Propargyl/methyl furanosides as potential glycosyl donors

Transfuranosylations are not well studied though many similar studies exist for transpyranosylation; herein, we report that propargyl/methyl D-ribf- and D-lyxf- give only 1,2-trans glycosides whereas D-araf- and D-xylf- result in a mixture of 1,2-trans and 1,2-cis glycosides; observed facts are rationalised by computational studies.     

Stereospecific synthesis of two metabolites of LTB4

The stereospecific synthesis of the recently identified metabolites of LTB₄ 1: 5(S),12(R),20-trihydroxy-6-cis,8,10-trans,14-cis-eicosatetraenoic acid 2 and 5(S),12(R),dihydroxy-6-cis,8,10-trans,14-cis-eicosatetraen-1,20-dioic acid 3, via the synthon 4 has been accomplished; identity of synthetic and natural products has been confirmed.     

Divinylketone: Conformational preferences. Molecular geometry and vibrational force field in the ground state

The conformational preferences of the molecule 1,4-pentadien-3-one (divinylketone) have been studied by ab initio molecular orbital calculations and discussed in terms of interaction between molecular fragments. The calculations predict a molecular ground state having a fully coplanar s-cis, s-cis conformation. In addition, we find three other structures that represent local minima on the energy surface. These are a fully coplanar s-cis, s-trans form, and two nonplanar s-trans, s-trans forms having symmetries C₂ and C_1h, respectively. The energies of these forms relative to the ground state are 7.5, 19.2, and 35.9 kJ/mol, respectively. The coplanar s-trans, s-trans form represents a saddle point on the energy surface. All conformers and the saddle point have been completely geometry optimized by the gradient technique. For the ground state a complete in-plane harmonic force field has been evaluated.     

Facile synthesis of unusual glycosyl carbamates and amino acid glycosides from propargyl 1,2-orthoesters as glycosyl donors

Propargyl 1,2-O-orthoesters are exploited for the synthesis of 1,2-trans O-glycosides of protected amino acids. N-Fmoc- and N-Cbz protected serine/threonine - benzyl/methyl esters reacted well with glucosyl-, galactosyl-, mannosyl- and lactosyl- derived propargyl 1,2-orthoesters affording respective 1,2-trans glycosides in good yields under AuBr(3)/4 ? MS Powder/CH(2)Cl(2)/rt. t-Boc serine derivative gave serine 1,2-orthoester and glycosyl carbamate. Optimized conditions enabled preparation of new glycosyl carbamates from N-Boc protected amines in a single step using gold catalysts and propargyl 1,2-orthoesters in excellent yields.     

Radical C-glycosylation reaction of pyranosides with the 2,3-trans carbamate group

Radical-mediated C-glycosylation of pyranosides with the 2,3-trans carbamate group was investigated. C-Glycosylation was achieved with high α-selectivity.     

How the arming participating moieties can broaden the scope of chemoselective oligosaccharide synthesis by allowing the inverse armed-disarmed approach

A new method for stereocontrolled glycosylation and chemoselective oligosaccharide synthesis has been developed. It has been determined that complete 1,2-trans selectivity can be achieved with the use of a 2-O-picolyl moiety, a novel neighboring group that is capable of efficient participation via a six-membered intermediate. The application of the picolyl concept to glycosidations of thioimidoyl, thioglycosyl, and trichloroacetimidoyl glycosyl donors is demonstrated. The picolyl moiety also retains the glycosyl donor in the armed state, as opposed to conventional acyl participating moieties. We name this new approach the "inverse armed-disarmed" strategy, because it allows for the chemoselective introduction of a 1,2-trans glycosidic linkage prior to other linkages. In the context of the oligosaccharide synthesis, the strategy provides trans-trans and trans-cis patterned oligosaccharides as opposed to classic Fraser-Reid's armed-disarmed approach leading to cis-trans and cis-cis linkages.     

1,2和1,3-缩水内醚糖衍生物的制备, 构象及其糖苷化反应

吡喃糖的1,2-及1,3-缩水内醚苄醚由相应的吡喃糖的C-2氧负离子(对1,3-缩水内醚是C-3氧负离子)与连有氯原子的C-1经分子内关环反应而制备, 而有些吡喃糖的1,2-缩水内醚苄醚是由相应的吡喃糖的C-1氧负离子与连有对甲苯磺酰氧基的C-2经分子内关环(倒关环)反应而得。呋喃糖的1,2-缩水内醚苄醚只能用倒关环法合成。1,2-(或1,3-)缩水内醚糖的开环反应通常给出1,2-反式连接(对1,3缩水内醚是1,3反式连接)的糖苷键。1,2-及1,3-缩水内醚糖的构象分析是通过^1H NMR测定及分子力学计算的方法而完成的。     

Total synthesis of (+)-astrophylline

The first total synthesis of (+)-astrophylline (2) has been achieved, starting from readily available enantiomerically pure (+)-(1R,4S)-4-hydroxycyclopent-2-enyl acetate (11). A novel ruthenium-catalyzed ring-closing ring-opening ring-closing metathesis of carbocyclic olefins of general type 5 was the key step, providing the stereochemically well-defined bis-piperidyl skeleton of the target molecule. A [2,3]-Wittig-Still rearrangement of 9 was also employed as the critical transformation in the stereocontrolled generation of the 1,2-trans configuration of the cyclopentene intermediate 6c. Our early synthetic efforts toward 1,2-trans cyclopentene derivatives of type 6, as well as the synthetic pathway to an optimized 13-step total synthesis of 2 (12% overall yield), are reported.     

Oxidative cyclisation of cinnamyl ethers mediated by CAN: a stereoselective synthesis of 3,4-trans disubstituted tetrahydrofuran derivatives

The oxidative cyclisation of cinnamyl ethers mediated by cerium(IV) ammonium nitrate results in the stereospecific formation of 3,4-trans disubstituted tetrahydrofuran derivatives in moderate to good yields.     

A diastereoselective ring contraction of 1,3-dioxepins to 2,3,4-trisubstituted and tetrasubstituted tetrahydrofurans

A modular and diastereoselective approach to 2,3,4-trisubstituted and tetrasubstituted tetrahydrofurans is reported. The use of dioxepins containing an embedded vinyl acetal functionality leads to a Lewis acid-mediated [1,3] ring contraction to afford tetrahydrofurans in good yield and excellent diastereoselectivity. The use of TMSOTf in MeCN leads to the 2,3-cis/3,4-trans diastereomer while SnCl4 in CH2Cl2 provides the 2,3-trans/3,4-cis diastereomer. A variety of substituents are tolerated at each position. The presence of Lewis basic functionality under the SnCl4 conditions alters the reaction favoring the diastereomer formed under the TMSOTf conditions. We present conclusive evidence that the products of each of these reactions are formed under kinetic control. We further provide stereochemical models consistent with each of these rearrangement reactions that account for the formation of the major diastereomer in each case.     

Reagent-controlled stereoselective synthesis of lignan-related tetrahydrofurans

The reaction of ring-closing metathesis-derived cyclic allylsiloxanes 3 with aldehydes in the presence of a Lewis acid gives 2,3,4-trisubstituted tetrahydrofurans related to the furanolignan family of natural products. The reactions proceed with complete 3,4-trans stereoselectivity, whereas the C-2 stereochemistry depends on both the aldehyde and Lewis acid used. When boron trifluoride etherate is used with aliphatic or electronically neutral aryl aldehydes, the reactions favor the production of the 2,3-cis isomer 8, whereas electron-rich aryl aldehydes lead to the 2,3-trans isomer 9 by Lewis acid-mediated isomerization of the kinetically favored cis isomer. The isomerization can be avoided by use of TMSOTf as a promoter, and hence, the stereochemistry can be tuned by appropriate choice of reagent. Cleavage of the pendant 3-ethenyl group installs the 3-hydroxymethyl group common to the furanolignans.     

A rapid and sensitive method for measurement of leukotrienes based on HPLC

A rapid and sensitive method for simultaneous detection of leukotriene (LT) B4, LTC4, LTD4, LTE4, 5S,12S-dihydroxy-eicosatetraenoic acid, 6-trans LTB4, 12-epi-6-trans LTB4 and the omega-metabolites of LTB4 has been developed. Leukocyte incubations were terminated by the addition of one volume of methanol, and the mixture was centrifuged. Analysis was then directly performed by reverse phase high performance liquid chromatography on a column packed with 3 microns Nucleosil C18 guarded by a precolumn. Quantitation of eluted compounds was carried out by UV-monitoring at 280 nm and integration of the elution profile. The sensitivity of the method was approximately 1 ng for both LTB4 and LTC4. Accuracy was 101.6% (SD +/- 9.8%) for LTB4 and 99.9% (SD +/- 8.7%) for LTC4 in the interval 5-160 ng.     

Functionalized tetrahydrofurans from alkenols and olefins/alkynes via aerobic oxidation-radical addition cascades

Aerobic oxidation of alkyl- and phenyl-substituted 4-pentenols (bishomoallyl alcohols), catalyzed by cobalt(II) complexes in solutions of γ-terpinene or cyclohexa-1,4-diene, stereoselectively gave tetrahydrofurylmethyl radicals. Cyclized radicals were trapped with monosubstituted olefins (e.g., acrylonitrile, methyl acrylate), (E)- and (Z)-1,2-diacceptor-substituted olefins (e.g., dimethyl fumarate, fumarodinitrile, N-phenyl maleic imide), and ester-substituted alkynes (e.g., ethyl propynoate). Oxidation-addition cascades thus furnished side-chain-substituted (CN, CO(2)R, COR, or SO(2)R) di- and trisubstituted tetrahydrofurans in stereoselective reactions (2,3-trans, 2,4-cis, and 2,5-trans). A diastereomerically pure bistetrahydrofuran was prepared in a cascade consisting of two aerobic oxidations, one alkyne addition, and one final H-atom transfer.     

Glycosidation-anomerisation reactions of 6,1-anhydroglucopyranuronic acid and anomerisation of beta-D-glucopyranosiduronic acids promoted by SnCl(4)

The reaction of silylated nucleophiles with 6,1-anhydroglucopyranuronic acid (glucuronic acid 6,1-lactones) catalysed by tin(IV) chloride provides 1,2-trans or 1,2-cis (deoxy)glycosides in a manner dependent on the donor structure. The alpha-glycoside was obtained for reactions of the donor with the 2-acyl group and 2-deoxydonors, whereas the 2-deoxy-2-iodo donor gave the beta-glycoside. Experimental evidence shows that when 1,2-cis-glycoside formation occurs, the anomerisation of initially formed 1,2-trans-glycosides catalysed by SnCl(4) is possible. The anomerisation of beta-D-glucopyranosiduronic acids was found to be faster, in some cases, than anomerisation of related beta-D-glucopyranosiduronic acid esters and beta-D-glucopyranoside derivatives and the rates are dependent on the structure of the aglycon. Moreover, the rates of anomerisation of beta-D-glucopyranuronic acid derivatives can be qualitatively correlated with rates of hydrolysis of beta-D-glucopyranosiduronic acids. Mechanistic possibilities for the reactions are considered.     

Simple Methods for Determining Relative Stereochemistry of Kainoid Amino Acids by (1)H NMR Chemical Shifts

The kainoid amino acids are biologically important compounds because they show remarkable neuroexcitatory and excitotoxic activities. For exhibiting potent activity, the stereochemical relationship of the substituents on the pyrrolidine ring is crucial. We found simple methods for determining the relative stereochemistry of these compounds on the basis of the (1)H NMR chemical shifts of H-2 and H-4 in D(2)O solution. The signals of H-2 appear at fields higher than 4.2 ppm when the compounds have 2,3-trans stereochemistry whereas, in the 2,3-cis compounds, they appear lower than 4.2 ppm, irrespective of the C-4 substituent. This criterion holds when the solution is in the range of pD 3-8. Moreover, when an epimeric pair at C-2 is available and the spectra are recorded at the same or nearly equal pD, the H-2 chemical shift of the 2,3-trans isomer is higher than that of the corresponding 2,3-cis isomer. Similarly, the relative stereochemistry between C-3 and C-4 can be determined from the chemical shift of H-4. The signals of H-4 of the 3,4-cis isomers appear at lower fields than those of the corresponding 3,4-trans isomers in each pair of C-4 epimers when the spectra are recorded at the same or nearly equal pD. This holds for the compounds bearing an unsaturated substituent at C-4. All these phenomena can be rationalized by the anisotropic effect of the pi-electron system in the C-2 and C-4 substituents.     

Fully Chemical Recyclable Poly(γ-butyrolactone)-based Copolymers with Tunable Structures and Properties

Chinese Journal of Polymer Science - The emerging chemical recyclable polymers, such as poly(γ-butyrolactone) (PGBL) and poly((R)-3,4-trans six-membered ring-fused GBL) (P((R)-M)), provide a...     

Synthesis of amphiphilic phenylazophenyl glycosides and a study of their liquid crystal properties

Several 4-(4'-N,N-didodecylaminophenylazo)phenyl 1,2-trans glycosides 5a-e with various carbohydrate heads (beta-D-gluco, beta-D-galacto, beta-lacto, beta-D-xylo, and alpha-D-manno) have been synthesized. The key step was the formation of phenyldiazonium tetrafluoroborates 2a-e from the per-O-acetylated 4-aminophenyl glycosides 1a-e. These salts were condensed with N,N-didodecylaniline under phase transfer conditions and the per-O-acetylated 4-(4'-N,N-didodecylaminophenylazo)phenyl 1,2-trans glycosides 4a-e were fully de-O-acetylated by the Zemplén method. The self-organizing liquid crystal properties of the compounds were investigated by a variety of techniques, including polarized light microscopy, differential scanning calorimetry, and X-ray diffraction. All but one of the materials exhibited smectic A, lamellar phases. Remarkably, the glucose derivative exhibited a rectangular disordered columnar phase. This result has implications with respect to the induced curvature created by the recognition processes of the glucose headgroup relative to the other sugar moieties and to the prevalence of various glycolipids in cell membranes     

Stereospecific synthesis of 11S, 12S-oxido 5Z, 7E, 9E, 14Z-eicosatetraenoic acid

The first stereospecific synthesis of 11S,12S-oxido 5Z, 7E, 9E, 14Z-eicosatetraenoic acid has been achieved from 2-deoxy-D-ribose using either a Horner-Emmons or Wittig condensation to form the 9,10-trans or the 5,6-cis-double bond respectively.     

Endocyclic cleavage in glycosides with 2,3-trans cyclic protecting groups

An endocyclic pathway is proposed as a reaction mechanism for the anomerization from the β (1,2-trans) to the α (1,2-cis) configuration observed in glycosides carrying 2,3-trans cyclic protecting groups. This reaction occurs in the presence of a weak Lewis or Br?nsted acid, while endocyclic cleavage (endocleavage) in typical glycosides was observed only when mediated by protic media or strong Lewis acids. To rationalize the behavior of this class of compounds, the reaction mechanism and the promoting factors of the endocleavage are investigated using quantum-mechanical (QM) calculations and experimental studies. We examine anomerization reactions of thioglycosides carrying 2,3-trans cyclic protecting groups, employing boron trifluoride etherate (BF(3)·OEt(2)) as a Lewis acid. The estimated theoretical reactivity, based on a simple model to predict transition state (TS) energies from the strain caused by the fused rings, is very close to the TS energies calculated by the TS search along the C1-C2 bond rotation after the endo C-O bond breaking. Excellent agreement is found between the predicted TS energies and the experimental reactivity ranking. The series of calculations and experiments strongly supports the predominance of the endocyclic rather than the exocyclic mechanism. Furthermore, these investigations suggest that the inner strain is the primary factor enhancing the endocleavage reaction. The effect of the cyclic protecting group in restricting the pyranoside ring to a (4)C(1) conformation, extensively discussed in conjunction with the stereoelectronic effect theory, is shown to be a secondary factor.     

Synthesis and structure of gold complexes [Ph3PR]+[Au(CN)2I2-trans]−, R = Et,CH2Ph,Ph

Russian Chemical Bulletin - Gold complexes [Ph3PR]+[Au(CN)2I2-trans]?, where R = Et (1), CH2Ph (2), Ph (3), were synthesized by the reaction of potassium dicyanodiiodoaurate with...