Chemistry Reference
In-Depth Information
t
Bu
Cl
H
2
O
NO
O
Y
Cr
+
Cr
+
23a
Y =
t
Bu
23b
Y = OTIPS
23c
Y = Br
Y
O
O
N
OH
2
Cl
t
Bu
O
X
OMe
O
H
X
OMe
23a
(5 mol %)
+
H
BaO
Acetone, 4
°
C
X = H 88% ee (82%)
X = Br 96% ee (97%)
O
H
r
O
H
r
OMe
O
H
Br
O
3
HCl
H
3
CO
H
3
C
94%
97%
Scheme 8D.27.
O
OTMS
23b
(5 mol %
)
OH
OTMS
1) TBSOTf, Et
3
N
+
HR
DIPEA
MS 4A
R
2) Isobutyraldehyde
BF
3
OEt
2
4°C
R =
n
Pr
89% ee (87%)
OH
O
OH
I
Pr
TBDPSOCH
2
CH
2
93% ee (90%)
90% ee (83%)
i
Pr
TBDPSO
83% (3 steps)
syn
/
anti
59/41
Scheme 8D.28.
oselectivities under solvent-free conditions. These products are inactive as enophiles
under the reaction conditions. Additionally, after protection as the TBS ether of the
product (R = TBDPSOCH
2
CH
2
), a Mukaiyama aldol reaction is attempted in the pres-
ence of BF
3
· OEt
2
to afford enantiopure product in a 59/41 (
syn
/
anti
) ratio.
Enantioselective glyoxylate-ene reactions catalyzed by hindered salen-Cr complexes,
which are readily accessible on a multigram scale [53], are also reported (Scheme 8D.29)
[54]. It is demonstrated that introduction of the adamantyl group is important to give a
good enantioselectivity. For example, the reaction with isobutene and ethyl glyoxylate
in the presence of complex
24
is improved to 79% ee from 15% ee obtained by the
Jacobsen - type complex.
