Allylstannation III. Synthesis of homoallylic alcohols and 4-chloro-2,6-dialkyl-3-methyltetrahydropyrans by reactions between (E/Z)-2-butenyldichloro-n-butyltin and aldehydes

ABSTRACT: 2-Butenyldichloro-n-butyltin (in various cis/trans isomer ratios) reacts readily with neat RCHO (R = CH3, C2H5, (CH3)2CH, and C6H5) at 25\ub0C to give (a) linear alcohols, RCH(OH)CH2CHCHCH3 in the E and Z forms, (b) branched alcohols, RCH(OH)CH(CH3)CHCH2 in the threo and erythro forms, and (c) 2,3,4,6-tetra-substituted tetrahydropyrans (A) as a mixture of cis/trans isomers arising from the CH(CH3)CHCl bond. The maximum yields of these tetrahydropyrans were obtained{A figure is presented} by the use of 3-3.5 molar ratios RCHO/tin compound in the absence of solvent, whereas work-up after reactions in CH2Cl2 gave linear, alcohols as the main products. The formation of linear alcohols appears to be stereospecific, as the ratio of E/Z isomers obtained is the same as that in the organotin compound. Tetrahydropyrans are formed preferentially as the trans isomer