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

562 NITRONATES
References to Scheme 3.141 exemplify that the selectivity decreases or even
the stereodirection is inverted in going from the ISOC process to the INOC
process (see entry 8, Scheme 3.141).
An interesting example was cited in Reference 196 (Scheme 3.142).
The authors demonstrated that the stereocontrol in the ISOC process as applied
to nitro compound (192) isinßuenced by the phenyl substituent at the C* atom
to give, respectively, stereoisomers 193a and (193b), whereas the stereocontrol
in the INOC process is provided by the substituent at the C** atom. This corresponds
to a decrease in the effect of the “dipole” in going to the INOC process.
The INOC and ISOC processes, taken together, cover a wide range of substrates
and generally give the target bicyclic products in yields from high to
moderate. It should be noted that the reaction conditions for the INOC process
are milder and the yields of bicyclic products are somewhat higher, while the
ISOC process is substantially more selective.
An increase in the tether length requires substantially more severe reaction
conditions. For tethers containing more than Þve atoms between the reacting
fragments, intramolecular [3 + 2]-cycloaddition reactions remain unknown.
Recently, β-bromo-β-nitrostyrene has been successfully involved in the ISOC
process (Scheme 3.143) (390).
In this case, unlike the above considered examples, Bu t Me2SiCl was used as
the silylating agent and the target isoxazolidine (194) was isolated in high yield
Ph
O
∗
∗∗
Ph 192
Ph
+
OH
Br
NO2
NO 2
Me3SiCl/Et3N
r.t., 48 h, benzene
PhNCO/Et 3N
r.t., 48 h, benzene
Ph
NO 2
Ph
Ph
O
∗
N
O + O
∗
Ph
H
193a
Ph
N
O O
∗∗
Ph
H
193a
Scheme 3.142
Me 2Bu t SiCl/Et 3N
+
Ph
Ph
O
∗∗
Ph
H
193b
H
193c
N
O
N
O
38:62
47:53
O Br
O O
N
194
OSiMe2But base yield 83%
1 diastereomer
Ph
Br
Scheme 3.143
H