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

NITRONE REACTIONS 273
Table 2.14 Synthesis of α-iminonitriles using Amberlite IRA-400 [CN]
R 1 R 2 Yield % Time/h
Ph Ph 90 3
p-Cl-C6H4 Me 80 3.5
p-Me-C6H4 C6H11 75 4
o-Cl-C6H4 p-NO2-C6H4 78 3.6
p-NO2-C6H4 m-NO2-C6H4 82 4
PhCH = CH Ph 76 6
PhCH = CH p-Br-C6H4 77 5
PhCH = CH p-OMe-C6H4 81 5.5
2.6.6.1.7. Addition of Ketene Acetals and Enoles In recent years, much attention
has been given to the synthesis of optically active nitrogen-containing compounds,
with the key step being the highly stereoselective nucleophilic addition
of ketene silyl acetals to nitrones (Scheme 2.174). Similar to nitrone cyanations,
in ketene silyl acetal reactions one observes an accelerating effect with thiourea
derivatives (633).
Silyl ketene acetal addition to nitrone (292) is likely to generate four diastereomers.
Depending on the employed Lewis acid, either O-silylhydroxylamines or
free hydroxylamines (Scheme 2.175) are obtained (635).
The reaction of silyl ketene acetal addition to nitrones has been used for
the synthesis of optically active (2S ,3S )-benzoyl- and N-boc-phenylisoserine
(636a) of isoxazolidine nucleoside–analog of thymine polyoxine C(636b) and of
R 1
+ O
N
−
C 6H 5
TBDMSO
C 6H 5
+
R 3 O
N R1
O
OR 3
OR 2
OR 2
OTBDMS
R 1 and/or R 2 = chiral auxiliaries
Scheme 2.174
R 1 CONH
C6H5
O
OCH 3
OH
R1 = C6H5 R1 = t-C4H9O