D3h‐Symmetrical Shape‐Persistent Macrocycles Consisting of Pyridine–Acetylene–Phenol Conjugates as an Efficient Host Architecture for Saccharide Recognition

Hexagonal shape‐persistent macrocycles (SPMs) consisting of three pyridine and three phenol rings linked with acetylene bonds were developed as a preorganized host for saccharide recognition by push–pull‐type hydrogen bonding. Three tert‐butyl or 2,4,6‐triisopropylphenyl substituents were introduced on the host to suppress self‐aggregation by steric hindrance. In spite of the simple architecture, association constants K_a of the host with alkyl glycoside guests reached the order of 10⁶ m ^?1 on the basis of UV/Vis titration experiments. This glycoside recognition was much stronger than that in the cases of acyclic equivalent hosts because of the entropic advantage brought by preorganization of the hydrogen‐bonding sites. Solid–liquid extraction and liquid–liquid transport through a liquid membrane were demonstrated by using native saccharides, and much preference to mannose was observed.     

Cucurbit[7]uril: A High‐Affinity Host for Encapsulation of Amino Saccharides and Supramolecular Stabilization of Their α‐Anomers in Water

Cucurbit[7]uril (CB[7]), an uncharged and water‐soluble macrocyclic host, binds protonated amino saccharides (D ‐glucosamine, D ‐galactosamine, D ‐mannosamine and 6‐amino‐6‐deoxy‐D ‐glucose) with excellent affinity (K_a=10³ to 10⁴ M ^?1). The host–guest complexation was confirmed by NMR spectroscopy, isothermal titration calorimetry (ITC), and MALDI‐TOF mass spectral analyses. NMR analyses revealed that the amino saccharides, except D ‐mannosamine, are bound as α‐anomers within the CB[7] cavity. ITC analyses reveal that CB[7] has excellent affinity for binding amino saccharides in water. The maximum affinity was observed for D ‐galactosamine hydrochloride (K_a=1.6×10⁴ M ^?1). Such a strong affinity for any saccharide in water using a synthetic receptor is unprecedented, as is the supramolecular stabilization of an α‐anomer by the host.     

Combined 1H, 13C and 11B NMR and mass spectral assignments of boronate complexes of D‐(+)‐glucose,D‐(+)‐mannose,methyl‐α‐D‐glucopyranoside,methyl‐β‐D‐galactopyranoside and methyl‐α‐D‐mannopyranoside

Complex formation between N‐butylboronic acid and D ‐(+)‐glucose, D ‐(+)‐mannose, methyl‐α‐D ‐glucopyranoside, methyl‐β‐D ‐galactopyranoside and methyl α‐D ‐mannopyranoside under neutral conditions was investigated by ¹H, ¹³C and ¹¹B NMR spectroscopy and gas chromatography–mass spectrometry (GC–MS) D ‐(+)‐Glucose and D ‐(+)‐mannose formed complexes where the boronates are attached to the 1,2:4,6‐ and 2,3:5,6‐positions of the furanose forms, respectively. On the other hand, the boronic acid binds to the 4,6‐positions of the two methyl derivatives of glucose and galactose. Methyl α‐D ‐mannopyranoside binds two boronates at the 2,3:4,6‐positions. ¹¹B NMR was used to show the ring size of the complexed sugars and the boronate. GC–MS confirmed the assignments. Copyright © 2003 John Wiley & Sons, Ltd.     

Structural and Kinetic Dissection of the endo‐α‐1,2‐Mannanase Activity of Bacterial GH99 Glycoside Hydrolases from Bacteroides spp.

Glycoside hydrolase family 99 (GH99) was created to categorize sequence‐related glycosidases possessing endo‐α‐mannosidase activity: the cleavage of mannosidic linkages within eukaryotic N‐glycan precursors (Glc_1–3Man₉GlcNAc₂), releasing mono‐, di‐ and triglucosylated‐mannose (Glc_1–3‐1,3‐Man). GH99 family members have recently been implicated in the ability of Bacteroides spp., present within the gut microbiota, to metabolize fungal cell wall α‐mannans, releasing α‐1,3‐mannobiose by hydrolysing αMan‐1,3‐αMan→1,2‐αMan‐1,2‐αMan sequences within branches off the main α‐1,6‐mannan backbone. We report the development of a series of substrates and inhibitors, which we use to kinetically and structurally characterise this novel endo‐α‐1,2‐mannanase activity of bacterial GH99 enzymes from Bacteroides thetaiotaomicron and xylanisolvens. These data reveal an approximate 5 kJ mol^?1 preference for mannose‐configured substrates in the ?2 subsite (relative to glucose), which inspired the development of a new inhibitor, α‐mannopyranosyl‐1,3‐isofagomine (ManIFG), the most potent (bacterial) GH99 inhibitor reported to date. X‐ray structures of ManIFG or a substrate in complex with wild‐type or inactive mutants, respectively, of B. xylanisolvens GH99 reveal the structural basis for binding to D ‐mannose‐ rather than D ‐glucose‐configured substrates.     

NMR Study of Hetero‐Association Accompanying Self‐Association of 2‐Hydroxyl‐4‐methyl‐pyridine and 2‐Pyrrolidone in Acetonitrile

This work in vestigated not only the equilibrium of self‐association of 2‐pyrrolidone (A) and that of 2‐hydroxyl‐4‐methyl‐pyridine (B), but also the hetero‐association between A and B in [²H₃]acetonitrile through hydrogen bonding using high‐resolution nuclear magnetic resonance spectroscopy. Dilution shift data for the protons of the NH group of A and OH group of B were measured over a wide range of temperatures and concentrations. In addition, the monomer shifts, dimer shifts and dimerization constants of self‐association and hetero‐association were evaluated using a graphic method operating on the dilution chemical shift data. The enthalpy and entropy of dimerization of self‐association and hetero‐association were also obtained from the van't Hoff plot.     

Synthesis of Branched Ribo‐Polysaccharides with Defined Structures by Ring‐Opening Polymerization of 1,4‐Anhydro Ribo‐Disaccharide Monomer

Ring‐opening polymerization of a new 1,4‐anhydro‐disaccharide monomer, 1,4‐anhydro‐2‐O‐benzyl‐3‐O‐(2,3,4,6‐tetra‐O‐benzyl‐β‐D ‐galactopyranosyl)‐α‐D ‐ribopyranose, which was prepared by the glycosylation of 1,4‐anhydro‐2‐O‐benzyl‐α‐D ‐ribopyranose with 2,3,4,6‐tetra‐O‐acetyl‐1‐O‐trichloroacetimidoyl‐α‐D ‐galactopyranose, was performed for the first time with boron trifluoride etherate to give stereoregular branched ribofuranans having high molecular weights of M̄_n = 43.0×10³ and positive specific rotation of [α]_D²⁵ = +25.1 deg·dm^–1· g^–1·cm³. The repalcement of the benzyl group by a hydroxyl group gave stereoregular 1,5‐α‐D ‐ribofuranans having a β‐D ‐galactopyranose branch in every repeating unit. The copolymerization of the ribo‐disaccharide monomer with 1,4‐anhydro‐2,3‐di‐O‐benzyl‐α‐D ‐ribopyranose was also carried out to afford stereoregular 1,5‐α‐D ‐ribofuranans having randomly distributed galactopyranose branches on the main chain.     

An AAA–DDD Triply Hydrogen‐Bonded Complex Easily Accessible for Supramolecular Polymers

For a complementary hydrogen‐bonded complex, when every hydrogen‐bond acceptor is on one side and every hydrogen‐bond donor is on the other, all secondary interactions are attractive and the complex is highly stable. AAA–DDD (A=acceptor, D=donor) is considered to be the most stable among triply hydrogen‐bonded sequences. The easily synthesized and further derivatized AAA–DDD system is very desirable for hydrogen‐bonded functional materials. In this case, AAA and DDD, starting from 4‐methoxybenzaldehyde, were synthesized with the Hantzsch pyridine synthesis and Friedländer annulation reaction. The association constant determined by fluorescence titration in chloroform at room temperature is 2.09×10⁷ M ^?1. The AAA and DDD components are not coplanar, but form a V shape in the solid state. Supramolecular polymers based on AAA–DDD triply hydrogen bonded have also been developed. This work may make AAA–DDD triply hydrogen‐bonded sequences easily accessible for stimuli‐responsive materials.     

Electron Transfer between Hydrogen‐Bonded Pyridylphenols and a Photoexcited Rhenium(I) Complex

Two pyridylphenols with intramolecular hydrogen bonds between the phenol and pyridine units have been synthesized, characterized crystallographically, and investigated by cyclic voltammetry and UV/Vis spectroscopy. Reductive quenching of the triplet metal‐to‐ligand charge‐transfer excited state of the [Re(CO)₃(phen)(py)]⁺ complex (phen=1,10‐phenanthroline, py=pyridine) by the two pyridylphenols and two reference phenol molecules is investigated by steady‐state and time‐resolved luminescence spectroscopy, as well as by transient absorption spectroscopy. Stern–Volmer analysis of the luminescence quenching data provides rate constants for the bimolecular excited‐state quenching reactions. H/D kinetic isotope effects for the pyridylphenols are on the order of 2.0, and the bimolecular quenching reactions are up to 100 times faster with the pyridylphenols than with the reference phenols. This observation is attributed to the markedly less positive oxidation potentials of the pyridylphenols with respect to the reference phenols (≈0.5 V), which in turn is caused by proton coupling of the phenol oxidation process. Transient absorption spectroscopy provides unambiguous evidence for the photogeneration of phenoxyl radicals, that is, the overall photoreaction is clearly a proton‐coupled electron‐transfer process.     

Facilitated Transfer of Alkali and Alkaline‐Earth Metal Ions by a Calix[4]arene Derivative Across Water/1,2‐Dichloroethane Microinterface: Amperometric Detection of Ca2+

The host–guest complexation reactions between 5,11,17,23‐tetra‐tert‐butyl‐25,27‐diethoxycarbonylmethoxy‐26,28‐dimethoxy calix[4]arene (BDDC4) and alkali and alkaline‐earth metal ions were investigated by facilitated ion transfer processes across water/1,2‐dichloroethane microinterface by using steady‐state cyclic voltammetry and differential pulse voltammetry. The obtained facilitated transfers for Li⁺, Na⁺, K⁺, Rb⁺ and Ca²⁺ were evaluated under the different experimental conditions, at the excess concentrations of metal ions with respect to BDDC4 and vice versa. The association constants having 1 : 1 stoichiometry for Li⁺, Na⁺, K⁺ and Rb⁺ in 1,2‐DCE were determined. Also, we demonstrated that BDDC4 can play an important role for the development of highly selective chemical sensor for Ca²⁺ among alkaline‐metal ions in the concentration range of 0.1–1.0 mM in aqueous solution.     

Reaction‐path calculations and crystal structures of 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dichloride dihydrate and 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dibromide

The design of new organic–inorganic hybrid ionic materials is of interest for various applications, particularly in the areas of crystal engineering, supramolecular chemistry and materials science. The monohalogenated intermediates 1‐(2‐chloroethyl)pyridinium chloride, C₅H₅NCH₂CH₂Cl⁺·Cl⁻, (I′), and 1‐(2‐bromoethyl)pyridinium bromide, C₅H₅NCH₂CH₂Br⁺·Br⁻, (II′), and the ionic disubstituted products 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dichloride dihydrate, C₁₂H₁₄N₂²⁺·2Cl⁻·2H₂O, (I), and 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dibromide, C₁₂H₁₄N₂²⁺·2Br⁻, (II), have been isolated as powders from the reactions of pyridine with the appropriate 1,2‐dihaloethanes. The monohalogenated intermediates (I′) and (II′) were characterized by multinuclear NMR spectroscopy, while (I) and (II) were structurally characterized using powder X‐ray diffraction. Both (I) and (II) crystallize with half the empirical formula in the asymmetric unit in the triclinic space group P. The organic 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dications, which display approximate C2h symmetry in both structures, are situated on inversion centres. The components in (I) are linked via intermolecular O—H…Cl, C—H…Cl and C—H…O hydrogen bonds into a three‐dimensional framework, while for (II), they are connected via weak intermolecular C—H…Br hydrogen bonds into one‐dimensional chains in the [110] direction. The nucleophilic substitution reactions of 1,2‐dichloroethane and 1,2‐dibromoethane with pyridine have been investigated by ab initio quantum chemical calculations using the 6–31G** basis. In both cases, the reactions occur in two exothermic stages involving consecutive S_N2 nucleophilic substitutions. The isolation of the monosubstituted intermediate in each case is strong evidence that the second step is not fast relative to the first.     

Polyol‐Metall‐Komplexe.47 Kristalline D‐Mannose‐Kupfer(II)‐Komplexe aus Fehlingscher Lösung

Polyol Metal Complexes.47¹⁾ Crystalline D ‐Mannose‐Copper Complexes from Fehling Solutions Blue, unstable crystals of K₃[Cu₅(β‐D ‐Manp)₄H_—13] · α‐D ‐Manp · 16.5 H₂O ( 1 ), which contain a pentanuclear cupric complex of the reducing sugar D ‐mannose in its β‐pyranose form (β‐D ‐Manp), have been obtained from ice‐cold aqueous alkaline solutions. The homoleptic pentacuprate contains bridging mannopyranose ligands, which are charged 4— and 2.5—. Addition of ethylenediamine (en) to such Fehling solutions yields N, N′‐Bis(β‐D ‐mannopyranosyl)‐ethylenediamine (L) as a condensation product of the diamine and mannopyranose. Crystals of [(en)₂Cu₇(β‐D ‐Manp1, 2, 3, 4H_—4)₂(L2, 3, 4H_—3)₂] · 26.6 H₂O ( 2 ) could be isolated. The heptanuclear cupric complex is a structural derivative of the homoleptic mannose complex.     

Synthesis of glycosyl esters from phosphoroates derivatives of 2‐bromosugars

This work presents the synthesis of glycosyl esters of 2‐bromo‐2‐deoxy‐D ‐hexopyranose, having the α‐D ‐manno ( 10a–cα ), β‐D ‐gluco ( 11a–dβ ) and α‐D ‐gluco ( 11a,bα ) configuration, by a stereoselective reaction between phosphoroates 3–8 and carboxylic acids 9a–d. Derivatives of 10a–c and 11a–d are formed in an overall quantitative yield, in an aprotic solvent in the presence of silver salts as a leaving group activator. The phosphoroselenoate of 3 was obtained by the condensation reaction of the triethylammonium salt of phosphoroseleno acid 2 with α‐1,2‐D ‐manno‐pyranosyl dibromide 1 with high stereoselectivity. The structures of the compounds 3,10a–c and 11a–d were established by ¹H and ¹³C NMR spectra and by elemental analyses. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:292–298, 2000     

Aminodiphenylphosphanes: Isotope‐induced chemical shifts 1Δ14/15N(31P), coupling constants 1J(31P,15N), and chemical shifts δ15N and δ31P

A series of aminodiphenylphosphanes 1 [Ph₂P‐N(H)tBu ( a ), ‐NEt₂ ( b ), ‐NiPr₂ ( c )], 2 [Ph₂P‐NHPh ( a ), ‐NH‐2‐pyridine ( b ), ‐NH‐3‐pyridine ( c ), ‐NH‐4‐pyridine ( d ), NH‐pyrimidine ( e ), NH‐2,6‐Me₂‐C₆H₃ ( f ), NH‐3‐Me‐2‐pyridine ( g )], 3 [Ph₂P‐N(Me)Ph ( a ), ‐NPh₂ ( b )], and N‐pyrrolyldiphenylphosphane 4 (Ph₂P‐NC₄H₄) was prepared and studied by NMR (¹H, ¹³C, ³¹P, ¹⁵N NMR) spectroscopy. The isotope‐induced chemical shifts ¹Δ^14/15N(³¹P) were determined at natural abundance of ¹⁵N by using HEED INEPT experiments. A dependence of ¹Δ^14/15N(³¹P) on the substituents at nitrogen was found (alkyl < H < aryl; increasingly negative values). The magnitude and sign of the coupling constants ¹J(³¹P,¹⁵N) (positive sign) are dominated by the presence of the lone pair of electrons at the phosphorus atom. The X‐ray structural analysis of 2b is reported, showing the presence of dimers owing to intermolecular hydrogen bridges in the solid state. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:542–550, 2001     

Sugar‐Alcohol Complexes of Palladium(II): On the Variable Rigidity of Open‐Chain Carbohydrate Ligands

The C₄, C₅, and C₆ sugar alcohols erythritol (Eryt), D ‐threitol (D ‐Thre), D ‐arabitol (D ‐Arab), ribitol (Ribt), xylitol (Xylt), dulcitol (Dulc), and D ‐mannitol (D ‐Mann) form chelate complexes upon dissolution in Pd‐en, an aqueous solution of [Pd^II(en)(OH)₂]. Stability rules are derived from the proportion of a respective species in the solution equilibrium. Crystal‐structure analysis supports the NMR spectroscopic results for a series of binuclear compounds that contain the sugar alcohols as tetraanionic polyolato ligands: [Pd₂(en)₂(ErytH_?4)]? 10H₂O, [Pd₂(en)₂(D ‐Arab1,2;3,4H_?4)]? 7H₂O, [Pd₂(en)₂(Xylt1,2;3,4H_?4)]?4H₂O, [Pd₂(en)₂(D ‐Mann1,2;3,4H_?4)]?5H₂O, and [Pd₂(en)₂(Dulc2,3;4,5H_?4)]?6H₂O. In the case of the pentitols and hexitols, the metalated tetraanions are stabilized by intramolecular hydrogen bonds. The hydrogen bonds uniformly connect an alkoxide acceptor to the hydroxy donor group located at the δ carbon atom. As a consequence of hydrogen bonding, the open‐chain carbohydrate ligands become rigid. Crystal‐structure analysis provides information on the configurational requirements for rigidity. According to these rules, the hydrogen‐bond‐supported Dulc2,3;4,5H_?4 tetraanion provides a geometrically persistent ligating pattern. Intramolecular hydrogen bonding seems to be the most‐competitive variable to metalation of a polyol. [Pd₂(tm‐2,1:3,2‐tet)(OH)₃]OH (tm‐2,1:3,2‐tet=1,3‐bis(2′‐dimethylaminoethyl)hexahydropyrimidine) is a metallizing agent that can force full metalation even in a case as intractable as that of dulcitol. Accordingly, [Pd₄(tm‐2,1:3,2‐tet)₂‐(DulcH_?6)]Cl₂?16H₂O contains the fully deprotonated hexitol as the ligand.     

Aminocyclopentitol Inhibitors of α‐L‐Fucosidases

Aminocyclopentitol analogs of α‐L ‐fucose were synthesized stereoselectively from D ‐ribose. Alkyl substituents were attached at the NH₂ group to mimic the glycosidic leaving group. The resulting (alkylamino)cyclopentitols inhibited α‐L ‐fucosidases selectively with inhibition constants in the range of K_i=10⁻⁷ M . Comparisons with stereoisomers and acyclic analogs showed that this inhibition only occurs with N‐alkyl substitution and proper configuration at the cyclopentane, as expected for transition‐state‐analog‐type inhibition. These observations were supported by molecular‐modeling comparisons between inhibitor and transition state.     

Optically Active Hexaazamacrocycles: Protonation Behavior and Chiral‐Anion Recognition

The protonation features of two optically active 22‐membered hexaazamacrocycles possessing one ( L1 ) or two ( L2 ) (R,R)‐cyclohexane‐1,2‐diamine moieties have been studied by means of potentiometric ¹H‐ and ¹³C‐NMR techniques. This study allows the determination of the basicity constants and the stepwise protonation sites. The presence of the cyclohexane decreases the protonation ability, and this effect can be explained in terms of conformational and electrostatic factors. Binding of different chiral dicarboxylates has been studied by potentiometry. Macrocycle L2 presents higher anion‐complexation equilibrium constants than L1 . The stability of the diastereoisomeric complexes depends on the pH, and the structures of the macrocycles and anions. Receptor L1 ⋅6 H⁺ shows moderate D ‐selectivity towards tartrate anion, whereas L2 ⋅6 H⁺ exhibits a good preference for N‐Ac‐D ‐aspartate. Both protonated L1 and L2 form strong complexes with N‐Ac‐glutamate, and the stoichiometry of the complex depends on the degree of protonation and the absolute configuration of the anion. For this last anion, both azamacrocycles exhibit a clear D ‐preference.     

Three‐dimensional hydrogen‐bonded structures in the 1:1 proton‐transfer compounds of l‐tartaric acid with the associative‐group monosubstituted pyridines 3‐aminopyridine, 3‐carboxypyridine (nicotinic acid) and 2‐carboxypyridine (picolinic acid)

The 1:1 proton‐transfer compounds of l ‐tartaric acid with 3‐aminopyridine [3‐aminopyridinium hydrogen (2R,3R)‐tartrate dihydrate, C₅H₇N₂⁺·C₄H₅O₆⁻·2H₂O, (I)], pyridine‐3‐carboxylic acid (nicotinic acid) [anhydrous 3‐carboxypyridinium hydrogen (2R,3R)‐tartrate, C₆H₆NO₂⁺·C₄H₅O₆⁻, (II)] and pyridine‐2‐carboxylic acid [2‐carboxypyridinium hydrogen (2R,3R)‐tartrate monohydrate, C₆H₆NO₂⁺·C₄H₅O₆⁻·H₂O, (III)] have been determined. In (I) and (II), there is a direct pyridinium–carboxyl N⁺—H...O hydrogen‐bonding interaction, four‐centred in (II), giving conjoint cyclic R₁²(5) associations. In contrast, the N—H...O association in (III) is with a water O‐atom acceptor, which provides links to separate tartrate anions through O_hydroxy acceptors. All three compounds have the head‐to‐tail C(7) hydrogen‐bonded chain substructures commonly associated with 1:1 proton‐transfer hydrogen tartrate salts. These chains are extended into two‐dimensional sheets which, in hydrates (I) and (III) additionally involve the solvent water molecules. Three‐dimensional hydrogen‐bonded structures are generated via crosslinking through the associative functional groups of the substituted pyridinium cations. In the sheet struture of (I), both water molecules act as donors and acceptors in interactions with separate carboxyl and hydroxy O‐atom acceptors of the primary tartrate chains, closing conjoint cyclic R₄⁴(8), R₃⁴(11) and R₃³(12) associations. Also, in (II) and (III) there are strong cation carboxyl–carboxyl O—H...O hydrogen bonds [O...O = 2.5387 (17) Å in (II) and 2.441 (3) Å in (III)], which in (II) form part of a cyclic R₂²(6) inter‐sheet association. This series of heteroaromatic Lewis base–hydrogen l ‐tartrate salts provides further examples of molecular assembly facilitated by the presence of the classical two‐dimensional hydrogen‐bonded hydrogen tartrate or hydrogen tartrate–water sheet substructures which are expanded into three‐dimensional frameworks via peripheral cation bifunctional substituent‐group crosslinking interactions.     

Microwave‐assisted synthesis and characterization of heterocyclic,and optically active poly(amide‐imide)s incorporating L‐amino acids

N,N′‐Pyromelliticdiimido‐di‐L ‐alanine ( 1 ), N,N′‐pyromelliticdiimido‐di‐L ‐phenylalanine ( 2 ), and N,N′‐pyromelliticdiimido‐di‐L ‐leucine ( 3 ) were prepared from the reaction of pyromellitic dianhydride with corresponding L ‐amino acids in a mixture of glacial acetic acid and pyridine solution (3/2 ratio) under refluxing conditions. The microwave‐assisted polycondensation of the corresponding diimide‐diacyl chloride monomers ( 5–7 ) with 4‐phenyl‐2,6‐bis(4‐aminophenyl) pyridine ( 10 ) or 4‐(p‐methylthiophenyl)‐2,6‐bis(4‐aminophenyl) pyridine ( 12 ) were carried out in a laboratory microwave oven. The resulting poly(amide‐imide)s were obtained in quantitative yields, and they showed admirable inherent viscosities (0.12–0.55 dlg^?1), were soluble in polar aprotic solvents, showed good thermal stability and high optical purity. The synthetic compounds were characterized by IR, MS, ¹H NMR, and ¹³C NMR spectroscopy, elemental analysis, and specific rotation. Copyright © 2008 John Wiley & Sons, Ltd.     

Bulk cyclopolymerization of 1,2:5,6‐diepithio‐3,4‐di‐O‐methyl‐1,2:5,6‐tetradeoxy‐D‐mannitol with quaternary ammonium salts leading to gel‐free thiosugar polymer

The bulk cyclopolymerization of diepisulfide, 1,2:5,6‐diepithio‐3,4‐di‐O‐methyl‐1,2:5,6‐tetradeoxy‐D ‐mannitol ( 1 ), was studied using R₄N⁺Br^? (R = ? CH₃, C₂H₅, C₃H₇, C₄H₉, and C₇H₁₅) and (C₄H₉)₄N⁺X^? (X = Cl, I, NO₃, and ClO₄) as the initiators. All the bulk polymerizations of 1 using quaternary tetraalkylammonium salts at 90 °C proceeded without gelation even at high conversion to produce gel‐free polymers consisting of 2,5‐anhydro‐1,5‐dithio‐D ‐glucitol (I) as the major cyclic repeating unit along with 1,5‐anhydro‐2,5‐dithio‐D ‐mannitol (II) and the desulfurized acyclic unit (III) as the minor units. The polymerization rate and molar fraction of the I unit increased with the increasing alkyl chain length of the tetraalkylammonium cation and the increasing nucleophilicity of the counteranion. Tetrabutylammonium chloride exhibited the highest catalytic activity and the highest stereoselectivity, that is, the thiosugar polymer with I:II:III = 81:15:4 and a number‐average molecular weight of 31.9 × 10³ was obtained in 85% yield for a polymerization time of 0.5 h. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 965–970, 2002     

Synthesis and Self‐Assembly of NCN‐Pincer Pd‐Complex‐Bound Norvalines

The NCN‐pincer Pd‐complex‐bound norvalines Boc‐D /L ‐[PdCl(dpb)]Nva‐OMe ( 1 ) were synthesized in multigram quantities. The molecular structure and absolute configuration of 1 were unequivocally determined by single‐crystal X‐ray structure analysis. The robustness of 1 under acidic/basic conditions provides a wide range of N‐/C‐terminus convertibility based on the related synthetic transformations. Installation of a variety of functional groups into the N‐/C‐terminus of 1 was readily carried out through N‐Boc‐ or C‐methyl ester deprotection and subsequent condensations with carboxylic acids, R¹COOH, or amines, R²NH₂, to give the corresponding N‐/C‐functionalized norvalines R¹‐D /L ‐[PdCl(dpb)]Nva‐R² 2 – 9 . The dipeptide bearing two Pd units 10 was successfully synthesized through the condensation of C‐free 1 with N‐free 1 . The robustness of these Pd‐bound norvalines was adequately demonstrated by the preservation of the optical purity and Pd unit during the synthetic transformations. The lipophilic Pd‐bound norvalines L ‐ 2 , Boc‐L ‐[PdCl(dpb)]Nva‐NH‐n‐C₁₁H₂₃, and L ‐ 4 , n‐C₄H₉CO‐L ‐[PdCl(dpb)]Nva‐NH‐n‐C₁₁H₂₃, self‐assembled in aromatic solvents to afford supramolecular gels. The assembled structures in a thermodynamically stable single crystal of L ‐ 2 and kinetically stable supramolecular aggregates of L ‐ 2 were precisely elucidated by cryo‐TEM, WAX, SAXS, UV/Vis, IR analyses, and single‐crystal X‐ray crystallography. An antiparallel β‐sheet‐type aggregate consisting of an infinite one‐dimensional hydrogen‐bonding network of amide groups and π‐stacking of PdCl(dpb) moieties was observed in the supramolecular gel fiber of L ‐ 2 , even though discrete dimers are assembled through hydrogen bonding in the thermodynamically stable single crystal of L ‐ 2 . The disparate DSC profiles of the single crystal and xerogel of L ‐ 2 indicate different thermodynamics of the molecular assembly process.