Efficient Synthesis of Rare Sugar D-Allal via Reversal of Diastereoselection in the Reduction of Protected 1,5-Anhydrohex-1-en-3-uloses: Protecting Group Dependence of the Stereoselection

D-Allal was selectively obtained by reducing bulky-silylprotected 1,5-anhydrohex-1-en-3-uloses using the NaBH4-CeCl3 center dot 7H(2)O system. The crucial point of this synthesis is the nature of the protecting group. When bulky silyl group such as t-butyldiphenylsilyl was used as substrate, protected D-allal was obtained in >99% selectivity. In contrast, when acetylated enone was used, protected D-glucal was obtained exclusively. The addition of CeCl3 center dot 7H(2)O was also found to influence selectivity.