Utility of a chiral 1,3-dioxane template in stereoselective intramolecular Diels-Alder reactions

The ethylidene acetal of d-erythrose was used as a template for stereoselective IMDA reactions: high endo selectivity and yields in favor of the cis product were observed with 1,3,9-trienes, resulting from a boat transition state. For natural product synthesis, the reaction was successfully applied to a diene with terminal Z-olefin.     

Bosonic helium clusters doped by alkali metal cations: interaction forces and analysis of their most stable structures

Ab initio potentials are computed for alkali metal cationic partners interacting with ⁴He and an overall many-body potential is constructed for each of the ionic dopants in helium clusters. The structures are then obtained via a genetic algorithm approach and results compared with Basin-Hopping Monte Carlo simulations. The classical arrangements are analyzed and quantum effects discussed in comparison with what has been found with Diffusion Monte Carlo calculations. Further corrections to the classical picture by including three-body forces and radial delocalization of the helium adatoms are also considered and their effects analyzed. This work is dedicated to the late Nando Bernardi, an eclectic and gifted scientist and a dear friend whose early departure has left a sad void in our community.     

An improved approximation algorithm for the asymmetric TSP with strengthened triangle inequality

We consider the asymmetric traveling salesperson problem with γ-parameterized triangle inequality for γ[1/2,1). That means, the edge weights fulfill w(u,v)γ(w(u,x)+w(x,v)) for all nodes u,v,x. Chandran and Ram [L.S. Chandran, L.S. Ram, Approximations for ATSP with parametrized triangle inequality, in: Proc. 19th Int. Symp. on Theoret. Aspects of Comput. Sci. (STACS), in: Lecture Notes in Comput. Sci., vol. 2285, Springer, Berlin, 2002, pp. 227–237] gave the first constant factor approximation algorithm with polynomial running time for this problem. They achieve performance ratio γ/(1−γ). We devise an approximation algorithm with performance ratio (1+γ)/(2−γ−γ³), which is better for γ[0.5437,1), that is, for the particularly interesting large values of γ.     

[Cs6Cl][Fe24Se26]: A Host–Guest Compound with Unique Fe–Se Topology

The novel host–guest compound [Cs₆Cl][Fe₂₄Se₂₆] (I4/mmm; a=11.0991(9), c=22.143(2) Å) was obtained by reacting Cs₂Se, CsCl, Fe, and Se in closed ampoules. This is the first member of a family of compounds with unique Fe–Se topology, which consists of edge‐sharing, extended fused cubane [Fe₈Se₆Se_8/3] blocks that host a guest complex ion, [Cs₆Cl]⁵⁺. Thus Fe is tetrahedrally coordinated and divalent with strong exchange couplings, which results in an ordered antiferromagnetic state below T_N=221 K. At low temperatures, a distribution of hyperfine fields in the Mössbauer spectra suggests a structural distortion or a complex spin structure. With its strong Fe–Se covalency, the compound is close to electronic itinerancy and is, therefore, prone to exhibit tunable properties.     

Characterisation of regioselectively functionalized 2,3-O-carboxymethyl cellulose by enzymatic and chemical methods

The determination of the molecular structure of 2,3-O-carboxymethyl cellulose (2,3-O-CMC), prepared via 6-O-(4-monomethoxy)triphenylmethyl cellulose, was carried out in detail by means of enzymatic and chemical methods. The 2,3-O-CMCs had degrees of substitution (DS) in the range of 0.5–1.2 showing a narrow molar mass distribution as revealed by SEC. As a result of an endoglucanase treatment, an intensive depolymerization of the samples occurred which was more pronounced for 2,3-O-CMC with comparatively low DS. All degraded samples could be separated into 18 fractions by preparative SEC and the proportion of each individual repeat unit was analysed by anion exchange chromatography (AEC) following complete hydrolytic chain degradation. The results indicated a homogeneous distribution of the functional groups within the polymer chain. Moreover, it became obvious that a preferred carboxymethylation of O-2 compared with O-3 occurred and that a preferred functionalization of already carboxymethylated units occurred as the reaction progressed. AEC with pulsed amperometric detection, which was used to separate and analyse the differently functionalized repeating units as well as glucose, had to be calibrated. Therefore, a method to determine the response factors of the individual carboxymethylated glucose units was developed using ¹³C NMR spectroscopic measurements (inverse gated decoupling) of depolymerised 2,3-O-CMC.