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

SYNTHESIS OF NITRONATES 469
3.2.2.3.2. Synthesis of Seven-membered Cyclic Nitronates Cyclic nitronates containing
more than four carbon atoms in the ring remain virtually unknown.
Convenient procedures for the synthesis of these compounds are lacking. In
particular, intramolecular alkylation of 5-bromo-1-nitropentane affords nitrocyclopentane
rather than the corresponding seven-membered cyclic nitronate (169)
(Scheme 3.48).
However, intramolecular O-alkylation can be performed under particular conditions
leading to of annelation of a seven-membered heterocycle. Japanese
researchers (170) prepared the corresponding seven-membered cyclic nitronates
(50a–c) in good yields by the reaction of triethylamine with brominated aryl
ketones (49a–c) containing the nitromethyl group in the ortho position.
3.2.3. Synthesis of Trialkylsilyl Nitronates
Owing to the fact that silyl esters of nitronic acids (SENAs) are readily available,
rather stable, and exhibit various reactivities, these compounds have attracted
considerable interest in the chemistry of nitronates.
First SENA was prepared by Klebe in 1964 by the reaction of nitromethane
with N-trimethylsilyl-N,N’-diphenylurea (171) (Scheme 3.49).
The resulting bis-silylated derivative of methazonic acid contained the SENA
fragment and, in the author’s opinion, was produced by condensation of two
Br NO 2
R′
MeNO 2
49a–c
O
Br
CH2NO2
R
SiMe3 H
N N
Ph
Ph
O
(DPSU)
(PhNH) 2CO
K2CO3/benzene
Et3N/C6H6
5h
R′
NO 2
Scheme 3.48
CH2 N
O
OSiMe3
Scheme 3.49
O N
O
R
a: R = H; R′ = Me (54%)
O
b: R = R′ = Me (59%)
N
c: R = Me; R′ = ΟMe ~100%)
O
50a–c
O
Me3SiO N N OSiMe3 76% O