Stereochemistry of the Wagner Hydroxylation of the Diastereomeric (4Z)- and (4E)-1,1,3-Trimethoxy-2-bromo-4-hexenes. Synthesis of the Acyclic Acetals of the 3-O-Methyl-2-bromo-2,6-didesoxy-D,L-hexoses and of the 3-O-Methyl-2,6-didesox