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

652 NITRONATES
Y
N
O
NO2
O
396
Y −
NO 2
6
R
O N O
A
NO2 −
O N O
R
80–90%
399 Y = N3, CN, AlkO
R = Ph, CO2Me, CN, MeO
+
+
R
ONO
Scheme 3.220
O
N
396′
ONO
O N O
397
OH
O N O
398
NO2
O
R
R'OH or Et2NH
R
R
R = Ph
approach of the nucleophile is typical also for catalytic C,C-coupling reactions
of six-membered cyclic nitronates with silyl-containing nucleophiles (478).)
Recently, it has been demonstrated (495) that the [3 + 2]-cycloaddition reactions
of 3-bromo-substituted six-membered cyclic nitronates (400), which are
constructed from oleÞns (401) and 1-bromo-1-nitro-2,4 ′ -methoxyphenylethylene,
with oleÞns (402) produce 3-vinylisoxazolines (403) orÞve-membered cyclic
nitronates (404) in good yields (Scheme 3.221).
To account for the formation of the Þnal products, a pathway involving the
initial generation of cycloadducts (405) followed by elimination of the bromide
anion to give cations A was proposed. The cations can stabilize by deprotonation
of the C-4 atom followed by retro-[4 + 2]-cycloaddition of intermediates B to
give isolable vinylisoxazolines (403).
The formation of Þve-membered cyclic nitronates (404) is explained in terms
of ring-chain tautomerism of cationic intermediates A (A⇄A ′ ). The presence of
the alkoxy substituent (R 4 ) at the C-6 atom could stabilize the open form (cation
A ′ ), which Þnally leads to the formation of functionalized Þve-membered cyclic
nitronates (404) probably with the participation of water.
In processes (400 + 402)→(403) and (400 + 402)→(404), the possibility of
introducing of auxillaries at the C-6 atom is advantageous for the development
of new strategies for asymmetric synthesis with participation of six-membered
3-bromo-substituted six-membered cyclic nitronates.