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

566 NITRONATES
OSiMe 3
+
203 MeO 204
O
[a] r.t. = −57,1
D
H
NO 2
O
N
67%
206
Cl2Ti – TADDOL
toluene – 90˚ → 75˚
OSiMe 3
OMe
Scheme 3.146
80˚C, 10 h
benzene
H
205
H
O
O
H
NO2
OMe
59%
BSA (i-Pr2NEt,
3 drops)
benzene,
20˚C, 1 h
H
N
OMe
89% de
82% ee
O
OSiMe 3
isoxazoline E, which undergoes isomerization to form conjugated nitrosoalkene
F. The 1,5-proton shift in the latter gives rise to isolable oxime (208).
Not all of the steps of this scheme are justiÞed. However, it was successfully
extended to the synthesis of 2-nitrobenzothiophene (392) (Scheme 3.148).
It is known that an attempt to use this strategy for the preparation of unfused
2-nitro-3-(trimethylsilylmethyl)dihydrofuran led to the selective formation of
another product.
The behavior of SENAs in intramolecular [3 + 2]-cycloaddition was studied
also with a Si-containing tether (193, 194).
In particular, a series of isoxazolines (211) fused to the Þve-membered ring
was synthesized (193) (Scheme 3.149, Eq. 1). As a rule, the reactions are characterized
by high stereoselectivity (dr > 20:1).
Products (211) can be hydrogenated over Ni/Ra, which is accompanied by
opening of both rings, the conÞgurations of the stereocenters being retained.
In the ISOC process, a longer Si-containing tether was also used (194)
(Scheme 3.149, Eq. 2). As expected, the use of a longer tether requires more
severe reaction conditions and leads to a decrease in stereoselectivity of the process.
However, the results of the study (194) show that the stereoselectivity of
the reaction additionally depends on the bulkiness of the substituent R (cf. the
2/2 ′ ratio for (212a) and (212b) respectively).