Methyl -pyranosides

Write structural formulas for the a and 3 methyl pyranosides formed by reaction of o galactose with methanol in the presence of hydrogen chloride... 

In their study of Bronsted acid induced cleveage of a [with axial C(l)-OMe] and (3 [with equatorial C(l)-OMe] glycopyranosides, Fraser-Reid et al.46 demonstrated that the (3/a rate ratios for hydrolysis of methyl pyranosides (Table 5) can be explained by the different intermediates and transition-state structures through which proceed the heterolysis of a and [3 isomers (Fig. 18). 

Table 5 p/a rate ratios for hydrolysis of methyl pyranosides... 

By considering the reversibility of the acetalforming reactions, it is apparent that treatment of either of the two methyl pyranosides with acidic methanol will prodnce the same eqnilibrium mixture. A related equilibration occurs with the anomers of glucose, as seen earlier (see Box 7.1, mutarotation of glucose). 

D-Mannofuranotetrazole (409) was prepared analogously from methyl pyrano-side (406) (75b) (Scheme 9.77). Methyl pyranoside (406) was smoothly converted into the azido nitrile 407. On heating in DMSO, the azido nitrile 407 gave the tetrazole 408 in 92% yield. The ketal unit in 408 was cleaved using 1 1 TFA/H2O to give the target molecule D-mannofuranotetrazole (409) in 85% yield. 

D-Rhamnofuranotetrazole (412) and L-rhamnofuranotetrazole (416) were similarly prepared from methyl pyranoside (406) and L-rhamnose (413) in 8 and 15% overall yields, respectively (Scheme 9.78). 

Bond lengths collected from experimental data in X-ray crystal-structure determination of methyl pyranosides (11) and other products (77) strongly support the above discussion. Oxygen atoms with an electron pair antiperi-planar to a C — 0 R bond definitely have shorter C—0 bonds whereas the C - 0 R bond is longer than normal. Interestingly, theoretical calculations have been carried out (11, 78) and the results are consistent with the above discussion and experimental observations. 

A-5. Write structural formulas for the a- and jS-methyl pyranosides formed from the reaction of D-mannose (see Problem A-2 for its structure) with methanol in the presence of hydrogen chloride. How are the two products related—are they enantiomers Diastereomers ... 

P. A. McNicholas, M. Batley, and J. W. Redmond, Synthesis of methyl pyranosides and furanosides of 3-deoxy-D-marcrco-oct-2-ulosonic acid (KDO) by acid-catalysed solvolysis of the acetylated derivatives, Carbohydr. Res., 146 (1986) 219-231. 

The ring structure of methyl pyranosides was found by Criegee to be stable to lead tetraacetate oxidation,43 for methyl a-D-glucopyranoside and methyl a-D-mannopyranoside consume about two moles of oxidant per mole without concomitant production of formaldehyde. It was possible, therefore, to distinguish between aldohexofuranosides and aldohexopyranosides by this method. 

Oxidation of Methylglycosid e s.8 4 The following directions for the oxidation of a-methyl-D-glucopyranoside apply generally to the methyl-pyranosides of the aldohexoses and aldopentoses. 

Trimethyl-L-rhamnopyranose has been isolated from methylated gum arabic on hydrolysis, and from the specific somatic polysaccharide of M. tvbercubsis.tb This derivative was originally prepared either by the direct methylation of the free sugar or of the methyl pyranoside, followed by hydrolysis.67 Oxidation gave L-arabo-trimethoxyglutaric acid. 

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