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

558 NITRONATES
R
PhCOCH 2SCH 2R
177
hv
r.t.
Ph
RCH=S S S
178
•
S
O
MeCH=N(O)OSi
N
179
OSi
O
H
S
CHR
R S R
• •
R
H
But PhCH2CH2 CH2=CH +
S S
PhCOMe
R S R
• •
R
R
Si – SiMe2But Yield %
78
93•
91* * Mixture of stereoisomers
92*
Scheme 3.137
+
RCH=S
It was reported (387) that decomposition of nitronate (183) afforded isoxadiazole
184 (Scheme 3.139).
It was suggested that this product was formed by [3 + 2]-cycloaddition of
nitronate (183) and oxime of benzaldehide, which was generated upon decomposition
of the above mentioned nitronate, followed by elimination of water and
methanol. However, direct evidence for the occurrence of cycloaddition of nitronates
to the C,N double bond of oximes is lacking.
3.4.3.3. Intramolecular [3 + 2]-Cycloaddition of Nitronates These reactions are
more efÞcient than analogous intermolecular transformations of nitronates as
[1,3]-dipoles, and, consequently, activation of the “dipolarophilic” fragment is
not required. However, another problem arises, that is, the construction of the
starting substrate combining the nitronate fragment and the C,C double bond in
the required positions.
A rather general strategy was developed for the construction of such substrates
(Scheme 3.140).
This approach involves the Michael reaction of readily available α-nitroalkenes
(185) with various nucleophiles (186) containing the C,C double bond. (Generally,
it is necessary to activate the nucleophile by particular bases, such as
amines, metal hydrides, organomagnesium compounds, etc.) As a result, a wide
range of target nitro derivatives 187, which differ primarily by the length and
composition of the tether between the > CH(NO2) fragment and the C,C double
bond, can be obtained. This tether (n = 1 or 2) can consist only of carbon atoms
(X = > C) (186, 191, 199) or include one oxygen atom (X=O) (192), (194–196,
198), a nitrogen atom (X = N–Alk or NH) (200), or the sulfur atom (X=S) (195,
388, 389). Nitro compounds (187) are transformed into nitrile oxides A, which
are subjected to intramolecular cycloaddition to give bicyclic derivatives 188
178