Synthesis and ring-chain-ring tautomerism of bisoxazolidines, thiazolidinyloxazolidines, and spirothiazolidines

The synthesis of fused heterocycles such as thiazolidinyl-oxazolidine 3 is described starting from Tris·HCl. The mercaptomethyl bisoxazolidine 8 was found to convert to the corresponding thiazolidinyloxazolidine 3 and the spiro-heterocycle 4 by a ring-chain-ring tautomerism, depending on the electronic nature of the ring substituents as well as the reaction conditions. This equilibration pathway is absent in the hydroxymethyl bisoxazolidines 2. Computational studies confirm that both kinetic and thermodynamic control features play a role in the product distribution.     

Scattering from particles on surfaces: visibility factor and polydispersity

We have developed an experimental method based on the visibility factor of light-scattering minima to obtain size-polydispersity information from contaminants upon a flat substrate. We verify the method by using double-interaction-model calculations and use this technique to examine experimentally the radial variation of a micrometer-sized fiber and the size polydispersity of spherical particles upon a substrate.     

Directionality in scattering by nanoparticles: Kerker's null-scattering conditions revisited

Since the first studies made by Kerker in the 1970s stating the conditions for null light scattering in certain directions by particles, such conditions have remained unquestioned. The increasing interest in scattering directionality by tuning the optical properties of materials demands a new analysis of this problem. In addition, as has been shown recently, one of Kerker's statements does not comply with the optical theorem. We propose corrected expressions for the null-scattering conditions that satisfy the optical theorem.     

Influence of the substrate optical properties on the backscattering of contaminated microstructures

The presence of small defects in micron-sized structured surfaces introduces small changes in the backscattering that can be assessed by means of an integrated parameter. In this work the influence of the optical properties of the substrate on this technique is analyzed for two different configurations: defect on the microstructure and defect on the substrate beside the microstructure.     

Synthesis of pyrrolo[1,3]diazepines by a dipolar cycloaddition—retro-Mannich domino reaction

Microwave irradiation facilitated the synthesis of 4-arylthio-3-oxazolin-5-ones from ethyl cyanoformate, thiophenol, and cyclic ketones. Subsequent decarboxylation and in situ [3+2] cycloaddition provided novel 2,3,4,5-tetrahydro-1H-pyrrolo[1,2-c][1,3]diazepine scaffolds after a spontaneous retro-Mannich domino reaction.     

Bisphosphine‐Functionalized Cyclic Decapeptides Based on the Natural Product Gramicidin S: A Potential Scaffold for Transition‐Metal Coordination

The natural product Gramicidin S is a promising scaffold for novel oligopeptide‐based bisphosphine ligands, combining the advantageous rigid chiral backbone with the close proximity of phosphine substituents. The required unnatural, phosphine‐containing, amino acid building blocks were synthesized by means of a novel protocol that involves the enantioselective alkylation of a chiral nickel Schiff base template. Three Ni complexes were prepared with different alkyl chains between the phosphine group and the α‐carbon atom of the incorporated glycine; the absolute stereochemistry of two of them was determined by single‐crystal X‐ray structure analysis. By detaching the template, enantiopure L ‐phosphine amino acids resulted enabling the solid‐phase, stepwise construction of a linear sequence of the phosphine‐modified oligopeptides. On cyclization three bisphosphine‐substituted Gramicidin S analogues resulted, differing only in the size and shape of the linkage between the phosphine groups and the oligopeptides backbone. Their crystal structures suggest these species to have potential as chelating ligands.     

Molecular Weight Distribution of a Clinic Dextran

A sample of clinic dextran (M_n = 54,000) has been fractionated by precipitation at 25° C in the system water/ethanol. Seven fractions were isolated in the usual way and freezedried. Fitting of the molecular weight distribution to the exponential functions of Tung and Schulz gives a good approximation, but it is improved by using a linear combination of the exponential, logarithmic, and normal functions, according to a method recently developed.     

Application of a Laplace Transform Method to the Study of Binary Mixtures of Spherical and Rod-Like Particles for Low Intensity Levels.

A Laplace Transform (LT) method is used to improve the results obtained in a quasi-elastic scattering (QELS) experiment. This is designed to determine the characteristic parameters of a bimodal solution of spherical and rod-like macromolecules. The theoretical study and the experimental simulation are carried out for low light levels and the results show that the LT technique provides a real alternative to the second order correlation function, g⁽²⁾(τ), in QELS measurements for these particularly difficult experimental conditions.