Acetal Formation Research Articles

Hydrothermal synthesis of chiral inorganic-organic CoII complex: Structural, thermal and catalytic evaluation

By heating the cobalt chloride hexahydrate (CoCl2·6H2O) with the R form of the organic amine α-methylbenzylamine (C8H11N) under hydro(solvo)thermal conditions, we have successfully generated the corresponding non-centrosymmetric homochiral hybrid tris (α-methylbenzylammonium tetrachloridocobaltate chloride [R-(C8H12N)3][CoCl4]Cl abbreviated R(MBA)Co. We present the growth conditions together with a characterization of the single crystals by means of X-ray single-crystal diffraction, Fourier-transform infrared, TG/TDA thermal decomposition and catalytic properties. This inorganic–organic hybrid compound crystallizes in the chiral space group P21 and exhibits a supramolecular-layered organization wherein a double layer of (R)-methylbenzylammonium cations and the uncoordinated chloride anions are sandwiched between anionic layers, formed by isolated tetrachloridocobaltate tetrahedra. The crystal packing is governed by a three-dimensional network of N/CH⋯Cl hydrogen bonds between the inorganic and organic moieties and CH···π interactions between the aromatic rings of the organic moieties themselves. Thermal analysis discloses a phase transition at the temperature 130 °C. The Co complex was also employed as suitable catalyst activating the acetal formation reaction of aldehydes using MeOH as solvent and as the unique source of acetalization.

Mechanism of the Escherichia coli MltE lytic transglycosylase, the cell-wall-penetrating enzyme for Type VI secretion system assembly

Lytic transglycosylases (LTs) catalyze the non-hydrolytic cleavage of the bacterial cell wall by an intramolecular transacetalization reaction. This reaction is critically and broadly important in modifications of the bacterial cell wall in the course of its biosynthesis, recycling, manifestation of virulence, insertion of structural entities such as the flagellum and the pili, among others. The first QM/MM analysis of the mechanism of reaction of an LT, that for the Escherichia coli MltE, is undertaken. The study reveals a conformational itinerary consistent with an oxocarbenium-like transition state, characterized by a pivotal role for the active-site glutamic acid in proton transfer. Notably, an oxazolinium intermediate, as a potential intermediate, is absent. Rather, substrate-assisted catalysis is observed through a favorable dipole provided by the N-acetyl carbonyl group of MurNAc saccharide. This interaction stabilizes the incipient positive charge development in the transition state. This mechanism coincides with near-synchronous acetal cleavage and acetal formation.

Triflic acid‐Mediated Expedient Synthesis of Benzo[a]fluorenes and Fluorescent Benzo[a]fluorenones

AbstractFluorene‐based polyaromatic hydrocarbons are renowned compounds for materials applications. Herein, a straightforward route via in situ acetal formation has been presented to access benzo[a]fluorenes by a triflic acid promoted cationic cycloisomerization of enynones in presence of trimethyl orthoformate under metal‐free conditions. In the absence of trimethyl orthoformate, the same reaction results in benzo[a]fluorenones. All the synthesized benzo[a]fluorenones are highly fluorescent in solution phase with high Stokes shift while the corresponding benzo[a]fluorenes are not fluorescent.magnified image

A Simple Emulsification-Assisted Extraction Method for the GC–MS/SIM Analysis of Wine Markers of Aging and Oxidation: Application for Studying Micro-Oxygenation in Madeira Wine

Sotolon and the heterocyclic acetals of glycerol are known as potential aging and oxidation markers in fortified wines such as Madeira, Port, and Sherry. Thus, determining the evolution of these compounds under different oxidative aging conditions is important for fortified wine quality purposes. This study proposes a new methodology based on a miniaturized emulsification extraction followed by GC–MS/SIM, which was developed and optimized to follow the formation of sotolon and heterocyclic acetals in fortified wines that were submitted to traditional accelerated aging and micro-oxygenation. The optimization was achieved by means of a mixed-level factorial design, considering 3 factors: sample volume, extractant volume, and concentrated extract volume, by performing 19 experiments in duplicate. The extraction was optimized using 8 mL of wine sample, 5 mL of dichloromethane, concentrating the extract up to 10-fold. The method performance was evaluated for sotolon, using a matrix-matched calibration between 10 and 2000 μg/L. The selectivity was confirmed through the analysis of real samples. The methodology showed good linearity (R2 = 0.999), high sensitivity (LOQ = 6.8 μg/L), recovery about 105%, and good precision (less than 8 and 9%, evaluated by the variation of intra- and inter-day measurements, respectively). This is the first methodology that revealed to be an excellent tool to simultaneously follow the formation of sotolon and heterocyclic acetals in Madeira wines, using an inexpensive, simple, efficient, and effective experimental layout. Indeed, it was shown that traditional accelerated aging and micro-oxygenation have impact on the formation of such molecules.

Highly Selective Continuous Flow Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol in a Pt/SiO2 Coated Tube Reactor

A novel continuous flow process for selective hydrogenation of α, β-unsaturated aldehyde (cinnamaldehyde, CAL) to the unsaturated alcohol (cinnamyl alcohol, COL) has been reported in a tube reactor coated with a Pt/SiO2 catalyst. A 90% selectivity towards the unsaturated alcohol was obtained at the aldehyde conversion of 98.8%. This is a six-fold improvement in the selectivity compared to a batch process where acetals were the main reaction products. The increased selectivity in the tube reactor was caused by the suppression of acid sites responsible for the acetal formation after a short period on stream in the continuous process. In a fixed bed reactor, it had a similar acetal suppression phenomenon but showed lower product selectivity of about 47–72% due to mass transfer limitations. A minor change in selectivity and conversion caused by product inhibition was observed during the 110 h on stream with a turnover number (TON) reaching 3000 and an alcohol production throughput of 0.36 kg gPt−1 day−1 in the single tube reactor. The catalysts performance after eight reaction cycles was fully restored by calcination in air at 400 °C. The tube reactors provide an opportunity for process intensification by increasing the reaction rates by a factor of 2.5 at the reaction temperature of 150 °C compared to 90 °C with no detrimental effects on catalyst stability or product selectivity.

Transition metal oxide supported on alumina catalysts: a comparative study for the hydrogenation of octanal

Monometallic (10 wt.%) Co, Ni and Cu nanoparticles supported on alumina catalysts were prepared using an ultrasonic impregnation- cavitation method and characterized using ICP, XRD, physisorption, chemisorption and temperature programmed techniques. The copper catalyst showed higher metal dispersion and greater hydrogen and CO chemisorption capacity when compared to the nickel and cobalt catalysts. Hydrogenation of octanal carried out in a continuous flow high pressure fixed bed reactor showed that the rate of reaction and turnover number of octanol depended on the amount of hydrogen chemisorbed. The copper catalyst showed the lowest activation energy, as well as best catalytic activity. The Cu-Al catalyst which showed higher metal dispersion and low acidity, showed the highest selectivity towards octanol with no C24 acetal formation, when compared to the Ni-Al and Co-Al catalysts. Keywords: Octanal hydrogenation, octanol, copper, nickel, cobalt

Metal-Free, Brønsted Acid-Catalyzed Formal [3+2] Annulation of Quinone Monoacetals with 2-Naphthols.

An operationally simple and metal-free cross-coupling of quinone monoacetals (QMAs) with 2-naphthols catalyzed by triflic acid is reported. This formal [3+2] annulation allowed the synthesis of diverse naphtho[2,1-b]benzofuran derivatives in moderate to good yields. Preliminary mechanistic studies reveal the initial nucleophilic substitution of QMAs with 2-naphthols in preference to the mixed acetal formation and subsequent [3,3] sigmatropic rearrangement.

Multifunctional self-assembling hydrogel from guar gum

Facile and “green” techniques for hydrogel fabrication are in high demand to satisfy a wide range of practical applications. To the best of our knowledge, this report presents the first description of an efficient, one-flask method for preparing a guar gum (GG)-based self-assembling hydrogel, which takes advantage of the formation of cyclic acetals between the vicinal hydroxyl groups and carbonyl groups, as introduced via the regioselective oxidation of GG using sodium periodate (NaIO4). The final product was achieved without the addition of any crosslinking agents. The self-assembling hydrogel was rapidly formed under ambient conditions, and demonstrated remarkable self-healing performance, as well as thermally responsive properties associated with sol-gel transitions. Furthermore, the GG hydrogel had the ability to remove copper ions (Cu2+) from the surrounding aqueous solution. This removal can be accounted for due to the large number of active sites (i.e., hydroxyl groups) within its porous three-dimensional structure. The adsorption equilibrium was achieved after 5h, and the amount of adsorbed Cu2+ was 944mg/g GG when an initial concentration of 25mg/mL CuSO4 in solution was used. The adsorption isotherm data correlated well with the Freundlich model (R2>0.99), suggesting that the Cu2+ was removed through multilayered and complex adsorption mechanisms.

Double Diastereoselective Approach to Chiral syn- and anti-1,3-Diol Analogues through Consecutive Catalytic Asymmetric Borylations.

Homoallylic boronate carboxylate esters derived from unsaturated aldehydes via an imination, β-borylation, imine hydrolysis, and Wittig trapping sequence, were subjected to a second boryl addition to give 1,3-diborylated carboxylate esters. Control of the absolute and relative stereochemistry of the two new 1,3-stereogenic centers was achieved through: (1) direct chiral catalyst controlled asymmetric borylation of the first stereocenter on the unsaturated imine with high e.e.; and (2) a double diastereoselectively controlled borylation of an unsaturated ester employing a chiral catalyst to largely overcome directing effects from the first chiral boryl center to give poor (mismatched) to good (matched) diastereocontrol. Subsequently, the two C-B functions were transformed into C-O systems to allow unambiguous stereochemical assignment of the two borylation reactions involving oxidation and acetal formation.

The Effect of Juice Clarity and Several Conditions Promoting Yeast Growth on Fermentation Rate, the Production of Aroma Components and Wine Quality

In cellar practice grape juice is preferably made as clear as possible, and fermentations take place under strictly anaerobic conditions. In the laboratory, however, the fermentation rate of EK-filtered and de-aerated juices was always so much slower than that of juices obtained by normal cellar routine that no dry wines could be prepared unless fermentation stimulating methods were applied. Apparently in most cellar fermentations sufficient amounts of oxygen and/or of yeast growth promoting steroids from the grapes are present to complete fermentation, but the exact amounts present are unknown. In practice problems with lagging fermentation could occur in cases where clarifying and de-aeration of juice were performed too rigorously. Grape juice was clarified by diverse methods, and white wines were prepared from the juices under various conditions in order to study the effect of de·aeration, addition of settled juice with residual turbidity, micro-aeration and application of ergosterol suspensions on fermentation rate. EK-filtered juices were used to experiment with methods for reactivating lagging fermentation and also for investigating the effect of fermentation conditions on the production of wine volatiles. With the aid of the above-mentioned fermentation stimulating means the fermentation of EK-filtered juices could be completed within an acceptable period of time. Steps to reactivate lagging fermentations should be taken as early as possible, although additions of settled juice were often successful, even when applied at a late stage during fermentation. Filtered juice from different cultivars and origins behaved in a markedly similar way with regard to fermentation and the production of volatile components. Filtration of juice caused partial loss of the cultivar character of the wine but wines from filtered juice with additions of ergosterol were nevertheless allotted the highest scores during organoleptic evaluation. The production of higher alcohols depends on the degree of juice turbidity and fermentation temperature. In clear filtered juice, however, the production of higher alcohols was independent of fermentation temperature. The formation of acetates of higher alcohols is greatly enhanced, by adding ergosterol to the filtered juice.

Total syntheses of Prelactone V and Prelactone B

The total syntheses of natural products Prelactone-V and Prelactone-B have been accomplished by a novel Chiron approach starting from d-glucose. The synthesis involves isopropylidene acetal formation of d-glucose using Poly(4-vinylpyridine) supported iodine as a catalyst, Tebbe olefination, Grignard reaction, Wittig olefination, selective mono deprotection of acetal using PMA/SiO2, hydrogenation and anti-1,3-diol formation are as key steps.

Nano-fiber/net structured PVA membrane: Effects of formic acid as solvent and crosslinking agent on solution properties and membrane morphological structures

A technique of electro-spinning/netting (ESN) has been investigated to prepare poly(vinyl alcohol) (PVA) nanofibrous membrane with unique morphological structure of nano-fiber/net (NFN). The prepared NFN membrane consists of both common electrospun nanofibers (with diameters in hundreds of nanometers) and two-dimensional nano-nets (with nanofibril diameters smaller than 50nm). It is revealed that formic acid has dual effects (i.e., as solvent and crosslinking agent) on solution properties and membrane morphological structures, and systematic analyses on rheological properties of three different 10wt% PVA solutions with solvents being water, formic acid, and their 1/1 (wt./wt.) mixture have been conducted. Results indicate that the molecular interactions (particularly hydrogen bonding) among water, formic acid, and PVA macromolecular chains, as well as the acetal formation (i.e., chemical crosslinking), have significant influences on macromolecular conformation, solution rheological properties, and morphological structure of membranes. The acetal formation, which leads to improved tensile strength of PVA NFN membrane, has been verified by FT-IR and DSC. Additionally, the PVA NFN membrane has been assembled into a gravity-driven filtration device; and the membrane exhibits high rejection ratio (>99%) for polystyrene microspheres (with diameters of ~0.2μm) and high water flux (3773L/m2∙h) under the pressure of 40kPa.

An alumina-supported silver catalyst with high water tolerance for H2 assisted C3H6-SCR of NOx

Water vapor is typically present in diesel engine exhausts, and thus the design of catalysts with high water-tolerance is highly desired. The addition of water vapor was shown to have quite different influences on the activity of Ag/Al2O3 catalysts with different Ag loadings during the H2-assisted C3H6-SCR of NOx (H2-C3H6-SCR). The 2wt% Ag/Al2O3 catalyst showed the best activity for H2-C3H6-SCR, with excellent water resistance over the whole temperature range. An enhancement in NOx conversion was observed after water vapor was introduced, particularly at low temperatures. Over this catalyst, kinetic studies confirmed that H2O addition did not change the apparent activation energy for NOx reduction, while it increased the reaction order of C3H6 from −0.62 to 0.73. This result indicated that the reaction pathway for NOx reduction was hardly changed by the introduction of water vapor, while a poisoning effect related to C3H6 oxidation was decreased. In situ DRIFTS studies and DFT calculations revealed that water vapor significantly inhibits the formation of inert formate during the H2-C3H6-SCR process, and thus more sites are available for the formation of active enolic species and acetates, finally leading to increased activity for 2wt% Ag/Al2O3 in NOx reduction.

Solution-phase automated synthesis of an α-amino aldehyde as a versatile intermediate.

A solution-phase automated synthesis of the versatile synthetic intermediate, Garner’s aldehyde, was demonstrated. tert-Butoxycarbonyl (Boc) protection, acetal formation, and reduction of the ester to the corresponding aldehyde were performed utilizing our originally developed automated synthesizer, ChemKonzert. The developed procedure was also useful for the synthesis of Garner’s aldehyde analogues possessing fluorenylmethyloxycarbonyl (Fmoc) or benzyloxycarbonyl (Cbz) protection.

ChemInform Abstract: Donor—Acceptor Substituted 2‐Phenylpyridines by Means of Reductive C,C‐Cross Coupling Reaction.

The preparation of 2-phenylpyridines decorated with an electron donating and accepting unit, which are potentially suitable to induce second harmonic generation, was performed by means of reductive nickel catalyzed C,C-cross coupling reaction, developed by Gosmini et. al. We were successful to obtain the desired 2-phenylpyridines directly from donor-functionalized arylhalides and acceptor-modified 2-halopyridines with a cheap and easy to prepare Ni catalyst. It turned out that even the presence of hydroxy groups and carboxylic acids are tolerated in this reaction, whereas in the case of a formyl group containing pyridine substrate no C,C-coupled product was obtained. However, the coupling was successful when the formyl function was protected by an acetal formation.

Hydroformylation-hydrogenation and hydroformylation-acetalization reactions catalyzed by ruthenium complexes

In this work, the catalytic activity of ruthenium II and III complexes containing chloride, pyridine, phosphine and CO ligands was investigated in the hydroformylation – hydrogenation and hydroformylation – acetalization reactions. The complexes mer-[RuCl3(dppb)(H2O)](1), mer-[RuCl3(dppb)(4-Vpy)](2), mer-[RuCl3(dppb)(4-tBupy)](3), mer-[RuCl3(dppb)(py)](4), mer-[RuCl3(dppb)(4-Phpy)](5), mer-[RuCl3(dppb)(4-Mepy)](6), cis-[RuCl2(CO)2(dppb)](7), trans-[RuCl2(CO)2(dppb)](8), RuCl3·xH2O(9), [RuCl2(PPh3)3](10) and [RuCl2(PPh3)2(dppb)](11) were used as supplied or synthesized as previously described in the literature {Where PPh3=triphenylphosphine, dppb=1,4-bis(diphenylphosphino)butane, py=pyridine, 4-Mepy=4-methylpyridine, 4-Vpy=4-vinylpyridine, 4-tBupy=4-tert-butylpyridine and 4-Phpy=4-phenylpyridine}. These complexes were used as a pre-catalysts in a hydroformylation catalytic system to produce CC, CO and CO bonds, where 1-decene resulted in a formation of respective alcohol and dimethyl acetals. Several reactions were performed in order to find the best reaction conditions presenting the best conversion (64% after 24h). The 1-decene was also used as a substrate in two type tandem reactions labeled as: hydroformylation – hydrogenation (HH) and hydroformylation – acetalization (HA) reactions. The relationship between Ru – catalyst/substrate was 1:100, without free ligands or additives, in a controlled temperature and pressure. All the products of catalytic reactions HH and HA were analyzed by CG-FID with good yields.

Design, Synthesis, and Bioactivation of O-Glycosylated Prodrugs of the Natural Nitric Oxide Precursor N(ω)-Hydroxy-l-arginine.

Naturally occurring N(ω)-hydroxy-l-arginine (NOHA, 1) is the best substrate of NO synthases (NOS). The development of stable and bioavailable prodrugs would provide a pharmacologically valuable strategy for the treatment of cardiovascular diseases that are associated with endothelial dysfunction. To improve NOHAs druglike properties, we demonstrate that O-substitution by (glycosylic) acetal formation greatly increased the chemical stability of the hydroxyguanidine moiety and provided a nontoxic group that could be easily bioactivated by glycosidases. A straightforward synthetic concept was devised and afforded a series of diversely substituted prodrugs by O-conjugation of the hydroxyguanidine moiety with different monosaccharides. Systematic exploration of their bioactivation profile revealed that glucose-based prodrugs were more efficiently bioactivated than their galactose counterparts. NOS-dependent cytosolic NO release was quantified by automated fluorescence microscopy in a cell-based assay with murine macrophages. Glucose-based prodrugs performed particularly well and delivered cellular NO levels comparable to 1, demonstrating proof-of-concept.

Donor‐Acceptor Substituted 2‐Phenylpyridines by Means of Reductive C,C–Cross Coupling Reaction

AbstractThe preparation of 2‐phenylpyridines decorated with an electron donating and accepting unit, which are potentially suitable to induce second harmonic generation, was performed by means of reductive nickel catalyzed C,C‐cross coupling reaction, developed by Gosmini et. al. We were successful to obtain the desired 2‐phenylpyridines directly from donor‐functionalized arylhalides and acceptor‐modified 2‐halopyridines with a cheap and easy to prepare Ni catalyst. It turned out that even the presence of hydroxy groups and carboxylic acids are tolerated in this reaction, whereas in the case of a formyl group containing pyridine substrate no C,C‐coupled product was obtained. However, the coupling was successful when the formyl function was protected by an acetal formation.

Synthesis and characterization of Zwitterionic Zn(II) and Cu(II) coordination compounds with ring-substituted 2,2′-biimidazole derivatives

Zwitterionic coordination compounds with strongly asymmetrical charge distribution were synthesized and characterized. Ring-substituted biimidazoles were used as the primary ligands for Zn and Cu compounds. Formation of Zwitterionic coordination compound was found to be strongly dependent on the pH of the reaction medium as well as on the ring and nitrogen substituents of the ligand. Reaction of the Df-R2biim (Df-R2biim=2,2′-bi-1R-imidazole-5,5′-dicarboxaldehyde, R=Me, Et or Pr) with ZnCl2 in neutral conditions led to binuclear compounds [Zn2Cl4(Df-R2biim)2] with two bridging ligands (1a–c). Reaction with CuCl2·2H2O gave neutral mononuclear compound [CuCl2(Df-Me2biim)] (1d) with chelating biimidazole ligand. In acidic conditions (pH=3–4) the imidazole nitrogens were either fully or partially protonated, which prevented the bidentate coordination of the ligands. Furthermore, aldehyde substituents of Df-R2biim ligands were involved in acetal formation with methanol solvent. Under acidic conditions the primary products were either ion pairs [MCl4][C14H24N4O4]·(M=Zn 3a, M=Cu 3b) with fully protonated H2L2+ counter cation or desired Zwitterionic coordination compounds [ZnCl3(C16H27N4O4)] (2a), [ZnCl3(C18H31N4O4)] (2b), [CuCl3(C16H27N4O4)] (2c), or [CuCl3(C18H31N4O4)] (2d) with partially protonated monodentate HL+ ligand. Zwitterions were obtained only with ligands having both aldehyde groups as ring-substituents and longer alkyl chains (ethyl or propyl) as N-substituents. In other cases ion pairs were formed as final products. Zwitterions were found to decompose in alcoholic solutions. In methanol solution, Zwitterionic compounds released neutral ligand and HZnCl3. All main products were characterized by NMR, elemental analysis and single-crystal X-ray diffraction.

Cation-exchange resins in the hydroformylation–acetalization tandem reaction

Cation-exchange resins in the acid form (Amberlyst-16, Wofatit KPS 200, Nafion, KU-2) have been tested as the acidic component of a catalyst system for the hydroformylation–acetalization tandem process. It has been shown that the hydroformylation of octene-1 in the presence of ethylene glycol and glycerol using a catalyst system comprising a rhodium complex, a phosphine ligand, and a solid acid leads to the formation of acetals with a yield of up to 99%.