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137ChemInform Abstract The title reaction is carried out in the presence of aqueous 0.1-4.0 M HClO4. The stereochemical course of the reaction depends on the number, n, of the chlorine atoms bonded to tin and on the concentration of perchloric acid. In 0.1 M HClO4 the threo/erythro isomer pair is obtained. With increasing acid concentration this isomer pair is gradually replaced by the E/Z isomer pair, which is the sole product in 4 M HClO4. However, in most cases the ratio anti/syn = (threo + E)/(erythro + Z) remains constant for any given aldehyde (I) and crotyl compound (II). Stereoselectivity is determined by the nature of the aldehyde: syn-convergence occurs with pivalaldehyde, anti-convergence with isobutyraldehyde; no stereoselection is found for propanaldehyde.