Occurrence of Cationic Intermediates and Deficient Control during the Enzymatic Cyclization of Squalene to Hopanoids

Tetracyclic triterpenes (for example, 3 ) are formed as minor products of the cyclization of squalene ( 1 ) by squalene/hopene cyclase; they have also been found in vivo. The identification of these side products offers new insight into the complex biosynthesis of the hopane skeleton, and proves that the enzyme has no absolute control of the conformation of the acyclic substrate squalene and that tetra- and pentacyclic carbocationic intermediates are involved (for example, 2 ) in the cyclization process.     

Stereoselective hydrogenation of methylcyclohex-2-ene-1,4-diols used in the synthesis of ampelomins and deoxy-carbasugars

Stereoselective hydrogenation of methylcyclohex-2-ene-1,4-diols used as important intermediates for the preparation of ampelomins and deoxy-carbasugars was studied. These olefins were obtained in few steps from a chiral cis-diol resulting from microbial oxidation of toluene. Although the stereoselective hydrogenation of this type of substrates is difficult, high yields were obtained for heterogeneous hydrogenation using Adam’s catalyst, where steric hindrance controlled the stereochemical outcome of the process. On the other hand, for homogeneous hydrogenation of similar olefins using Crabtree’s catalyst, coordination with the allylic alcohols allowed for a controlled hydrogen addition from the more hindered face. In this manner two protocols for the hydrogenation of these types of substrates resulting in complementary stereoselectivities are described.     

Carbon Dioxide as a Protecting Group: Highly Efficient and Selective Catalytic Access to Cyclic cis‐Diol Scaffolds

The efficient and highly selective formation of a wide range of (hetero)cyclic cis‐diol scaffolds using aminotriphenolate‐based metal catalysts is reported. The key intermediates are cyclic carbonates, which are obtained in high yield and with high levels of diastereo‐ and chemoselectivity from the parent oxirane precursors and carbon dioxide. Deprotection of the carbonate structures affords synthetically useful cis‐diol scaffolds with different ring sizes that incorporate various functional groups. This atom‐efficient method allows the simple construction of diol synthons using inexpensive and accessible precursors and green metal catalysts and showcases the use of CO₂ as a temporary protecting group.     

Influence of the soft segment length on the properties of water‐cured poly(carbonate‐urethane‐urea)s

Poly(carbonate‐urethane‐urea)s (PCUU) based on oligocarbonate diols (M_n ≈ 2000) with different length of the hydrocarbon chain as soft segments were synthesized and investigated. Carbonate oligomerols were obtained in a two‐step method from dimethyl carbonate (DMC) and linear α,ω‐diols (1,4‐butanediol, 1,5‐pentanediol, 1,6‐hexanediol, 1,9‐nonanediol, 1,10‐dekanediol and 1,12‐dodecanediol). Oligo(trimethylene carbonate) diol was synthesized using ring‐opening polymerization of trimethylence carbonate. PCUUs were obtained by prepolymer method using isophorone diisocyanate (IPDI) and water as a chain extender. Changes in polymers properties with increase of methylene group number between carbonate linkages were investigated by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), tensile strength and hardness measurements. The thermal stability was also analyzed by means of thermogravimetric analysis (TGA). Based on FTIR analysis influence of methylene groups number between carbonate linkages on phase separation and concentration of allophanate and biuret groups in the samples were investigated. The obtained poly(carbonate‐urethane‐urea)s exhibited very good mechanical properties. Tensile strength and elongation at break were 40 MPa and 400–600%, respectively, depending on the oligocarbonate used. Copyright © 2014 John Wiley & Sons, Ltd.     

A facile method to synthesize high‐molecular‐weight biobased polyesters from 2,5‐furandicarboxylic acid and long‐chain diols

In this study, biobased furan dicarboxylate polyesters have been prepared using 2,5‐furandicarboxylic acid (FDCA) and diols with high number of methylene groups (long‐chain diols), namely, 8, 9, 10, and 12. Because of the high boiling points of these diols, a modified procedure of the well‐known melt polycondensation was applied in this work. According to this, the dimethyl ester of FDCA (DMFD) reacted in the first transesterification stage with the corresponding diols forming bis‐hydroxy‐alkylene furan dicarboxylates (BHFD). In the second stage, the BHFD reacted with DMFD again at temperatures of 150–170 °C (for 4–5 h), and in the final stage, the temperature was raised to 210–230 °C (vacuum was applied for 2–3 h). The molecular weight of the polyesters and the content of oligomers, as was verified by gel permeation chromatography analysis, depend on the polycondensation time and temperature. The chemical structure of the polyesters was verified from ¹H NMR spectroscopy. All the polymers were found to be semicrystalline, with melting temperatures from 69 to 140 °C depending on the diol used. In addition, the mechanical properties also varied with the type of diol. The higher values were observed for poly(octylene 2,5‐furanoate), whereas the lowest values were observed for poly(dodecylene 2,5‐furanoate) with the higher number of methylene groups in its repeating unit. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2617–2632     

Inverse gas chromatography in the study of polymer degradation. Part 4: A study of model systems for reinforced vulcanizates

Summary Inverse gas chromatography with olefinic stationary phases, impregnated with silica gel or carbon black, and air as carrier gas has been found to yield characteristic shifts of retention data and concomitant changes in peak shape. The plots of retention index versus oxygen flow obtained indicate carbon black to have significant antioxidant properties, which are not altogether lost on solvent extraction. On the other hand, silica gel yields results consistence with the existence of a novel chain scission repair process.     

手性芳基邻二醇的不对称合成

具有特定功能基团的手性芳基邻二醇是许多具有特殊功能的药物、农药和信息素的重要中间体,近年来手性芳基邻二醇类化合物的合成与应用研究引起了人们的广泛关注。本文从生物催化不对称合成和化学催化不对称合成两方面综述了近年来手性芳基邻二醇的合成进展,概述了前手性底物上取代基的电子效应和空间效应、手性催化剂的种类和反应体系等因素对合成手性芳基邻二醇产率及光学活性的影响,并对手性芳基邻二醇不对称合成的发展趋势进行了展望。     

Facile Synthesis and Unusual Methanesulfonylation Reaction of (2R,3R)‐1,4‐Dimethoxy‐1,1,4,4‐tetrasubstituted‐2,3‐butanediols

A facile method for the synthesis of (2R,3R)‐1,4‐dimethoxy‐1,1,4,4‐tetrasubstituted‐2,3‐butanediols involving oxidative cleavage of benzylidene acetal as a key step is described. These sterically hindered diols unusually formed cyclic sulfites as the major product under methanesulfonylation reaction conditions.     

Solute-solute-solvent interactions in dilute aqueous solutions of aliphatic diols. Excess enthalpies and gibbs free energies

The heats of dilution and the osmotic coefficients for some aliphatic diols (1,3-propanediol; 1,2-butanediol; 1,3-butanediol; 1,4-butanediol; 2,3-butanediol; 1,5-pentanediol; 1,6-hexanediol) in water at 25°C are reported. The experimental free energy and enthalpy pairwise interaction coefficients were evaluated and are discussed in terms of the hydrophobic-hydrophilic properties of the solutes. The effect of the mutual position of the polar hydrophilic groups in the molecule on the experimental interaction coefficients transposed to the McMillan-Mayer (MM) state is emphasized. The Sawage-Wood additivity of groups (SWAG) approach has been used and critically discussed.Paper presented at J.C.A.T. '86-Ferrara 27–30 Ottobre 1986.     

High performance bio-based polyurethane elastomers: Effect of different soft and hard segments

In this work, a series of high performance bio-based polyurethanes(bio-PUs) were synthesized from polylactide(PLA)-based diols, different diisocyanates(TDI, MDI, HDI, IPDI) and chain extender 1,4-butanediol, in which different soft and hard segments are used to adjust their transition temperatures and mechanical properties. Poly(lactide-co-caprolactone)copolymer diols(co-PLAols) instead of PLA diols as the soft segment improved the thermal stability and mechanical properties of the synthesized bio-PUs. Among them, MDI-based bio-PUs have the highest T_g(43.8 °C), tensile strength(23.5 MPa) and modulus(380.8 MPa), while HDI-based bio-PUs have the lowest T_g(21.4 °C) and highest elongation at break(580%). Especially, the bio-PUs synthesized from co-PLAols and MDI demonstrate better mechanical properties,closed to petroleum-based commodities. Furthermore, the obtained bio-PUs display good shape memory properties at body temperature and cytocompatibility. Therefore, these bio-PUs are promising for applications in biomedical fields.