Configuration of pyruvic acid ketals, 4,6-O-linked to d-galactose units, in bacterial and algal polysaccharides

Methyl 4,6-O-(hydroxyisopropylidene)-α-d-glucopyranosides (X and XI) were synthesized and the ketal configuration of X was established by a Smith degradation to 1,3-O-(hydroxyiso-propylidene)-l-erythritol (XIII), which was prepared from 2,8-anhydro-1-deoxy-d-glycero-β-d-gulo-octulopyranose (XVI). Methyl 4,6-O-(hydroxyisopropylidene)-α-d-galactopyranosides (II and V) were prepared and their ketal configurations determined by comparison of molecular rotations. Methyl 4,6-O-(1-carbomethoxyethylidene)-α-d-galactopyranosides (IX and XII) from II and V and methyl 4,6-O-(1-carbomethoxyethylidene)-α-d-glucopyranosides (XIV and XV) from X and XI were utilized for this purpose. Also, the hydrogen-bonding characteristics of methyl 4,6-O-(hydroxyisopropylidene)-2,3-di-O-methyl-α-d-galactopyranosides (III and VI) from II and V were compared with those of methyl 4,6-O-(hydroxyisopropylidene)-2,3-di-O-methyl-α-d-glucopyranosides (VII and VIII) from X and XI. Based on the finding that X contains an axial C-methyl group, the results from the two approaches show that methyl 4,6-O-(hydroxyisopropylidene)-α-d-galactopyranoside (II) contains an axial C-methyl group in the conformations depicted. The stereoisomer of this compound was prepared from 4,6-O-(1-carboxyethylidene)-d-galactose (IV), a partial hydrolysis product of Corynebacterium insidiosum polysaccharide and Difco agar and establishes that the natural products contain equatorial C-methyl ketal groups.