A-Pyranosides, methyl

In the methyl a-pyranosides, the glycosidic bond is still the shortest of the four C-O bonds, but the difference is about 1 pm less. The bond-shortening appears to be shared more equally between the two C-O bonds on either side of the anomeric carbon. As in the 3-pyranosides, the external C-O bonds of the acetal sequence are the longest. The differences between the valence angles are similar to the 3 anomers, except that the C-l value is tetrahedral rather than less than tetrahedral. 

At reflux, the action of 3 equivalents of mercury(II) chloride in methanol converts the diethyl dithioacetal of D-glucose into a 5 1 mixture of methyl /3- and a-D-glucopyranoside299 extension of this reaction to the dibenzyl dithioacetal s of L-arabinose, L-rhamnose, and D-galactose afforded, as major products, the methyl /3-l-pyranoside of the L-pentose and the methyl a-pyranosides of the two hexoses, respectively.300... 

The new antitumour antibiotic pyrrolosporin A (11) is a macrolide with an unusual spiro-a-acyl tetronic acid moiety. It also contains a 4-amino-2,4,6-trideoxy-P-D-aru6mo-hexopyranose unit, a sugar not previously found in nature (see Vol. 24, p. 148-9 for the synthesis of this sugar in studies of antibiotic analogues), and methanolysis of pyrrolosporin A gave the methyl a-pyranoside, with the pyrrole unit still linked to the sugar. 

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