The Mechanism of 2-Furaldehyde Formation from d-Xylose Dehydration in the Gas Phase. A Tandem Mass Spectrometric Study

The mechanism of reactions occurring in solution can be investigated also in the gas phase by suited mass spectrometric techniques, which allow to highlight fundamental mechanistic features independent of the influence of the medium and to clarifying controversial hypotheses proposed in solution studies. In this work, we report a gas-phase study performed by electrospray triple stage quadrupole mass spectrometry (ESI-TSQ/MS) on the dehydration of d-xylose, leading mainly to the formation of 2-furaldehyde (2-FA). It is generally known in carbohydrate chemistry that the thermal acid catalyzed dehydration of pentoses leads to the formation of 2-FA, but several aspects on the solution-phase mechanism are controversial. Here, gaseous reactant ions corresponding to protonated xylose molecules obtained from ESI of a solution containing d-xylose and ammonium acetate as protonating reagent were allowed to undergo collisionally activated decomposition (CAD) into the triple stage quadrupole analyzer. The product ion mass spectra of protonated xylose are characterized by the presence of ionic intermediates arising from xylose dehydration, which were structurally characterized by their fragmentation patterns. As expected, the xylose triple dehydration leads to the formation of the ion at m/z 97, corresponding to protonated 2-FA. On the basis of mass spectrometric evidences, we demonstrated that in the gas phase, the formation of 2-FA involves protonation at the OH group bound to the C1 atom of the sugar, the first ionic intermediate being characterized by a cyclic structure. Finally, energy resolved product ion mass spectra allowed to obtain information on the energetic features of the d-xylose→2-FA conversion.

Front Cover: Katritzky Salts for the Synthesis of Unnatural Amino Acids and Late-Stage Functionalization of Peptides (Eur. J. Org. Chem. 12/2023)

Peptide drug discovery often benefits from the large structural diversity permitted by unnatural amino acids (UAAs). Indeed, numerous approved peptide drugs include UAAs in their sequences. Therefore, innovative chemical approaches either to synthesize UAAs or to allow late-stage functionalization of peptides are emerging themes in peptide drug discovery. Thanks to the recent advances in deaminative strategies using alkylpyridiniums salts, often referred to as Katritzky salts, a variety of radical alkylation methods have been developed. In recent years the use of Katritzky salts have become popular in peptide chemistry due to their ease of preparation from a primary amine, which is a predominant functional group in amino acids. This review highlights the progress that has been made by using Katritzky salts in the synthesis of UAAs, late-stage peptide functionalization, and peptide macrocyclization.     

Microplate-Based Evaluation of the Sugar Yield from Giant Reed,Giant Miscanthus and Switchgrass after Mild Chemical Pre-Treatments and Hydrolysis with Tailored <Emphasis Type="Italic">Trichoderma</Emphasis> Enzymatic Blends

Giant reed, miscanthus, and switchgrass are considered prominent lignocellulosic feedstocks to obtain fermentable sugars for biofuel production. The bioconversion into sugars requires a delignifying pre-treatment step followed by hydrolysis with cellulase and other accessory enzymes like xylanase, especially in the case of alkali pre-treatments, which retain the hemicellulose fraction. Blends richer in accessory enzymes than commercial mix can be obtained growing fungi on feedstock-based substrates, thus ten selected Trichoderma isolates, including the hypercellulolytic strain Trichoderma reesei Rut-C30, were grown on giant reed, miscanthus, or switchgrass-based substrates. The produced enzymes were used to saccharify the corresponding feedstocks, compared to a commercial enzymatic mix (6 FPU/g). Feedstocks were acid (H₂SO₄ 0.2–2%, w/v) or alkali (NaOH 0.02–0.2%, w/v) pre-treated. A microplate-based approach was chosen for most of the experimental steps due to the large number of samples. The highest bioconversion was generally obtained with Trichoderma harzianum Or4/99 enzymes (78, 89, and 94% final sugar yields at 48 h for giant reed, miscanthus, and switchgrass, respectively), with significant increases compared to the commercial mix, especially with alkaline pre-treatments. The differences in bioconversion yields were only partially caused by xylanases (maximum R ² = 0.5), indicating a role for other accessory enzymes.     

Synthesis,characterization and properties of a soluble polymer with a poly(phenylenevinylene) structure

Soluble poly(2,5-dialkoxy-1,4-phenylenevinylene) has been prepared via Stille coupling reaction between 2,5-dialkoxy-1,4-diiodobenzene and E-1,2-bis(tributylstannyl)-ethene in the presence of palladium complexes. Characterization of this material by means of ¹H and ¹³C nuclear magnetic resonance (NMR), ultraviolet/visible (UV/VIS) and infrared (IR) spectra is described. Molecular weights, determined by means of gelpermeation chromatography (GPC) analysis and referred to standard polystyrene, were in the range number-average molecular weights M _n = 2061–2544 and weight-average molecular weights M _w = 3347–3878. X-ray diffraction (XRD) analysis of the polymer showed semicrystalline structure. T_g = 57°C, transition to a stable smectic mesophase at 115°C and clearing point at 210°C were revealed by differential scanning calorimetry analysis, optical microscopy observation and XRD of the annealed polymer.     

Essential oil extraction,chemical analysis and anti-Candida activity of Foeniculum vulgare Miller – new approaches

A comprehensive study on essential oil samples of Foeniculum vulgare Miller from Tarquinia (Italy) is reported. A 24-h systematic steam distillation was performed on different harvested samples applying different extraction times. The GC-MS analysis of the residue outcome showed o-cymene, α-phellandrene, α-pinene and estragole as the major constituents. The predominance and continued presence of o-cymene makes this fennel oil a rather unique chemotype. An evident correlation between the antifungal activity and phenological stage is demonstrated. The most active fractions were particularly rich in estragole, as well as a significant amount of fenchone that possibly exerts some additive effect in the expression of overall antifungal potency. Pre-fruiting material produced oil particularly rich in o-cymene. With reference to the duration of the extraction, the maximum amount of oil was released within the first 3 h, whereas the reproductive phase material needed at least 6 h for the extraction.     

Quantitative Prediction of Rate Constants for Aqueous Racemization To Avoid Pointless Stereoselective Syntheses

Racemization has a large impact upon the biological properties of molecules but the chemical scope of compounds with known rate constants for racemization in aqueous conditions was hitherto limited. To address this remarkable blind spot, we have measured the kinetics for racemization of 28 compounds using circular dichroism and ¹H NMR spectroscopy. We show that rate constants for racemization (measured by ourselves and others) correlate well with deprotonation energies from quantum mechanical (QM) and group contribution calculations. Such calculations thus provide predictions of the second‐order rate constants for general‐base‐catalyzed racemization that are usefully accurate. When applied to recent publications describing the stereoselective synthesis of compounds of purported biological value, the calculations reveal that racemization would be sufficiently fast to render these expensive syntheses pointless.     

A Jocic-type approach for a practical and scalable synthesis of pyrrolonaphthoxazepine (PNOX)-based potent proapoptotic agents

We developed a Jocic-type protocol for the construction of the pyrrolonaphthoxazepine (PNOX) core. After an initial investigation based on the isolation of a trichloromethyl carbinol derivative, we shifted our attention towards a multicomponent single-step protocol. Screening of a variety of bases and solvents led to the identification of the optimum conditions for the preparation of the key α-aryloxy carboxylic acids to undergo intramolecular cyclization. The novel chemical route significantly improved overall yields for the preparation of PNOX-based compounds and was successfully extended to the preparation of 1,4-benzoxazinone-based templates.     

Integrated Conversion of Lignocellulosic Biomass to Bio-Based Amphiphiles using a Functionalization-Defunctionalization Approach

Concerns over the sustainability and end-of-life properties of fossil-derived surfactants have driven interest in bio-based alternatives. Lignocellulosic biomass with its polar functional groups is an obvious feedstock for surfactant production but its use is limited by process complexity and low yield. Here, we present a simple two-step approach to prepare bio-based amphiphiles directly from hemicellulose and lignin at high yields (29 % w/w based on the total raw biomass and >80 % w/w of these two fractions). Acetal functionalization of xylan and lignin with fatty aldehydes during fractionation introduced hydrophobic segments and subsequent defunctionalization by hydrogenolysis of the xylose derivatives or acidic hydrolysis of the lignin derivatives produced amphiphiles. The resulting biodegradable xylose acetals and/or ethers, and lignin-based amphiphilic polymers both largely retained their original natural structures, but exhibited competitive or superior surface activity in water/oil systems compared to common bio-based surfactants.     

On the sonic composites with scatterers made from auxetic material

A finite size periodic array of resonators made from auxetic material embedded into an epoxy matrix is analyzed in this paper. According to the Bragg's theory, the sound attenuation band is due to the superposition of multiple reflected waves inverse proportional to the central distance between resonators. The sound attenuation in such composites is studied using a method that combines the features of the cnoidal method and the genetic algorithm [1-3]. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A general framework for pricing Asian options under stochastic volatility on parallel architectures

In this paper, we present a transform-based algorithm for pricing discretely monitored arithmetic Asian options with remarkable accuracy in a general stochastic volatility framework, including affine models and time-changed Lévy processes. The accuracy is justified both theoretically and experimentally. In addition, to speed up the valuation process, we employ high-performance computing technologies. More specifically, we develop a parallel option pricing system that can be easily reproduced on parallel computers, also realized as a cluster of personal computers. Numerical results showing the accuracy, speed and efficiency of the procedure are reported in the paper.