethylene dimerization is the result of a facile first ethylene insertion into the O‐coordinated PdMe isomer and a subsequent favored β‐H elimination from the N‐coordinated isomer formed by isomerization of the insertion product. Steric hindrance by the N‐aryl substituent in the neutral systems ^CF3 1 and ^H 1 appears to contribute significantly to a higher barrier of insertion, which accounts for the experimentally observed low activity towards ethylene oligomerization.