Structural Phase Transitions of Aliphatic Nylons Viewed from the Simultaneous Measurements of WAXD and SAXS

Structural changes occurring in the high temperature region of doubly-oriented nylon 10/10 sample have been investigated through the temperature-dependent simultaneous measurements of wide-angle and small-angle X-ray scatterings, and the results were compared with the infrared spectral data as well as the molecular dynamics simulation results. In the Brill transition region of 150–180 °C the methylene segments are conformationally disordered with keeping the intermolecular hydrogen bonds. During this phase transition the stacked lamellar structure did not change very much: the lamellae are tilted by ca. 34° from the draw axis and the long period is almost 160 Å. In the temperature region immediately below the melting point the molecular chains were found to be contracted by ca. 10% the original repeating period and the intermolecular hydrogen bonds were almost broken, causing the violent rotational and translational motions of the chains around the chain axis. At the same time the long period increased remarkably from 160 Å to 410 Å and the originally tilted lamellae stood up in parallel to the draw axis.     

Solvation structure of magnesium,zinc, and alkaline earth metal ions in N,N‐dimethylformamide,N,N‐dimethylacetamide,and their mixtures studied by means of Raman spectroscopy and DFT calculations—Ionic size and electronic effects on steric congestion

The solvation structure of magnesium, zinc(II), and alkaline earth metal ions in N,N‐dimethylformamide (DMF) and N,N‐dimethylacetamide (DMA), and their mixtures has been studied by means of Raman spectroscopy and DFT calculations. The solvation number is revealed to be 6, 7, 8, and 8 for Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, respectively, in both DMF and DMA. The δ (O C N) vibration of DMF shifts to a higher wavenumber upon binding to the metal ions and the shift Δν(= ν_bound − ν_free) becomes larger, when the ionic radius of the metal ion becomes smaller. The ν (N CH₃) vibration of DMA also shifts to a higher wavenumber upon binding to the metal ions. However, the shift Δν saturates for small ions, as well as the transition‐metal (II) ions, implying that steric congestion among solvent molecules takes place in the coordination sphere. It is also indicated that, despite the magnesium ion having practically the same ionic radius as the zinc(II) ion of six‐coordination, their solvation numbers in DMA are significantly different. DFT calculations for these metalsolvate clusters of varying solvation numbers revealed that not only solvent–solvent interaction through space but also the bonding nature of the metal ion plays an essential role in the steric congestion. The individual solvation number and the Raman shift Δν in DMF–DMA mixtures indicate that steric congestion is significant for the magnesium ion, but not appreciable for calcium, strontium, and barium ions, despite the solvation number of these metal ions being large. Copyright © 2006 John Wiley & Sons, Ltd.     

Polyhydroxybutyrate production from carbon dioxide by cyanobacteria

Genetic characterization and enhancement of polyhydroxybutyrate (PHB) accumulation in cyanobacteria were investigated for efficient PHB production from CO₂. The genome DNAs in the PHB-accumulating strains Synechococcus sp. MA19 and Spirulina platensis NIES46 retained the highly homologous region to phaC of Synechocystis PCC6803, whereas low homology was detected in the nonaccumulating strains Synechococcus sp. PCC7942 and Anabaenacylindrica NIES19. Synechococcus sp. MA19, which accumulates PHB up to 30% of dry cell weight from CO₂ as the sole carbon source, was mutated by insertion of transposon Tn5 to enhance the PHB accumulation. Genetic and physiological analysis of the mutant indicated that decreased phosphotransacetylase activity could trigger an increase of acetyl coenzyme A leading to enhancement of PHB accumulation. PHB synthase in Synechococcus sp. MA19 was probably attached to thylakoid membrane since PHB granules were associated with pigments. A genetically engineered cyanobacteria retaining soluble PHB synthase from Ralstonia eutropha accumulated pigment-free PHB granules, which is an advantage for the purification of PHB.     

Radical C?H Functionalization of Heteroarenes under Electrochemical Control

Electrochemical reactions are shown to be effective for the C H functionalization of a number of heterocyclic substrates that are recalcitrant to conventional peroxide radical initiation conditions. Monitoring reaction progress under electrochemical conditions provides mechanistic insight into the C H functionalization of a series of heterocycles of interest in medicinal chemistry.     

Synthesis of both enantiomers of cyclic methionine analogue: (R)- and (S)-3-aminotetrahydrothiophene-3-carboxylic acids

A method of synthesizing an optically active cyclic methionine analogue, 3-aminotetrahydrothiophene-3-carboxylic acid (At₅c), is described. A Bucherer–Bergs reaction of 4,5-dihydro-3(2H)-thiophenone and the subsequent alkaline hydrolysis of a hydantoin, followed by Cbz protection of the amine, afforded racemic Cbz-At₅c (±)-3 in excellent yield. Diastereomeric esters derived from Cbz-At₅c (±)-3 and (R)-BINOL could be separated by column chromatography to give both diastereomers with >99% de. X-ray crystallographic analysis revealed the absolute configuration of the synthesized amino acid derived from the less polar diastereomeric ester to be (S).     

Total Synthesis and Complete Stereostructure of a Marine Macrolide Glycoside, (−)‐Lyngbyaloside B

We have described in detail the total synthesis of both the proposed and correct structures of (?)‐lyngbyaloside B, which facilitated the elucidation of the complete stereostructure of this natural product. Our study began with the total synthesis of 13‐demethyllyngbyaloside B, in which an esterification/ring‐closing metathesis (RCM) strategy was successfully used for the efficient construction of the macrocycle. We also established reliable methods for the introduction of the conjugated diene side chain and the l ‐rhamnose residue onto the macrocyclic framework. However, the esterification/RCM strategy proved ineffective for the parent natural product because of the difficulties in acylating the sterically encumbered C‐13 tertiary alcohol; macrolactionization of a seco‐acid was also extensively investigated under various conditions without success. We finally completed the total synthesis of the proposed structure of (?)‐lyngbyaloside B by means of a macrolactonization that involves an acyl ketene as the reactive species. However, the NMR spectroscopic data of our synthetic material did not match those of the authentic material, which indicated that the proposed structure must be re‐examined. Inspection of the NMR spectroscopic data of the natural product and molecular mechanics calculations led us to postulate that the configuration of the C‐10, C‐11, and C‐13 stereogenic centers had been incorrectly assigned in the proposed structure. Finally, our revised structure of (?)‐lyngbyaloside B was unambiguously verified through total synthesis.     

Coupling Reaction of Enol Derivatives with Silyl Ketene Acetals Catalyzed by Gallium Trihalides

A cross‐coupling reaction between enol derivatives and silyl ketene acetals catalyzed by GaBr₃ took place to give the corresponding α‐alkenyl esters. GaBr₃ showed the most effective catalytic ability, whereas other metal salts such as BF₃?OEt₂, AlCl₃, PdCl₂, and lanthanide triflates were not effective. Various types of enol ethers and vinyl carboxylates as enol derivatives are amenable to this coupling. The scope of the reaction with silyl ketene acetals was also broad. We successfully observed an alkylgallium intermediate by using NMR spectroscopy, suggesting a mechanism involving anti‐carbogallation among GaBr₃, an enol derivative, and a silyl ketene acetal, followed by syn‐β‐alkoxy elimination from the alkylgallium. Based on kinetic studies, the turnover‐limiting step of the reaction using a vinyl ether and a vinyl carboxylate involved syn‐β‐alkoxy elimination and anti‐carbogallation, respectively. Therefore, the leaving group had a significant effect on the progress of the reaction. Theoretical calculations analysis suggest that the moderate Lewis acidity of gallium would contribute to a flexible conformational change of the alkylgallium intermediate and to the cleavage of the carbon?oxygen bond in the β‐alkoxy elimination process, which is the turnover‐limiting step in the reaction between a vinyl ether and a silyl ketene acetal.