The role of the double bond geometry (E/Z stereochemistry) in reactions of crotylstannanes with aldehydes has been examined for representative ''simple'', alpha-alkoxy, and beta-alkoxy aldehydes. For the reaction of crotylstannane with simple achiral aliphatic, aromatic, or alpha,beta unsaturated aldehydes mediated by BF3.Et(2)O, use of the E crotylstannane gives much enhanced syn selectivity over that obtained with Z (e.g., 43:1 vs 4:1 with benzaldehyde). A synclinal transition state in which the CH(2)SnBu(3) group is gauche to oxygen is proposed to explain these results. For alpha-alkoxy aldehydes, use of the E stannane with MgBr2 gives the highest syn selectivity, while the Z stannane gives slightly better stereoselectivity with beta-alkoxy substrates. In contrast, the use of TiCl4 gives anti products preferentially from the E stannane and either alpha or beta-alkoxy substrates.

The preparation of allylstannanes 1a–c, all of which bear an oxygen substituent at the allylic terminus, is described. These reagents react with alpha- and beta-alkoxyaldehydes in the presence of MgBr2 as Lewis acid to give SE′, products with diastereofacial selectivity consistent with “chelation control”; a syn disposition of substituents about the newly formed bond is highly favored in all cases.

Proper choice of Lewis acid and hydroxyl protecting group allows for selective addition of crotyltri-n-butylstannane to either face of the aldehyde carbonyl in derivatives of 2-methyl-3-hydroxypropanal with preservation of erythro selectivity for the bond construction; allyl additions are limited to diastereofacial selectivity consistent with “chelation control”.

For additions crotyltri-n-butylstannane to simple aldehydes mediated by BF3·Et2O, erythro selectivity may be increased considerably (25:1 vs. 9:1, for example) by simply using 2 eq. of stannane, TiCl4 mediated additions give high erythro or threo selectivity depending simply on the order in which reactants are mixed; and other Lewis acids are also evaluated in such reactions.