Diastereoselective hydroboration of substituted exo-glucals revisited. A convenient route for the preparation of l-iduronic acid derivatives

Suitably protected derivatives of methyl 3-O-benzyl-6-deoxy-α-d-xylo-hex-5-enopyranoside, readily prepared from 3-O-benzyl-d-glucopyranose, were reacted with various boranes to prepare the corresponding l-idose components. Reaction of methyl 2-O-benzoyl-3-O-benzyl-6-deoxy-4-O-tert-butyldimethylsilyl-α-d-xylo-hex-5-enopyranoside with 9-borabicyclo[3.3.1]nonane (9-BBN) afforded an easily separable 9:1 l-ido:d-gluco mixture. The l-ido isomer was then transformed in a straightforward manner into suitably protected l-iduronic acid glycosyl donors (chloride, bromide, trichloroacetimidate) which could serve as highly elaborated building blocks in syntheses of l-iduronic acid containing glycosaminoglycan fragments. An unexpected side-reaction occurring in the preparation of 1-O-trichloroacetimidoyl derivatives is also reported.