Regio- and stereoselective synthesis of β-D-gluco-, α-L-ido-, and α- L-altropyranosiduronic acids from Δa⁴-uronates

The stereoselective synthesis of β-D-glucopyranosiduronic, α-L- idopyranosiduronic, and α-L-altropyranosiduronic acids has been performed from different Δ⁴-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 α-L C-4 ketopyranosides adopting the ¹C₄ chair conformation. Hydride reduction afforded the α-L-idopyranosiduronic or the α-L- altropyranosiduronic acids, the stereoselectivity of the reduction being controlled by the appropriate substitution pattern.