Palladium-catalyzed decarboxylation-carbonylation of allylic carbonates to give β,γ-unsaturated esters under mild conditions

Allylic carbonates undergo facile palladium-catalyzed decarboxylation-carbonylation under mild conditions to give β,γ-unsaturated esters in high yields using palladium-phosphine complex as a catalyst.     

Three-Dimensional H-Bonded Networks Based on Mono- and Tetranuclear Metal-Pteridine Complexes

Abstract

The crystal structure of the transition metal-pteridine complexes having three-dimensional hydrogen-bonded (H-bonded) network and stacking interactions between pteridine ligands are described; (1) Mononuclear copper(II)-pteridine complex and (2) Tetranuclear palladium(II)-pteridine complex.     

Cleavage of esters under nearly neutral conditions at high pressure

Abstract

Hydrolysis of esters proceeded at room temperature under high pressure in the presence of iPr₂, NEt or N-methylmorpholine using CH₃CN—H₂O (60:1) as the solvent. This very mild procedure enables the smooth hydrolysis of biologically important compounds such as amino esters, aliphatic unsaturated fatty esters, and β-hydroxy ester; no racemization, no isomerization, and no side reactions take place. Selective hydrolysis of two ester groups is also accomplished with alcohol exchange reactions. This new synthetic technique can be utilized for the mild and selective hydrolysis of sophisticated molecules.

Hydrolysis of esters is one of the most essential transformations in organic synthesis. Normally, a basic or acidic aqueous solution is used for ester hydrolysis. However, the hydrolysis of biologically related molecules such as amino esters, peptides, …, or unstaturated fatty esters, under such conditions is accompanied by side reactions, loss of chirality, or isomerization.¹ We report an entirely new approach to this problem via high pressure induced hydrolysis. Hydrolysis is carried out at room temperature under > 8 Kbar in CH₃CN—H₂, O.²     

Chemically-induced geometrical isomerization of stilbenes during peroxyoxalate chemiluminescence reaction: revisit to ‘photochemistry without light’

The chemically-induced isomerization of stilbenes during the peroxyoxalate chemiluminescence (PO-CL) reactions was reinvestigated. The PO-CL reactions using bis(2,4,6-trichlorophenyl) oxalate in the presence of several stilbenes (type A reaction) produced cis-stilbenes in 0–4% yields, which was dependent on the singlet excitation energy of the stilbenes. On the other hand, the PO-CL reactions of the oxalates, containing the stilbene moieties in the molecules (type B reaction), produced cis-stilbenes 0–9.3% yields, some of which were much more effective than the type A reactions considering the amount of the oxalate moiety as the energy supplier.     

A structural basis for the antibiotic resistance conferred by an A1408G mutation in 16S rRNA and for the antiprotozoal activity of aminoglycosides

Resistance explained: The crystal structures of the ribosomal decoding A?site with an A1408G antibiotic-resistance mutation were solved in the presence and absence of the aminoglycoside geneticin (see structure, geneticin carbon framework in yellow). These structures show how bacteria acquire high-level resistance against aminoglycosides by the mutation.     

Two-dimensional surface chirality control by solvent-induced helicity inversion of a helical polyacetylene on graphite

We report the direct evidence for the macromolecular helicity inversion of a helical poly(phenylacetylene) bearing l- or d-alanine pendants with a long alkyl chain in different solvents by atomic force microscopy observations of the diastereomeric helical structures. The diastereomeric helical poly(phenylacetylene)s induced in polar and nonpolar solvents self-assembled into ordered, two-dimensional helix bundles with controlled molecular packing, helical pitch, and handedness on graphite upon exposure of each solvent. The macromolecular helicity deposited on graphite from a polar solvent further inverted to the opposite handedness by exposure to a specific nonpolar solvent, and these changes in the surface chirality based on the inversion of helicity could be visualized by atomic force microscopy with molecular resolution, and the results were quantified by X-ray diffraction of the oriented liquid crystalline, diastereomeric helical polymer films.     

Hierarchical amplification of macromolecular helicity of dynamic helical poly(phenylacetylene)s composed of chiral and achiral phenylacetylenes in dilute solution, liquid crystal, and two-dimensional crystal

Optically active poly(phenylacetylene) copolymers consisting of optically active and achiral phenylacetylenes bearing L-alanine decyl esters (1L) and 2-aminoisobutylic acid decyl esters (Aib) as the pendant groups (poly(1L(m)-co-Aib(n))) with various compositions were synthesized by the copolymerization of the optically active 1L with achiral Aib using a rhodium catalyst, and their chiral amplification of the macromolecular helicity in a dilute solution, a lyotropic liquid crystalline (LC) state, and a two-dimensional (2D) crystal on the substrate was investigated by measuring the circular dichroism of the copolymers, mesoscopic cholesteric twist in the LC state (cholesteric helical pitch), and high-resolution atomic force microscopy (AFM) images of the self-assembled 2D helix-bundles of the copolymer chains. We found that the macromolecular helicity of poly(1L(m)-co-Aib(n))s could be hierarchically amplified in the order of the dilute solution, LC state, and 2D crystal. In sharp contrast, almost no chiral amplification of the macromolecular helicity was observed for the homopolymer mixtures of 1L and Aib in the LC state and 2D crystal on graphite.     

Crystal structure of a NIR‐Emitting DNA‐Stabilized Ag16 Nanocluster

DNA has been used as a scaffold to stabilize small, atomically monodisperse silver nanoclusters, which have attracted attention due to their intriguing photophysical properties. Herein, we describe the X‐ray crystal structure of a DNA‐encapsulated, near‐infrared emitting Ag₁₆ nanocluster (DNA–Ag₁₆NC). The asymmetric unit of the crystal contains two DNA–Ag₁₆NCs and the crystal packing between the DNA–Ag₁₆NCs is promoted by several interactions, such as two silver‐mediated base pairs between 3′‐terminal adenines, two phosphate–Ca²⁺–phosphate interactions, and π‐stacking between two neighboring thymines. Each Ag₁₆NC is confined by two DNA decamers that take on a horse‐shoe‐like conformation and is almost fully shielded from the solvent environment. This structural insight will aid in the determination of the structure/photophysical property relationship for this class of emitters and opens up new research opportunities in fluorescence imaging and sensing using noble‐metal clusters.     

Photochromism of a radical diffusion-inhibited hexaarylbiimidazole derivative with intense coloration and fast decoloration performance

We report the synthesis and the photochromic behavior of a newly designed, photochromic, radical diffusion-inhibited hexaarylbiimidazole (HABI) derivative with markedly improved photochromic performance in coloration and decoloration rates as well as greater optical density in the colored state. The thermal bleaching rate (tau1/2 = 260 ms at 295 K) is the fastest among the reported ones for HABI derivatives.