Narayanaswamy Jayaraman

Branching out at C-2 of septanosides. Synthesis of 2-deoxy-2-C-alkyl/aryl septanosides from a bromo-oxepine.

Summary This paper deals with the synthesis of 2-deoxy-2-C-alkyl/aryl septanosides. A range of such septanoside derivatives was synthesized by using a common bromo-oxepine intermediate, involving C–C bond forming organometallic reactions. Unsaturated, seven-membered septanoside vinyl bromides or bromo-oxepines, obtained through a ring expansion methodology of the cyclopropane derivatives of oxyglycals, displayed a good reactivity towards several acceptor moieties in C–C bond forming Heck, Suzuki and Sonogashira coupling reactions, thus affording 2-deoxy-2-C-alkyl/aryl septanosides. Whereas Heck and Sonogashira coupling reactions afforded 2-deoxy-2-C-alkenyl and -alkynyl derivatives, respectively, the Suzuki reaction afforded 2-deoxy-2-C-aryl septanosides. Deprotection and reduction of the 2-deoxy-2-alkenyl derivative afforded the corresponding 2-deoxy-2-C-alkyl septanoside free of protecting groups. The present study illustrates the reactivity of bromo-oxepine in the synthesis of hitherto unknown septanosides, branching out at C-2, through C–C bond formation with alkyl and aryl substituents.

Glycosidic bond hydrolysis in septanosides: a comparison of mono-, di-, and 2-chloro-2-deoxy-septanosides

A kinetic study of the hydrolytic stabilities of mono-, di-, and 2-chloro-2-deoxy septanosides, under acid-catalysis, is reported herein. A comparison of mono- and diseptanosides, shows that the glycosidic bond in the disaccharide is more stable than the monosaccharide. Further the glycosidic bond at the reducing end hydrolyzes almost twice as faster than that of the non-reducing end of the disaccharide. 2-Chloro-2-deoxy septanoside is found to be the most stable and its glycosidic bond hydrolysis occurs at elevated temperatures only. The orientation of the exo-cyclic hydroxymethyl group and the inductive effect are suggested to play a role in the rates of hydrolysis.

Synthesis and Structure of Cyclic Trisaccharide with Expanded Glycosidic Linkages

A new cyclic trisaccharide is synthesized by cycloglycosylation of a linear trisaccharide, modified with hydroxymethyl moiety at C4 of glucopyranose moiety. The cyclic trisaccharide possesses a rarely observed perfect trigonal symmetry in the P3 space group, in a narrow cone shape, and a brick-wall type arrangement of molecules in the solid state, and exhibits a significantly enhanced binding affinity to 1-aminoadamantane in aqueous solution.

Facial selectivities in the nucleophilic additions of 2,3-unsaturated 3-arylsulfinyl pyranosides

2,3-Unsaturated 3-arylsulfinyl pyranosides undergo nucleophilic additions at C-2, with facial selectivities depending on the nucleophile and the substituent on sulfinyl sulfur. The reactions of such sugar vinyl sulfoxides lead to the addition of nucleophile preferring an axial orientation at C-2, with concomitant formation of an allylic bond at C-3 to C-4. This trend in the addition pattern is observed for primary amine, carbon and sulfur nucleophiles, whereas secondary amines prefer an equatorial addition at C-2. The effect of p-tolylthio- versus (p-isopropylphenyl)thio vinyl sulfoxide is that the equatorial nucleophilic addition is preferred even more with the latter vinyl sulfoxide.

Chemical and enzymatic approaches to the synthesis of cyclic oligosaccharides

Cyclic oligosaccharides, such as naturally-occurring cyclomaltodextrins or cyclodextrins, cycloisomaltooligosaccharides or cyclodextrans, cycloinulooligosaccharides, have attracted a repertoire of interest cutting across various fields, as a result of their immense industrial importance arising from their molecular and supramolecular properties. As opposed to the production of cyclic oligosaccharides biosynthetically in an exquisite manner, their chemical synthesis remains a challenge. Whereas macrocyclization and cyclooligomerization have provided necessary tools to approach their synthesis, modifications in their monosaccharide moieties add further to protract the synthesis overall. Yet significant progress has been made in chemical and enzymatic synthesis during the last three decades, particularly on the synthesis of tailor-made or made-to-order cyclic oligosaccharides. Manifold values of cyclic oligosaccharide macrocycles are imminent, resulting from their reduced hydrolytic rates to hydrophobic cavities, backbone modifications assume even greater importance. Such backbone modified cyclic oligosaccharides have been demonstrated to alter functions in several instances. Focusing on synthesis of cyclic oligosaccharides alone, there is a continuous evolution of methods and strategies that facilitate their synthesis. Both chemical and enzymatic routes are fore-runners in the advancements towards fully synthetic cyclic oligosaccharides. This review compiles logical advancements in cyclic oligosaccharide synthesis, particularly over the last decade, with emphasis on modifications of glycosidic bonds or individual sugar moieties.