Armed-disarmed effect of remote protecting groups on the glycosylation reaction of 2,3-dideoxyglycosyl donors

The armed-disarmed effect of remote protecting groups at the C-4 and/or C-6 position(s) on the glycosylation reactions of 2,3-dideoxyglycosyl donors was investigated. It was found that under various glycosylation conditions, 4- or 6-O-Bn 2,3-dideoxyglycosyl donors were much more reactive than the corresponding 4,6-di-O-Bz 2,3-dideoxyglycosyl donors. Based on these results, an effective and chemoselective glycosylation reaction using 4,6-di-O-Bn glycosyl acetate and 4-OH-6-O-Bz glycosyl acetate was realized, producing a 2,3-dideoxydisaccharide in good yield with high α-stereoselectivity.     

Cerium ammonium nitrate: a new catalyst for regioselective protection of glycols

The regioselective introduction of a methoxymethyl (MOM) group on different type of glycols via an orthoester intermediate was investigated. The novelty presented in this study is the use of ceric ammonium nitrate instead of the previously employed camphorsulfonic acid as catalyst. The monoprotection reaction was revealed to be highly selective when the glycol moiety was in the presence of an ether functionality.     

Discovery and application of iminotriphenylphosphorane as a formal aromatic primary amine protecting group

During our efforts to selectively synthesise N-arylated benzotriazole fragments, we developed a new primary aromatic amine protecting group strategy showing significant advantages over recognised protecting groups.     

Directing-protecting groups for carbohydrates. Design, conformational study, synthesis and application to regioselective functionalization

A novel concept of regioselective transformation of secondary hydroxyl groups in carbohydrates is presented. First, the relative reactivity of the free hydroxyl groups of onoprotected d-glucose derivatives was assessed using acetylation as a model reaction. As a result, acylation of these polyols gave a mixture of monosubstituted products in which the 3-O functionalized derivatives predominated. Novel hydrogen bond acceptor protecting groups were next designed to modulate the 4-OH and 3-OH reactivity in the hope to mediate higher regioselective transformations. A molecular modeling study later validated by spectroscopic analysis predicted additional intramolecular hydrogen bonds between the hydroxyl groups and pyridyl-containing protecting groups. Taking advantage of this induced hydrogen bond network, we achieved regioselective acetylation of the hydroxyl group at position 3 without protecting any secondary hydroxyl groups of the carbohydrate moiety. This designed protecting/directing group increased the nucleophilicity and the steric hindrance of position 3. As a result, optimization of the reaction conditions enabled the monoacetylation (not affected by steric hindrance) of 6-O-protected glucopyranosides at position 3 and selective silylation (affected by steric hindrance) of position 2 in high isolated yields and regioselectivities. This result certainly opens doors to the regioselective open glycosylation of carbohydrates.     

Wavelength-selective cleavage of o-nitrobenzyl and polyheteroaromatic benzyl protecting groups

Evaluation of the wavelength-selective cleavage of five photolabile protecting groups from two different families has been performed. Alanine, as a model bifunctional target molecule was masked at the amino terminal with o-nitrobenzyl group and at the carboxylic terminal with benzyl-type nitrogen and oxygen polyheteroaromatics, namely acridine, (thioxo)benzocoumarin and a coumarin built on the julolidine nucleus. The photosensitivity of the corresponding alanine conjugates was studied at selected wavelengths with HPLC/UV and ¹H NMR monitoring. The release of the fully deprotected molecule could be achieved by sequential irradiation in variable irradiation times, which were dependent on the heteroaromatic group used.     

Aryl groups,supplement of amino protecting group chemistry!

Amino group protective strategy has consequently emerged in multistep organic synthesis. Easy and selective deprotection procedures are crucial to facilitate the chemical transformation. Recently, Zhang's group from Henan Normal University collaborating with Chen's group of Nankai University developed a novel strategy for the regiospecific cleavage of inert aryl C-N bonds in N-aryl amides by hypervalent iodine(V) reagents. These procedures allow removal of sort of aryl groups under mild conditions to give primary amides in high efficiency. It bestows these aryl groups with the characteristics of amino protecting groups that might be the supplement of amino protecting group chemistry.     

Influence of functional groups on the self-assembly of liquid crystals

Functional groups in the molecule play an important role in the molecular o rganization process.To reveal the influence of functional groups on the self-assembly at interface,herein,the self-assembly structures of three liquid crystal molecules,which only differ in the functional groups,are explicitly characterized by using scanning tunneling microscopy(STM).The high-resolution STM images demonstrate the difference between the supramolecular assembly structures of three liquid crystal molecules,which attribute to the hydrogen bonding interaction and π-π stacking interaction between different functional groups.The density functional theory(DFT) results also confirm the influence of these functional groups on the self-assemblies.The effort on the self-assembly of liquid crystal molecules at interface could enhance the understanding of the supramolecular assembly mechanism and benefit the further application of liquid crystals.     

A simple PdCl2/O2/DMF catalytic system for highly regioselective cyclotrimerization of olefins with electron-withdrawing groups

A highly regioselective cyclotrimerization of olefins with electron-withdrawing groups in a PdCl₂/O₂/DMF catalytic system is disclosed, and a possible mechanism has also been proposed, which reveals the PdCl₂-catalyzed cyclotrimerization of olefins with electron-withdrawing groups goes through a quite different pathway from that of alkynes.     

Use of p-nitrobenzyloxycarbonyl (pNZ) as a permanent protecting group in the synthesis of Kahalalide F analogs

p-Nitrobenzyloxycarbonyl (pNZ) is used for the permanent protection of ornithine in the synthesis of derivatives of the anti-tumor cyclodepsipeptide Kahalalide F that contain acid labile residues.     

Structure and properties of hydrogen bonded complexes of pyridine-N-oxide and its derivatives

Molar Kerr constants and electric dipole moments of hydrogen bonded complexes that pyridine-N-oxide and its derivatives form with phenols are studied (a) experimentally, (b) by the vector/tensor addition scheme, and (c) by AM1 and PM3 semi-empirical quantum chemical methods. The data are used to establish the geometry of the complexes. It is shown that for a series of pyridine-N-oxide derivatives, the logarithm of the complex formation equilibrium constant correlates with the charge on the oxygen atom of the N–O group. A method to calculate the first and second complex formation constants based on the electric properties of the individual components of the complex is proposed. A relationship between the polarity of the hydrogen bond and the sum of the charges on the hydrogen and oxygen atoms forming the bond is established for the complexes under study.     

Total synthesis of bidensyneosides A2 and C: remarkable protecting group effects in glycosylation

Bidensyneosides are a group of five recently identified polyacetylenic glucosides from Bidens parviflora WILLD, a traditional Chinese medicinal plant that contains rich bioactive natural products. It was shown that bidensyneosides inhibited both histamine release and nitric oxide production. The synthesis of bidensyneoside A2 (2) and C (4) as well as 3-deoxybidensyneoside C (5) are described. These syntheses establish a synthetic entry to the bidensyneosides and confirm the stereochemistry at C3. Furthermore, a remarkable protecting group effect on orthoester formation was observed during the glycosylation reaction.     

BH3 as a protecting group for phosphonic acid: a novel method for the synthesis of dinucleoside H-phosphonate

A BH₃ group is found to be an effective protecting group for phosphonic acid esters. This new phosphonic acid protecting group was applied to the synthesis of a dithymidine H-phosphonate derivative from a dithymidine boranophosphate derivative. Triarylmethyl cations were found to be effective for the deprotection of the BH₃ group in the dithymidine boranophosphate diester to afford the corresponding H-phosphonate derivative in excellent yield.     

Effect of hydrogen bonds on intermolecular crosslinking reaction by introduction of carboxyl groups in free-radical crosslinking monomethacrylate/dimethacrylate copolymerizations

The effect of physical interaction through hydrogen bonds on the intermolecular crosslinking reaction leading to the promoted gelation in free-radical crosslinking monovinyl/divinyl copolymerizations was discussed from the standpoint of the control of network formation. The solution copolymerizations of benzyl methacrylate (BzMA) with 2 mol% of 1,6-hexanediol dimethacrylate in t-butylbenzene were conducted in the absence and presence of different amounts of mono(2-methacryloyloxyethyl) succinate (MMOES). Gelation was promoted by the addition of MMOES and the ratio of the actual gel point to the theoretical one became smaller; this would be related to the formation of hydrogen bonds between carboxyl groups introduced into prepolymer and growing polymer radical. As an extension of the above discussion, we treated the effect of hydrogen bonds on the gelation in the crosslinking BzMA/triicosaethylene glycol dimethacrylate copolymerization. The addition of MMOES obviously promoted the gelation. The ratio of the actual gel point to the theoretical one calculated according to Stockmayer's equation [J. Chem. Phys. 12 (1944) 125] was obtained as 1.9, very close to unity.     

Sulfonate protecting groups. Synthesis of O- and C-methylated inositols: d- and l-ononitol, d- and l-laminitol, mytilitol and scyllo-inositol methyl ether

Syntheses of d- and l-ononitol, d- and l-laminitol, mytilitol and scyllo-inositol methyl ether starting from myo-inositol are described. One or two of the myo-inositol 1,3,5-orthoformate hydroxyl groups were protected as tosylates. These mono or ditosylates served as key intermediates for the preparation of O- and C-methyl inositols. Racemic 2,4-di-O-tosyl-myo-inositol 1,3,5-orthoformate was resolved as its diastereomeric camphanates. Use of sulfonate groups for the protection of inositol hydroxyl groups resulted in substantial improvement in the overall yield of O- and C-methyl inositols.     

Ring conformations of d-glucose derivatives possessing two bulky silyl protecting groups at the 3,4-positions; the first observation of a stable full-axial chair conformer without bridge structures

The ring conformations of 3,4-bis-O-tert-butyldimethylsilyl- and 3,4-bis-O-tert-butyldiphenylsilyl-d-glucopyranoses as well as the corresponding phenyl 1-thio-d-glucopyranosides were investigated. Observations showed that the introduction of the two tert-butyldiphenylsilyl groups can flip the pyranose-ring into the ¹C₄ conformation possessing more axial substituents. All the substituents of the 3,4-bis-O-tert-butyldiphenylsilyl-β-d-glucopyranose were axially oriented.