D-Pyranosides

When the closure of the furanoside ring is not possible, as in the 2,4-dimethyl derivative of LYI (there being no longer a hydroxyl group available on C4), then non-aqueous acid simply catalyzes the interconversion of the a and /3 isomers of the methyl D-pyranoside. The... 

The selectivity for the formation of the p-D-pyranoside (equatorial) stereoisomer in the reaction originated from a thermodynamic control. Indeed, starting from D-glucose, after 24 h at room temperature, a mixture of the four possible a,p-furanosides 46 and 47, and a,p-pyranosides stereoisomers 48 and 45a, in which... 

Extrapolation of rates back to zero time also showed a small but definite formation of pyranosides from the free sugar, in addition to conversion from the furanosides. The reversibility of the conversion of furanosides into pyranosides was definitely demonstrated by obtaining the final equilibrium composition from two directions, namely, from the a-D-pyranoside and from the free sugar. 

Fig. 2.—Rates of Production49 of Formic Acid (CO2 Evolution) during Oxidation of the Methyl a-D-Pyranosides of Glucose (1), Galactose (2), and Mannose (3), with Lead Tetraacetate in Glacial Acetic Acid Containing Potassium Acetate (Temp. 27°).

Scheme 4.1 (a) Structure of the lipopoly saccharides of Gram-negative bacteria, (b) The structure of the lipid A from E. coli. (c) The structure of Kdo residue (a-3-deoxy-D-manno-oct-2-ulopyranosonic acid). For all structures, where not stated otherwise, sugars are a-D-pyranosides. Residues in bold are present as non-stoichiometric substitutions. Common abbreviations P, Phosphate, PPEtn, 2-aminoethanol diphosphate, PEtN, 2-aminoethanol phosphate, PCho, 2-trimethylaminoethanol phosphate Gly, glycine, Ac, Acetyl, Cm, Carbamoyl, Pyr, Pyruvic Acid... 

The quantitative data obtained by Isbell and Frush on hydrolysis by dilute hydrochloric acid of methyl glycosides are of interest in this connection. From these studies, it may be deduced that the rates of hydrolysis of the methyl /3-D-pyranosides of the wood sugars are in the following order D-xylose = D-galactose > n-mannose > D-glucose. The extreme acid-... 

The reactivity and S mthetic utility of l,3,2X -dioxaphospholanes are discussed in a paper presented at Tallinn by Evans et al. The abstract includes a summary of the kinetics of Lewis add-mediated decomposition and a report on the use of dioxaphospholanes to prepare a-D-pyranosides mentioned in Section 3 (reference 17). 

Figure 1. Theoretical potential-energy curves for rotation about the anomeric bond from ah initio Hartree-Fock 431-G calculations on dimethoxymethane. is the glycosidic O-5-C-l-O-l-CH i torsion angle. is the C-5-0-5-C-1-0-1 torsion angle and is 60° for a-D-pyranosides, 180° for fl-v-pyranosides (see...

D-furanoside (after acetylation, 144, 32%) and the free sugar (142, 24%) are formed in almost equal amounts, minor components being the starting material (12%), the l-thio-/3-D-pyranoside 143 (4%), and the 1-thio-jS-D-furanoside (after acetylation, 145, 3%). 

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