MECHANISMS OF NUCLEOPHILIC SUBSTITUTIONS OF ACETALS

The nucleophilic substitution of (+)-(R)-benzaldehyde methyl isopropyl acetal (alpha R)-1 (93% eel and (+)-(R)-o-anisaldehyde methyl isoprop yl acetal (alpha R)-2 (homochiral) with Me2CuLi-BF3 . OEt2 occurs comp letely chemoselectively to afford (+)-(alpha R)-alpha-methyl(benzyl) i sopropyl ether (+)-(alpha R)-3 (40% eel and (+)-(alpha R)-alpha-methyl (o-methoxybenzyl) isopropyl ether (+)-(alpha R)-4 (34% eel respectivel y, demonstrating that the mechanism of the former reaction involves a free oxonium ion to the extent of 56% and the latter to the extent of 66%.