Chemistry Reference
In-Depth Information
t
Bu
O
O
P
O
OH
MeO
2
C
O
t
Bu
H
3
O
+
26a
(5 mol %)
NH
+
R
1
H
2
Et
R
2
MS 4A, CH
2
Cl
2
r.t.
O
O
H
O
O
H
+
R
2
R
2
CO
2
Et
CO
2
Et
R
1
R
1
anti
syn
R
1
= H, R
2
= Ph
R
1
= Me, R
2
= Ph (
E
)
R
1
= Me, R
2
= Ph (Z)
R
1
, R
2
= -(CH
2
)
4
-
(1 h)
(2 h)
(24 h)
(1 h)
99% (98% ee)
>73% (>99% ee)
8% (72% ee)
79% (99% ee)
:
:
:
:
-
<1% (53% ee)
3% (88% ee)
10% (98% ee)
Ar
O
HCO
2
Et
O
P
O
O
O
H
Ar
Scheme 8D.31.
Highly enantioselective intermolecular ene reactions with an alkene instead of alde-
hyde are also reported. Diarylprolinol silyl ether derivatives [59] are employed as effec-
tive organocatalysts (Scheme 8D.32) [60]. The reaction of cyclopentadiene and
- enals
in the presence of
27
and
p
-nitrophenol as an additive gives two ene products without
the formation of (hetero) D-A products in good yield and enantioselectivity. The Wittig
reaction of the ene product followed by an intramolecular D-A reaction is examined to
provide tricyclic compound in high
endo
- selectivity.
α
,
β
8D.4. INTERMOLECULAR IMINE - ENE REACTION
The enantioselective imine-ene reaction with
- imino glyoxylate, fi rst used as precur-
sors to amino acid by Borzilleri and Weinreb [61], provides a direct and simple
method for the asymmetric synthesis of both natural and unnatural
α
- amino acids
[62] such as cyclohexylalanine [63]. However, there has been no successful example
α
