Paterno-Buchi photocyclization of 2-siloxyfurans and carbonyl compounds. Notable substituent and carbonyl (Aldehyde vs ketone and singlet- vs triplet-excited state) effects on the regioselectivity (Double-bond selection) in the formation of bicyclic exo-oxetanes

Paterno-Buchi coupling, photochemical [2 + 2] cycloaddition, of carbonyl compounds 2a-f with 2-siloxyfurans 1a-d has been investigated in detail. The stereoselective formations of exo-oxetanes 3 and 4 were observed in high yields. The regioselectivity (double-bond selection, 3 vs 4) was found to be largely dependent upon the carbonyls, the substituents at the furan ring, and the excited state of the carbonyls (singlet vs triplet). The photoreaction of aldehydes 2a-c gave bicyclic exo-oxetanes 3 and 4 at regio-random, independent upon their excited states and the substituents at furan ring. However, the photoreaction of the triplet state of ketones 2d-f was found to give regioselectively exo-oxetanes 4, except for the 4-methyl-2-siloxyfurane 1d case. The singlet-excited state of acetone 2f gave both oxetanes 3 and 4 at regio-random. For the singlet-state photochemistry, the approach direction of the electrophilic oxygen of the excited carbonyls to the furan ring is proposed to be an important factor for the exo-stereoselection. The Griesbeck model can rationalize the regio- and exo-selective formation of oxetanes in the triplet-state photoreaction.     

Syntheses of linear ethoxysiloxanes by the oxidative condensation of triethoxysilane

Linear ethoxysiloxanes were synthesized by the oxidative condensation of hydrosilane. Triethoxysilane was subjected to oxidation to form triethoxysilanol, and pentaethoxydisiloxane was formed by the condensation reaction of triethoxysilane with triethoxysilanol. Pentaethoxydisiloxane was also subjected to oxidative condensation to form a mixture of nona- and decaethoxytetrasiloxanes. In contrast, pentaethoxydisiloxane, heptaethoxytrisiloxane, and nonaethoxytetrasiloxanes were subjected to the reaction with ethanol in the presence of zinc to isolate hexaethoxydisiloxane, octaethoxytrisiloxane, and decaethoxytetrasiloxane, respectively.     

Sol-Gel Processing of Transparent PLZT((Pb,La)(Zr,Ti)O3) Fibers

PLZT((Pb,La)(Zr,Ti)O₃) ceramic fibers were prepared by the sol-gel method from a solution of lead acetate trihydrate, lanthanum isopropoxide, zirconium n-propoxide and titanium isopropoxide containing 2-methoxyethanol as the solvent. The as-drawn gel fibers were pretreated in an autoclave and preheated in flowing H₂O vapor prior to the heat-treatment for crystallization. The effects of autoclaving and preheating in flowing H₂O vapor on the structure and composition of the resultant fibers were investigated. Optically transparent single phase perovskite PLZT fibers of 5–15 m in thickness and 5 cm in length were successfully prepared when the autoclave-treatment and the preheating in flowing H₂O vapor were conducted prior to crystallization. These treatments are thought to promote hydrolysis of the organic groups and polycondensation reaction in the gel fibers, leading to the successful formation of dense, crack-free transparent PLZT fibers.     

Isotope fractionation of molecular oxygen during adsorption/desorption by molecular sieve zeolite

The isotope fractionation of O(2) gas during adsorption onto, and desorption from, molecular sieve zeolite (MSZ) has been confirmed. It has been found that the magnitude of fractionation during gas adsorption for the determination of isotope ratio measurements by dual inlet mass spectrometry is significant during desorption but not during adsorption. The isotope shift will take place during insufficient desorption from MSZ and its magnitude varies with the amount of MSZ and its identity. In order to obtain complete desorption and to remove isotope shifts, the use of the minimum amount of 5A MSZ and additional heating at a temperature of 60 degrees C for a period of 10 min just before gas admission into the mass spectrometer are recommended.     

Comparison of the characteristics of cellulose microfibril aggregates of wood, rice straw and potato tuber

The focus of this study has been to isolate cellulose microfibril aggregates by the one-time grinding treatment from wood, rice straw and potato tuber, and to compare their morphological and mechanical properties. Field emission scanning electron microscopy images showed that the diameter range of isolated microfibril aggregates from wood, 12–20 nm, was smaller than those from rice straw and potato tuber, 12–35 nm and 12–55 nm, respectively. These differences were observed even in the purified rice straws and potato tuber before the grinder treatment, but were hardly observed in the purified wood. The results of X-ray analysis and tensile tests indicated that there were no significant differences among the sources in the cellulose crystallinity and Young’s modulus of the isolated microfibril aggregates in the dry state. These results suggest that the inherent characteristics of cellulose microfibril aggregates in the dry state are very similar regardless of plant sources and tissue functions.     

Biosynthetic pathway for high structural diversity of a common dilactone core in antimycin production

We herein report comparative analysis of two versions of the biosynthetic gene clusters of antimycins, a natural product family possessing up to 44 distinct entities. The biosynthetic pathway of antimycins is amenable to the high structural variation of the substrates, supported by successes in heterologous expression of the ant cluster and in fluorine incorporation. The latter facilitated the investigation of the structure-activity relationship into the usually invariable 3-formamidosalicylic acid moiety of the molecules.     

Separation of perfluoroalkylsulfinic acids and perfluoroalkylsulfonic acids by ion-exclusion chromatography

Ion-exclusion chromatography has been successfully applied to the separation of a number of perfluoroalkylsulfinic acids and perfluoroalkylsulfonic acids. The separation of various perfluoroalkylsulfinic and perfluoroalkylsulfonic acids, with different alkyl groups, was investigated on a polymethacrylate-based, weakly acidic, cation-exchange resin (TSK gel OApak-A) in the H⁺-form and using conductimetric detection. When water was used as the eluent, these perfluoroalkylsulfinic and perfluoroalkylsulfonic acids could not be resolved. When an aqueous solution of benzoic acid and o-phthalic acid was used, the separation of each of these acids occurred. In order to improve their separation, the effect of the addition of methanol and 2,2,2-trifluoroethanol, as organic modifiers, was also investigated.     

Effect of peracylation of beta-cyclodextrin on the molecular structure and on the formation of inclusion complexes: an X-ray study.

The molecular structures of peracylated beta-cyclodextrins (CDs)--heptakis(2,3,6-tri-O-acetyl)-beta-CD (TA), heptakis(2,3,6-tri-O-propanoyl)-beta-CD (TP), and heptakis(2,3,6-tri-O-butanoyl)-beta-CD (TB)--have been determined by single crystal X-ray structure analysis. Due to the lack of O2...O3' hydrogen bonds between adjacent glucose units of the peracylated CDs, the macrocycles are elliptically distorted into nonplanar boat-shaped structures. The glucose units are tilted with respect to the O4 plane to relieve steric hindrance between adjacent acyl chains. In TB, all glucose units adopt the common (4)C(1)-chair conformation and one butanoyl chain intramolecularly penetrates the cavity, whereas, in TA and TP, one glucose unit each occurs in (O)S(2)-skew-boat conformation and one acyl chain closes the O6 side like a lid. In each of the three homologous molecules the intramolecular self-inclusion and lidlike orientation of acyl chains forces the associated O5-C5-C6-O6 torsion angle into a trans-conformation never observed before for unsubstituted CD; the inclusion behavior of TA, TP, and TB in solution has been studied by circular dichroism spectroscopy with the drug molsidomine and several organic compounds. No inclusion complexes are formed, which is attributed to the intramolecular closure of the molecular cavity by one of the acyl chains.     

Syntheses of amphiphilic block copolymers by living cationic polymerization of vinyl ethers and their selective solvent‐induced physical gelation

Amphiphilic diblock copolymers were prepared by the living cationic polymerization of vinyl ethers in the presence of added bases, and their selective solvent‐induced physical gelation behavior was investigated. The block copolymerization of 2‐phenoxyethyl vinyl ether (PhOVE) and 2‐methoxyethyl vinyl ether (MOVE) was carried out in the presence of ethyl acetate with Et_1.5AlCl_1.5 in toluene at 0 °C. Despite the rate difference, diblock copolymers with a very narrow molecular weight distribution were obtained, quantitatively. By adding the selective solvent, water, to the acetone solution of the diblock copolymer, PhOVE₂₀₀‐b‐MOVE₄₀₀, physical gelation occurred suddenly and the system ceased to flow, maintaining transparency. Viscoelastic measurements and transmission electron microscopic observations were performed to examine the characteristic gelation behavior and structure of the obtained gels. Various gelation conditions and physical gelation by other amphiphilic block copolymers were also designed on the basis of the solubility of each block segment. Further, new forms of physical gelation, accompanied by the solubilization of immiscible organic compounds, were achieved using similar diblock copolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3190–3197, 2001