Gold-catalyzed glycosidations: unusual cleavage of the interglycosidic bond while studying the armed/disarmed effect of propargyl glycosides

Armed/disarmed effect of propargyl glycosides in the presence of AuBr₃ is studied. Observed that oxophilic AuBr₃ cleaves interglycosidic bond of an armed disaccharide resulting in the formation of a disaccharide and a 1,6-anhydro sugar. Trisaccharides were obtained after fine tuning the reactivity of the glycosyl donor with different protecting groups.     

Low‐Concentration 1,2‐trans β‐Selective Glycosylation Strategy and Its Applications in Oligosaccharide Synthesis

This study develops an operationally easy, efficient, and general 1,2‐trans β‐selective glycosylation reaction that proceeds in the absence of a C2 acyl function. This process employs chemically stable thioglycosyl donors and low substrate concentrations to achieve excellent β‐selectivities in glycosylation reactions. This method is widely applicable to a range of glycosyl substrates irrespective of their structures and hydroxyl‐protecting functions. This low‐concentration 1,2‐trans β‐selective glycosylation in carbohydrate chemistry removes the restriction of using highly reactive thioglycosides to construct 1,2‐trans β‐glycosidic bonds. This is beneficial to the design of new strategies for oligosaccharide synthesis, as illustrated in the preparation of the biologically relevant β‐(1→6)‐glucan trisaccharide, β‐linked Gb₃ and isoGb₃ derivatives.     

A Stereoselective Glycosylation Approach to the Construction of 1,2-trans-β-d-Glycosidic Linkages and Convergent Synthesis of Saponins

Conventional syntheses of 1,2-trans-β-d - or α-l -glycosidic linkages rely mainly on neighboring group participation in the glycosylation reactions. The requirement for a neighboring participation group (NPG) excludes direct glycosylation with (1→2)-linked glycan donors, thus only allowing stepwise assembly of glycans and glycoconjugates containing this type of common motif. Here, a robust glycosylation protocol for the synthesis of 1,2-trans-β-d - or α-l -glycosidic linkages without resorting to NPG is disclosed; it employs an optimal combination of glycosyl N-phenyltrifluroacetimidates as donors, FeCl₃ as promoter, and CH₂Cl₂/nitrile as solvent. A broad substrate scope has been demonstrated by glycosylations with 12 (1→2)-linked di- and trisaccharide donors and 13 alcoholic acceptors including eight complex triterpene derivatives. Most of the glycosylation reactions are high yielding and exclusively 1,2-trans selective. Ten representative, naturally occurring triterpene saponins were thus synthesized in a convergent manner after deprotection of the coupled glycosides. Intensive mechanistic studies indicated that this glycosylation proceeds by S_N2-type substitution of the glycosyl α-nitrilium intermediates. Importantly, FeCl₃ dissociates and coordinates with nitrile into [Fe(RCN)_nCl₂]⁺ and [FeCl₄]⁻, and the ferric cationic species coordinates with the alcoholic acceptor to provide a protic species that activates the imidate, meanwhile the poor nucleophilicity of [FeCl₄]⁻ ensures an uninterruptive role for the glycosidation.     

Efficient and direct synthesis of saccharidic 1,2-ethylidenes, orthoesters, and glycals from peracetylated sugars via the in situ generation of glycosyl iodides with I2/Et3SiH

Peracetylated sugars can be efficiently converted into the corresponding 1,2-ethylidenes, -orthoesters, and -glycals via the in situ generation of glycosyl iodides promoted by I₂/Et₃SiH. The approach is straightforward and avoids isolation of the sensitive iodinated intermediates.     

AuBr3-catalyzed cyclization of o-(alkynyl)nitrobenzenes. Efficient synthesis of isatogens and anthranils

The cyclization of o-(arylalkynyl)nitrobenzenes was catalyzed by AuBr₃ to produce the corresponding isatogens in good to high yields together with small amounts of anthranils. On the other hand, anthranils were obtained selectively when the AuBr₃-catalyzed reaction was carried out using o-(alkylalkynyl)nitrobenzenes.     

Stereoselective Glycosidic Coupling Reactions of Fully Benzylated 1,2-Anhydro Sugars with N-Tosyl- or N-Benzyloxycarbonyl-l-serine Methyl Ester

Abstract

The glycosidic coupling reaction of 1,2-anhydro-3,4,6-tri-O-benzyl-β-d-mannopyranose (7), 1,2-anhydro-3,4,6-tri-O-benzyl-α-d-galactopyranose (21), and 1,2-anhydro-3,4-di-O-benzyl-α-d-xylopyranose (18) with N-tosyl- (10) or N-benzyloxycarbonyl- (11) L-serine methyl ester provides a new stereocontrolled synthesis of 1,2-trans linked glycopeptides. The 1,2-anhydro sugars are shown to react smoothyl with 10 or 11 in the presence of Lewis acid (ZnCl₂ or AgOTf) as well as powdered 4A molecular sieves in CH₂Cl₂ at room temperature to afford glycosyl serine derivatives with high stereoselectivity and high yield in less than 30 min. An improved method using 2-O-acetyl-3,4,6-tri-O-benzyl-α-d-mannopyranosyl chloride (6) as the key intermediate for ring closure was applied for the synthesis of 1,2-anhydro-3,4,6-tri-O-benzyl-β-d-mannopyranose.     

Glycosylation with 1,2-Anhydromannofuranose Benzyl Ether as the Glycosyl Donor: A Comparison Between Sugar Pyranose and Furanose Acceptors for their Primary Hydroxy Activity

ABSTRACT

Some 1→5 and 1→6 α-linked furano-disaccharide derivatives were synthesized using 1,2-anhydromannofuranose as the glycosyl donor. Comparison of the glycosyl acceptors indicated that the activity of primary hydroxyl groups of glycofuranoses was much lower than that of glycopyranoses.     

Regioselective and 1,2‐cis‐α‐Stereoselective Glycosylation Utilizing Glycosyl‐Acceptor‐Derived Boronic Ester Catalyst

Regioselective and 1,2‐cis‐α‐stereoselective glycosylations using 1α,2α‐anhydro glycosyl donors and diol glycosyl acceptors in the presence of a glycosyl‐acceptor‐derived boronic ester catalyst. The reactions proceed smoothly to give the corresponding 1,2‐cis‐α‐glycosides with high stereo‐ and regioselectivities in high yields without any further additives under mild reaction conditions. In addition, the present glycosylation method was successfully applied to the synthesis of an isoflavone glycoside.     

Thermal decomposition of 1,2 butadiene

The thermal decomposition of 1,2 butadiene has been studied behind reflected shock waves over the temperature and total pressure ranges of 1300–2000 K and 0.20–0.55 atm using mixtures of 3% and 4.3% 1,2 butadiene in Ne. The major products of the pyrolysis are C₂H₂, C₄H₂, C₂H₄, CH₄ and C₆H₆. Toluene was observed as a minor product in a narrow temperature range of 1500–1700 K. In order to model successfully the product profiles which were obtained by time-of-flight mass spectrometry, it was necessary to include the isomerization reaction of 1,2 to 1,3 butadiene. A reaction mechanism consisting of 74 reaction steps and 28 species was formulated to model the time and temperature dependence of major products obtained during the course of decomposition. The importance of C₃H₃ in the formation of benzene is demonstrated.     

The synthesis of 1,2-diacetyl- and 1,2-diethylferrocenes

A method is described for the preparation of 1,2-diacetylferrocene, in which ferrocene is acetylated with acetyl chloride in the presence of AlCl₃ in methylene chloride. Addition of the ferrocene to an excess of the acetylation mixture over a prolonged period was found to be most favourable. The 1,2-diacetylferrocene formed proved to be free of the 1,3-isomer. It was reduced with LiAlH₄/AlCl₃ to 1,2-diethylferrocene.     

Macrocycles with 1,2-dicyano-1,2-dithioethene units

Abstract

The new unsaturated macrocyclic tetrathioethers (Z,Z)-4 (n = 0), (Z,Z)-5 (n = 1), (Z,Z)-6 (n = 2) and (Z,Z)-7 (n = 3) were synthesized by the cyclization of (Z)-disodium-1,2-dicyanoethene-1,2-dithiolate (Z)-3 with ω,ω'-dibromoalkanes BrCH₂CH₂(CH₂)nCH₂Br (n = 0;1;2;3) on refluxing in dioxane in yields up to 15%. By reaction of the dithiolate (Z)-3 with 1,3-dibromopropane the unsaturated hexathioether (Z,Z,Z)-6 was also obtained. By the cyclization of dithiolate (Z)-3 with 1,5-dibromopentane and 1,6-dibromohexane the (Z,E)- and (E,E)-isomers, respectively, were formed in addition to the (Z,Z)-isomers. The (E,E)- and (Z,E)-isomers are photochemically convertable to the corresponding themodynamically more stable (Z,Z)-isomers by irradiation with UV-light. The (E,E)-isomers can be synthesized in a straightforward manner using the (E)-disodium-1,2-dicyanoethene-1,2-dithiolate (E)-3. Crystal structures of (Z,Z)-5, (Z,Z)-6, (E,E)-6, (Z,E)-7 and (E,E)-7 are reported.     

Dissolution of gold in the DMSO—RX systems. The concept of a donor-acceptor electron transport system

Summary The liquid phase oxidation of gold in donor-acceptor organic and aqueous-organic media has been studied. The compounds [AuCl(Me₂S)], [AuBr(Me₂S)], [AuBr₃(Me₂S)], [Me₃S][AuBr₄], [Me₃S][AuBr₄(Me₂S)]·H₂O, [Me₃SO]-[AuBr₄]·H₂O, [Me₃S][Au₂Br₇(Me₂S)₂]·3H₂O, [Me₃S]₂-[Au₂Br₈]·2DMSO·H₂O, [Me₂(Bu)SO][AuBr₄]·H₂O and [Me₃S]Br were isolated by dissolution of Au⁰ in DMSO-RX mixtures (R = H or Bu; X = Cl or Br). The products were characterized by elemental analysis and i.r. spectroscopy. The nature of the Au⁰-DMSO-RX systems and the oxidant species are discussed in terms of a newly-developed concept of donor-acceptor electron transport (DAET) systems.     

1,2-Bis(dimethylamino)-3,4,5-triphenyl-3,4,5-triphospha-1,2-diborolan und 1,2-Bis(dimethylamino)-3,4,5,6-tetra-tbutyl-3,4,5,6-tetraphospha-1,2-diborin: Synthese und Struktur sowie Berechnungen zur Molekülstruktur

1,2-Bis(dimethylamino)-3,4,5-triphenyl-3,4,5-triphospha-1,2-diborolane and 1,2-Bis(dimethylamino)-3,4,5,6-tetra-tbutyl-3,4,5,6-tetraphospha-1,2-diborine: Synthesis and Structure as well as Calculations on the Molecular Structure The diphosphides K₂[(C₆H₅)P? (C₆H₅)P? P(C₆H₅)], 4 or K₂[(tBuP)? (tBuP)₂? P(tBu)], 5 , react with (ClBNMe₂)₂ to form the binary 5-membered ring system 1,2-bis(dimethylamino)-3,4,5-triphenyl-3,4,5-triphospha-1,2-diborolane (C₆H₅P)₃(BNMe₂)₂, 2a , and the 6-membered ring system 1,2-bis(dimethylamino)-3,4,5,6-tetra-tbutyl-3,4,5,6-tetraphospha-1,2-diborine, (tBuP)₄(BNMe₂)₂, 3a , respectively. 2a and 3a could be obtained in a pure form and characterized NMR spectroscopically and by X-ray structure analyses. The two ring systems are folded; 2a exists in the ?envelope”?- 3a in the ?boat”?-conformation. Ab initio computations for 3,4,5-triphospha-1,2-diborolane M5 show that the global minimum is characterized by one B? P double bond. The parent compound geometry M6 is characterized by transannular bonding in the PH? BH? BH? PH moiety which differs in character from those in the four- and five-membered rings (BH)₂(PH)₂ and (BH)₂(PH)₃ M5 d , respectively. Explicit calculation of the influence of amino substituents on boron improved agreement of the bond length between computed and X-ray data.     

Formation of Heptaleno[1,2-c]furans and Heptaleno[1,2-c]furanones

The dehydrogenation reaction of the heptalene-4,5-dimethanols 4a and 4d , which do not undergo the double-bond-shift (DBS) process at ambient temperature, with basic MnO₂ in CH₂Cl₂ at room temperature, leads to the formation of the corresponding heptaleno[1,2-c]furans 6a and 6d , respectively, as well as to the corresponding heptaleno[1,2-c]furan-3-ones 7a and 7d , respectively (cf. Scheme 2 and 8). The formation of both product types necessarily involves a DBS process (cf. Scheme 7). The dehydrogenation reaction of the DBS isomer of 4a , i.e., 5a , with MnO₂ in CH₂Cl₂ at room temperature results, in addition to 6a and 7a , in the formation of the heptaleno[1,2-c]-furan-1-one 8a and, in small amounts, of the heptalene-4,5-dicarbaldehyde 9a (cf. Scheme 3). The benzo[a]heptalene-6,7-dimethanol 4c with a fixed position of the C?C bonds of the heptalene skeleton, on dehydrogenation with MnO₂ in CH₂Cl₂, gives only the corresponding furanone 11b (Scheme 4). By [²H₂]-labelling of the methanol function at C(7), it could be shown that the furanone formation takes place at the stage of the corresponding lactol [3-²H₂]- 15b (cf. Scheme 6). Heptalene-1,2-dimethanols 4c and 4e , which are, at room temperature, in thermal equilibrium with their corresponding DBS forms 5c and 5e , respectively, are dehydrogenated by MnO₂ in CH₂Cl₂ to give the corresponding heptaleno[1,2-c]furans 6c and 6e as well as the heptaleno[1,2-c]furan-3-ones 7c and 7e and, again, in small amounts, the heptaleno[1,2-c]furan-1-ones 8c and 8e , respectively (cf. Scheme 8). Therefore, it seems that the heptalene-1,2-dimethanols are responsible for the formation of the furan-1-ones (cf. Scheme 7). The methylenation of the furan-3-ones 7a and 7e with Tebbe's reagent leads to the formation of the 3-methyl-substituted heptaleno[1,2-c]furans 23a and 23e , respectively (cf. Scheme 9). The heptaleno[1,2-c]furans 6a, 6d , and 23a can be resolved into their antipodes on a Chiralcel OD column. The (P)-configuration is assigned to the heptaleno[1,2-c]furans showing a negative Cotton effect at ca. 320 nm in the CD spectrum in hexane (cf. Figs. 3–5 as well as Table 7). The (P)-configuration of (–)- 6a is correlated with the established (P)-configuration of the dimethanol (–)- 5a via dehydrogenation with MnO₂. The degree of twisting of the heptalene skeleton of 6 and 23 is determined by the Me-substitution pattern (cf. Table 9). The larger the heptalene gauche torsion angles are, the more hypsochromically shifted is the heptalene absorption band above 300 nm (cf. Table 7 and 8, as well as Figs. 6–9).     

Preparation and characterization of some structural variants of Cp*Ti(1,2-propandiolato) complexes

1,2-Propandiol reacts with Cp*Ti(CH₃)₃ by rapid liberation of methane to yield a dimetallic complex 6 of the net composition (Cp*Ti)₂(1,2-propandiolato)₃. The X-ray crystal structure analysis revealed an unsymmetrical bridging between the [Cp*Ti(1,2-propandiolato)] and [Cp*Ti(1,2-propandiolato)₂] subunits. Cp*TiCl₃ reacts with 1,2-propandiol in a 1:1 stoichiometry in the presence of excess pyridine by replacement of two chlorides by a 1,2-propandiolato ligand. The resulting product was isolated as a dimer 8 and characterized by X-ray diffraction. It exhibits a central Ti₂O₂ ring that was formed by bridging between the two [Cp*TiCl(1,2-propandiolato)] subunits using the oxygen atoms of the primary end of the ligand. From the reaction mixture a more complicated condensation product 9 was isolated in a small yield that contains two [Cp*TiCl(1,2-propandiolato)] units connected in a similar way by a Cp*-free [Ti(1,2-propandiolato)₂] moiety as revealed by its X-ray crystal structure analysis. Complex [Cp*TiCl(1,2-propandiolato)]₂ (8) gives an active catalyst for the syndiotactic polymerization of styrene upon treatment with excess methylalumoxane in toluene solution.     

Trinuclear Gold-Catalyzed 1,2-Difunctionalization of Alkenes

Activated alkyl halides have been extensively explored to generate alkyl radicals with Ru- and Ir- photocatalysts for 1,2-difunctionalization of alkenes, but unactivated alkyl bromides remain challenging substrates due to their strong reduction potential. Here we report a three-component 1,2-difunctionalization reaction of alkenes, unactivated alkyl bromides and nucleophiles (e.g., amines and indoles) using a trinuclear gold catalyst [Au₃(tppm)₂](OTf)₃. It can achieve the 1,2-aminoalkylation and 1,2-alkylarylation readily. This protocol has a broad reaction scope and excellent functional group compatibility (>100 examples with up to 96 % yield). It also affords a robust formal [2+2+1] cyclization strategy for the concise construction of pyrrolidine skeletons under mild reaction conditions. Mechanistic studies support an inner-sphere single electron transfer pathway for the successful cleavage of inert C−Br bonds.     

Syndiotactic 1,2-polybutadiene with Co-CS2 catalyst system. I. Preparation,properties, and application of highly crystalline syndiotactic 1,2-polybutadiene

Highly crystalline syndiotactic 1,2-polybutadiene (s-PB) having melting point (mp) up to 216°C was obtained by using a Co(acac)₃-AIEt₃-CS₂ catalyst. The polymer with mp 208°C was found to have 99.7% 1,2 content and 99.6% syndiotacticity by ¹H and ¹³C-NMR measurements. The s-PB can be molded by addition of a stabilizer such as 2,6-di-t-butyl-4-hydroxymethylphenol into fiber, film, and various shaped articles. The physical properties presented in the present article include stress-strain and dynamic mechanical behavior. The highly crystalline syndiotactic 1,2-polybutadiene was applied to a carbon fiber and UBEPOL VCR (cis-1,4-polybutadiene reinforced by fibrous syndiotactic 1,2-polybutadiene).     

Glycosyl Formates: Glycosylations with Neighboring-Group Participation

Protected 2-O-benzyolated glycosyl formates were synthesized in one-step from the corresponding orthoester using formic acid as the sole reagent. Glucopyranosyl, mannopyranosyl and galactopyranosyl donors were synthesized and their glycosylation properties studied using model glycosyl acceptors of varied steric bulk and reactivity. Bismuth triflate was the preferred catalyst and KPF₆ was used as an additive. The 1,2-trans-selectivities resulting from neighboring-group participation were excellent and the glycosylations were generally high-yielding.     

Bimolecular nature of boron trifluoride catalyzed glycosylation of a galactosyl donor: The role of the acceptor

Density functional theory (DFT) computations disclose the mechanism of a crucial neighboring participation step in BF₃ catalyzed stereoselective glycosylation of 1,2‐cyclopropaneacetylated galactosyl donor. Two tandem S_N2 displacements comprise this step: first, the glycosyl acceptor attacks the BF₃‐activated donor to break the donor's 1,2‐cyclopropane ring; then, the donor's 2‐acetyl oxygen substitutes the acceptor to accomplish the neighboring participation. A donor–acceptor hydrogen bond has been found to lower the overall activation free energy. This mechanism is preferred over a 2‐acetyl oxygen coface S_N2 displacement mechanism, in which no glycosyl acceptor is involved. © 2013 Wiley Periodicals, Inc.     

Intercalation of 1,2-Alkanediols into α-Zirconium Hydrogenphosphate

Intercalation compounds of α-Zr(HPO₄)₂ · H₂O with 1,2-alkanediols (from C3 to C16) have been prepared by replacing 1-propanol in α-Zr(HPO₄)₂ · 2C₃H₇OH with the desired 1,2-alkanediols by a treatment in a microwave field. It was found that the intercalates contain 1.5 molecules of diol per formula unit. The diol molecules are placed between the host layers in a bimolecular way with their aliphatic chains tilted at an angle of 51°. The diol molecules are anchored in the interlayer space by H-bonds. A mixed intercalate, containing 1,2-butanediol and 1,2-decanediol in a roughly equimolar ratio, is formed when the α-Zr(HPO₄)₂ · 2C₃H₇OH intercalate, suspended in a mixture of 1,2-butanediol and 1,2-decanediol, is exposed to microwave radiation. No new phase containing both types of the guest molecules was observed when the 1-propanol intercalate, suspended in a mixture of 1-propanol and 1,2-octanediol, is exposed to microwave radiation.