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OPiv
PivO
CHO
OPiv
PivO
O
i
-PrOH/r.
t
.
acetic acid
O
NH
2
OPiv
N
OPiv
PivO
+
PivO
R
R
3
4
5
OPiv
PivO
R =
p-
NO
2
, 88%, 88 de
R =
p-
Br, 85%, 88 de
R =
p-
F, 82%, 86 de
R =
o-
Br, 90%, 62 de
R =
p-
Cl, 82%, 86 de
R =
p-
CH
3
, 85%, 84 de
R =
p-
OCH
3
, 80%, 77 de
R = H, 87%, 80 de
O
OEt
OEt
N
OPiv
O
HP(O)(OEt
)
2
P
PivO
BF
3
Et
2
O
97
R
Scheme 1.30
Synthesis of
N
-galactosyl
α
-aminoalkylphosphonates
97.
The diastereomerically pure compounds
97
were obtained by simple recrystal-
lization from
n
-hexane and diethyl ether. To determine the absolute configuration
of the main isomer of the diethyl phosphite addition to
N
-galactosyl aldimines
5,
a single-crystal X-ray diffraction study of
97
(R
=
p
-Cl) was performed. The molecu-
lar structure of
97
(R
p
-Cl), shown in Figure 1.4, proves that the absolute con-
figuration of the main product is
(S)
[42].
Figure 1.5 shows a possible mechanism for the reaction. The preferred forma-
tion of the
(S)-
configured diastereomers of
97
can be rationalized by an attack of
diethyl phosphite from the
Si
side of
N
-galactosylaldimines
5.
Initially, the Lewis
acid boron trifluoride is coordinated to the imine nitrogen of
5.
The
Re
-face of the
imine is shielded by the 2-
O
-pivaloyl group, leaving the
Si
-face exposed. Upon
attack of the diethyl phosphite in the transition state, one fluoride may be removed
from the Lewis acid and the vacant coordination site may then be filled by the
carbonyl oxygen of the 2-
O
-pivaloyl group.
=
OPiv
PivO
O
OEt
OEt
N
OPiv
O
P
PivO
Cl
Figure 1.4
ORTEP presentation of the crystal structure of
97
(R
=
p
-Cl).
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