E. Thomas Strom and Vera V. Mainz (eds): The posthumous Nobel Prize in chemistry. Volume 1. Correcting the errors and oversights of the Nobel Prize committee

Structural Chemistry -     

Synthesis and Structural Assignment of 1-O-Acetyl-2,3,5-tri-O-benzoyl-4-(thymin-1-yl)-α-l-lyxofuranose

Abstract

Analogues of nucleosides in which the nucleobase is fixed onto the C-4 of the sugar moiety are generally prepared either from 4,5-unsaturated sugar derivatives or via a formaldehyde condensation.¹ We tested the furanosyl bromide reactivity of 1 ² towards a series of nucleophiles, mostly azides or cyanides, without success. Conversely, the nucleosidation of 1 using 5-methyl-2,4-bis(trimethyl-silyloxy)pyrimidine in the presence of stannic chloride took place at the second anomeric position (C-4) and led to the isolation in acceptable yield (47%) of a unique anomer 2 (Scheme 1).     

Derivatives of 3-Deoxy-3-(N-Hydroxyamino)-D-Ribose

Abstract

A series of 3-(N-arylmethyl-N-hydroxyamino)-l,2-O-cyclopentylidene-3-deoxy-5-O-toluoyl-α-D-riboses has been prepared. The blocking groups used were chosen to allow an easy nucleosidation of these compounds to spin labelled analogs of natural nucleosides. The conformational behavior of the N-arylmethyl-N-hydroxyamino group has been studied using ³/_CH NMR coupling data and molecular mechanics computations. Upon spontaneous oxidation, these hydroxylamines led to the corresponding aminoxyl free radicals which were submitted to EPR spectroscopy and quantum mechanical computations at a semiempirical level (PM3).     

Synthesis of Sialyl Lewis X Analogues

Abstract

Sialyl Lewis X (SLe^x) analogs 2a and 2b were synthesised, where the N-acetyl-D-glucose and the D-galactose units of SLe^x 1 were replaced with an alkyl and a heteroalkyl spacer. Sulphate ester 6i was also synthesised from alcohol 6b and chlorosulphonic acid. A novel promoter, silver mercaptoethanesulphonate, was used to synthesise α-sialosides 2c. 7b and 7c.     

In celebration of 30 years of Structural Chemistry: a “delocalized” special issue

Structural Chemistry -     

Some aspects of the living isobutylene polymerization

The possibilities of increasing the livingness in cationic isobutylene polymerizations has been investigated. It has been assumed that the living chain-end is polarized but non-dissociated and relatively stable, therefore, i) shifting of the dissociation equilibrium by adding salts with common or non-common anion in excess, and ii) increasing of the stability of the chain-end by complexing it with electrondonors, may increase the livingness of the system. Indeed, the addition of n-Bu₄NCl or n-Bu₄NI to systems with BCl₃ or TiCl₄ coinitiators, as well as the addition of dimethyl sulphoxide as electrondonor in a broad concentration range, improved the living character. The possibility of complexation of the chain-end by the monomer has also been supported     

A Multicolumn Ion Chromatographic Determination of Nitrate and Sulfate in Waters Containing Humic Substances

Abstract

A three-column ion chromatographic system for the removal of humic substances from natural waters, and subsequent on-line concentration and determination of nitrate and sulfate using non-suppressed ion chromatography is presented. Humic substances are removed using disposable adsorption columns packed with chemically bonded amine silica material. The sample is directly transfered to an ion exchange column where the anions are concentrated ca 10 times. After reversing the flow, the ions are transferred to a third column where they are separated and quantified. The detection limit is less than 1 mg L^?1 of nitrate or sulfate in water containing 45mgL^?1 of humic acid.     

Populations of coupled electrochemical oscillators

Experiments were carried out on arrays of chaotic electrochemical oscillators to which global coupling, periodic forcing, and feedback were applied. The global coupling converts a very weakly coupled set of chaotic oscillators to a synchronized state with sufficiently large values of coupling strength; at intermediate values both intermittent and stable chaotic cluster states occur. Cluster formation and synchronization were also obtained by applying feedback and forcing to a moderately coupled base state. The three cases differ, however, in other details. The feedback and forcing also produce periodic cluster states and more than two clusters. Configurations of two (chaotic) clusters and two, three, or four (periodic) clusters were observed. (c) 2002 American Institute of Physics.     

Detecting embedded pure network structures in LP problems

Recently, linear programming problems with special structures have assumed growing importance in mathematical programming. It is well known that exploiting network structures within linear programs can lead to considerable improvement of the computational solution of large-scale linear programming problems. A linear program is said to contain an embedded network structure provided that some subset of its constraints can be interpreted as specifying conservation of flow. If a column of the constraint matrix has at most two non-zeros, then it leads to embedded generalized network structure and if these non-zeros are unit elements and of opposite signs, then it leads to embedded pure network structure. In this paper, we are concerned with algorithms for detecting embedded pure network structures within linear programs. The network extraction methods are presented in two groups. The first group covers deletion and addition based algorithms and the second group covers GUB based algorithms. We have extended the GUB based algorithm appearing in the second group by introducing Markowitz merit count approach for exploiting matrix non zeros. A set of well known test problems has been used to carry out computational experiments which show that our extensions to the GUB based algorithms give better results than the algorithms reported earlier.