Regio- and stereoselective synthesis of β-D-gluco-, α-L-ido-, and α-L-altropyranosiduronic acids from Δ4-uronates

The stereoselective synthesis of beta-D-glucopyranosiduronic, alpha-L-idopyranosiduronic, and alpha-L-altropyranosiduronic acids has been performed from different Delta(4)-uronate monosaccharides. Bromination of the C-4,5 double bond provided the trans-diaxial bromohydrin derivatives, which were converted to the corresponding epoxides in high yields. Direct reduction of the epoxides using boranetetrahydrofuran complex led to the corresponding glucuronic acids in low to good yields. Glucuronic acids were also obtained in satisfactory yields through a two-steps procedure involving bromination of the epoxide with titanium(IV) bromide followed by reduction using tributyltin hydride. Lewis acid-catalyzed rearrangement of these epoxides led to the corresponding alpha-L C-4 ketopyranosides adopting the C-1(4) chair conformation. Hydride reduction afforded the alpha-L-idopyranosiduronic or the alpha-L-altropyranosiduronic acids, the stereoselectivity of the reduction being controlled by the appropriate substitution pattern.