Chemistry Reference
In-Depth Information
affords the active peracid. The peracid oxidizes the olefi n to yield the epoxide along
with the regeneration of the carboxylic acid catalyst
36
. Although the formation of
diacyl peroxide delays the oxygen atom transfer, the addition of DMAP as an acyl
transfer catalyst accelerates the entire process and renders the reaction highly catalytic.
While DMAP is oxidized to DMAP
N
-oxide under the reaction conditions,
N
- oxide
may also serve as acyl transfer catalysts. Indeed, NMO instead of DMAP could promote
the epoxidation, albeit with diminished productivity. With an effective acid/peracid
shuttle, the authors introduced tripeptide catalyst
37
— the sequence L - Pro - D - Val is well
known to induce a
-turn structure that provides an effective asymmetric environment
for other asymmetric reactions [79]—and found that trisubstituted olefi ns bearing car-
bamate functionality effectively underwent epoxidation with high enantioselectivity
(Scheme 11.46). While allylic alcohol-derived substrates showed high enantioselectivity,
the elongation of the tether led to the signifi cant loss of the enantioselectivity. The limi-
tation of the substrate scope suggests that hydrogen bonding between the catalyst and
the substrate is essential for the enantiocontrol. Three transition state models have been
proposed (Fig. 11.5). This study demonstrates that better understanding of peptide-
substrate-binding structure including hydrogen-bonding network will offer a potent
approach to highly enantioselective epoxidation of other olefi ns.
β
37
(10 mol
%)
DIC (2.0 equiv.)
DMAP (10 mol
%)
O
O
O
N
O
HN
BocHN
R
O
NHAr
+ 30% H
2
O
2
(2.5 equiv.)
R
O
NHAr
n
n
O
CH
2
Cl
2
-H
2
O
R
R
HN
O
HO
2
C
Ph
37
O
O
O
O
O
O
O
NHPh
Ph
O
NHPh
O
NHPh
O
80%, 76% ee (RT, 5 h)
97%, 89% ee (-10°C, 78 h)
76%, 92% ee (4°C, 33 h, with UHP in toluene)
99%, 8% ee (-10°C, 79 h)
95%, 86% ee (-10°C, 43 h)
73%, 10% ee (RT, 3.5 h)
Scheme 11.46.
O
O
Ar
R
N
H
R
O
H
O
H
O
H
O
O
O
H
H
Ar
O
O
O
O
H
N
i
Pr
O
H
H
O
N
Ph
O
ArHN
O
O
N
H
O
H
Me
NHBoc
Figure 11.5.
