Reactions of [(Z)-gamma-[(diisopropylidene-alpha-D-mannopyranosyl)oxy]allyl]tributylstannane (6) with several chiral and achiral aldehydes are described. This reagent was designed in anticipation that significant diastereofacial bias in reactions with aldehydes would be exerted by the mannosyl auxiliary as a consequence of the exo anomeric effect. In fact, chiral reagent 6 displays especially useful diastereoselectivity in BF3.OEt(2)-promoted matched double asymmetric reactions with chiral aldehydes (S)-7 (18:1 selectivity), (R)-19 (greater than or equal to 20:1 selectivity), and (R)-26 (greater than or equal to 20:1 selectivity). Reagent 6 also gives good to excellent selectivity in mismatched double asymmetric reactions with (R)-7 (16:1 selectivity), (S)-19 (5:1 selectivity), (S)-20 (7:1 selectivity), but with (S)-26 the mismatched double diastereoselectivity falls to 2:1. Reagent 6 also participates in MgBr2-promoted reactions with alpha-alkoxy aldehydes (e.g., 19), although it proved incapable of overriding th intrinsic diastereofacial bias of the MgBr2-complexed aldehyde. In all cases, it appears that the aldehyde-Lewis acid complexes approach the allylstannane unit of 6 on the side opposite to the pyran C-O bond with the vinyl ether C-O bond oriented anti to the pyranoside C(1)-C(2) bond, as dictated by the exo anomeric effect. However, reactions of 6 with alpha-(benzyloxy)acetaldehyde (45) demonstrate that the enantioselectivity of the reagent is attenuated by the tendency of reactions to occur via transition states with the enol ether either in the s-trans (e.g., 53, 56) or the less stable s-cis rotamer (e.g., 54, 57), which exhibit opposite enantiofacial selectivities. It is suggested that double asymmetric reactions involving 6 display synthetically useful levels of enantioselectivity because the chiral aldehydes are able to discriminate between the s-cis/s-trans rotamer pool such that the matched pair double asymmetric reactions proceed almost exclusively via transition states with s-trans enol ether rotamers. Pathways involving s-cis enol ether rotamers (cf., 32, 43) become significant in mismatched double asymmetric reactions of aldehydes with very large intrinsic diastereofacial preferences, as in the reactions of 6 with (S)-26-BF3.Et(2)O and (R)-19-MgBr2.