Per J. Garegg

A novel, reductive ring-opening of carbohydrate benzylidene acetals☆

Abstract Further examples are given of a facile, highly regioselective, reductive opening of benzylidene acetals of hexopyranosides using sodium cyanoborohydride-hydrogen chloride. For 4,6-benzylidene acetals, the benzyl group in the product is at O-6 and HO-4 is free. For dioxolane benzylidene acetals, the direction of reductive opening of the five-membered ring depends on the stereochemistry at the asymmetric, benzylidene acetal carbon.

Thioglycosides as Glycosyl Donors in Oligosaccharide Synthesis

Publisher Summary This chapter analyzes the importance of thioclycosides as the glycosyl donors in oligosaccharide synthesis. The presence and biological importance of oligosaccharide structures, usually as the components of glycolipids and glycoproteins, in bacterial capsular and cell-wall polysaccharides, in mammalian cell membranes, in cytoplasm, and in extra-cellular fluids are well documented. Acylated glycosyl thiocyanates are made by the treatment of acylated glycopyranosyl halides with potassium thiocyanate. An attractive feature of thioglycosides as the glycosyl donors in oligosaccharide synthesis is that they are readily converted into all of the other glycosyl donors. The glycosyl bromide can also be converted into a 4-pentenyl glycoside, which becomes a glycosyl donor by promotion with an iodonium compound. Tris(4-bromophenyl)ammoniurnyl hexachloroantimonate (TBPA) differs from the other promoters in the context that its cation is a radical and produces radical cationic sulfonium ions such as glycosylating species from thioglycosides. In acetonitrile as solvent, and with a nonparticipating 2-substituent in the glycosyl donor, 1,2-trans glycosides are formed as a result of solvent participant. The transfer of the Armed–Disarmed concept to thioglycoside glycosylation reactions is also elaborated in the chapter.

Thioglycosides as glycosylating agents in oligosaccharide synthesis

A review is given of the use of thioglycosides as glycosyl donors in oligosaccharide synthesis. Both indirect use, by conversion of the thioglycoside into a glycosyl halide and direct use, by electrophilic activation of the thioglycoside, are discussed.

Nucleoside H-phosphonates. III. Chemical synthesis of oligodeoxyribonucleotides by the hydrogenphosphonate approach

Abstract A rapid synthesis of oligodeoxyribonucleotides on solid support is described via coupling of the deoxyribonucleoside 3′- H-phosphonates in the presence of various condensing reagents.

Reductive Ring Openings of Carbohydrate Benzylidene Acetals Using Borane-Trimethylamine and Aluminium Chloride. Regioselectivity and Solvent Dependance

Regioselective reductive ring openings of 4, 6-O-benzylidene acetals of hexopyranosides are described using borane trimethylamine-aluminium chloride. Using toluene as solvent, 4-O-benzyl ethers with the 6-OH free are obtained. Using tetrahydrofuran as solvent. 6-O-benzyl ethers with the 4-OH free are obtained.

Nucleoside H-phosphonates. IV. Automated solid phase synthesis of oligoribonucleotides by the hydrogenphosphonate approach

Abstract A rapid and efficient synthesis of oligoribonucleotides on solid support is described via coupling of 5′- O -dimethoxytrityl-2′- O -t-butyldimethylsilyl ribonucleoside 3′-H-phosphonates II to the polymer bound nucleoside in the presence of pivaloyl chloride as coupling agent.

Some aspects of regio-, stereo-, and chemoselective reactions in carbohydrate chemistry

Selective transformations in carbohydrate chemistry will be illustrated with examples chosen from three distinct areas: 1. Phase transfer reactions are useful in obtaining partially substituted carbohydrate intermediates. The substitution patterns frequently are quite different to those obtained in homogeneous phase. 2. Various reagent systems have been developed for the conversion of hyd roxy compounds to chlorodeoxy b romodeoxy and deoxyiod o compounds as well as to olefinic derivatives. A number of partial substitutions have been achieved starting from unsubstituted hexopyranosides providing easy access to some compounds of biological significance. 3. Benzylidene acetals, phenyl substituted derivatives of these and also 2—propenylidene acetals may be reductively cleaved in the presence of proton or Lewis acids to give partially substituted derivatives useful as intermediates in various synthetic work. The regioselectivity may be manipulated by change in reagent or solvent.

Synthesis of 1,2-cis-linked glycosides using dimethyl(methylthio) sulfonium triplate as promoter and thioglycosides as glycosyl donors

Abstract Stereoselective, halide-assisted, 1,2- cis -glycosylations may be obtained using stable thioglycosides containing a non-participating 2-substituent as glycosyl donors, in the presence of dimethyl(methythio)sulfonium triflate and tetrabutylammonium bromide as promoters.

Novel reagent system for converting a hydroxy-group into an iodo-group in carbohydrates with inversion of configuration. Part 2

Isolated primary and secondary hydroxy-groups in carbohydrate derivatives are transformed into iodo-groups with inversion of configuration on treatment with either triphenylphosphine, iodine, and imidazole or triphenyl-phosphine and 2,4,5-tri-iodoimidazole at elevated temperatures. At lower temperatures, primary hydroxy-groups may be selectively replaced by iodo-groups.

Preparation and N.M.R. studies of pyruvic acid and related acetals of pyranosides: configuration at the acetal carbon atoms

Abstract Stereoisomeric pairs of pyruvic acid and related acetals linked to the 3,4- and 4,6-positions, respectively, of the anomeric methyl d -galactopyranosides and the corresponding acetals linked to the 4,6-positions of the anomeric methyl d -glucopyranosides have been prepared by conventional methods, and their structures have been assigned. Their 1 H- and 13 C-n.m.r. spectra have been recorded. The differences in chemical shifts obtained for stereoisomeric pairs of acetalic CH 3 groups are of sufficient magnitude to make possible the unequivocal determination of the stereo-chemistry of pyruvic acid acetals in naturally occurring polysaccharides.