Chemistry Reference
In-Depth Information
R
1
OH
O
Yb
t
Bu
N
N
N
O
H
N
H
N
t
Bu
R
2
R
3
N
Figure 3.7
Supposed transition state without triflate anion
78
C in the presence of a catalytic amount of copper salts
to afford the corresponding optically active homoallyl alcohols in high yield (81-99%) and
high enantioselectivities (89-94%ee) [78].
aldehydes or reactive ketones at
3.3.14.9 Ytterbium
Achiral ytterbium Lewis acid was prepared from ytterbium triflate [Yb(OTf)
3
], (R)-(
)-
1,1
0
-bis(2-naphthol) (BINOL) and DBU (1), and subjected to aza Diels-Alder reactions of
achiral imines (N-benzylidene-2-hydroxyanilines) and achiral dienophiles [79]. In this
reaction the use of a chiral Lewis acid containing 1,3,5-trimethylpiperidine instead of 1
resulted in a lowering of the enantiomeric excess of adduct. Thus, the phenolic hydrogen of
the imine interacts with DBU (1) in transition state, as shown in Figure 3.7, to increase the
selectivity.
รพ
3.3.15 Michael Reaction
Four enantiopure hydroxyamidines were prepared from (S)-pyroglutamic acid by coupling
of an (S)-malic acid derived N-allyliminium ion with
-naphthol, and from an (S)-serine-
derived imide [80] (Figure 3.8). Unfortunately, their application to the catalytic Michael
b
H
H
H
H
Ph
NN
NN
NN
OH
NN
OH
OH
Ph
HO
Figure 3.8
Structures of chiral hydroxyamidines
