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The reaction of 4, 6-O-benzylidene-D-pyranosides with a stoichiometric amount of TsCl, AcCl, and BzCl in the presence of silver(I) oxide and a catalytic amount of potassium iodide led to monosubstituted derivatives in high regioselectivity and in good yields.

An oligosaccharides library of small tri- and tetrasaccharides is prepared by one-pot sequential glycosylation strategies using thioglycosides as glycosylation reagents. The methodology facilitates rapid assembly of 33 linear or branched protected oligosaccharides.

A novel method for the synthesis of unsymmetrically 3,4-disubstituted thiophenes was developed based on the intermolecular thiazole-alkyne Diels-Alder reactions involving 3,4-bis(trimethylsilyl)thiophene. The synthetic strategy was characterized by the formation of the [2+2]- and [4+2]-cycloaddition adducts of 3,4-didehydrothiophene via 1,3-dipolar cycloadditions. Furthermore, the synthetic process was characterized by 3,4-bromothiophene modification and intermolecular thiazole-alkyne Diels-Alder reactions with 3,4-bis(trimethylsilyl)thiophene.

Two asymmetrically branched oligosaccharides, [Lewis.sup.x] and dimeric [Lewis.sup.x], are assembled in one pot with high yields and exclusive regio- and stereoselectivities, where p-tolyl thioglycosides are used as the sole type of building blocks. The reactivity-independent nature of the preactivation based method has allowed modular assembly of the dimeric Lewis octasaccharide.

Two efficient routes are described for the rapid assembly of the tumor-associated carbohydrate antigen Globo-H hexasaccharide by a preactivation based iterative one-pot strategy. Both the methods have shown that oligosaccharides containing [alpha] and [beta] linkages within the same molecule can be constructed in one pot through a preactivation based approach with higher glyco-assembly efficiencies than the automated solid-phase synthesis strategy.

Reports that the assembly of an oligosaccharide library has been achieved in a practical and efficient manner. Application of a one-pot sequential approach; Rapid assembly of 33 linear or branched fully-protected oligosaccharides using designed building blocks; Deprotection of the oligosaccharides either partially or completely to create 29 more structures.

Carbohydrates play essential roles in various physiological and pathological processes. Trifluoromethylated compounds have wide applications in the field of medicinal chemistry. Herein, we report a practical and efficient trifluoromethylation of glycals by an electrochemical approach using CF 3 SO 2 Na as the trifluoromethyl source and MnBr 2 as the redox mediator. A variety of trifluoromethylated glycals bearing different protective groups are obtained in 60-90% yields with high regioselectivity. The successful capture of a CF 3 radical indicates that a radical mechanism is involved in this reaction.

An efficient protocol for the O-sialylation using thiosialoside donors under visible light photocatalysis was developed. Thiosialosides were activated under the irradiation with blue light in the presence of Ru(bpy)3(PF6)2 as photocatalyst, Umemoto's reagent as CF3 radical source and Cu(OTf)2 as an additive in acetonitrile/dichloromethane at -30 °C, and the subsequent reaction with glycosyl acceptors generally produced the desired sialosides in good to excellent yields with the satisfactory α-selectivity.

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-Δ(2,3)-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 manner.

The glycan portion of ganglioside HLG-2, which was identified in the extracts of the sea cucumber Holothuria leucospilota , was synthesized in a highly efficient and stereoselective manner. The unusual sequence of the trisaccharide moiety, α-N-glycolylsialyl-(2,4)-α-N-acetylsialyl-(2,6)-glucoside, was assembled by stereoselective coupling of a 5-N,4-O-carbonyl-protected sialyl phosphate donor, a N-2,2,2-trichloroethoxycarbonyl (Troc)-protected sialyl acceptor, and a (trimethylsilyl)ethyl-β-glucosyl acceptor in high yield. The synthesis featured the high-yielding construction of two α-sialyl linkages.

Catalytic or stoichiometric amounts of Lewis acids were found to be very effective α-directing additives in the stereoselective glycosylations of diverse 2,3-O-carbonate-protected glucose and galactose thioglycoside donors by preactivation protocol. The poor stereoselectivities of 4,6-di-O-acetyl-2,3-O-carbonate protected thioglycoside donors in glycosyl coupling reactions were greatly improved, and excellent α-stereoselectivities were achieved by the addition of 0.2 equiv of BF(3)·OEt(2). On the other hand, the β-selectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thioglucoside donor toward glycosylations were reversed completely to the α-selectivities by the use of 1 equiv of SnCl(4), making the stereoselectivity controllable. Furthermore, the poor stereoselectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thiogalactoside donor in glycosylations were also improved by using SnCl(4) as additive.

A versatile and concise synthesis of N-alkylated 1,4-dideoxy-1,4-imino-D-arabinitol and 1,4-dideoxy-1,4-imino-L-xylitol derivatives is described. These were prepared using pseudohemiketal lactams as key intermediates, which in turn were obtained from sucrose. The key intermediates were prepared by a diastereospecific tandem reaction which facilitated the introduction of various substituents on the nitrogen atom of the iminosugars.

Two efficient routes for the rapid assembly of the tumor-associated carbohydrate antigen Globo-H hexasaccharide 2 by a preactivation based iterative one-pot strategy are reported. The first method involves the sequential coupling of four glycosyl building blocks, leading to the desired hexasaccharide in 47% overall yield in one-pot synthesis. Although model studies on constructing the challenging Gal-alpha-(1-4)-Gal linkage in Gb3 trisaccharide yielded the desired alpha linkage almost exclusively, a similar approach to assemble the hexasaccharide led to the formation of a significant amount of beta anomer. As an alternative, the second synthesis utilized three components in one pot with the Gal-alpha-(1-4)-Gal linkage preformed, producing the desired hexasaccharide in a similar overall yield as the four component approach. Both methods demonstrate that oligosaccharides containing alpha and beta linkages within the same molecule can be constructed in one pot via a preactivation based approach with higher glyco-assembly efficiencies than the automated solid-phase synthesis strategy. Furthermore, because glycosylations can be carried out independent of anomeric reactivities of donors, it is not necessary to differentiate anomeric reactivities of building blocks through extensive protective group adjustment for chemoselective glycosylation. This confers great flexibilities in the building block design, allowing matching of the donor with the acceptor, leading to improved overall yield.

The reaction of 4, 6-O-benzylidene-d-pyranosides with a stoichiometric amount of TsCl, AcCl, and BzCl in the presence of silver(I) oxide and a catalytic amount of potassium iodide led to monosubstituted derivatives in high regioselectivity and in good yields., (Copyright 2004 American Chemical Society)