Chemistry Reference
In-Depth Information
O
OH
29
, (cat.), KBr (cat.), electrolysis,
silica gel, NaHCO
3
, H
2
O,
−
15
°
(61 %)
MeO
MeO
O
H
+
(37 %),
S
=
5.3
MeO
Scheme 12.27
O
29
(cat.), NaBr, silica gel,
electrolysis, NaHCO
3
, H
2
O,
OH
OH
+
O
O
−
15
°
C
O
(89 %), 74 % ee
Scheme 12.28
Alternatively, one of two alcohol functionalities of a
meso
-compound may be selectively oxidized
(Scheme 12.28).
111
The latter reaction theoretically allows a stereochemical reaction with 100 % conversion
to a chiral product and is, therefore, of greater interest, though little explored.
41,111,112
Again, the ee's
achieved to date are not practical.
Most stereoselective nitroxide-catalyzed oxidations were carried out using optically active nitroxides,
examples of which are shown below,
29,37,113 - 120
or by using an achiral nitroxide in the presence of an
optically active auxiliary (such as sparteine).
121,122
Similarly, an electrode surface can be modified with an
optically active nitroxide to perform stereoselective electrochemical oxidations.
109,112,123,124
In all cases,
however, the ee values achieved are modest, at best. In cases in which the chiral element is fairly remote
to the oxidizing site (e.g., TEMPO derivative
26
125
), this may not be too surprising. However, even in
cases in which the chirality is much closer to the active site of the nitroxide (such as in nitroxide
28
41,126
)
or in cases in which the nitroxide is altogether
C
2
-symmetric (
29
118
or the derivatives
30
to
32
116,127
), the
outcomes in terms of stereoselectivity are by and large disappointing.
O
NHBu
-t
O
H
O
NHAc
N
O
H
Pr
-i
Bn
*
*
N
N
O
N
HO
n
-Bu
O
26
27
28
O
O
O
N
MeO
N
N
N
O
MeO
OMe
OMe
29
30
31
32
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