Diastereoselective Reductions of E-Ketoesters
25
R 3 O 2 C
O
Cl 3 Ce
H
R 2
H
CO 2 R 3
CeCl 3
R 1
OR 3
OH
R 2
O
H
R 2
H
Cl 3 Ce
R 1
OR 3
O
R 2
O
7
anti-3
1
syn-2
O
R 1
R 1
Scheme 4.3
Considering the previous discussion, we can predict the diastereoselectivity in
the reduction of 2,3,3-trimethyl-1-phenyl-1-butanone (8) either with BH 3 .
py/TiCl 4 or with LiEt 3 BH/CeCl 3 .
The 2,3,3-trimethyl-1-phenyl-1-butanone is an D-substituted ketone, but the Dsubstituents have no ability to chelate metals. In that case, the role of the Lewis
acid will be just to coordinate the oxygen atom, enhancing its reactivity towards
the nucleophilic attack. The same open Felkin-Ahn’s model previously commented will be valid for both, TiCl 4 and CeCl 3 -mediated reductions, as it is shown
in Scheme 4.4. Again, the first step of the reaction is the coordination of the Lewis
acid with the carbonyl group. Although a priori, conformations 9 and 10 could be
considered, hydride attack to the carbonyl group in 9 should be favored by steric
effects leading to the anti-alcohol 11 as the main reaction product in both cases.
Me
O
Me
Me
Me
e e
Ph
Me Me
C(CH 3 ) 3
Me
OH
Ph
O
LA
H
Me
H
H
Me
H
C(CH 3 ) 3
Me
OH
Ph
Me
Me
anti-11
10
TiCl 4 / BH 3 .py
or
CeCl 3 /LiEt 3 BH
syn-11
maj a or isomer
8
LA = Lewis acid
9
Ph
Ph
O
LA
Scheme 4.4