110
2
General Synthetic Methods
⊡ Scheme 3
Regioselective benzylation by reductive ring opening of benzylidene acetals
or toluene. Subsequent treatment of the preformed tin intermediates dissolved in a polar aprotic
solvent with alkyl halides in the presence of added nucleophiles such as tetrabutylammonium
halides [35] or CsF [36] yields the corresponding alkyl (benzyl) ethers. Regarding regioselectivity, this is much the same irrespective of which type of tin derivative is used, the primary hydroxyl group and equatorial hydroxyl group in a vicinal cis-dioxygen configuration
are preferentially benzylated. As exemplified in > Scheme 4, this rule is generally correct, but
the degree of selectivity is also dependent on structural features and other factors such as the
presence of additives.
Regioselective de-O-benzylation. An alternative strategy to partially benzylated carbohydrates has been accomplished by selective de-O-benzylation of easily available polybenzylated
precursors. This has been achieved in limited cases by catalytic hydrogenolysis [37], catalytic
hydrogen-transfer cleavage [38], acetolysis [39], hypoiodite fragmentation [40], iodine-mediated addition-elimination sequences [41], or use of Lewis acids [42].
Recently, isobutylalanes [43,44,45,46] or the combination CrCl 2 /LiI [47] have been shown as
efficient agents for the selective deprotection of poly-benzylated carbohydrates. The reaction
with isobutylalanes is assumed to proceed through the formation of a penta-coordinated com-
2
General Synthetic Methods
⊡ Scheme 3
Regioselective benzylation by reductive ring opening of benzylidene acetals
or toluene. Subsequent treatment of the preformed tin intermediates dissolved in a polar aprotic
solvent with alkyl halides in the presence of added nucleophiles such as tetrabutylammonium
halides [35] or CsF [36] yields the corresponding alkyl (benzyl) ethers. Regarding regioselectivity, this is much the same irrespective of which type of tin derivative is used, the primary hydroxyl group and equatorial hydroxyl group in a vicinal cis-dioxygen configuration
are preferentially benzylated. As exemplified in > Scheme 4, this rule is generally correct, but
the degree of selectivity is also dependent on structural features and other factors such as the
presence of additives.
Regioselective de-O-benzylation. An alternative strategy to partially benzylated carbohydrates has been accomplished by selective de-O-benzylation of easily available polybenzylated
precursors. This has been achieved in limited cases by catalytic hydrogenolysis [37], catalytic
hydrogen-transfer cleavage [38], acetolysis [39], hypoiodite fragmentation [40], iodine-mediated addition-elimination sequences [41], or use of Lewis acids [42].
Recently, isobutylalanes [43,44,45,46] or the combination CrCl 2 /LiI [47] have been shown as
efficient agents for the selective deprotection of poly-benzylated carbohydrates. The reaction
with isobutylalanes is assumed to proceed through the formation of a penta-coordinated com-