Preparative route to glucuronyl donors bearing temporary protecting group at O-3 via 6,3-lactonisation by Bz(2)O or Piv(2)O.

Heating of non-substituted beta-D-glucopyranuronic acids in the presence of pivalic or benzoic anhydride in DMF gives 4-O-monoacylated or 2,4-di-O-acylated 6,3-lactones that can be easily transformed into corresponding methyl uronates bearing free OH-groups at C-2 and C-3 or C-3 only, respectively. This method was used for preparation of selectively 3-O-protected glucuronyl donors of thioglycoside and trichloracetimidate types.