A palladium-catalyzed glycosylation reaction: the de novo synthesis of natural and unnatural glycosides

A highly stereoselective and sterospecific palladium-catalyzed glycosylation reaction of a variety of alcohols is reported. The reaction selectively converts alpha-2-substituted 6-carboxy-2H-pyran-3(6H)-ones into alpha-2-substituted 6-alkoxy-2H-pyran-3(6H)-ones with complete retention of configuration and similarly converts the pyranones with beta-carboxy groups into pyranones with beta-alkoxy groups. The reaction works equally well with both amino acid- and carbohydrate-based alcohols. To demonstrate the utility of this process for carbohydrate chemistry several of the products were selectively converted into alpha-manno-pyranosides in two additional steps. Because the 2-substituted 6-carboxy-2H-pyran-3(6H)-ones are prepared by asymmetric synthesis, this reaction can be used for the preparation of either d- or l-pyranones.     

Oxidatively induced glycosylation starting from hydroquinone glycosides

As a new class of glycosyl donors, hydroquinone glycosides can be used for glycosylation reactions. Their activation can be performed either electrochemically or under homogeneous chemical conditions. Conventionally, several glucosides were produced with yields greater than 77% using DDQ in CH₂Cl₂ as oxidizing agent. For electrolyses, glycosides of trimethylhydroquinone are preferably used because their low oxidation potentials allow the utilization of an undivided cell. The synthesis of the glycosyl donors was achieved with high efficiency by direct coupling of the phenols with peracetylated monosaccharides employing boron trifluoride etherate as the catalyst. The oxidation of hydroquinone derivatives can also be applied to the generation of other stabilized cations.     

Practical synthesis of latent disarmed S-2-(2-propylthio)benzyl glycosides for interrupted Pummerer reaction mediated glycosylation

Practical synthetic methods to latent disarmed S-2-(2-propylthio)benzyl (SPTB) glycosides for interrupted Pummerer reaction mediated glycosylation have been discovered. Among them, both coupling reaction of PTB-Cl with glycosyl thiols and BF₃·OEt₂ promoted reaction of peracylated glycosides with PTB-SH produced peracylated SPTB glycosides in large scales and with high efficiency.     

Palladium-catalyzed reaction of allyl halides with alpha-diazocarbonyl compounds

The Pd(OAc)(2)-catalyzed reaction between alpha-diazocarbonyl compounds and allyl bromides or chlorides leads to the formation of 1,3-diene derivatives.     

Rates of reaction of allyl radicals: A reassessment

Values of the rate constants for allyl-radical abstraction from toluene and allyl-radical recombination have been recalculated in the light of recent data on butene-1 decomposition. The rate of recombination is in good agreement with that found by Van den Bergh and Callear [13] and the cross-combination ratio for the allyl + methyl system has a “normal” value of almost 4. It is concluded that allyl radicals behave like alkyl radicals in respect of their rates of reaction with other radicals.     

Intramolecular allyl transfer reaction from allyl ether to aldehyde groups: experimental and theoretical studies

The intramolecular transfer of the allyl group of functionalized allyl aryl ethers to an aldehyde group in the presence of Ni0 complexes was studied from chemical, electrochemical and theoretical points of view. The chemical reaction involves the addition of Ni0 to the allyl ether followed by stoichiometric allylation. The electrochemical process is catalytic in nickel and involves the reduction of intermediate eta3-allylnickel(II) complexes.     

Building addressable libraries: a site-selective allyl alkylation reaction

A Pd(0) catalyst has been used to effect allyl alkylation reactions at pre-selected locations on an addressable microelectrode array. The reactions provide a new approach to site-selective carbon-carbon bond formation and a new method for placing substrates on the surface of a microelectrode array.     

Features of the reaction of polyfluoroalkyl chlorolsulfites with allyl alcohol

Unlike the saturated aliphatic and aromatic alcohols, allyl alcohol under the same conditions reacts with polyfluoroalkyl chlorosulfites to form not ethers, but polyfluorinated alcohols. The exception is polyfluoroalkyl chlorosulfites with the chain length of more than five carbon atoms. Allyl ethers of polyfluorinated alcohols of general formula CH₂=CHCH₂OCH₂(CF₂CF₂)nH (n = 1–3) were obtained, when the reaction proceeded in the presence of potassium carbonate, owing to its participation in a specific orientation of the reaction centers in the resulting intermediate structure, which is easily transformed into allyl ethers of polyfluorinated alcohols.     

Kinetics of the reaction between H atoms and allyl radicals

The reaction between H and C₃H₅ has been studied at 291 K. Exciplex laser flash photolysis at 193.3 nm of hexa-1,5-diene-He mixtures generated both H and C₃H₅ ([H] ? [C₃H₅]), which were detected in time-resolved mode by resonance fluorescence and absorption spectroscopy, respectively. Rate coefficients are presented at four pressures in the range 98 ? P/torr ? 400; no clear pressure-dependence is found in this range of pressures and the mean rate coefficient is (2.8 ± 1.0) × 10^?10 cm³ molecule^?1 s^?1. Calculations based on the Troe factorization method confirm that this reaction is near its high-pressure limit under the experimental conditions.     

Molecular details from computational reaction dynamics for the cellobiohydrolase I glycosylation reaction

Glycosylation of cellobiose hydrolase I (CBHI), is a key step in the processing and degradation of cellulose. Here the pathways and barriers of the reaction are explored using the free energy from adaptive reaction coordinate forces (FEARCF) reaction dynamics method coupled with SCC-DFTB/MM. In many respects CBHI follows the expected general GH7 family mechanism that involves the Glu-X-Asp-X-X-Glu motif. However, critical electronic and conformational details, previously not known, were discovered through our computations. The central feature that ensures the success of the glycosylation reaction are the Glu212 nucleophile's hydrogen bond to the hydroxyl on C2, of the glucose in the -1 position of the cellulosic strand. This Glu212 function restricts the C2 hydroxyl in such a way as to favor the formation of the (4)E ring pucker of the -1 position glucose. A frontier molecular orbital analysis of the structures along the reaction surface proves the existence of an oxocarbenium ion, which has both transition state and intermediate character. The transition state structure is able to descend down the glycosylation pathway through the critical combination of Asp214 (HOMO), ring oxygen (LUMO), and Glu212 (HOMO), anomeric carbon (LUMO) interactions. Using the fully converged FEARCF SCC-DFTB/MM reaction surface, we find a barrier of 17.48 kcal/mol separating bound cellulose chain from the glycosylated CBHI. Taking recrossing into account gives k(cat) = 0.415 s(-1) for cellobiohydrolase glycosylation.     

Short synthesis of methylenecyclopentenones by intermolecular Pauson-Khand reaction of allyl thiourea

N,N,N′-Trimethylallylthiourea promotes the intermolecular Pauson-Khand reaction with alkynes in the presence of Co₂(CO)₈ and moderate pressure of CO followed by thiourea elimination allowing the formation of methylenecyclopentenone derivatives.     

Effect of catalysts on the reaction of allyl esters with hydrosilanes

The reaction of hydrosilylation of allyl esters XOCH₂CH=CH₂ (X = MeCO, CF₃CO, C₃F₇CO) and PhOCH₂CH=CH₂ with hydrosilanes HSiY₃ (Y = Cl, OEt) in the presence of the Speier catalyst, the Speier catalyst with additives, and of various nickel complexes was studied. The catalytic hydrosilylation reaction in the presence of the Speier catalyst is accompanied by the reduction. Additives to the Speier catalyst (vinyltriethoxysilane and some ethers) allow to suppress considerably the reduction reaction. In the presence of the studied nickel complexes mainly reduction and isomerization reactions occurred. The best nickel catalysts of hydrosilylation were the mixtures of NiCl₂ or Ni(acac)₂ with phosphine oxides. In contrast to allyl esters, the hydrosilylation of simple olefins proceeds easier, the content of the product of hydrosilylation in the reaction mixture reaches 94.3%.     

Synthesis of tetrazole-fused glycosides by a tandem fragmentation-cyclization reaction

The fragmentation of anomeric alkoxyl radicals (ARF) and the subsequent intramolecular cyclization promoted by hypervalent iodine reagents provide an excellent method for the synthesis of tetrazolo-sugars. This new reaction offers additional advantages for the synthesis of these compounds, including the ready availability of the starting materials, experimental simplicity, mild conditions, and good yields.     

Synthesis of 2,3-unsaturated glycosides via metal-free Ferrier reaction

Hexafluoroisopropanol (HFIP) is explored as an effective medium for the synthesis of 2,3-unsaturated glycosides through allylic rearrangement of 3,4,6-tri-O-acetyl glucal. This metal-free, exclusively solvent-promoted Ferrier glycosylation, affords products in good to excellent yields and with good α-selectivity.     

Non-selectivity in the reaction of levoglucosenone with the sulfinyl allyl carbanion

Condensation of levoglucosenone with the carbanion ofrac-allyl phenyl sulfoxide, in contrast with reactions of this anion with the majority of other unsaturated ketones, proceeds without regio- or enantioselectivity to give a (1.0–1.8): 1 mixture of products of both 1,2- and 1,4-γ-addition of the allylic residue. Each product is a (1.2–1.6): 1 mixture of epimers at the asymmetric sulfur atom. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 157–159, January, 1999.     

The reaction of tetra(allyl)uranium with Lewis bases

The reaction of the thermally unstable U(π-C₃H₅)₄ with an excess of 2,2′-bipyridine leads to incorporation of three 2,2′-bipyridine ligands in the thermally more stable product in which transfer of two of the four allyl groups to one or two of the 2,2′-bipyridine ligands has occurred. The product has been characterized analytically, spectroscopically, magnetically, and by detailed studies of hydrolysis products.     

Palladium-catalyzed cross-coupling reaction of allyl acetates with pinacol aryl- and vinylboronates

Pinacol boronates 2 couple efficiently with allyl acetates 1 in the presence of a palladium catalyst prepared in situ from PdCl₂ and TFP to give the coupled products 3 in moderate to good yields under mild conditions.