Level 2 – Case 19
126
i-Pr
Me
O
H
Me 2 AlCl
t-Bu
OTMS
i-Pr
OR
OH
t-Bu
O
i-Pr
Me
OH
t-Bu
O
2
3
4
R = Bn, 3:4 1:99
R = TBS, 3:4 3:97
1
OR
Me
OR
Scheme 19.1b
Furthermore, during the reaction of 2 (R = Bn) with enolsilane 1 in presence of
SnCl 4 , alcohol 5 was obtained as by-product in significant yield.
OH
Me
O
i-Pr
Ph O
t-Bu
5
1. Explain the stereochemical outcome of the reactions in Scheme 19.1, specially
the inversion of the selectivity in function of the Lewis acid.
2. Propose a reasonable reaction pathway to explain the formation of 5.
E Ex xp pe er ri im me en nt ta al l D Da at ta a
Aldehyde 6 reacts with enolsilane 1 in the presence of Me 2 AlCl to yield 8 as the
major reaction products (the stereoselectivity 7:8 is higher than 1:9) (Scheme
19.2).
Me
O
H
OR t-Bu
OTMS
Me 2 AlCl 2
Me
t-Bu
O
O R
OH
Me
t-Bu
O
O R
OH
6
7
R = Bn, 7:8 10:90
R = TBS, 7:8 3:97
8
1
Scheme 19.2
D Di is sc cu us ss si io on n
S St te te t te te te te t ter re re r re re re re r reo oc ch he em mi is is i is i is is i i t tr t tr tr tr tr t t y
ry ry y
The experimental data in Scheme 19.1 point to the metal in the Lewis acid as directly responsible for the stereochemical outcome of the reaction. This metal-