Seema Mehta

Phenylselenoglycosides as novel, versatile glycosyl donors. Selective activation over thioglycosides

Abstract Selective activation of phenylselonoglycosides over ethylthioglycosides with silver trifluoromethanesulfonate and anhydrous potassium carbonate gives an efficient synthesis of disaccharides from selenoglycoside donors and thioglycoside acceptors. Activation is quenched by addition of 1,1,3,3,-tetramethylurea or collidine.

Synthesis of sulfur analogues of methyl and allyl kojibiosides and methyl isomaltoside and conformational analysis of the kojibiosides

Abstract The synthesis of methyl and allyl 5′-thio-α-D-kojibiosides and methyl 5′-thio-α-D-isomaltoside is described. The phenylselenoglycoside and trichloroacetimidate of 2,3,4,6-tetra- O -acetyl-5-thioglucose have been employed as glycosyl donors to glycosylate glucopyranosyl acceptors with 2-OH and 6-OH positions free. The disaccharides thus obtained are potential glucosidase inhibitors. The conformational preferences of allyl 5′-thiokojibioside ( 34 ) were studied by comparison of experimental NOE curves with the theoretical counterparts for the corresponding methyl glycoside 25 , derived from a Boltzmann-averaged grid search using the program PIMM91. Very good agreement of experimental NOE curves derived from selective NOE measurments with the theoretical curves is found. The data are consistent with the population of a global minimum structure (Φ=−43, Ψ=−39 degrees) to the extent of 90%, and a second local minimum (Φ=−36, Ψ=−173 degrees) to the extent of 6%. An X-ray crystal structure of 34 at 190 K (R=4.2%) indicates a conformation (Φ=−46, Ψ=−23 degrees) that is similar to that of the global minimum.

Unprecedented chemical glycosidation of 5-thioglucose to give disaccharides.

Abstract The glycosyl trichloroacetimidate of 2,3,4,6-tetra-O-acetyl-5-thioglucopyranose when used to glycosylate selectively protected glucopyranosyl acceptors with the 2-OH and 6-OH positions free affords the 1,2-linked and the 1,6-linked disaccharides as a 3:1 and 1.5:1 α:β mixture, respectively.

Tris(4-bromophenyl)aminium hexachloroantimonate-mediated glycosylations of selenoglycosides and thioglycosides. Evidence for single electron transfer?11This paper is dedicated, with respect and gratitude, to the memory of Margaret A. Clark.

Abstract Radical cation-initiated glycosylation reactions of phenyl selenoglycosides are described. Glycosylations of phenyl selenoglycosides effected by the single-electron-transfer (SET) reagent, tris(4-bromophenyl)aminium hexachloroantimonate (BAHA), are examined with primary and secondary hydroxyl acceptors. The corresponding reaction of an ethyl thioglycoside with a primary hydroxyl acceptor is also examined. Reactions are performed in the presence of the SET quenching reagent, 1,2,4,5-tetramethoxybenzene, to assess whether BAHA-mediated glycosylation reactions involve SET. These experiments indicate that the reactions are completely quenched in dichloromethane but only partially in acetonitrile. The results provide support for the SET mechanism but an alternative mechanism involving electrophilic activation cannot be discounted. The oxidation potentials of various selenoglycosides are determined by cyclic voltammetry.