Bromohydroxylation of Glycals - An Investigation into the Reaction of Some 4-N-Acylated Derivatives of Methyl 5-Acetamido-7,8,9-tri-O-acetyl-2,6-anhydro-3,4,5-trideoxy-D-glycero-D-ga lacto-non-2-enonate and Its 4-Epimer.