Reductive openings of benzylidene acetals. Kinetic studies of borane and alane activation by Lewis acids

The reaction kinetics for a number of reductive openings of methyl 2,3-di-O-benzyl-4,6-O-benzylidene-α-d-glucopyranoside have been investigated. Openings to give free HO-6 (using BH3·THF–AlCl3–THF or LiAlH4–AlCl3–Et2O) follow first order kinetics, while reactions yielding free HO-4 (using BH3·NMe3–AlCl3–THF or BH3·NMe3–BF3·OEt2–THF) follow higher order kinetics. The addition of water to the BH3·NMe3–AlCl3–THF results in faster reactions. The BH3·SMe2–AlCl3–THF system constitutes a borderline case, yielding both free HO-6 (by a first order reaction) and free HO-4 (by a higher order reaction). These results correlate well with the concept of regioselectivity by activation of borane complexes.