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A stereoselective Koenigs-Knorr glycosylation reaction under the catalysis of urea is described. This method is characterized by urea-mediated hydrogen-bond activation and subsequent glycosylation with glycosyl chlorides or bromides. Excellent yields and high anomeric selectivity can be achieved in most cases. Moreover, the low α-stereoselectivity of glycosylations observed when using perbenzylated glucosyl donors can be greatly improved by the addition of tri-(2,4,6-trimethoxyphenyl)phosphine (TTMPP).

Methods for trifluoromethylthiolation are very limited. To tackle this problem, a mild and efficient approach to the 2-trifluoromethylthiolation of glycals has been developed. This protocol employed N-trifluoromethylthiosaccharin as the trifluoromethylthiolating reagent, TMSCl as the activator, and DBU as the base, affording 2-trifluoromethylthioglycals in good yields. These trifluoromethylthiolated carbohydrates may benefit the development of carbohydrate-based drugs.

The 3-butenyloxycarbonyl group can be introduced into carbohydrates such as (I) under mild conditions.

A monoselective synthesis of aryl-C-Δ1,2-glycosides from 1-iodoglycals via palladium-catalyzed ortho-C–H activation of N-quinolyl benzamides has been developed. An amino acid derivative was used as a crucial ligand to improve the yield and monoselectivity of the coupling reaction. The utility of this protocol was demonstrated by a concise synthesis of key moieties of some natural products.

A new useful method for the synthesis of phenolic glycosides by the reaction of 1,2-anhydrosugars with aryl boronic acids is reported.

The HIV envelope (Env) is heavily glycosylated, facilitating the spread and survival of HIV in many ways. Some potent broadly neutralizing antibodies (bnAbs) such as 2G12, PG9, PG16, and PGTs can recognize the conserved glycan residues on Env. The bnAbs, which often emerge after many years of chronic infection, provide insight into the vulnerability of HIV and can therefore guide the design of vaccines. Many carbohydrate-conjugated vaccines have been designed to induce bnAb-like antibodies, but none have yet been successful. The low antigenicity of these vaccines is one possible explanation. New strategies have been applied to obtain high-affinity antigens of glycan-dependent and other bnAbs. However, when used as immunogens in vivo, high-affinity antigens are still insufficient in eliciting bnAb-like antibodies. bnAbs generally possess some unusual features and may therefore be suppressed by the host immune system. In view of this situation, some immunization regimens based on the affinity maturation of antibodies have been tested. Herein we summarize recent studies into the design of carbohydrate-based HIV vaccines and some valuable experiences gained in work with other bnAb-based HIV vaccines.

Fac-Ir(ppy)3 is used as photocatalyst, Umemoto′s reagent as CF3-source, and a household blue LED or sunlight as light source.

Activation of thioglycosides upon UV irradiation followed by cupric triflate oxidation leads to the in situ formation of an oxacarbenium ion which is able to undergo glycosylation reactions with a wide variety of substrates such as sugars, amino acids, terpenes, and sterols.

An efficient method for the construction of cyclopropane-fused dihydroquinolines was established by a novel Povarov-type three-component reaction of aromatic aldehydes, anilines and cyclopropenes in the presence of trifluoromethanesulfonic acid. The reaction proceeded in a regioselective and diastereoselective manner. Furthermore, the dihy-droquinolines were smoothly converted to benzazepine derivatives via a ring opening/isomerization sequence, further expanding the synthetic scope.

Iminosugars are an extremely interesting and versatile class of compounds and have many potential therapeutic applications. However, the use of iminosugar derivatives as immunomodulating agents is an area that is less explored. This review describes recent advances in the field of iminosugars as immunomodulating agents with a particular emphasis on 1-deoxynojirimycin analogues and related compounds. Both chemical and biological aspects concerning their preparation and/or occurrence in nature, as well as their immunomodulatory properties, are discussed.

A new “ring-opening–ring closure” strategy for the synthesis of aryl-C-glycosides was described. This strategy exploited the nickel-catalyzed regioselective β-O elimination of glycals by reactions with various aryl boronic acids or potassium aryltrifluoroborates to yield the ring-opened products, which underwent the Lewis acid, protonic acid, PhSeCl, or NBS mediated ring closure reactions to afford diverse aryl-C-glycosides. After Lewis acids and protonic acids were screened, it was found that, starting from the ring-opened substrates, the Ph3PHBr or Sc(OTf)3 mediated ring closure reaction provided α- or β-preferred aryl-C-Δ2,3-glycosides, respectively. Furthermore, β-d-phenyl-C-glycosides were successfully prepared via the PhSeCl-mediated cyclization reaction, whereas the α-d-phenyl-C-glycoside was obtained via the NBS-mediated cyclization reaction. After removal of the 2-substituted functionalities by Bu3SnH/AIBN, the synthesis of 2-deoxy-aryl-C-glycosides was ultimately realized in a stereoselective ma...

The multifaceted biological importance of carbohydrates has made them very popular targets in modern synthetic chemistry. Great progress has been made in stereoselective glycosylation methods and the construction of complex oligosaccharides. This Focus Review highlights the versatile applications of preactivation strategies in stereoselective glycosylation and oligosaccharide synthesis. Recent advances in synthetic methods, such as 4,6-O-benzylidene-acetal-directed β-mannosylation, the 1,2-cis-glycosylation mediated by a chiral auxiliary, the use of an oxazolidinone or carbonate protecting group, and glycosylation with participating additives, are described. Various examples of sequential glycosylation based on preactivation protocols for efficient oligosaccharide assembly are also summarized.

Wittig reaction with dipeptide (I) is highly chemoselective and is used to modify peptides in a site-specific manner.

It is found that the carbohydrates can undergo highly α-selective glycosylations in the presence of BF3 or SnCl4.

Optimized conditions are elaborated for the diastereoselective preparation of the target compounds.

An efficient preactivation protocol for the highly α-stereo-selective glycosylation of2-deoxy- and 2,6-dideoxysugars has been developed using acetyl-protected 2-deoxy- and 2,6-dideoxythioglycosides as glycosyl donors. The approach allows a wide range of glycosyl acceptors and donors to be used.

3,4-Bis(trimethylsilyl)thiophene and several other 3,4-disubstituted thiophenes are synthesised by an unprecedented intermolecular cycloaddition–cycloreversion procedure between disubstituted acetylenes and 4-methyl- or 4-phenyl-thiazole; 3,4-bis(trimethylsilyl)thiophene undergoes consecutive regiospecific mono-ipso-iodination and palladium-catalysed reactions to provide unsymmetrically 3,4-disubstituted thiophenes.

In the present work a novel improved and convenient procedure for the synthesis of oxazoles has been developed. Acetamide, instead of ammonium acetate, was used to react with phenacyl benzoates in boiling xylene in the presence of BF 3 Et 2 O as the catalyst. According to this new synthetic route, nine oxazole compounds were readily prepared as single products in reasonably high yields. On the basis of the experimental results, a plausible mechanism of this reaction is proposed.

The regiospecific synthesis of 3,4-disubstituted furans and thiophenes is reviewed. Our initial approach used 3,4-bis(tri-n-butylstannyl)furan as a building block, which led to many structurally diverse 3,4-disubstituted furans. Another avenue was explored by using 3,4-bis(trimethylsilyl)furan and 3,4-bis(trimethylsilyl)thiophene as precursors. A variety of unsymmetrically 3,4-disubstituted furans and thiophenes were prepared in this way. The advantages of our strategy lie in its stepwise manner as well as its prospect of yielding 3,4-disubstituted furans and thiophenes with rather different substitution pattern.

3,4-Bis(trimethylsilyl)thiophene (1a) was synthesized by three routes: (a) 1,3-dipolar cycloaddition; (b) modification of 3,4-dibromothiophene; and (c) intermolecular thiazole−alkyne Diels−Alder reaction. 3,4-Bis(trimethylsilyl)thiophene (1a) can function as a versatile building block for the construction of unsymmetrically 3,4-disubstituted thiophenes utilizing its stepwise regiospecific mono-ipso-substitution followed by palladium-catalyzed cross-coupling reactions. In this manner, thiophenes 15, 16, 17a−j, 19a,b, 20, 22a−c, 23a,b, 24a−d, 25a−c, and 27a−j were prepared. The thiophene-3,4-diyl dimer 28 and thiophene-3,4-diyl tetramer 29 were also realized by palladium-catalyzed self-coupling reaction of organoboroxines. The stannylthiophene 31, formed by conversion of the C−Si bond to a C−Sn bond via boroxine 26c underwent both carbonylative coupling and lithiation followed by quenching with electrophiles to afford unsymmetrically 3,4-disubstituted thiophenes 33 and 36a−c as well. Moreover, 3,4-bis(trim...

The assembly of an oligosaccharide library has been achieved in a practical and efficient manner employing a' one-pot sequential approach. With the help of the anomeric reactivity values of thioglycosides, using a thioglycoside (mono- or disaccharide) with one free hydroxyl group as acceptor and donor coupled with another fully protected thioglycoside, a di- or trisaccharide is selectively formed without self-condensation and subsequently reacted in situ with an anomerically inactive glycoside (mono- or disaccharide) to form a tri- or tetrasaccharide in high overall yield. The approach enables the rapid assembly of 33 linear or branched fully protected oligosaccharides using designed building blocks. These fully protected oligosaccharides have been partially or completely deprotected to create 29 more structures to further increase the diversity of the library.

Oxidative Heck-type C-glycosylations of glycals with various arylboronic acids using Pd(OAc)2 as catalyst in the presence of oxidant were developed. The corresponding ketone, enol ether, and enone types of C-glycosides were predictably obtained with benzoquinone (BQ), Cu(OAc)2/O2, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as oxidants, respectively. This method provides a simple, mild, and stereoselective synthesis of aryl 2-deoxy-C-glycosides.

Under pre-activation glycosylation conditions, the 4,6-di-O-acetyl-N-acetyloxazolidinone protected donor afforded either excellent β- or α-stereoselectivity simply by means of the addition of hindered base TTBP or the absence of base, leading to the controllable stereochemistry of coupling reactions.

Two asymmetrically branched oligosaccharides, LewisX and dimeric LewisX, were assembled in one pot with high yields and exclusive regio- and stereoselectivities. p-Tolyl thioglycosides were utilized as the sole type of building blocks, thus simplifying the overall synthetic design. The reactivity-independent nature of the preactivation based method allows modular assembly of the dimeric LewisX octasaccharide without the need for tedious protective group manipulation to achieve exact anomeric reactivities.

NaHSO 4 ·SiO 2 was used as an efficient heterogeneous catalyst for both the formation and the cleavage of benzylidene -acetals. This catalyst is compatible with many functional or protective groups. Under different solvent systems, either the formation or the cleavage of benzylidene acetals was carried out smoothly in -excellent yields and with good chemoselectivity.

The oligosaccharide residue in glycoconjugates located in cell membranes is responsible for intercellular recognition and interaction: it acts as a receptor for proteins, hormones, and microorganisms and governs immune reactions. These significant activities have stimulated great interest in the field of oligosaccharides and glycoconjugates. Although many advances have been made in the synthesis of oligosaccharides, more convenient and efficient methods are still needed. This review describes one of these new methods-the one-pot sequential glycosylation approach as a potent tool for oligosaccharide assembly. The oligosaccharide library construction in a one-pot fashion is also summarized.

Gb3 and isoGb3 are both biologically important oligosaccharides. A new efficient synthesis of Gb3 and isoGb3 trisaccharide analogues has been achieved by one-pot sequential glycosylation strategy starting from simple monosaccharide building blocks promoted by N-(phenylthio)-ɛ-caprolactam at room temperature.

A regioselective benzylation or p-methoxybenzylation of diols in azido-containing monosaccharide derivatives was reported, and the reaction was performed under the conventional reaction conditions (NaH/BnBr in DMF). The hydroxyl functionality adjacent to the azido group was benzylated selectively in good yields.

An efficient and improved procedure for preparation of triflyl azide and application in catalytic diazotransfer reaction are described. The amount of highly toxic reagents NaN3 and Tf2O, is significantly reduced comparing with the previous protocol.