Design and synthesis of a novel class of sugar-peptide hybrids: C-linked glyco beta-amino acids through a stereoselective "acetate" Mannich reaction as the key strategic element

A new type of sugar-amino acid hybrid, which is comprised of a sugar unit (gluco-, galacto-, or mannopyranose) linked through a C-glycosidic linkage to the beta-position of an alpha-unsubstituted beta-amino acid unit, is presented. It is hypothesized that these new compounds, or the oligomeric peptides derived therefrom, might possess the structural features of beta-amino acid oligomers and the chemical and enzymatic resistance of C-glycosides to hydrolysis. The synthetic strategy is based on a new Mannich-type reaction between a chiral acetate enolate equivalent and alpha-amido sulfones derived from the corresponding sugar-C-glycoside aldehydes. While the sugar-C-glycoside aldehyde partner is prepared from well-established transformations on known sugar precursors, the lithium enolate derived from (1R)-endo-2-acetylisoborneol 3 is employed as the key element. This Mannich approach proceeds with essentially perfect diasteromeric control leading to the new beta-amino carbonyl adducts in good yields. Further, cleavage of the camphor auxiliary is smoothly performed by oxidative treatment with ammonium cerium nitrate (CAN). Complementarily, direct peptide-type coupling of the beta-amino carbonyl Mannich adducts with an alpha- or beta-amino acid residue and subsequent CAN-promoted detachment of the auxiliary yields dipeptide fragments bearing a sugar-containing aliphatic side chain and is a process that can be iterated. A preliminary conformational study based on the combination of experimental NMR data and molecular mechanics and molecular dynamics (MD) of one particular adduct is also provided.     

Stereoselective formal [3 + 2] cycloaddition of N-alkylidene glycine ester anions to chiral Fischer alkenylcarbene complexes. Asymmetric synthesis of 3,4,5-trisubstituted prolines

The reaction between N-alkylidene glycine ester enolates, generated from glycine esters aldimines with LDA in THF at low temperature, and chiral alkoxyalkenylcarbene complexes of chromium provided directly 2,4,5-trisubstituted-3-pyrrolidinylcarbene complexes with total exo selectivity and very high syn and facial diastereoselectivity when carbene complexes bearing the (-)-8-phenylmenthyloxy group were employed. Oxidation of the metal carbene moiety followed by basic hydrolysis of the esters afforded enantiomerically highly enriched syn,exo-3,4,5-trisubstituted prolines, whereas acidic hydrolysis of the same functional groups proceeded with epimerization at the alpha-amino acid center leading to anti,exo-3,4,5-trisubstituted prolines of very high enantiomeric purity as well.     

Thermal decomposition of copolymers from ethylene with some vinyl derivatives

The thermal degradation of ethylene-vinyl acetate (EVA), ethylene-vinyl-3,5-dinitrobenzoate (EVDNB) and ethylene-vinyl alcohol (EVAL) copolymers have been studied using differential thermal analysis (DTA) and thermogravimetry (TG) under isothermal and dynamic conditions in nitrogen. Thermal analysis indicates that EVA copolymers are thermally more stable than EVDNB samples. The degradation of the copolymers considered occurs as an additive degradation of each component polyethylene (PE) and poly(vinyl acetate) (PVA), poly(vinyl-3,5-dinitrobenzoate) (PVDNB) or poly(vinyl alcohol) (PVAL). The apparent activation energy of the decomposition was determined by the Kissinger and Flynn-Wall methods which agree well.     

Theoretical evidence for pyramidalized bicyclic serine enolates in highly diastereoselective alkylations

A new chiral serine equivalent and its enantiomer have been synthesized from (S)- and (R)-N-Boc-serine methyl esters (Boc: tert-butyloxycarbonyl). The use of these compounds as chiral building blocks has been demonstrated in the synthesis of alpha-alkyl alpha-amino acids by diastereoselective potassium enolate alkylation reactions and subsequent acid hydrolyses. Theoretical studies were performed to elucidate the stereochemical outcome of both the formation of five-membered cyclic N,O-acetals and the subsequent alkylation process, which occurs with total retention of configuration. This feature could be explained in terms of the high degree of pyramidalization of enolate intermediates.     

Characterization and optimization by experimental design of a liquid chromatographic method for the separation of hydroxylated polychlorinated biphenyls on a polar-embedded stationary phase

Traditionally, the determination of hydroxylated polychlorinated biphenyls (OH-PCBs) has been carried out by gas chromatography (GC). However, the gas chromatographic behavior and sensitivity of this type of hydroxylated compounds are not always satisfactory, hence a prior derivatization of the OH-PCBs must be performed. Therefore, the development of liquid chromatographic methods should prove to be a very interesting task aimed at dealing with the instrumental determination of OH-PCBs. Taking into account that octadecylsilane stationary phases are not the most adequate for the separation of isobaric compounds, an amide-type column has been tested. For the development of the method, the Response Surface Methodology was used, based on a Box-Wilson Central Composite experimental design. The initial content of methanol in the mobile phase, the gradient time, and the concentration and the pH value of the buffer were chosen as relevant experimental parameters. A global optimum was obtained by selecting the elution time, the sensitivity and the overall resolution as responses to optimize. The developed method for liquid chromatography presented a very good resolution and sensitivity, and a reasonably short analysis time. In addition, a retention study was conducted in order to survey the different interactions that take place in the separation process, showing that hydrogen bonding is the main interaction between OH-PCBs and the amide-type stationary phase. However, a substantial contribution of dispersion forces was present in methanol contents in the mobile phase below 65%. Besides, the pH value of the mobile phase was found to be the most important parameter to control the hydrogen bond forces and therefore, to regulate the OH-PCBs separation.     

The Role of Solvent on the Mechanism of Proton Transfer to Hydride Complexes: The Case of the [W3PdS4H3(dmpe)3(CO)]+ Cubane Cluster

The kinetics of reaction of the [W₃PdS₄H₃(dmpe)₃(CO)]⁺ hydride cluster ( 1 ⁺) with HCl has been measured in dichloromethane, and a second‐order dependence with respect to the acid is found for the initial step. In the presence of added BF₄^? the second‐order dependence is maintained, but there is a deceleration that becomes more evident as the acid concentration increases. DFT calculations indicate that these results can be rationalized on the basis of the mechanism previously proposed for the same reaction of the closely related [W₃S₄H₃(dmpe)₃]⁺ cluster, which involves parallel first‐ and second‐order pathways in which the coordinated hydride interacts with one and two acid molecules, and ion pairing to BF₄^? hinders formation of dihydrogen bonded adducts able to evolve to the products of proton transfer. Additional DFT calculations are reported to understand the behavior of the cluster in neat acetonitrile and acetonitrile–water mixtures. The interaction of the HCl molecule with CH₃CN is stronger than the W? H???HCl dihydrogen bond and so the reaction pathways operating in dichloromethane become inefficient, in agreement with the lack of reaction between 1 ⁺ and HCl in neat acetonitrile. However, the attacking species in acetonitrile–water mixtures is the solvated proton, and DFT calculations indicate that the reaction can then go through pathways involving solvent attack to the W centers, while still maintaining the coordinated hydride, which is made possible by the capability of the cluster to undergo structural changes in its core.     

Delineating Binding Modes of Gal/GalNAc and Structural Elements of the Molecular Recognition of Tumor‐Associated Mucin Glycopeptides by the Human Macrophage Galactose‐Type Lectin

The human macrophage galactose‐type lectin (MGL) is a key physiological receptor for the carcinoma‐associated Tn antigen (GalNAc‐α‐1‐O‐Ser/Thr) in mucins. NMR and modeling‐based data on the molecular recognition features of synthetic Tn‐bearing glycopeptides by MGL are presented. Cognate epitopes on the sugar and matching key amino acids involved in the interaction were identified by saturation transfer difference (STD) NMR spectroscopy. Only the amino acids close to the glycosylation site in the peptides are involved in lectin contact. Moreover, control experiments with non‐glycosylated MUC1 peptides unequivocally showed that the sugar residue is essential for MGL binding, as is Ca²⁺. NMR data were complemented with molecular dynamics simulations and Corcema‐ST to establish a 3D view on the molecular recognition process between Gal, GalNAc, and the Tn‐presenting glycopeptides and MGL. Gal and GalNAc have a dual binding mode with opposite trend of the main interaction pattern and the differences in affinity can be explained by additional hydrogen bonds and CH–π contacts involving exclusively the NHAc moiety.     

Quantum stochastic synchronization

We study, within the spin-boson dynamics, the synchronization of a quantum tunneling system with an external, time-periodic driving signal. As a main result, we find that at a sufficiently large system-bath coupling strength (i.e., for a friction strength alpha > 1) the thermal noise plays a constructive role in yielding forced synchronization. This noise-induced synchronization can occur when the driving frequency is larger than the zero-temperature tunneling rate. As an application evidencing the effect, we consider the charge transfer dynamics in molecular complexes.     

On the growth and range of functions in Möbius invariant spaces

This paper is a continuation of our earlier work and focuses on the structural and geometric properties of functions in analytic Besov spaces, primarily on univalent functions in such spaces and their image domains. We improve several earlier results.     

Diastereoselective and enantiospecific synthesis of gamma-substituted alpha,beta-unsaturated nitriles from O-protected allylic cyanohydrins

gamma-Functionalized alpha,beta-unsaturated nitriles are prepared diastereoselectively and enantiospecifically from enantioenriched cyanohydrin-O-phosphates and carbonates derived from alpha,beta-unsaturated aldehydes, either by palladium or iridium-catalyzed nucleophilic allylic substitution reactions with different nucleophiles. Appropriate reaction conditions for dibenzylamine, benzylamine, sodium azide, NaOAc, tetra-n-butylammonium acetate (TBAA), the corresponding sodium salts of phenol and N-hydroxysuccinimide and the carbonucleophile sodium dimethyl malonate are described. Different substituted O-protected cyanohydrins, such as carbonates and phosphates, derived from crotonaldehyde, (E)-hex-2-enal, oct-2-enal, 2-methylbut-2-enal, and cinnamaldehyde are used as allylic substrates. The substitution takes place with total retention of the configuration for the (E)-gamma-functionalized nitriles and with inversion of the configuration for the Z-isomers. In general, cyanohydrin-O-phosphates are the materials of choice to get the highest E-diastereoselectivity. Dibenzylamine is the best nucleophile for the synthesis of gamma-nitrogenated alpha,beta-unsaturated nitriles in the presence of either palladium or iridium catalysts when aliphatic compounds and cinnamaldehyde derivative are used (up to 98% dr). For the synthesis of gamma-oxygenated alpha,beta-unsaturated nitriles sodium or TBAA the reagents are selected to avoid epimerizations in up to 76% dr. Finally, the Tsuji-Trost reaction with sodium malonate works only under palladium catalysis in up to 70% dr.