Methanol methyl glycoside from

The mechanism for formation of the 3 methyl glycoside is shown The mechanism for for mation of the a isomer is the same except that methanol approaches the carbocation from the axial direction... 

In about 1936, sialic acid was discovered by Blix, who found it to be a component of submaxillary-gland proteins, and who described many of its properties. However, little notice was taken of this work at the time it was published. In 1941, Klenk, who was working on glycolipids of the brain, described a compound, later shown to be a methyl glycoside of sialic acid, that had been obtained by treatment of a lipid fraction with 5% methanolic hydrogen chloride at 105°. In 1954, Klenk and Faillard reported the first isolation of pure N-acetyl-neuraminic acid from animal sources. 

The preparation of several glucofuran [2, l-t/] oxazolines 35 and 36 from reaction of 2-amino-2-deoxy-D-glucose 34 with HF has been described. Compounds 35a and 35b are formed when the reaction is carried out in formic acid, whereas the orthoesters 36a-c are formed when the reaction is carried out using anhydrides. Further reaction of 35 and 36 with methanol gives methyl glycosides. Thus, 35 and 36 may hnd use as potential glycosyl donors for the synthesis of 2-amino-2-deoxy sugars (Scheme 8.14). " ... 

Glycosides from the Dihalides.—By the decomposition of the dihalides with silver carbonate or oxide in methanol, followed by deacetylation of the product, the corresponding methyl 2-desoxy-2-halogeno-glycosides are formed. 

Table II also has some geometric parameters that assist in the characterization of such complexes. For example the bond between the anomeric carbon atom and methanol nucleophile, C-1-Om, is close to 1.46 A for all F and G species, and thus indicative of the hydronium ion character. Since typical methyl glycosides have C-1-Om bond lengths close to 1.38 A, then protonation causes a bond lengthening of only <0.1 A. It is noteworthy that t5 deviates appreciably from planarity, an indication of little similarity to the isolated oxacarbenium ion B species. Thus, in terms of conformation, such hydronium ion species can be quite close to the parent glycosides. However, except for GO, none of these species have ring conformations close to 4Ci, and this is probably a discriminating factor among these species.

A branched tetrasaccharide methyl glycoside (85) was isolated from the Alcaligenes faecalis O-specific polysaccharide (84) by the action of hydrogen fluoride-methanol for 30 min at —78°. 

If solvolysis of CMP- NeuAc required nucleophilic participation of the carboxylate, this would proceed via a transient a-lactone, and in a second step give net retention to afford the p-anomcr. A direct displacement of CMP by solvent without carboxylate participation would yield the a-anomer, while a transition state that resulted in formation of a sialyl oxocarbenium ion would proceed with results ranging from inversion to racemization depending on whether the oxocarbenium ion lived long enough to equilibrate with solvent. Solvolysis of CMP-NeuAc in meth-anol/water mixtures at pH 5.0, resulted in 1 1 ratios of a- and p-methyl glycosides of NeuAc as determined by H-NMR.31 The a/p-methyl glycoside ratio was invariant when the mol fraction of methanol was increased by over 50%, which shows... 

---