Oxidation of ethylbenzene to styrene oxide in the presence of cellulose‐supported Pd magnetic nanoparticles

Palladium and Fe₃O₄ nanoparticles were deposited on N‐(2‐aminoethyl)acetamide‐functionalized cellulose for use in a catalytic reaction. The catalyst was characterized using Fourier transform infrared spectroscopy, thermogravimetric analysis, X‐ray diffraction, energy‐dispersive X‐ray analysis and transmission electron microscopy, and applied in the oxidation reaction of ethylbenzene at 100 °C using H₂O₂. Styrene oxide was obtained as the sole product of the oxidation reaction during 24 h. This reaction has some advantages such as one‐pot transformation of ethylbenzene to styrene oxide, high yield, excellent selectivity and magnetically recoverable catalyst. Also, the recovered catalyst could be used in the oxidation reaction four times without decrease in yield. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis,characterization and catalytic performance of core‐shell structure magnetic Fe3O4/P(GMA‐EGDMA)‐NH2/HPG‐COOH‐Pd catalyst

A strategy has been developed for the synthesis, characterization and catalysis of magnetic Fe₃O₄/P(GMA‐EGDMA)‐NH₂/HPG‐COOH‐Pd core‐shell structure supported catalyst. The P(GMA‐EGDMA) polymer layer was coated on the surface of hollow magnetic Fe₃O₄ microspheres through the effect of KH570. The core‐shell magnetic Fe₃O₄/P(GMA‐EGDMA) modified by ‐NH₂ could be grafted with HPG. Then, the hyperbranched glycidyl (HPG) with terminal ‐OH were modified by ‐COOH and adsorbed Pd nanoparticles. The hyperbranched polymer layer not only protected the Fe₃O₄ magnetic core from acid–base substrate corrosion, but also provided a number of functional groups as binding sites for Pd nanoparticles. The prepared catalyst was characterized by UV–vis, TEM, SEM, FTIR, TGA, ICP‐OES, BET, XRD, DLS and VSM. The catalytic tests showed that the magnetic Fe₃O₄/P(GMA‐EGDMA)‐NH₂/HPG‐COOH‐Pd catalyst had excellent catalytic performance and retained 86% catalytic efficiency after 8 consecutive cycles.     

Covalent bonding of homochiral metal‐organic framework in capillaries for stereoisomer separation by capillary electrochromatography

In this work, a [Cu(mal)(bpy)]?H₂O (mal, l ‐(?)‐malic acid; bpy, 4,4′‐bipyridyl) homochiral metal‐organic frameworks (MOFs) was synthesized and used for modifying the inner walls of capillary columns by utilizing amido bonds to form covalent links between the MOFs particles and capillary inner wall. The synthesized [Cu(mal)(bpy)]?H₂O and MOFs‐modified capillary column were characterized by X‐ray diffraction, thermogravimetric analysis, particle size distribution analysis, nitrogen absorption characterization, FTIR spectroscopy, SEM, and energy‐dispersive X‐ray spectroscopy (EDX). The MOFs‐modified capillary column was used for the stereoisomer separation of some drugs. The LODs and LOQs of six analytes were 0.1 and 0.25 μg/mL, respectively. The linear range was 0.25–250 μg/mL for ephedrine, 0.25–250 μg/mL for pseudoephedrine, 0.25–180 μg/mL for d ‐penicillamine, 0.25–120 μg/mL for l ‐penicillamine, 0.25–180 μg/mL for d ‐phenylalanine, and 0.25–160 μg/mL for l ‐phenylalanine, all with R² > 0.999. Finally, the MOFs‐modified capillary column was applied for the analysis of active ingredients in a real sample of the traditional Chinese medicine ephedra.     

Conformational analysis of the disaccharide methyl α‐d‐mannopyranosyl‐(1→3)‐2‐O‐acetyl‐β‐d‐mannopyranoside monohydrate

The crystal structure of methyl α‐d ‐mannopyranosyl‐(1→3)‐2‐O‐acetyl‐β‐d ‐mannopyranoside monohydrate, C₁₅H₂₆O₁₂·H₂O, ( II ), has been determined and the structural parameters for its constituent α‐d ‐mannopyranosyl residue compared with those for methyl α‐d ‐mannopyranoside. Mono‐O‐acetylation appears to promote the crystallization of ( II ), inferred from the difficulty in crystallizing methyl α‐d ‐mannopyranosyl‐(1→3)‐β‐d ‐mannopyranoside despite repeated attempts. The conformational properties of the O‐acetyl side chain in ( II ) are similar to those observed in recent studies of peracetylated mannose‐containing oligosaccharides, having a preferred geometry in which the C2—H2 bond eclipses the C=O bond of the acetyl group. The C2—O2 bond in ( II ) elongates by ～0.02 Å upon O‐acetylation. The phi (?) and psi (ψ) torsion angles that dictate the conformation of the internal O‐glycosidic linkage in ( II ) are similar to those determined recently in aqueous solution by NMR spectroscopy for unacetylated ( II ) using the statistical program MA′AT, with a greater disparity found for ψ (Δ = ～16°) than for ? (Δ = ～6°).     

Dual‐Functional Small Molecules for Generating an Efficient Cytochrome P450BM3 Peroxygenase

We report a unique strategy for the development of a H₂O₂‐dependent cytochrome P450BM3 system, which catalyzes the monooxygenation of non‐native substrates with the assistance of dual‐functional small molecules (DFSMs), such as N‐(ω‐imidazolyl fatty acyl)‐l ‐amino acids. The acyl amino acid group of DFSM is responsible for bounding to enzyme as an anchoring group, while the imidazolyl group plays the role of general acid–base catalyst in the activation of H₂O₂. This system affords the best peroxygenase activity for the epoxidation of styrene, sulfoxidation of thioanisole, and hydroxylation of ethylbenzene among those P450–H₂O₂ system previously reported. This work provides the first example of the activation of the normally H₂O₂‐inert P450s through the introduction of an exogenous small molecule. This approach improves the potential use of P450s in organic synthesis as it avoids the expensive consumption of the reduced nicotinamide cofactor NAD(P)H and its dependent electron transport system. This introduces a promising approach for exploiting enzyme activity and function based on direct chemical intervention in the catalytic process.     

Structural properties of D‐mannopyranosyl rings containing O‐acetyl side‐chains

The crystal structures of 1,2,3,4,6‐penta‐O‐acetyl‐α‐d ‐mannopyranose, C₁₆H₂₂O₁₁, and 2,3,4,6‐tetra‐O‐acetyl‐α‐d ‐mannopyranosyl‐(1→2)‐3,4,6‐tri‐O‐acetyl‐α‐d ‐mannopyranosyl‐(1→3)‐1,2,4,6‐tetra‐O‐acetyl‐α‐d ‐mannopyranose, C₄₀H₅₄O₂₇, were determined and compared to those of methyl 2,3,4,6‐tetra‐O‐acetyl‐α‐d ‐mannopyranoside, methyl α‐d ‐mannopyranoside and methyl α‐d ‐mannopyranosyl‐(1→2)‐α‐d ‐mannopyranoside to evaluate the effects of O‐acetylation on bond lengths, bond angles and torsion angles. In general, O‐acetylation exerts little effect on the exo‐ and endocyclic C—C and endocyclic C—O bond lengths, but the exocyclic C—O bonds involved in O‐acetylation are lengthened by ～0.02 Å. The conformation of the O‐acetyl side‐chains is highly conserved, with the carbonyl O atom either eclipsing the H atom attached to a 2°‐alcoholic C atom or bisecting the H—C—H bond angle of a 1°‐alcoholic C atom. Of the two C—O bonds that determine O‐acetyl side‐chain conformation, that involving the alcoholic C atom exhibits greater rotational variability than that involving the carbonyl C atom. These findings are in good agreement with recent solution NMR studies of O‐acetyl side‐chain conformations in saccharides. Experimental evidence was also obtained to confirm density functional theory (DFT) predictions of C—O and O—H bond‐length behavior in a C—O—H fragment involved in hydrogen bonding.     

Cationic ring‐opening polymerization of 1,6‐anhydro‐2,3,4‐tri‐O‐allyl‐β‐D‐glucopyranose as a convenient synthesis of dextran

For the convenient synthesis of (1→6)‐α‐D ‐glucopyranan, i. e., dextran ( 4 ), ring‐opening polymerization of 1,6‐anhydro‐2,3,4‐tri‐O‐allyl‐β‐D ‐glucopyranose ( 1 ) has been carried out using BF₃·OEt₂. With a ratio of [BF₃·OEt₂]/[ 1 ] = 0.5 at 0 °C for 140 h, the yield and M_n of the obtained polymer are 84.0% and 21 700, respectively. The polymer consists of (1→6)‐α‐linked 2,3,4‐tri‐O‐allyl‐D ‐glucopyranose ( 2 ) which is similar to the results for the cationic ring‐opening polymerization of 1,6‐anhydro‐2,3,4‐tri‐O‐methyl‐β‐D ‐glucopyranose and 1,6‐anhydro‐2,3,4‐tri‐O‐ethyl‐β‐D ‐glucopyranose. Polymer 2 was isomerized using tris(triphenylphosphine)‐chlororhodium as the catalyst in toluene/ethanol/water to yield polymeric 2,3,4‐tri‐O‐propenyl‐(1→6)‐α‐D ‐glucopyranan ( 3 ). Deprotection of the propenyl ether linkage of 3 was then performed using hydrochloric acid in acetone to give 4 .     

Tuning the Redox Properties of a Nonheme Iron(III)–Peroxo Complex Binding Redox‐Inactive Zinc Ions by Water Molecules

Redox‐inactive metal ions play important roles in tuning chemical properties of metal–oxygen intermediates. Herein we report the effect of water molecules on the redox properties of a nonheme iron(III)–peroxo complex binding redox‐inactive metal ions. The coordination of two water molecules to a Zn²⁺ ion in (TMC)Fe^III‐(O₂)‐Zn(CF₃SO₃)₂ ( 1 ‐Zn²⁺) decreases the Lewis acidity of the Zn²⁺ ion, resulting in the decrease of the one‐electron oxidation and reduction potentials of 1 ‐Zn²⁺. This further changes the reactivities of 1 ‐Zn²⁺ in oxidation and reduction reactions; no reaction occurred upon addition of an oxidant (e.g., cerium(IV) ammonium nitrate (CAN)) to 1 ‐Zn²⁺, whereas 1 ‐Zn²⁺ coordinating two water molecules, (TMC)Fe^III‐(O₂)‐Zn(CF₃SO₃)₂‐(OH₂)₂ [ 1 ‐Zn²⁺‐(OH₂)₂], releases the O₂ unit in the oxidation reaction. In the reduction reactions, 1 ‐Zn²⁺ was converted to its corresponding iron(IV)–oxo species upon addition of a reductant (e.g., a ferrocene derivative), whereas such a reaction occurred at a much slower rate in the case of 1 ‐Zn²⁺‐(OH₂)₂. The present results provide the first biomimetic example showing that water molecules at the active sites of metalloenzymes may participate in tuning the redox properties of metal–oxygen intermediates.     

Methyl 4‐O‐β‐d‐galactopyranosyl α‐d‐mannopyranoside methanol 0.375‐solvate

Methyl β‐d ‐galactopyranosyl‐(1→4)‐α‐d ‐mannopyranoside methanol 0.375‐solvate, C₁₃H₂₄O₁₁·0.375CH₃OH, (I), was crystallized from a methanol–ethanol solvent system in a glycosidic linkage conformation, with ϕ′ (O5_Gal—C1_Gal—O1_Gal—C4_Man) = −68.2 (3)° and ψ′ (C1_Gal—O1_Gal—C4_Man—C5_Man) = −123.9 (2)°, where the ring is defined by atoms O5/C1–C5 (monosaccharide numbering); C1 denotes the anomeric C atom and C6 the exocyclic hydroxymethyl C atom in the βGalp and αManp residues, respectively. The linkage conformation in (I) differs from that in crystalline methyl α‐lactoside [methyl β‐d ‐galactopyranosyl‐(1→4)‐α‐d ‐glucopyranoside], (II) [Pan, Noll & Serianni (2005). Acta Cryst. C 61 , o674–o677], where ϕ′ is −93.6° and ψ′ is −144.8°. An intermolecular hydrogen bond exists between O3_Man and O5_Gal in (I), similar to that between O3_Glc and O5_Gal in (II). The structures of (I) and (II) are also compared with those of their constituent residues, viz. methyl α‐d ‐mannopyranoside, methyl α‐d ‐glucopyranoside and methyl β‐d ‐galactopyranoside, revealing significant differences in the Cremer–Pople puckering parameters, exocyclic hydroxymethyl group conformations and intermolecular hydrogen‐bonding patterns.     

Efficient Selective Oxidation of Aromatic Alkanes by Double Cobalt Active Sites over Oxygen Vacancy-rich Mesoporous Co3O4

The development of efficient catalyst for selective oxidation of hydrocarbon to functional compounds remains a challenge. Herein, mesoporous Co₃O₄ (mCo₃O₄-350) showed excellent catalytic activity for selective oxidation of aromatic-alkanes, especially for oxidation of ethylbenzene with a conversion of 42 % and selectivity of 90 % for acetophenone at 120 °C. Notably, mCo₃O₄ presented a unique catalytic path of direct oxidation of aromatic-alkanes to aromatic ketones rather than the conventional stepwise oxidation to alcohols and then to ketones. Density functional theory calculations revealed that oxygen vacancies in mCo₃O₄ activate around Co atoms, causing electronic state change from Co³⁺_(Oh)→Co²⁺_(Oh). Co²⁺_(Oh) has great attraction to ethylbenzene, and weak interaction with O₂, which provide insufficient O₂ for gradual oxidation of phenylethanol to acetophenone. Combined with high energy barrier for forming phenylethanol, the direct oxidation path from ethylbenzene to acetophenone is kinetically favorable on mCo₃O₄, sharply contrasted to non-selective oxidation of ethylbenzene on commercial Co₃O₄.     

Ein neuer Zugang zu 2′‐O‐(2‐Methoxyethyl)ribonucleosiden ausgehend von D‐Glucose

A New Access to 2′‐ O ‐(2‐Methoxyethyl)ribonucleosides Starting from D ‐Glucose A new synthesis of 2′‐O‐(2‐methoxyethyl)ribonucleosides, building blocks for second‐generation antisense oligonucleotides, starting from D ‐glucose is presented. The key‐step is the transformation of 3‐O‐methoxyethylallofuranose to 2‐O‐(2‐methoxyethyl)ribose by NaIO₄ oxidation. Together with the 4′‐phenylbenzoyl protecting group, which results in crystalline intermediates, this synthesis provides an easy and cheap access to 2′‐O‐(2‐methoxyethyl)‐substituted ribonucleosides.     

Megastigmane Glycosides from Docynia indica and Their Anti‐inflammatory Activities

Using various chromatographic methods, three new megastigmane glycosides, docynicasides A – C ( 1  –  3 ) and ten known, (6S,9R)‐vomifoliol 9‐O‐β‐d ‐xylopyranosyl‐(1′′→6′)‐O‐β‐d ‐glucopyranoside ( 4 ), hyperin ( 5 ), quercitrin ( 6 ), quercetin 3‐α‐l ‐arabinofuranoside ( 7 ), naringenin 7‐O‐β‐d ‐glucopyranoside ( 8 ), phloridzin ( 9 ), phloretin 2′‐O‐β‐d ‐xylopyranosyl‐(1→6)‐β‐d ‐glucopyranoside ( 10 ), pinosylvin 3‐O‐β‐d ‐glucopyranoside ( 11 ), tormentic acid ( 12 ), and chlorogenic acid methyl ester ( 13 ) were isolated from the fruits of Docynia indica. Their chemical structures were elucidated by physical and chemical methods. All the isolated compounds were evaluated for the inhibitory activity on NO production in LPS‐stimulated BV2 cells. As the results, compounds 3  –  5 showed significant inhibitory activity on LPS‐stimulated NO production in BV2 cells with the IC₅₀ values ranging from 21.0 to 29.3 μm .     

Efficient conversion of fructose to 5‐hydroxymethylfurfural by functionalized γ‐Al2O3 beads

Millimeter size γ‐Al₂O₃ beads were prepared by alginate assisted sol–gel method and grafting organic groups with propyl sulfonic acid and alkyl groups as functionalized γ‐Al₂O₃ bead catalysts for fructose dehydration to 5‐hydroxymethylfurfural (5‐HMF). Experiment results showed that the porous structure of γ‐Al₂O₃ beads was favorable to the loading and dispersion of active components, and had an obvious effect on the properties of the catalyst. The lower calcination temperature of γ‐Al₂O₃ beads increased the specific surface area, the hydrophobicity and the activity of catalysts. Competition between the reaction of alkyl groups and ‐SH groups with surface hydroxyl during the preparation process of the catalyst influenced greatly the acid site densities, hydrophobic properties and activity of the catalyst. With an increase in the alkyl group chain, the hydrophobicity of catalysts increased obviously and the activity of the catalyst was enhanced. The most hydrophobic catalyst C₁₆‐SO₃H‐γ‐Al₂O₃–650°C exhibited the highest yield of 5‐HMF (84%) under the following reaction conditions: reaction medium of dimethylsulfoxide/H₂O (V/V, 4:1), catalyst amount of 30 mg, temperature of 110°C and reaction time of 4 hr.     

Simultaneous quantification and rat pharmacokinetics of formononetin‐7‐O‐β‐d‐glucoside and its metabolite formononetin by high‐performance liquid chromatography–tandem mass spectrometry

Formononetin‐7‐O‐β‐d ‐glucoside has been proved to have significant anti‐inflammatory effect. To evaluate its rat pharmacokinetics, a rapid, sensitive, and specific liquid chromatography–tandem mass spectrometry method has been developed and validated for the quantification of formononetin‐7‐O‐β‐d ‐glucoside and its main metabolite formononetin in rat plasma. Samples were pretreated using a simple protein precipitation and the chromatographic separation was performed on a C₁₈ column by a gradient elution using a mobile phase consisting of water and acetonitrile both containing 0.1% formic acid. Both analytes were detected using a tandem mass spectrometer in positive multiple reaction monitoring mode. The assay showed wide linear dynamic ranges of both 0.10–100 ng/mL, with acceptable intra‐ and inter‐batch accuracy and precision. The lower limits of quantification were both 0.10 ng/mL using 50 μL of rat plasma for two analytes. The method has been successfully used to investigate the oral pharmacokinetic profiles of both analytes in rats. After oral administration of formononetin‐7‐O‐β‐d ‐glucoside at the dose of 50 mg/kg, it was rapidly absorbed in vivo and metabolized to its metabolite formononetin. The plasma concentration‐time profiles both showed double‐peak phenomena, which would be attributed to the strong enterohepatic circulation of formononetin‐7‐O‐β‐d ‐glucoside.     

l‐Isoleucyl‐l‐serine 0.33‐hydrate,l‐phenyl­alanyl‐l‐serine and l‐methionyl‐l‐serine 0.34‐hydrate

The structures of the title dipeptides, C₉H₁₈N₂O₄·0.33H₂O, C₁₂H₁₆N₂O₄ and C₈H₁₆N₂O₄S·0.34H₂O, complete a series of investigations focused on l ‐Xaa‐l ‐serine peptides, where Xaa is a hydro­phobic residue. All three structures are divided into hydro­philic and hydro­phobic layers. The hydro­philic layers are thin for l ‐phenyl­alanyl‐l ‐serine, rendered possible by an unusual peptide conformation, and thick for l ‐isoleucyl‐l ‐serine and l ‐methionyl‐l ‐serine, which include cocrystallized water mol­ecules on the twofold axes.     

Polyisobutylene‐based polyurethanes. VIII. Polyurethanes with ‐O‐S‐PIB‐S‐O‐ soft segments

We describe the synthesis, characterization, and select properties of a novel polyurethane (PU) prepared using a new polyisobutylene diol, HO‐CH₂CH₂‐S‐PIB‐S‐CH₂CH₂‐OH, soft segment and conventional hard segments. The diol is synthesized by terminal functionalization of ally‐telechelic PIB followed by low‐cost thiol‐ene click chemistry. Properties of ‐S‐ containing PU (PIB_S‐PU) containing 72.5% PIB were investigated and compared to similar PUs made with HO‐PIB‐OH (PIB_O‐PU). Hydrolytic resistance was studied by contact with phosphate‐buffered saline, oxidative resistance by immersing in concentrated HNO₃, and metal ion oxidation resistance by exposure to CoCl₂/H₂O₂. Hydrolytic and oxidative resistances of PIB_S‐PU and PIB_O‐PU are similar and superior to a commercial PDMS‐based PU, Elast‐Eon? E2A. According to ¹H NMR spectroscopy the ‐S‐ in PIB_S‐PUs remained unchanged upon treatment with HNO₃, however, oxidized mainly to ‐SO₂‐ by CoCl₂/H₂O_2. Static mechanical properties of PIB_S‐PU and PIB_O‐PU are similar, except creep resistance of PIB_S‐PU is surprisingly superior. The thermal stability of PIB_S‐PUs is ～15 °C higher than that of PIB_O‐PU. FTIR spectroscopy indicates H bonded S atoms (N‐H…S) between soft and hard segments, which noticeably affect properties. DSC and XRD studies suggest random low‐periodicity crystals dispersed within a soft matrix. Energy dispersive X‐ray spectroscopy–scanning electron microscopy indicates homogeneous distribution of S atoms on PIB_S‐PU surfaces. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1119–1131     

Rose Bengal‐Sensitized Photooxidation of the Dipeptides l‐Tryptophyl‐l‐Phenylalanine,l‐Tryptophyl‐l‐Tyrosine and l‐Tryptophyl‐l‐Tryptophan: Kinetics,Mechanism and Photoproducts¶

The Rose Bengal‐sensitized photooxidations of the dipeptides l ‐tryptophyl‐l ‐phenylalanine (Trp‐Phe), l ‐tryptophyl‐l ‐tyrosine (Trp‐Tyr) and l ‐tryptophyl‐l ‐tryptophan (Trp‐Trp) have been studied in pH 7 water solution using static photolysis and time‐resolved methods. Kinetic results indicate that the tryptophan (Trp) moiety interacts with singlet molecular oxygen (O₂(¹Δ_g)) both through chemical reaction and through physical quenching, and that the photooxidations can be compared with those of equimolecular mixtures of the corresponding free amino acids, with minimum, if any, influence of the peptide bond on the chemical reaction. This is not a common behavior in other di‐ and polypeptides of photooxidizable amino acids. The ratio between chemical (k_r) and overall (k_t) rate constants for the interaction O₂(¹Δ_g)‐dipeptide indicates that Trp‐Phe and Trp‐Trp are good candidates to suffer photodynamic action, with k_rlk_t values of 0.72 and 0.60, respectively (0.65 for free Trp). In the case of Trp‐Tyr, a lower k_rlk_t value (0.18) has been found, likely as a result of the high component of physical deactivation of O₂(¹Δ_g) by the tyrosine moiety. The analysis of the photooxidation products shows that the main target for O₂(¹Δ_g) attack is the Trp group and suggests a much lower accumulation of kynurenine‐type products, as compared with free Trp. This is possibly because of the occurrence of another accepted alternative pathway of oxidation that gives rise to 3a‐oxidized hydrogenated pyrrolo[2,3‐b]indoles.     

2,3,3′,6‐Tetra‐O‐acetyl‐4,1′,6′‐trichloro‐4,1′,4′,6′‐tetradeoxygalactosucrose

At 160 K, one of the Cl atoms in the furanoid moiety of 3‐O‐acetyl‐1,6‐di­chloro‐1,4,6‐tri­deoxy‐β‐d ‐fructo­furan­osyl 2,3,6‐tri‐O‐acetyl‐4‐chloro‐4‐deoxy‐α‐d ‐galacto­pyran­oside, C₂₀H₂₇­Cl₃O₁₁, is disordered over two orientations, which differ by a rotation of about 107° about the parent C—C bond. The conformation of the core of the mol­ecule is very similar to that of 3‐O‐acetyl‐1,4,6‐tri­chloro‐1,4,6‐tri­deoxy‐β‐d ‐tagato­furanos­yl 2,3,6‐tri‐O‐acetyl‐4‐chloro‐4‐deoxy‐α‐d ‐galacto­pyran­oside, particularly with regard to the conformation about the glycosidic linkage.     

Synthesis of star‐shaped poly(D,L‐lactic acid‐alt‐glycolic acid)‐b‐poly(L‐lactic acid) with the poly(D,L‐lactic acid‐alt‐glycolic acid) macroinitiator and stannous octoate catalyst

Two types of three‐arm and four‐arm, star‐shaped poly(D,L ‐lactic acid‐alt‐glycolic acid)‐b‐poly(L ‐lactic acid) (D,L ‐PLGA50‐b‐PLLA) were successfully synthesized via the sequential ring‐opening polymerization of D,L ‐3‐methylglycolide (MG) and L ‐lactide (L ‐LA) with a multifunctional initiator, such as trimethylolpropane and pentaerythritol, and stannous octoate (SnOct₂) as a catalyst. Star‐shaped, hydroxy‐terminated poly(D,L ‐lactic acid‐alt‐glycolic acid) (D,L ‐PLGA50) obtained from the polymerization of MG was used as a macroinitiator to initiate the block polymerization of L ‐LA with the SnOct₂ catalyst in bulk at 130 °C. For the polymerization of L ‐LA with the three‐arm, star‐shaped D,L ‐PLGA50 macroinitiator (number‐average molecular weight = 6800) and the SnOct₂ catalyst, the molecular weight of the resulting D,L ‐PLGA50‐b‐PLLA polymer linearly increased from 12,600 to 27,400 with the increasing molar ratio (1:1 to 3:1) of L ‐LA to MG, and the molecular weight distribution was rather narrow (weight‐average molecular weight/number‐average molecular weight = 1.09–1.15). The ¹H NMR spectrum of the D,L ‐PLGA50‐b‐PLLA block copolymer showed that the molecular weight and unit composition of the block copolymer were controlled by the molar ratio of L ‐LA to the macroinitiator. The ¹³C NMR spectrum of the block copolymer clearly showed its diblock structures, that is, D,L ‐PLGA50 as the first block and poly(L ‐lactic acid) as the second block. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 409–415, 2002     

The monohydrates of the four polar dipeptides l‐seryl‐l‐asparagine,l‐seryl‐l‐tyrosine,l‐tryptophanyl‐l‐serine and l‐tyrosyl‐l‐tryptophan

The crystal structures of the four dipeptides l ‐seryl‐l ‐asparagine monohydrate, C₇H₁₃N₃O₅·H₂O, l ‐seryl‐l ‐tyrosine monohydrate, C₁₂H₁₆N₂O₅·H₂O, l ‐tryptophanyl‐l ‐serine monohydrate, C₁₄H₁₇N₃O₄·H₂O, and l ‐tyrosyl‐l ‐tryptophan monohydrate, C₂₀H₂₁N₃O₄·H₂O, are dominated by extensive hydrogen‐bonding networks that include cocrystallized solvent water molecules. Side‐chain conformations are discussed on the basis of previous observations in dipeptides. These four dipeptide structures greatly expand our knowledge on dipeptides incorporating polar residues such as serine, asparagine, threonine, tyrosine and tryptophan.