Indium trichloride promoted catalytically. the addition of alkynyltins or allylic tins to aldehydes in two ways: transmetallation with the tin compounds and activation of aldehydes as a Lewis acid. The stereochemical outcome in the addition of cinnamyltributyltin to an aldehyde strongly indicates that control of the process can be achieved simply by the choice of solvent acetonitrile or dichloromethane.