N,N‐dimethylaminopyridine as initiator in the copolymerization of diglycidylether of bisphenol A with six‐membered cyclic carbonates

N,N‐Dimethylaminopyridine (DMAP) was used as initiator to cure mixtures of diglycidylether of bisphenol A (DGEBA) and 1,3‐dioxan‐2‐one (TMC) or 5,5‐dimethyl‐1,3‐dioxan‐2‐one (DMTMC). The curing was studied by differential scanning calorimetry (DSC) and Fourier transform infrared in the attenuated‐total‐reflection mode (FTIR/ATR). FTIR/ATR was used to monitor the competitive reactive processes and to quantify the evolution of the groups involved in the curing. We observed the formation of five‐membered cyclic carbonates and anionic carbonate groups that remain unreacted at the chain ends. The formation of these groups was explained by the attack of the anionic propagation species on the methylene carbon of the carbonate group, which leads to an alkyl‐oxygen rupture. By performing the cure in the thermobalance we could evaluate the loss of CO₂ produced in the samples containing carbonates. The kinetics were studied by DSC and analyzed with isoconversional procedures. The addition of carbonates slows down the curing rate. Thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA) experiments were used to evaluate the properties of the materials obtained. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2873–2882, 2006     

Classification of Nonorientable 3-Manifolds Admitting Decompositions into ≤ 26 Coloured Tetrahedra

The present paper adopts a computational approach to the study of nonorientable 3-manifolds: in fact, we describe how to create an automaticcatalogue of all nonorientable 3-manifolds admitting coloured triangulationswith a fixed number of tetrahedra. In particular, the catalogue has been effectively produced and analysed for up to 26 tetrahedra, to reach the complete classification of all involved 3-manifolds. As a consequence, the following summarising result can be stated:THEOREM I. Exactly seven closed connected prime nonorientable3-manifolds exist, which admit a coloured triangulation consisting of atmost 26 tetrahedra.More precisely, they are the four Euclidean nonorientable 3-manifolds, the nontrivial S2 bundle overS1, the topological product between thereal projective plane RP2 andS1, and the torus bundle overS1, with monodromy induced by matrix(10 -11).     

MNDO study of reaction paths: Diboration of acetylene

The reaction of B₂H₄ with acetylene has been studied by the MNDO method. It is shown that the reaction is exothermic and proceeds in two steps. The first step is the formation of a three-center -complex and this is the rate-determining step of the reaction. The second step is the rearrangement of the -complex to the product and this step requires a very small amount of activation energy. The activation barrier for the diboration reaction is 12.8 kcal/mol.The proposed mechanism is significantly different from those proposed earlier and explains all experimental data relating to this reaction.     

Diasterodivergent synthesis of optically pure vinyl episulfides and β-hydroxy thiocyanates from a bacterial metabolite

Diasterodivergent episulfides were chemoenzymatically derived from bromobenzene by sequential toluene dioxygenase dihydroxylation, followed by chemical epoxidation and thiolysis. The epoxide ring-opening by thiocyanate ion under literature conditions rendered the corresponding hydroxy thiocyanates and not the thiiranes as usually observed. Ring closing under carefully optimized conditions allowed the preparation of optically pure thiiranes that are key precursors for the preparation of thioconduritols and pseudodisaccharides.     

Synthesis and hydrolysis of substituted tetrahydropyrimidinium salts. Behaviour of the degradation products on varying pH

Reaction of 1,2-diaryl-3-methyl-1,4,5,6-tetrahydropyrimidinium iodides 1a-i with alkaline solutions afforded N-aroyl-N-aryl-N'-methyltrimethylenediamines 2a-i . Compounds 2 are stable under acid conditions but in neutral or alkaline media spontaneously rearrange giving N-aroyl-N'-aryl-N-methyltrimethylenediamines 3a-i . Treating compounds 3 with concentrated acids reverse reaction takes place. Kinetic studies were performed on this intramolecular N → N' aroyl transfer over the H_o-pH range -0.9 to 2.30. Compounds 3 undergo acyl transfer to give 2 by a mechanism which involves a change in the rate determining step from formation to acid-catalysed decomposition of a six-membered heterocyclic intermediate on going from H_o to pH values. The existence of maxima in the pH rate profile allow to determine apparent pKa values of the hexahydropyrimidine intermediates which gave good correlation with the Swain F substituent constants. Stability of these heterocycles was also predicted by determination of thermodinamic parameters. Comparisons are made with the behaviour of five-membered heterocyclic intermediates (imidazolidine derivatives) which were studied in an earlier paper.     

Thermal studies of pre-formulates of metronidazole obtained by spray drying technique

Compatibility studies between active drugs and excipients are substantial in the pharmaceutical technology. The objective of the present work was to develop pre-formulated mixtures of metronidazole (MT) obtained by spray drying (SPDR) and their thermoanalytical characterization. Dynamic and isothermal TG, conventional DSC and DSC coupled to a photovisual system were used. DSC experiments with both techniques confirmed the homogeneity of the conventional and pre-formulated mixtures. The TG data made possible the comparison the thermal stability of the different mixtures. Similar thermal stabilities were found of the conventional and pre-formulated mixtures, with slower particles sizes of MT.     

Assembly of the hexameric Escherichia coli arginine repressor investigated by nano-electrospray ionization time-of-flight mass spectrometry

The arginine repressor (ArgR) from Escherichia coli regulates genes for L-arginine metabolism and is a required recombination factor for colE1 plasmid replication. Both functions require binding of L-arginine to the protein. In this work, nano-electrospray ionization time-of-flight mass spectrometry (nano-ESI-TOFMS) is used to study conformational and oligomeric states of intact ArgR and its isolated structural domains. In agreement with X-ray diffraction studies, it is shown that ArgR oligomerizes to form hexamers in both the presence and absence of L-arginine, and the basic unit of oligomerization appears to be the trimer. Higher-order assembly into dodecamers is also detected. The isolated C-terminal domain is found to associate into trimers and hexamers whereas the N-terminal domain is detected in its monomeric form. The observed species distributions suggest a role for the N-terminal domain in hexamer stabilization.     

Coordination chemistry of f-block elements with imine acids. Part IV. Synthetic and chiroptical studies on lanthanide(III) complexes of imine acids derived from L-alanine,L-valine,L-tyrosine and L-glutamic acid

Summary N-(orthovanillidene)L-alanine (OVAlaH₂), N-(salicylidene)L-valine (SValH₂), N-(orthovanillidene)L-valine (OVValH₂), N-(orthovanillidene)L-tyrosine (OVTyrH₃) and N-(salicylidene)L-glutamic acid (SGluH₃) react with La^III, Pr^III, Nd^III and Sm^III to yield new chiral complexes. Their dominant conformers were determined from c.d. spectra supported by¹H n.m.r. data, except for the L-glutamic acid derived complexes which were insoluble.     

Reactions of pyrrolidine enamines of cyclic and acyclic 3,4-dioxobutanoic acid Derivatives with dimethyl acetylenedicarboxylate. A new case of atropoisomerism.

Reaction conditions and structure of the starting enamines (cyclic or open-chain) determine greatly the final products of the title reactions. Whereas in benzene and acetonitrile, DMAD and 1 give a mixture of the diastereoisomeric dienamines 5, in methanol they afford pirrolizine 3. Enaminofuranones 2 and 10 furnish the corresponding “Michael adducts” 7a,b,c and 11a,b,c but fail to yield pirrolizines. It has been demonstrated that above b and c adducts differ exclusively on the arrangement of groups around a chiral axis.