EFFECTS OF OLEFIN GEOMETRY ON THE STEREOCHEMISTRY OF LEWIS-ACID MEDIATED ADDITIONS OF CROTYLSTANNANES TO ALDEHYDES

The role of the double bond geometry (E/Z stereochemistry) in reaction s of crotylstannanes with aldehydes has been examined for representati ve ''simple'', alpha-alkoxy, and beta-alkoxy aldehydes. For the reacti on of crotylstannane with simple achiral aliphatic, aromatic, or alpha ,beta unsaturated aldehydes mediated by BF3.Et(2)O, use of the E croty lstannane 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 exp lain these results. For alpha-alkoxy aldehydes, use of the E stannane with MgBr2 gives the highest syn selectivity, while the Z stannane giv es slightly better stereoselectivity with beta-alkoxy substrates. In c ontrast, the use of TiCl4 gives anti products preferentially from the E stannane and either alpha or beta-alkoxy substrates.