A HIGHLY ACTIVE NEODYMIUM CHLORIDE ISOPROPANOL COMPLEX/MODIFIED METHYLALUMINOXANE CATALYST FOR PREPARING POLYISOPRENE WITH HIGH CIS-1,4 STEREOSPECIFICITY AND NARROW MOLECULAR WEIGHT DISTRIBUTION

Neodymium chloride isopropanol complex （NdCl3.3＇prOH） activated by modified methylaluminoxane （MMAO） was examined in isoprene polymerization in hexane, with regards to Nd compounds, aluminum （Al） compounds, [Al]/[Nd] ratio, polymerization temperature and time. NdCl3＇3iprOH exhibited high activity producing polymers feasting high cis-l,4 stereospecificity （〉 96%）, very high molecular weight （Mn 〉 1.0 × 10^6） and fairly narrow molecular weight distribution （MWD, Mw/Mn 〈 2.0） simultaneously. In comparison, neodymium isopropoxide also showed high activity providing polymers with narrow MWD （Mw/Mn = 2.07）, but somewhat low cis-1,4 content （ca. 92%）, while neodymium chloride had no activity under present polymerization conditions. The Al compounds affected the polymer yield in the order of Al（i-Bu）3 〉 MMAO 〉 Al（i-Bu）2H. MMAO as cocatalyst afforded polyisoprene with high Mn over 1.0 × 10^6, whereas as stronger chain transfer agent than MMAO, AI（i-Bu）3 and AI（i-Bu）EH yielded polymers with low Mn （1.0 × 10^5-8.0 × 10^5）. NdCl3·3＇PrOH/MMAO catalyst showed a fairly good catalytic activity even at relatively low [Al]/[Nd] ratio of 30, and the produced polymer remained high cis-1,4 content of 95.8% along with high Mn over 1.0× 10^6 even at elevated temperatures up to 70℃. The polymerization rate is of the first order with respect to the concentration of isoprene. The mechanism of active species formation was discussed preliminarily.

Stereoselective Syntheses of Some β-D-1-Thioglycopyranosides and Bis(β-D-glycopyranosyl) Sulfides by Phase Transition Catalysis

The present paper covers the stereoselective syntheses of four 1 thiogalactopyranosides 1—4 and three thio disaccharides 5—7 through the PTC methods and their structures confirmed by means of elementary analysis, IR and NMR spectral data.

Higher Order Cuprate Species( Ⅱ )——Stereospecific Carbocupration of Terminal Alkynes and Its Synthetic Utility for ( ± )-Ipsenol and (E)-β-Farnesene

The stereospecific carbocupration of terminal alkynes via higher order cuprates to give 1,1'-disubstituted olefins with 99% configuration purity is described. Its synthetic utility as a general method for the preparation of substituted olefins is further illustrated by the direct synthesis of (±)-ipsenol and (E)-β-farnesene with highly geometric purity of the carbon-carbon double bond.

Stereocontrolled Synthesis of Optically Active Aggregation Pheromone of Lesser Grain Borer

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Molecular Basis for Stereospecific Hydrolysis of Ethyl Mandelate by Thermophilic Esterase

The stereospecific hydrolysis of mandelate can be effectively catalyzed by hyperthermophilic acylpeptide esterase APE 1547（Aeropyrum pernix esterase 1547）. APE 1547 used in this reaction showed a remarkable stereodi-scrimination in favour of R-mandelic acid（99% e.e.） with an enantiomeric ratio E〉200. The results of computer simulation are consistent with the experimental results. It can be inferred that the R-substrate adopted a binding mode productive of the reaction due to the formation of the hydrogen bond at the active site of APE 1547.

Synthesis and Steric Structure of 1, 5-Benzothiazepine-Phenyl-β-Lactams

1, 5-Benzothiazepines 1 react with phenylacetyl chloride to give the title compounds.The structures of these new compounds were confirmed by elemental analysis, ~1H NMR, ^(13)C NMRand MS spectroscopy, and their configuration (the mutual positions of the substituents relative to theβ-lactam ring) and conformation of the compounds were determined by X-ray crystal analysis.

Synthesis and Stereostructure of New β-Lactam Derivatives of1,5-Benzothiazepines

Reaction of 1,5-benzothiazepine with N-protected glycine gives new a-amino-β-lactamderivatives of 1.5-benzothiazepine. The configuration and conformation of the products wereconfirmed by x-ray diffraction. The result further reveals that the reaction of 1.5-benzothiazepineswith derivatives of carboxylic acid stereospecific.

NATURAL PRODUCTS SYNTHESIS BY RETRO-DIELS-ALDER REACTION Ⅷ~1 A CONCISE STEREOSPECIFIC FORMAL SYNTHESIS OF TETRAHYDROCHLOROVULONE

A potent antineoplastic tetrahydrochlorovulone 3 was synthesized with retro-Diels Alder reaction as a key step.

Synthesis of 1,2:4,5-di-O-Isopropylidene-3-C-Cyano-β-D-ribo-hex-2-ulo Pyranose by Phase Transition Catalysis

Introduction The cyano branched carbohydrates are useful, versatil intermediaries for the synthesis of branched sugars having biological activity in nature. Owing to the good yield, mild reaction conditions and high stereospecificity, the phase transition catalysis(PTC)method has been widely used to the syntheses of C-

Impact of Leucine 278 Residue on Fatty Acid Length Specificity of <i>Candida antarctica</i>Lipase B

Structural analysis of Candida antarctica lipase B (CALB) indicates that side chain of leucine at 278 site lies above the entrance of the catalytic pocket, which prognosticates its potential role on substrate specificity of the enzyme. To verify this presumption, shortened side chain of glycine or proline was rational designed and mutants were constructed by site-directed mutagenesis method. The colorimetric assay using p-nitrophenyl esters of fatty acids with various chain-lengths was used to study the substrate preference of lipases. Results indicated that L278G or L278P mutations both induced the drift of substrate specificity of CALB from p-nitrophenyl caprylate (pNP-C8) to longer carbon chain length of p-nitrophenyl caprate (pNP-C10). Meanwhile, Vmax value of two mutants to pNP-C10 was both higher than that of wild-type. Docking results also indicated that shortened side chain of glycine or proline residues substitution at this site could get rid of the space block present above the catalytic pocket, and made longer chain substrate (pNP-C10) enter into the catalytic pocket easier. The modulation of specificity observed allowed for building substrate binding model and opened new possibilities for designing ligand specific lipases.

Polymerization of 1,3-Butadiene with Catalysts Based on Cobalt Dichloride Complexes with Aminophosphines: Switching the Regioselectivity by Varying the MAO/Co Molar Ratio

Several novel cobalt dichloride complexes with amino-phosphine bidentate ligands were synthesized and characterized.For some of them single crystals were obtained and their molecular structure was determined by X-ray diffraction method.All the complexes were then used in combination with MAO for the polymerization of 1,3-butadiene,obtaining polymers with different structures(i.e.,predominantly 1,2 or cis-1,4)mainly depending on the type of ligand and on the MAO/Co molar ratio.The behavior of these novel catalysts was compared with that exhibited,in the polymerization of the same monomer,by the systems CoCl_(2)(PR3)2-MAO and CoCl_(2)(PRPh2)2-MAO(R=alkyl or cycloalkyl group),and by the systems based on cobalt dichloride complexes with various bi-and tridentate ligands(e.g.,diphosphines,bis-imines,pyridyl-imines,bis-iminopyridines).The comparison between the different systems allowed us to make some clarity about the actual and effective role played by the various types of ligands in the polymerization of conjugated dienes with catalytic systems CoCl_(2)(L)-MAO,in which L=mono-,bi-,or tri-dentate ligand.

Pd-Catalyzed Stereospecific Glycosyl Cross-Coupling of Reversed Anomeric Stannanes for Modular Synthesis of Nonclassical C‑Glycosides

Nonclassical C-glycosides,distinguished by their unique glycosidic bond connection mode,represent a promising avenue for the development of carbohydrate-based drugs.However,the accessibility of nonclassical C-glycosides hinders broader investigations into their structural features and modes of action.Herein,we present the first example of Pd-catalyzed stereospecific glycosylation of nonclassical anomeric stannanes with aryl or vinyl halides.This method furnishes desired nonclassical aryl and vinyl C-glycosides in good to excellent yields,while allowing for exclusive control of nonclassical anomeric configuration.Of significant note is the demonstration of the generality and practicality of this nonclassical C-glycosylation approach across more than 50 examples,encompassing various protected and unprotected saccharides,deoxy sugars,oligopeptides,and complex molecules.Furthermore,biological evaluation indicates that nonclassical C-glycosylation modifications of drug molecules can positively impact their biological activity.Additionally,extensive computational studies are conducted to elucidate the rationale behind differences in reaction reactivity,unveiling a transmetalation transition state containing silver(Ag)within a six-membered ring.Given its remarkable controllability,predictability,and consistently high chemical selectivity and stereospecificity regarding nonclassical anomeric carbon and Z/E configuration,the method outlined in this study offers a unique solution to the longstanding challenge of accessing nonclassical C-glycosides with exclusive stereocontrol.

Synthetic rubbers prepared by lanthanide coordination catalysts

China is rich in rare earth resources. Rare earth elements, also named lanthanides, are number 58 to number 81 elements in the elemental periodic table. They have unique electronic structures and may form various coordination compounds. In the early 1960s, researchers at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CIAC) found the catalytic activities of lanthanide compounds in stereospecific polymerization of conjugated dienes, and published the first paper on this topic in 1964. On the basis of this finding, CIAC launched extensive research activities on lanthanide compounds as diene polymerization catalysts, from a series of fundamental research to the efforts of industrializing the rare earth catalyzed cis-1,4-polybutatine rubber and cis-1,4-polyisoprene rubber. This review aims to summarize the progress in this field in the past half century.

PREPARATION OF POLYPROPYLENE/CLAY NANOCOMPOSITES BY in situ POLYMERIZATION WITH TiCl_4/MgCl_2/CLAY COMPOUND CATALYST

TiCl4/MgCl2/clay compound catalyst was prepared by chemical reaction. Exfoliated polypropylene （PP）/clay nanocomposites were synthesized by in situ polymerization with this compound catalyst. Effects of polymerization temperature, polymerization time, propylene pressure, solvent consumption and pre-treat time of catalyst on catalyst activity and catalytic stereospecificity were studied. Under optimal conditions, activity of the nano-compound catalyst is about 88.3 kg/（mol Ti-h）. Isotacticity of PP obtained in the nanocomposites is in the range of 89%-99%, and its melting temperature is about 159℃. The weight-average molecular weight of PP can reach 6.7 × 10^5 - 7.8 × 10^5, and the molecular weight distribution is between 7.7 and 7.9.

EHMO QUANTITATIVE VERIFICATION OF QUANTUM THEORY MECHANISM FOR ELECTROPHILIC ADDITIONS

Most of the electrophilic additions on carbon-carbon multiply bond can be summarized as: where Y are certain atoms on the fight of the periodic table; Z may be null, atom or group. Regardless of the rate law of Ad_E2 Ad_E3 or the more complex kinetics, the micromechanisms of transition state of electrophilic addition can be divided into two sorts, i.e. open ion and bridged ion mechanisms. The former one allows free rotation of

Stereospecific Synthesis of Drospirenone

A procedure for the stereospecific synthesis of drospirenone has been developed. The key steps included the stereospecific reduction of the C7-tertiary alcohol with ZnI2/Et3SiH, a novel mild and stereospecific tertiary alcohol reduction system, and the tandem oxidation/cyclopropanation reactions.

First Synthesis of a Series of New Natural Glucosides

Alkyl glucoside 1 and aryl glycosides 2-4 were highly stereospecifically synthesized over 4-6 steps from commercially available starting materials. The coupling reaction of the acetobromo-α-D-glucose with the unprotective dihydroxy aglycon in the presence of silver oxide, or with aromatic aglycon in the presence of sodium hydroxide produced the key intermediate. Only β-configuration glycosides were formed in this procedure. The synthesis of all these glycosides was reported for the first time.

A Scalable Synthesis of 6,19-Dihydroxyandrostenedione

Starting from the commercially available 19-hydroxyandrostenedione, a practical protocol for the preparation of 6,19-dihydroxyandrostenedione is reported. This compound is a key intermediate for the synthesis of cyclocitrinols. With the stereospecific epoxidation and following isomerization to allylic alcohol as key steps, a six-step procedure provided desired product in high yield. The sequence is easy to scale-up without the need of laborious chromatog- raphy.

Deammoniative Condensation of Primary Allylic Amines with Nonallylic Amines

An unprecedented deammoniative condensation reaction of primary allylic amines with nonallylic amines has been developed through C-N bond cleavage.In the presence of 5 mol%palladium diacetate,10 mol%1,4-bis(diphenylphosphino)butane(dppb),and 5 mol%p-toluenesulfonic acid(TsOH),a range of α-unbranched primary allylic amines smoothly underwent deammoniative condensation with nonallylic amines in an α-selective fashion to give structurally diverse secondary and tertiary amines in good to excellent yields and E selectivity.Replacing dppb with racemic 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl(BINAP)permitted the deammoniative condensation of enantioenriched α-chiral primary allylic amines with nonallylic amines to proceed with complete retention of configuration.Electrospray ionization(ESI)mass spectrometric analysis of the reaction mixture permitted the identification of some π-allylpalladium intermediates,and plausible mechanisms have been proposed to account for the regioselectivity and stereospecificity of the deammoniative condensation reaction.