O-ethylidene-D-allopyranoses: 1,2-O-,1,2:4,6-, and 2,3:4,6-di-O-ethylidene derivatives

Solutions of 1,2-O-acetoxonium chlorides derived from O-acetylated D-allopyranose derivatives were treated with sodium borohydride to give three pairs of previously unknown 1,2-O-ethylidene-α-D-allopyranose diastereoisomers: 3,4,6-tri-O-acetyl-1,2-O-ethylidene-α-D-allopyranoses; 4,6-di-O-acetyl-3-O-benzyl-1,2-O-ethylidene-α-D-allopyranoses; and 3-O-benzyl-1,2:4,6-di-O-ethylidene-α-D-allopyranoses. Examples of a second class of novel O-ethylidene-D-allopyranoses, the diastereoisomeric methyl 2,3:4,6-di-O-ethylidene-α-D-allopyranosides, were prepared by treating methyl 4,6-O-benzylidene-α-D-alloside with acetaldehyde-sulfuric acid. Assignments of dioxolane ring configurations and pyranose conformations were made by n.m.r. analyses.