Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

600 NITRONATES
X
O
R
Me
287
O
O
O N
R′
N
N
O
Ph
H
Me
L-selectride
X H
Bu
N
tMe2SiO O
Ph
X= XC or XY;
Scheme 3.177
X y
X y
O
H
O N
290 Ph
O
+
H
O
N
290′
dr 290/290′ ~ 13,3
Xc - N
X y -
Me
O
O
SO 2
N
N
70%
Ph
Ph
H
Me
in the enantioselectivity of the reaction. The conÞgurations of the stereocenters
were established by the NMR method and conÞrmed by X-ray diffraction data.
On the basis of these data and assuming that only the major E isomer of the
nitronate is involved in the reaction, it can be concluded that the Re-face attack of
nitronate on the dipolarophile is preferable, apparently, because of shielding from
the Si face due to the presence of an auxiliary. The reaction with lactam (287)
proceeds analogously (438) but is characterized by a somewhat higher enantioselectivity.
Reduction of isoxazolines with L-selectride makes it possible to restore
auxiliaries. This process was used in the asymmetric synthesis of nonactin and
its derivatives. (439–441)
In intramolecular [3 + 2]-cycloaddition reactions, silyl nitronates also
lead to substantially higher stereoselectivity than intermolecular reactions (see,
e.g., Scheme 3.178) (193).
SENAs containing auxiliaries were successfully used in the ISOC procedure
for the synthesis of enantiomerically pure polycyclic (96) (Scheme 3.146) and
natural (210, 211) (Scheme 3.145) compounds.
(4)