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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 635
the process. Target isoxazolidines (355) were prepared in rather high yields. The
diastereoselectivity of the transformation (354→355) substantially depends on
the nature of the substituents.
It should be emphasized that most of isoxazolidines shown in Scheme 3.209
cannot be prepared according to the alternative [3 + 2]-cycloaddition scheme with
the use of the corresponding acyclic nitronates (for more details, see Section
3.4.3.1.2).
The C,C-coupling reactions of six-membered cyclic nitronates were studied
in most detail (274, 478). Here silyl ketene acetal was also used as the test
nucleophile. The conÞgurations of most of the starting nitronates and the resulting
nitroso acetals were determined by NMR spectroscopy and X-ray diffraction, and
also a conformational analysis was performed (see Tables 3.24 and 3.25).
Table 3.24 C–C coupling of nitronates (356a–j) with silylketene acetal.
(An–4–MeO–C6H4)
R 3
R 4
R 2
R 1
R 2
O N O
R OTBS
3
N
+ − O +
R4 356a–j 357a–j
TBS – SiMe 2Bu t
TBSOTf
R 1
OTf −
OMe
OTBS
TBSOTf
Nitronate Oxazine Yield,
Entry 356 R 1 R 2 R 3 R 4 358 %
R 3
R 2
R4 O N
5
4
3
6
358a–j
CO2Me
R 1
OTBS
Ratio of
stereoisomers a
1 a H Ph Me Me a 91 -
2 b H Me Me Me b 88 -
3 c H Ph OMe Me c 91 -
4 d H An H OEt d 93 -
5 e H An OEt H e+e 88 1.5:1
6 f H OBz Me Me f + f’ + f 88 19:1:2b 7 g H OCO-C6H4- Me Me g + g’ + g 92 20:1.5:1c NO2-p
8 h Me H H H h 95 -
9 i Me H Me Me i 90 -
10 j Me Ph Me Me j 92 -
a Established by quantitative analysis of 1 H NMR spectra of crude product.
b After column chromatography pure (358f) (77%) as well as inseparable mixture of (358f’’) and
(358f’) (11%) were isolated (358f’ : 358f’’) ≈ 1:2. Upon the storage in CDCl3 at r.t. for 3 weeks
(358f) was partially converted into (358f’’) (358f : 358f’’= 3:2).
c After column chromatography pure (358f) (82%) and an inseparable mixture of its invertomer.
(358 g’’) and diastereoisomer (358 g’) was also isolated in 10% yield (358 g’ : 358 g’’ 3:2).
≈
Upon the storage in CDCl3 at r.t. for 4 weeks (358 g) was partially converted into (358 g’’)
(358 g : 358 g’’ = 1:1).