Constituents of regenerated plants of Ophiorrhiza pumila; formation of a new glycocamptothecin and predominant formation of (3R)-deoxypumiloside over (3S)-congener

The regeneration of plantlets was successful from Ophiorrhiza pumila callus cultures, from which a new glucosyloxy camptothecin, 9-β-glucosyloxycamptothecin, together with 15 metabolites including six camptothecin-related alkaloids was isolated. (3S)-Deoxypumiloside, one of the plausible biogenetic precursors of camptothecin, could not be found, whereas the (3R) epimer was isolated from the regenerated plants.     

Recent topics in enantioselective acyl transfer reactions with dialkylaminopyridine-based nucleophilic catalysts

This digest focuses on recent topics in intra- and intermolecular enantioselective acyl transfer reactions with dialkylaminopyridine-based catalysts over the past decade (2007–2017). The structures of the catalysts are divided into four categories according to the location of the chiral environment (C2, C3, or C4 position of pyridine, and miscellaneous). The role of the structure in the enantioselective acyl transfer reaction and the reaction mechanism will be discussed briefly.     

Post-synthetic modification of oligonucleotides containing 5-trifluoromethylpyrimidine bases

A practical and operationally simple post-synthetic modification of oligonucleotides containing 5-trifluoromethylpyrimidine bases is described. Trifluoromethyl group was used as a post-synthetic precursor and 5-trifluoromethylpyrimidine bases within oligonucleotides were converted into the corresponding 5-carboxy-, 5-cyano-, 5-amidinyl-, and 5-carbamoyl derivatives by treatment with an alkaline solution and amines. Moreover, post-synthetic treatment of fully protected and controlled pore glass (CPG)-attached oligonucleotides proceeded successfully with the simultaneous removal of all protecting groups, cleavage from CPG, and conversion of the trifluoromethyl group to afford the corresponding modified oligonucleotides.     

Superflexible Multifunctional Polyvinylpolydimethylsiloxane‐Based Aerogels as Efficient Absorbents,Thermal Superinsulators,and Strain Sensors

Aerogels are porous materials but show poor mechanical properties and limited functionality, which significantly restrict their practical applications. Preparation of highly bendable and processable aerogels with multifunctionality remains a challenge. Herein we report unprecedented superflexible aerogels based on polyvinylpolydimethylsiloxane (PVPDMS) networks, PVPDMS/polyvinylpolymethylsiloxane (PVPMS) copolymer networks, and PVPDMS/PVPMS/graphene nanocomposites by a facile radical polymerization/hydrolytic polycondensation strategy and ambient pressure drying or freeze drying. The aerogels have a doubly cross‐linked organic–inorganic network structure consisting of flexible polydimethylsiloxanes and hydrocarbon chains with tunable cross‐linking density, tunable pore size and bulk density. They have a high hydrophobicity and superflexibility and combine selective absorption, efficient separation of oil and water, thermal superinsulation, and strain sensing.     

Tailoring Spontaneous Pillar Structure Using Phase-Separating Organosiloxane Sol-Gel Systems in Micro-Fabricated Grooves

A sharp transition to pillar structure has been observed in simple sol-gel systems accompanying phase separation inside rectangular-sectioned open grooves which were fabricated on a silica glass chip. The structural variation was investigated using two contrastive systems; one with a rapid sol-gel reaction and the other with a sluggish reaction. In the slower system, transition from web-like bicontinuous structure to pillar structure, we call it as “web-to-pillar transition”, occurred when the bulk characteristic length Λ_m exceeded the width of the groove D. On the other hand, the transition did not occur in the same condition in the faster system; it occurred when Λ_m became much longer than D. A consistent formation mechanism model was also proposed using a relation between interfacial curvatures and pressure.     

Organic intercalation material: reversible change in interlayer distances by guest release and insertion in sandwich-type inclusion crystals of cholic acid

Cholic acid (CA) forms inclusion crystals that have a sandwich-type lamellar structure constructed by the alternative stacking of host bilayers and guest layers. Five disubstituted benzenes, o-toluidine, m-fluoroaniline, o-chlorotoluene, o-bromotoluene, and indene, are accommodated in the two-dimensional void space between the host bilayers at 1:2 host-guest stoichiometries. Thermal gravimetric analysis of the inclusion crystals revealed that all the guest molecules, except o-toluidine, are released in two separate steps, indicating the formation of intermediate crystals after the first guest release. Adequate heat treatment of the four inclusion crystals induces release of half or three quarters of the guest molecules. X-ray diffraction patterns of the intermediate crystals revealed that the crystals have a bilayer structure the same as those of the common CA inclusion crystals. They have one-dimensional cavities, in which the guest molecules are included at a 1:1 or 2:1 host-guest stoichiometry. These facts indicate that the host bilayers move 1.6-4.5 A perpendicular to the layer direction by desorption of the guest molecules. Furthermore, a reverse structural change is also achieved by absorption of the guest molecules to regenerate the starting sandwich-type inclusion crystals. This reversible change in the host bilayer by the guest sorption and desorption is a novel example of organic intercalation materials.     

Monolithic silica column for in-tube solid-phase microextraction coupled to high-performance liquid chromatography

In-tube solid-phase microextraction (SPME) has successfully been coupled to capillary LC, and further an automated in-tube SPME system has been developed using a commercially available HPLC auto-sampler. However, an open tubular capillary column with a thick film of polymer (stationary phase) is unfavorable because the ratio of the surface area of coating layer contacted with sample solution to the volume of the capillary column is insufficient for mass transfer. A highly efficient SPME column is. therefore, required. We introduced a C18-bonded monolithic capillary column that was used for in-tube SPME. The column consisted of continuous porous silica having a double-pore structure. Both the through-pore and the meso-pore were optimized for in-tube SPME, and the optimized capillary column was connected to an HPLC injection valve for characterization. The results demonstrated that the pre-concentration efficiency is excellent compared with the conventional in-tube SPME. The novel method for both introduction and concentration of the samples was effective. satisfactory and suitable for use in the SPME medium.     

Molecular recognition and fluorescence sensing of monophosphorylated peptides in aqueous solution by bis(zinc(II)-dipicolylamine)-based artificial receptors

The phosphorylation of proteins represents a ubiquitous mechanism for the cellular signal control of many different processes, and thus selective recognition and sensing of phosphorylated peptides and proteins in aqueous solution should be regarded as important targets in the research field of molecular recognition. We now describe the design of fluorescent chemosensors bearing two zinc ions coordinated to distinct dipicolylamine (Dpa) sites. Fluorescence titration experiments show the selective and strong binding toward phosphate derivatives in aqueous solution. On the basis of (1)H NMR and (31)P NMR studies, and the single-crystal X-ray structural analysis, it is clear that two Zn(Dpa) units of the binuclear receptors cooperatively act to bind a phosphate site of these derivatives. Good agreement of the binding affinity estimated by isothermal titration calorimetry with fluorescence titration measurements revealed that these two receptors can fluorometrically sense several phosphorylated peptides that have consensus sequences modified with natural kinases. These chemosensors display the following significant features: (i) clear distinction between phosphorylated and nonphosphorylated peptides, (ii) sequence-dependent recognition, and (iii) strong binding to a negatively charged phosphorylated peptide, all of which can be mainly ascribed to coordination chemistry and electrostatic interactions between the receptors and the corresponding peptides. Detailed titration experiments clarified that the phosphate anion-assisted coordination of the second Zn(II) to the binuclear receptors is crucial for the fluorescence intensification upon binding to the phosphorylated derivatives. In addition, it is demonstrated that the binuclear receptors can be useful for the convenient fluorescent detection of a natural phosphatase (PTP1B) catalyzed dephosphorylation.     

Asymmetric conjugate reduction of alpha,beta-unsaturated ketones and esters with chiral rhodium(2,6-bisoxazolinylphenyl) catalysts

New asymmetric conjugate reduction of beta,beta-disubstituted alpha,beta-unsaturated ketones and esters was accomplished with alkoxylhydrosilanes in the presence of chiral rhodium(2,6-bisoxazolinylphenyl) complexes in high yields and high enantioselectivity. (E)-4-Phenyl-3-penten-2-one and (E)-4-phenyl-4-isopropyl-3-penten-2-one were readily reduced at 60 degrees C in 95 % ee and 98 % ee, respectively, by 1 mol % of catalyst loading. (EtO)2MeSiH proved to be the best hydrogen donor of choice. tert-Butyl (E)-beta-methylcinnamate and beta-isopropylcinnamate could also be reduced to the corresponding dihydrocinnamate derivatives up to 98 % ee.     

Phase Separation Process of Polymer—Incorporated Silica-Zirconia Sol-Gel System

Gels with interconnected domain morphologies in the micrometer-range have been prepared in the silica-zirconia system. The domain formation kinetics in the gelling solution have been examined. Growth of an ordered structure on the length scale of micrometers, for which the kinetics are interpreted as spinodal decomposition, was observed by time-resolved light scattering measurements. The remarkable feature of the silica-zirconia system was that a time-dependent decrease of the wavelength of compositional fluctuations was observed. This occurred in the early stage, probably because the fast condensation reaction which was induced by the addition of zirconia, lead to a substantial change in quench depth on a timescale similar to that of the growth of concentration fluctuations. In the following stage, the coarsened domain structure was frozen-in by the sol-gel transition as the permanent morphology.