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

574 NITRONATES
The process is initiated by the reversible reaction of a nucleophile with isoxazolidine
(229) at the silicon atom. The resulting anion A is also reversibly
isomerized to form the anion B, which is accompanied by cleavage of the endocyclic
N–O bond. The anion B acts as a nucleophile toward isoxazolidine (229),
removes the trialkylsilyl group to give nitroso derivative (230), and initiates the
chain growth. If at least one of the substituents at the C-3 atom is a proton,
nitroso product (230) undergoes isomerization to give oximino derivative (231).
The corresponding β-hydroxy oxime (232) istheÞnal product in this sequence
of transformations. In this case, the relative conÞgurations of the stereocenters at
the C-4 and C-5 atoms are also retained.
Two procedures were developed for efÞcient isomerization (229→230→232).
One procedure is based on the reaction of ßuoride anion, which has pronounced
selectivity with respect to the silicon atom (189, 207). Another procedure involves
treatment of nitroso acetals (229) with methanol containing a catalytic amount
of triethylamine (174, 216).
Potassium methoxide in benzene also works well as a nucleophile in the transformation
(233→234), where R=H or alkyl (Scheme 3.157, Eq. 1) (204, 205).
Interesting results were obtained in the reaction of strong nucleophiles with
isoxazolidines (235) containing two EWG groups at the C-3 atom (Scheme
3.157, Eqs. 2 and 3). If R=NO2, stabilization of the anionic intermediate A is
R′
R′
R
H
Me
Me
Et
R
H
O N OSiMe3 233
R′
Ph
Ph
CO 2Me
Ph
R
R
O N OSiMe3 235
MeOK/C 6H 6
MeOK
R = NO2; R′ = Ph
MeONa
R = CO2Me R′
R′
R′
R
H
O N O −
R′
OH
NO 2
NO 2
O N O− K
A
+
CO 2Me
CO2Me
O N ONa
A′
Scheme 3.157
R′
R
NOK
MeOH
H +
H +
R′
MeOH
−NaOCO2Me
R
H
O − N O
R′
OH
236
R
OH NOH
(1)
234
65%–95%
NO 2
NO2
R = H 83%
R = Ph