Abstract
Recently, we have developed four selective cleavage methods for the glucuronide linkage which is contained in the oligoglycosides such as saponins. They are photolysis, lead tetraacetate degradation, acetic anhydride-pyridine degradation, and anodic oxidation. During the course of the studies on the reaction pathways and the scope and limitation of these degradation methods, we have found that aminocyclitols are conveniently synthesized from uronic acids by employing oxidative decarboxylation reactions which are brought about by lead tetraacetate treatment and anodic oxidation. via Lead Tetraacetate Degradation. Studies on methyl uronides (1, 2, 10, 12) have shown participation of "dialdehyde intermediate (i or ii)", which is readily trapped by nitromethane in the alkaline medium to furnish nitrocyclitols (14, 16, 18). By employing these reaction procedures, N-acetyl-D-glucosamine (20) has been transformed to hexaacetyl-streptamine (29) in 8% overall yield. Furthermore, starting from the glucuronide-saponin (eg. 30), the aminocyclitol oligoside (eg. 36) has been synthesized. via Anodic Oxidation. Studies on methyl glucuronide (1) have shown that the oxidative decarboxylation reaction induced by anodic oxidation can also be a favorable reaction step for the conversion of uronic acids leading to aminocyclitols. Constant current electrolysis, followed by nitromethane cyclization, reduction, and acetylation, have successfully converted D-glucuronic acid (39) to aminocyclitols (48, 50). It is noteworthy in this case that protection of the free hydroxyls in starting 39 is unnecessary. It should be mentioned here that present synthetic methods for aminocyclitols could be applicable for the conversion leading to the corresponding aminocyclitols not only from uronic acids, but also from neutral sugars and amino-uronic acids.
