Specific deuterated reference compounds were prepared to probe the stereoselectivity of the reductive ring opening of carbohydrate-based benzylidene-type acetals. AlD₃ revealed a retentive stereoselectivity probably through the rare S_Ni (internal nucleophilic substitution) mechanism. An S_N1-like mechanism occurs in the acid-promoted regioselective BD₃·THF- or Et₃SiD-reductive ring opening.