Reversible stereocontrol in the Lewis acid promoted reaction of alkoxyaldehydes toward various allyltins

In the reactions of variously substituted allyltin reagents toward achiral alkoxyaldehydes, one of the diastereomeric homoallyl alcohols was stereoselectively obtained by the help of BF₃, while TiCl₄ preferentially gave the other diastereomer, though (E)- and (Z)-3-monoalkylallyltin reagents were exceptional. This reversible diastereoselectivity can be explained by the coordination geometry (anti or syn) of the Lewis acids toward alkoxyaldehydes.