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R
R
O
R
R
O
O
O
O
O
O
O
O
O
O
O
R
18
24
R
R
R
R R
O
R
R
n= 0, 1
O
O
O
O
O
O
n
n
O
N
N
OH
HO
O
O
R
R'
R'
R
R
R
pseudo-enantiomers
Figure 13.2
Pseudo-enantiomeric structural relationship between ketones
18
and
24
and their
derivatives.
OH
O
ZnEt
2
/
20b, 21b, 26b
RH
R t
O
O
O
O
O
O
O
R=
H
H
H
H
H
H
H
Cl
Br
Me
MeO
OMe
Yield (%), ee (%)
20b
21b
26b
99, 80
95, 83
81, 88
99, 81
99, 82
99, 94
99, 81
99, 80
99, 92
98, 80
97, 79
99, 90
99, 78
96, 80
93, 92
78, 50
75, 41
78, 90
85, 34
85, 36
87, 35
R
S
Scheme 13.9
Addition of diethylzinc to aldehydes catalyzed by pyridyl-alcohols
20b, 26b,
and
quinolyl-alcohol
21b.
13.2.2
Addition of Trialkylaluminium Compounds to Aldehydes. P,P and P,O-Donor Ligands
More recently, Woodward and coworkers reported the asymmetric addition
of trialkylaluminium compounds to aldehydes employing nickel as a catalyst
with phosphoramidite ligands [17]. Trialkylaluminium compounds can be easily
obtained from aluminium hydride and olefins. Later on, this reaction was explored
using chelate ligands such as phosphite-oxazoline (
30-33
) [18] and phosphite-
phosphoramidite (
35
) ligands derived from glucosamine (Scheme 13.10) [19], and
monophosphite ligands (
41-45
) derived from glucose [20], galactose, and fructose
(Scheme 13.11).
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