Oxidative cleavage of vicinal diols: IBX can do what Dess-Martin periodinane (DMP) can

The fact that IBX exhibits reactivity akin to DMP is demonstrated from the results observed with strained and sterically hindered syn 1,2-diols, which undergo oxidative cleavage via a 12-I-5 spirobicyclic periodinane. The use of TFA, a protonating solvent, promotes the formation of the 12-I-5 intermediate for 1,2-diols of all types (sec,sec, sec,tert and tert,tert), leading to efficient oxidative fragmentation.     

Polymer supported ionic liquid phases (SILPs) versus ionic liquids (ILs): how much do they look alike

The fluorescence of pyrene has been used for the first time to measure the static dielectric constant of a series of supported ionic liquids phases (SILPs) based on polymeric polystyrene networks.     

Comparison of formation of reactive conformers (NACs) for the Claisen rearrangement of chorismate to prephenate in water and in the E. coli mutase: the efficiency of the enzyme catalysis

The Claisen rearrangements of chorismate (CHOR) in water and at the active site of E. coli chorismate mutase (EcCM) have been compared. From a total of 33 ns molecular dynamics simulation of chorismate in water solvent, seven diaxial conformers I-VII were identified. Most of the time (approximately 99%), the side chain carboxylate of the chorismate is positioned away from the ring due to the electrostatic repulsion from the carboxylate in the ring. Proximity of the two carboxylates, as seen in conformer I, is a requirement for the formation of a near attack conformer (NAC) that can proceed to the transition state (TS). In the EcCM.CHOR complex, the two carboxylates of CHOR are tightly held by Arg28 of one subunit and Arg11* of the other subunit, resulting in the side chain C16 being positioned adjacent to C5 with their motions restricted by van der Waals contacts with methyl groups of Val35 and Ile81. With the definition of NAC as the C5...C16 distance < or =3.7 A and the attack angle < or =30 degrees, it was estimated from our MD trajectories that the free energy of NAC formation is approximately 8.4 kcal/mol above the total ground state in water, whereas in the enzyme it is only 0.6 kcal/mol above the average of the Michaelis complex EcCM.CHOR. The experimentally measured difference in the activation free energies of the water and enzymatic reactions (Delta Delta G(++)) is 9 kcal/mol. It follows that the efficiency of formation of NAC (7.8 kcal/mol) at the active site provides approximately 90% of the kinetic advantage of the enzymatic reaction as compared to the water reaction. Comparison of the EcCM.TSA (transition state analogue) and EcCM.NAC simulations suggests that the experimentally measured 100 fold tighter binding of TSA compared to CHOR does not originate from the difference between NAC and the TS binding affinities, but might be due to the free energy cost to bring the two carboxylates of CHOR together to interact with Arg28 and Arg11* at the active site. The two carboxylates of TSA are fixed by a bicyclic structure. The remaining approximately 10% of Delta Delta G(++) may be attributed to a preferential interaction of Lys39-NH(3)(+) with O13 ether oxygen in the TS.     

Photocrosslinkable resins with benzylideneacetophenone (chalcone) structure in the repeat units

Syntheses, characterization and photoreactions of six diglycidyl ethers obtained by condensing 2,2′, 2′,4, and 4,4′-dihydroxychalcones with epichlorohydrin in two different molar proportions have been reported. It has been observed that the well known cyclodimerization and trans-cis isomerization of the chalcone moiety under photoirradiation could be significantly affected to the extent of completely supressing the isomerization in 2,2′, and 2′,4-chalconic resins by steric factors. Increase in molecular weight made the photodimerization difficult in the 2,2′, and 2′,4-chalocnic resins whereas the 4,4′-chalconic resin remained unaffected. It has been suggested that the chalconic resin could be used in photomechanical processes mixed with aryldiazonium fluoroborates. These additives are known to decompose under photoirradiation yielding BF₃ which is a crosslinking agent for the epoxide group.     

Double dehydrogenation of carbocyclic β-dicarbonyl compounds:Koser's reagent can do what iodine(V) reagents can

Koser's reagent is found to be effective in the oxidative double dehydrogenation of various carbocyclic β-dicarbonyl compounds,which constitutes the first example on dehydrogenation reactivity of hypervalent iodine(Ⅲ) reagents for carbocyclic carbonyl compounds. DFT calculations reveal that the rate-determining step is the electrophilic addition of PhI+OH onto enolate of monodehydrogenated product.     

Pt(IV) analogs of oxaliplatin that do not follow the expected correlation between electrochemical reduction potential and rate of reduction by ascorbate

In contrast to the Pt(IV) derivatives of cisplatin, Pt(IV) derivatives of oxaliplatin do not show the expected correlation between the electrochemical reduction potentials and rates of reduction by ascorbate. This is probably due to the lower ability of the amine and carboxylato ligands to form a bridge with the reducing agents to facilitate electron transfer.     

卓酚酮基础的查尔酮衍生物(E)-5-溴-3-(3-芳基丙烯酰基)卓酚酮的合成(英文)

3-乙酰基-5-溴-卓酚酮与不同取代的苯甲醛类化合物通过Claisen-Schmidt缩合反应合成一系列结构新颖的卓酚酮取代的查尔酮类衍生物。该方法使用5%KOH溶液作为催化剂,50%甲醇水溶液作为溶剂,具有操作简便、反应条件温和、收率高等优点。所合成标题化合物的结构经IR、1H NMR、MS和元素分析得以证实。     

4-(N-咔唑基)查尔酮的合成及其光学性质

4-N-咔唑基苯甲醛和苯乙酮经aldol反应合成了一种新型含咔唑基查尔酮——4-(N-咔唑基)查尔酮(2),其结构经1H NMR,IR,MS和元素分析表征。运用UV-Vis和单光子荧光光谱研究了2的光学性质。结果表明:2的λmax位于360 nm和320 nm附近;2在CH2Cl2和DMF中具有良好的荧光发射能力,λem位于520 nm。     

Breaking up is easy to do (if you're a DNA enzyme that cleaves RNA)

In this issue of Chemistry & Biology, Cruz et al. use in vitro selection to select deoxyribozymes that collectively cleave almost any RNA dinucleotide junction. More remarkable is the finding that the new enzymes are related to the 8-17 deoxyribozyme that cleaves AG dinucleotide junctions.     

Spectral,catalytic, and antifungal studies of ruthenium(II) chalcone complexes

Reactions of [RuHCl(CO)(B)(EPh₃)₂] (B = EPh₃ or Py; E = P or As) and chalcones in benzene with equal molar ratio led to the formation of new complexes of the type [RuCl(CO)(EPh₃)(B)(L^1?4)] (B = PPh₃, AsPh₃ or Py; E = P or As; L = chalcone). The new complexes have been characterized by analytical and spectroscopic (IR-, electronic, ¹H-, ³¹P-, and ¹³C-NMR) data. Based on these data, an octahedral structure has been assigned for all the complexes. The chalcones are monobasic bidentate (O,O) donors and coordinate to ruthenium via phenolic and carbonyl oxygen. The new complexes exhibit efficient catalytic activity for the transfer hydrogenation of carbonyl compounds. Antifungal properties of the ligands and their complexes have been examined and compared with standard Bavistin.     

5-Methylcytosine is Oxidized to the Natural Metabolites of TET Enzymes by a Biomimetic Iron(IV)-Oxo Complex

Ten-eleven-translocation (TET) methyl cytosine dioxygenases play a key role in epigenetics by oxidizing the epigenetic marker 5-methyl cytosine (5mC) to 5-hydroxymethyl cytosine (5hmC), 5-formyl cytosine (5fC), and 5-carboxy cytosine (5cC). Although much of the metabolism of 5mC has been studied closely, certain aspects—such as discrepancies among the observed catalytic activity of TET enzymes and calculated bond dissociation energies of the different cytosine substrates—remain elusive. Here, it is reported that the DNA base 5mC is oxidized to 5hmC, 5fC, and 5cC by a biomimetic iron(IV)-oxo complex, reminiscent of the activity of TET enzymes. Studies show that 5hmC is preferentially turned over compared with 5mC and 5fC and that this is in line with the calculated bond dissociation energies. The optimized syntheses of d₃-5mC and d₂-5hmC are also reported and in the reaction with the biomimetic iron(IV)-oxo complex these deuterated substrates showed large kinetic isotope effects, confirming the hydrogen abstraction as the rate-limiting step. Taken together, these results shed light on the intrinsic reactivity of the C−H bonds of epigenetic markers and the contribution of the second coordination sphere in TET enzymes.     

Synthesis, spectral characterization and catalytic studies of new ruthenium(II) chalcone thiosemicarbazone complexes

A series of new hexa-coordinated ruthenium(II) complexes of the type [Ru(CO)(EPh₃)(B)(L)] (E = P or As; B = PPh₃, AsPh₃ or Py; L = chalcone thiosemicarbazone) have been prepared by reacting [RuHCl(CO)(EPh₃)₂(B)] (E = P or As; B = PPh₃, AsPh₃ or Py) with chalcone thiosemicarbazones in benzene under reflux. The new complexes have been characterized by analytical and spectroscopic (IR, UV-vis, ¹H, ³¹P and ¹³C NMR) methods. On the basis of data obtained, an octahedral structure was assigned for all of the complexes. The chalcone thiosemicarbazones behave as dianionic tridentate O, N, S donors and coordinate to ruthenium via the phenolic oxygen of chalcone, the imine nitrogen of thiosemicarbazone and thienol sulfur. The new complexes exhibit catalytic activity for the oxidation of primary and secondary alcohols to their corresponding aldehydes and ketones and they were also found to be efficient catalysts for the transfer hydrogenation of carbonyl compounds.     

Biodegradable Polycaprolactone (PCL) Nanosphere Encapsulating Superoxide Dismutase and Catalase Enzymes

Biodegradable polycaprolactone (PCL) nanosphere encapsulating superoxide dismutase (SOD) and catalase (CAT) were successfully synthesized using double emulsion (w/o/w) solvent evaporation technique. Characterization of the nanosphere using dynamic light scattering, field emission scanning electron microscope, and Fourier transform infrared spectroscopy revealed a spherical-shaped nanosphere in a size range of 812?±?64 nm with moderate protein encapsulation efficiency of 55.42?±?3.7 % and high in vitro protein release. Human skin HaCat cells were used for analyzing antioxidative properties of SOD- and CAT-encapsulated PCL nanospheres. Oxidative stress condition in HaCat cells was optimized with exposure to hydrogen peroxide (H₂O₂; 1 mM) as external stress factor and verified through reactive oxygen species (ROS) analysis using H₂DCFDA dye. PCL nanosphere encapsulating SOD and CAT together indicated better antioxidative defense against H₂O₂-induced oxidative stress in human skin HaCat cells in comparison to PCL encapsulating either SOD or CAT alone as well as against direct supplement of SOD and CAT protein solution. Increase in HaCat cells SOD and CAT activities after treatment hints toward uptake of PCL nanosphere into the human skin HaCat cells. The result signifies the role of PCL-encapsulating SOD and CAT nanosphere in alleviating oxidative stress.     

Synthesis of alpha,beta-unsaturated ketones as chalcone analogues via a S(RN)1 mechanism

An electron-transfer chain reaction between 2-nitropropane anion and alpha-bromoketones derived from nitrobenzene and nitrothiophene was demonstrated by mechanistic study and a specific convenient synthetic protocol. Thus, 2-bromo-1-(5-nitrothiophen-2-yl)ethanone or 2-bromo-1-(4-nitrophenyl)ethanone were reacted with several cyclic nitronate anions to form alpha,beta-unsaturated ketones via a S(RN)1 mechanism. This new method can be used to synthesize a wide variety of chalcone analogues.     

Why copper is intrinsically more selective than silver in alkene epoxidation: ethylene oxidation on Cu(111) versus Ag(111)

The heterogeneously catalyzed epoxidation of alkenes is experimentally challenging, theoretically interesting, and technologically important. Although large-scale ethylene epoxidation is universally carried out with Ag catalysts, recent laboratory studies on single crystal surfaces show that Cu is intrinsically much more selective than Ag in the epoxidation of a variety of terminal alkenes. The reasons for this striking difference between Ag and Cu have been investigated by means of density functional theory. It is found that the fundamental cause is the inversion in the ordering of activation barriers for the competing pathways to epoxide formation versus acetaldehyde formation (the latter being the first step on the route to combustion). On Cu, epoxide formation is less activated than aldehyde formation; the opposite is true on Ag. This behavior is associated with a late transition state to epoxidation on Cu (i.e., product-like) compared to an early (reactant-like) transition state to epoxidation on Ag.     

微生物酶合成手性C_(3~-)和C_(4~-)单元

具有C_(3~-)和C_(4~-)单元并含有其它官能团的化合物，是合成多种有用化合 物很好的原材料。尤其是光学活性C_(3~-)和C_(4~-)有机合成单元对制备药物和精 细化学产品是非常重要的中间体。应用具有立体专一性酶的微生物转化过程，可不 对称合成光学活性物质。本文阐述了用微生物酶合成手性C_(3~-)和C_(4~-)单元的 最新进展。     

d(TG(n)T) DNA sequences do not necessarily form tetramolecular G-quadruplexes

Contrasting with the common belief that d(TG(n)T) DNA sequences would form tetramolecular G-quadruplex assemblies, mass spectrometry reveals that these sequences tend to fold into G-quadruplex trimers, dimers, and eventually monomers as the G-tract length increases. The final structure also depends on the ionic strength.     

Calixarene-Based Copper(I) Complexes as Models for Monocopper Sites in Enzymes

A cavity that acts as a molecular funnel is formed from calix[6]arene 1 and [Cu^I(NCCH₃)₄]PF₆ [Eq. (a)]. An exchange of the well-protected acetonitrile ligand for other nitriles RCN is only possible with small R groups. The protection of the copper ions precludes oxidative dimerization; thus, the complexes mimic the mononuclear site of copper enzymes.