Magnesium-mediated Wittig reagent-promoted Stereoselective synthesis of L-Sorbopyranoses from D-Glucopyranoses.

l-Sorbose is an important rare sugar that exists in some natural products and widely used in pharmaceutical and chemical industries. Herein, two simple and practical routes were developed using cheap magnesium (II) for the synthesis of 1,3,4,5-tetra-O-benzyl-l-sorbopyranose from 2,3,4,6-tetra-O-benzyl-d-glucopyranose with high stereoselectivity and yield. The first route involved the intramolecular hydride shift from C5 to the C1 of the glucopyranose precursor. Wittig reagent (PPh3CHCOOBn) was used to combined with Mg(II) to promote this isomerization reaction from d-glucopyranose to l-sorbopyranose in the alternative route.