Dispersion of Electric Dichroism for Macromolecules and Particles in Suspension

The wavelength dependence of the optical absorption of solids, liquids and solutions has long been recognised as an indicator of electronic transitions and hence the associated electronic structure of these materials. With solids and highly ordered systems, a useful extension is the study of the absorption at a given wavelength (λ) as a function of the state of polarisation of incident, linearly polarised light. Such ‘dichroism’ data enable one to locate the principal directions of the transition moments in the crystal should a single absorbing chromophore be the sole origin of the effect. This may be done by evaluating the principal components ε₁₁ with i = 1₁, 2 or 3 of the extinction coefficient tensor which describes the system. For cylindrically symmetric particles, ε₁₁ = ε₂₂. Dispersion data of dichroism is thus of double value.     

Novel Selectivity in Carbohydrate Reactions Ii. Selective 6-O-Glycosylation of a Partially Protected Lactoside 1 2

Abstract

A novel regioselectivity was observed in the silver salt promoted glycosylation of 2-(trimethylsilyl)ethyl 3′-O-benzyl-β-D-lactoside using acetobromogalactose as the glycosyl donor. The resulting trisaccharide, obtained in 67% yield, was shown to have the newly formed β-glycosidic linkage at the O-6 position of the lactoside. This was confirmed by synthesis of the authentic product by an alternate route. The novel regioselectivity observed is attributed to the presence of the axially disposed 4′-OH group in the lactoside acceptor.

     

Dicationic Sulfur Analogues of N‐Heterocyclic Silylenes and Phosphenium Cations

DABling with sulfur : Sulfur(II) dications can be prepared using α‐diimines to stabilize the positive charge (see scheme; DAB=diazabutadiene, Dipp=2,6‐diisopropylphenyl, OTf=CF₃SO₃). The bonding is best described as that of a N,N‐chelated sulfur(II) dication; these species represent the first sulfur‐based structural mimics of N‐heterocyclic silylene compounds and phosphenium cations.

     

Cover Picture: Dicationic Sulfur Analogues of N‐Heterocyclic Silylenes and Phosphenium Cations (Angew. Chem. Int. Ed. 12/2009)

Highly reactive electrophilic compounds centered around multicationic main‐group elements are notoriously difficult to isolate. In the Communication on page 2210 ff., P. J. Ragogna and co‐workers describe the synthesis and characterization of salts of sulfur(II) dications stabilized by two chelating nitrogen atoms. These species are the first sulfur‐based analogues of N‐heterocyclic silylene compounds and are the most recent addition to the family of compounds featuring divalent p‐block elements. Mitch Zimmer is acknowledged for the cover design.

     

Stereodynamics and edge-to-face CH-pi aromatic interactions in o-phenethyl-substituted biaryls

As part of a search for systems that exhibit intramolecular aromatic edge-to-face interactions, a series of four biaryl compounds containing a phenethyl side chain were prepared. These compounds exist as a slowly interconverting mixture of two atropisomers due to steric hindrance to rotation about the biaryl bond. The more thermodynamically stable isomer exhibits an abnormal shielding of an ortho-proton in solution indicative of an edge-to-face CH-pi interaction with the terminal phenyl ring on the side chain. This tilted-T type of geometry was observed in the X-ray crystal structure of one of the compounds. The edge-to-face conformation in solution is estimated by variable temperature NMR studies to be energetically favored by ca. 1.6 kcal mol(-1) but entropically disfavored by ca. 5.0 cal K(-1) mol(-1).     

Efficient total syntheses and structural verification of both diospongins A and B via a common delta-lactone intermediate

The total syntheses of the two aryl C-glycoside natural products diospongins A and B are described. The key reactions involved stereoselective reductions of the appropriate oxocarbenium cations that were derived from a common delta-lactone intermediate.     

An efficient total synthesis and absolute configuration determination of varitriol

The first total synthesis and the absolute configuration determination of varitriol are described.     

Fluorescent lifetime quenching near d = 1.5 nm gold nanoparticles: probing NSET validity

The fluorescence behavior of molecular dyes at discrete distances from 1.5 nm diameter gold nanoparticles as a function of distance and energy is investigated. Photoluminescence and luminescence lifetime measurements both demonstrate quenching behavior consistent with 1/d(4) separation distance from dye to the surface of the nanoparticle. In agreement with the model of Persson and Lang, all experimental data show that energy transfer to the metal surface is the dominant quenching mechanism, and the radiative rate is unchanged throughout the experiment.