Total Synthesis of α,β-<scp>DL</scp>-Allose and α,β-<scp>DL</scp>-Galactose. Stereoselective cis-Hydroxylation by Osmium Tetroxide

The reaction of osmic acid with 1,6-anhydro-3,4-dideoxy-β-DL-erythro-hex-3-enopyranose and its 2-O-methyl and 2-O-acetyl derivatives in either pyridine or dioxane solution has produced 1,6-anhydro-β-DL-allopyranose and 1,6-anhydro-β-DL-galactopyranose and the corresponding 2-O-methyl and 2-O-acetyl derivatives. These or their fully acetylated derivatives could be hydrolyzed to α,β-DL-allose and α,β-DL-galactose and the 2-O-methyl derivatives. The proportion of allo to galacto product obtained varies widely with the solvent used in hydroxylation, showing that attack by osmic acid is subject to steric control.