α-Hydrogen elimination in some carbohydrate triflates

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

Influence of the stereochemistry of sugars on the selectivity of formation of carbohydrate-derived cyclopentadienyl and indenyl ligands

Interaction of lithium cyclopentadienide with a suitable partially protected alpha-D-allofuranose triflate, 4, epimer at C3 of glucose, gives as a major product, besides the expected glucose-cyclopentadiene, 5, a glucose-disubstituted cyclopentadiene, 6. This unprecedented behaviour, which does not occur with alpha-D-glucofuranose and other sugars, is tentatively explained by a complexation of LiCp and the oxygen atoms of the isopropylidene function of one molecule of 4 and one of 5, giving a termolecular structure as the result of a template effect. The results of other experiments, such as the use of MgCp2 in place of LiCp or the complexation of oxygen atoms by lithium triflate, which changed the selectivity of the reaction largely in favour of the monosubstitution product 5, support this hypothesis. When lithium indenide is reacted with 4, glucose-monosubstituted and glucose-disubstituted indenes, 8 and 9, respectively, are formed, and 9 is obtained with almost total diastereoselectivity. This result can also be rationalised by a stereoselective complexation of lithium, as shown by separate experiments and by molecular mechanics calculations. Methyltricarbonyl molybdenum(II) complexes have been synthesised and characterised from glucose-monosubstituted 5 and glucose-disubstituted 6 cyclopentadienes.

Reactions of some 2- and 4-O-triflylglycopyranosides with MeLi, t-BuOK, and pyridine

As an extension of our previous work on secondary triflates of carbohydrates [El Nemr, A.; Tsuchiya, T. Tetrahedron Lett. 1995, 36, 7665-7668. El Nemr, A.; Tsuchiya, T.; Kobayashi, Y. Carbohydr. Res. 1996, 293, 31-59. El Nemr, A.; Tsuchiya, T. Carbohydr. Res. 1997, 301, 77-87. El Nemr, A.; Tsuchiya, T. Carbohydr. Res. 1997, 303, 267-281], the reaction modes of several methyl 2- and 4-O-triflyl-D-glycopyranosides with MeLi (strong base), t-BuOK (moderately strong base), and pyridine (weak base) have been studied. This paper describes the reactions of 3-O-benzyl-4,6-O-benzylidene-2-O-triflyl-D-gluco and -mannopyranosides with MeLi to give mainly the corresponding 2-C-methyl derivatives through alpha-elimination, with t-BuOK to give either the 2,3-unsaturated compounds through beta-elimination or detriflyl 2-ols, and with hot pyridine to give the corresponding 2-pyridinium salts with inversion (except for the 2-O-triflyl-alpha-D-mannopyranoside (8)). 2,3,6-Tri-O-benzyl-4-O-triflyl-alpha-D-gluco and -mannopyranosides were also examined similarly.