Chemistry Reference
In-Depth Information
high yield and enantioselectivity are observed at room temperature within 20 min. A
number of functionalized tetrahydofuran derivatives are obtained in high yield and
enantioselectivity. Even with lower catalyst loading (0.2 mol % Rh), the cyclization of
the substrate (R
1
= Me, R
2
= Me) is completed within 35 min with high enantioselectivity
retention (
99.9% ee).
Zhang et al. extended their study to the catalytic asymmetric synthesis of functional-
ized
>
- lactones and - lactams (Scheme 8D.45 ) [75] . As the
trans
- isomer predominates in
the equilibrium, unprotected enyne amides are unreactive as substrates (Scheme 8D.46).
Therefore, the protected enyne amides especially with benzyl group lead to excellent
yields and enantioselectivities.
γ
[Rh(cod)Cl]
2
(5 mol %)
R
1
R
2
(
R
)-BINAP (12 mol %)
R
1
R
2
AgSbF
6
(20 mol %)
(CH
2
Cl)
2
, r.t.
*
O
O
X
O
within 2-10 min
R
1
= Ph, R
2
= Me
R
1
= Me, R
2
= Me
R
1
= Ph, R
2
= OAc
X = O,
>99% ee (92%)
>99% ee (98%)
>99% ee (96%)
R
1
= Ph, R
2
= Et
R
1
= CH
2
OMOM, R
2
= H
R
1
= Ph, R
2
= OMe
X = NBn,
>99% ee (96%)
>99% ee (98%)
>99% ee (88%)
Scheme 8D.45.
R
1
R
2
R
2
O
H
H
O
R
1
c
i
t
a
n
s
Scheme 8D.46.
This Rh system can be applied to the formal synthesis of (+)-pilocarpine, one of the
most important imidazole alkaloids, which is obtained in excellent yield and enantiose-
lectivity (Scheme 8D.47). In the previous report, the key intermediate (4
R
) - (
Z
) -
dehydrohomophilopic aldehyde is synthesized from 2-acetylbutyrolactone in fi ve steps
in 92% ee and 20% total yield [76].
The chiral Rh catalyst system reported by Zhang et al. is an excellent example in
terms of the facile cyclization even at room temperature but limited to disubstituted
cis
- olefi nic substrates. We reported the ene-type cyclization of a trisubstituted olefi nic
ether using cationic Rh(I) catalyst with chiral bidentate diphosphine ligands (Scheme
8D.48) [77]. The ene-type cyclization in the presence of [Rh(diphosphine)]
2
(SbF
6
)
2
