Talopyranose conformation

The authors also evidenced an increase of the percentages of a-talo-pyranose and a-ia/ofuranose upon 2H substitution at C(l) concomitantly with a decrease of the percentages of (l-conformers. This observed shift on the talopyranose anomeric equilibrium was attributed to the preference of the shorter C-2H bond (relative to the C l l bond) to assume an equatorial orientation. 

Talose is the C4 epimer of mannose. Draw the chair conformation of D-talopyranose. 

Oxirans. - The synthesis of l,2-anhydro-3,4-di-0-benzyl-6-deoxy-a-D-glucopyranose and its conformational analysis have been reported. A range of epoxides have been prepared by base treatment of bromohydrins, which were made by reaction of hydrogen bromide with aldonolactones. A one-pot conversion of vicinal diols into epoxides employs halohydrin ester intermediates generated from cyclic orthoacetates and either acetyl bromide or trimethylsilyl chloride. Levoglucosenone has been transformed into l,6 3,4-dianhydro-p-D-talopyranose by way of a trn/w-iodo-acetoxylation of the alkene moiety... 

Other Anhydrides. - Stereoselective chemical and chemoenzymatic approaches to the preparation of 1,6-anhydrohexopyranoses have been reviewed, and base treatment of pentabromophenyl glycosides has been investigated as an improved procedure for the synthesis of 1,6-anhydro sugars. Treatment of O-benzylated-3-O-acetyl-glycosyl chlorides with base (KO Bu, THF) has allowed syntheses of l,3-anhydro-2,4,6-tri-0-benzyl-P-D-talopyranose, l,3-anhydro-2,4-di-C -benzyl-a-L-arabinopyranose and 1,3-anhydro-2,4-di-0-benzyl-6-deoxy-P-L-talopyranose. The theoretical solution conformation of methyl 3,6-anhydro-a-D-galactopyranoside has been studied using ab initio calculations. The computed conformation was similar to that adopted in the crystal (X-ray analysis). ... 

Draw the standard chair conformation of P-talopyranose and P-allopyranose, and compare the number of axial hydroxyl groups in each compound. 

Conformational analyses by n.m.r. and computational methods have been reported for the following compounds l,2-anhydro-3,4,6-tri-0-benzyl-P-D-talopyranose, anhydro sugars 13 and similar hexopyranose derivatives, methyl 3,4-0-isopropylidene-a- and P-D-galactopyranoside and their di-O-acetates and di-O-methyl ethers, 3,4-0-(/ )-benzylidene-D-ribono-1,5-lactone (see Chapter 6), and 6-deoxy-6-phosphonoyl-D-fructopyranoses (see Chapter 17 for synthesis). D-Glucospyranosylamine, its tetra-O-acetate and 4,6-0-benzylidene acetal and several N-acylated derivatives, together with D-[l- C]glucopyranosylamine, have been used in a conformational study by n.m.r. spectroscopy aimed at probing the existence of the reverse anomeric effect. ... 

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