Divergent Synthesis of L ‐Sugars and L ‐Iminosugars from D ‐Sugars

An efficient divergent synthesis of L ‐sugars and L ‐iminosugars from D ‐sugars is described. The important intermediate, δ‐hydroxyalkoxamate, prepared from D ‐glucono‐/galactono‐1,5‐lactone, was cyclized under Mitsunobu conditions to give the O‐cyclized oxime compound and the N‐cyclized lactam compound as mixtures. A more detailed investigation revealed that the appropriate protecting groups and solvents controlled the specificity for the O‐/N ‐ cyclization of the δ‐hydroxyalkoxamate. Suitable protection at the 6‐position of δ‐hydroxyalkoxamate, derived from D ‐glucono‐1,5‐lactone, afforded the corresponding O‐alkylation product alone. Thus we succeeded in applying this to the total synthesis of L ‐iduronic acid. In contrast, with both TBDMS as the protecting group and RCN as the solvent the efficient conversion of D ‐glucono/galactono‐1,5‐lactone into the corresponding L ‐iminosugars ( L ‐idonolactam and L ‐altronolactam) was achieved.