Hydride Reductions and 1,2-Additions of Nucleophiles to Carbonyl Compounds Using Carbohydrate-Based Reagents and Additives - Carbohydrates: Tools for Stereoselective Synthesis - page 125

Li +

H 1

O

Al

O

H 2

O

O

HO H

R

Li +

O

CH 3

R

H 1
X

O H 3 C

O

6b
X=H 2

O

Al

O
RO

( S )

O

O R

6a R=Alkyl
6b R=Bn
X=H 2 , OEt

O

O

CH 3

Li +

H 1

O

Al

O

EtO

O

O

H OH

R

H 3 C

O

6b
X=OEt

O

( R )

Figure 6.1 Cyclic diol complexes derived from ligands 2 and 3 formed during the stereoselec-
tive reduction of carbonyl compounds using LiAlH 4 .

Li +

O

O H

O O

Al H

Al
H

O

H

O

Li +

7 8

Figure 6.2 Cyclic diol complexes derived from ligands 4 and 5 formed during the stereoselec-
tive reduction of carbonyl compounds using LiAlH 4 .

which yielded the corresponding optically active primary (up to 56% ee) and sec-
ondary amines (up to 24% ee) with moderate enantioselectivities [18-21]. Table
6.1 summarizes the best results obtained for the asymmetric reduction of carbonyl
and other isoelectronic compounds with hydride-monosaccharide complex 6b.
Low enantioselectivity ( 16% ee) in acetophenone reduction (64% yield) was also

induced by chiral diols 4 and 5, obtained from d-xylose and d-glucose, even though
the two residual hydrides of corresponding complexes 7 and 8 are diastereotopic
(Figure 6.2) [22].

<

6.2.2
Modified Borohydrides

The first general strategy employed for the preparation of chirally modified boro-
hydrides is the reaction of simple monosaccharides with metal borohydrides, in