Synthesis of 3-Substituted Xylopyrnosides from 2,3-Anhydropentosides

Abstract

The facile regio- and stereoselective epoxide ring-opening of anhydropentosides described herein provides an attractive pathway to 3-substituted analogs of pentosides. Benzyl 2,3-anhydro-β-D-ribopyranoside (2) and benzyl 2,3-anhydro-β-L-ribopyranoside (7) were obtained from benzyl β-D-arabinopyranoside (1) and benzyl β-L-arabinopyranoside (3) respectively. The anhydropentosides were converted to the corresponding new 3-amino derivatives (8, 9, 10, and 11), alkoxy derivatives (12, 13, and 14), and deoxy sugar (15) in high yield. Every conversion was a one-step reaction of the anhydroglycoside with the appropriate nucleophile. Side-products due to epoxide migration were not observed.     

Crystal orientation switching in spherulites grown from miscible blends of poly(3‐hydroxybutyrate) with cellulose tributyrate

Fully miscible blends over the whole composition range are obtained by melt mixing bacterial poly(3‐hydroxybutyate) (PHB) and tri‐substituted cellulose butyrate [cellulose tributyrate (CTB)]. Blends containing up to 50 wt % CTB are partially crystalline. Isothermal crystallization experiments show formation of PHB spherulites that grow until impingement. Depending on composition, radial growth rate is either constant or it suddenly increases in a very unusual manner leading to peculiar morphologies. In the latter case, in concomitance to the crystal growth acceleration, the sign of birefringence changes and rotation of the PHB unit cell orientation is observed. These results are discussed in terms of the influence of both composition and T_c on the relative crystallization kinetics of the two blend components. A strong effect played by the not yet crystallized CTB component that in the presence of the highly mobile PHB component forms a liquid crystal‐like phase is proposed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Comparison of the Effects of Biofouling on Voltammetric and Potentiometric Measurements

Biofouling of sensors is a common problem when measuring biological samples. The adherence of proteins and biomolecules, called hemostasis, is the first of four steps that lead to biofouling and eventually a foreign body response. This typically occurs within the first hours after the exposure of the biosensor to a biological sample. The purpose of this study was to assess the effect of this initial step of biofouling on cyclic voltammetry and potentiometric measurements. The results show that biofouling occurred rapidly within minutes and strongly affected cyclic voltammetry measurements, while potentiometric measurements were minimally affected even after 24 hours.     

Kinetics of isopropanol decomposition and reaction with H atoms from shock tube experiments and rate constant optimization using the method of uncertainty minimization using polynomial chaos expansions (MUM‐PCE)

We have used the single‐pulse shock tube technique with postshock GC/MS product analysis to investigate the mechanism and kinetics of the unimolecular decomposition of isopropanol, a potential biofuel, and of its reaction with H atoms at 918‐1212 K and 183‐484 kPa. Experiments employed dilute mixtures in argon of isopropanol, a radical scavenger, and, for H‐atom studies, two different thermal precursors of H. Without an added H source, isopropanol decomposes in our studies predominantly by molecular dehydration. Added H atoms significantly augment decomposition, mainly by abstraction of the tertiary and primary hydrogens, reactions that, respectively, lead to acetone and propene as stable organic products. Traces of acetaldehyde were observed in some experiments above ≈ 1100 K and establish branching limits for minor decomposition pathways. To quantitatively account for secondary chemistry and optimize rate constants of interest, we employed the method of uncertainty minimization using polynomial chaos expansions (MUM‐PCE) to carry out a unified analysis of all datasets using a chemical model–based originally on JetSurF 2.0. We find: k(isopropanol → propene + H₂O) = 10^{(13.87 ± 0.69)} exp(?(33 099 ± 979) K/ T) s^?1 at 979‐1212 K and 286‐484 kPa, with a factor of two uncertainty (2σ), including systematic errors. For H atom reactions, optimization yields: k(H + isopropanol → H₂ + p‐C₃H₆OH) = 10^{(6.25 ± 0.42)} T^2.54 exp(?(3993 ± 1028) K /T) cm³ mol^?1 s^?1 and k(H + isopropanol → H₂ + t‐C₃H₆OH) = 10^{(5.83 ± 0.37)} T^2.40 exp(?(1507 ± 957) K /T) cm³ mol^?1 s^?1 at 918‐1142 K and 183‐323 kPa. We compare our measured rate constants with estimates used in current combustion models and discuss how hydrocarbon functionalization with an OH group affects H abstraction rates.