Bromination of 1,5-Anhydrohex-1-enitols (Glycals) Using Quaternary Ammonium Tribromides as Bromine Donors: Synthesis of α-1,2-<i>trans</i>-2-Bromo-2-deoxyglycopyranosyl Bromides and Fluorides
Addition of bromine to glycals using quaternary ammonium tribromides as bromine donors is realized with higher stereoselectivities (α-1,2-trans configurated products) compared to bromination with molecular bromine. The reaction is neither sensitive to the solvent nor to the nature of the protecting groups (acetyl, benzoyl, benzyl) of the glycals and very slightly affected by the orientation of the substituents of the different glycals. Some of the α-1, 2-trans dibromo adducts have been isolated in 60-75% yields and transformed into the corresponding α-1,2-trans-2-bromo-2-deoxyglycopyranosyl fluorides.