Allylstannation V. cis-stereoconvergent sysnthesis of homoallylic alcohols and 4-chloro-2,6-dialkyl-3-methyltetrahydro-pyrans by the addition reaction of 1-buten-3-yldichloro-n-butyltin and aldehydes

1-Buten-3-yl-n-butyldichlorotin, generated in situ by redistribution of (E/Z)-2-butenyltri-n-butyltin and BuSnCl3, reacts readily with neat RCHO (R = CH3, C2H5, (CH3)2CH) at 25°C to give linear alcohols RCH(OH)CH2CHCHCH3 and/or 2,3,4,6-tetrasubstituted tetrahydropyrans, CH2CH(R)OCH(R)CH(CH3)CH-(Cl), which are mainly in the cis-configuration with respect to the CH(CH3)-CH(Cl) bond. When R = (CH3)3C and C6H5, only the homoallylic alcohols are obtained. These cis-stereoconvergent syntheses are explained in terms of kinetic control of the formation of adducts obtained by insertion of one or two aldehyde molecules into the organotin substrate