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

478 NITRONATES
NO2 O
EtO
P X
Me
−
EtO
Ph
58a-d Li+
+
−78°C
O Ph
(EtO) 2P
NO2 X
H Me
Li
A
+
O Ph
(EtO)2P
X
Li
Me
+
A'
X = MeO2C (a) 82%; CN (b); SO2Me (c); PO(OEt)2
NO 2
SiMe3
Cl
−LiCl
Scheme G 3.58
A
N
OSiMe3
O
(EtO) 2 P
A
Ph
O
(EtO)2P
X
Ph
G
N
O
X
O N
59a-d
−Me3SiOH
Me3SiO
N
+
SiMe3
A – Me, G – SiMe3 (BSA); A – CF3, G – SiMe3 (BSTFA); A– PhNH,
G – Ph (DPSU)
Scheme 3.59
in Scheme 3.59 can be used as such agents. This approach was used for the
introduction of the trimethylsilyl group, although there are no principal obstacles
to the introduction of other triorganosilyl fragments.
All three amides are evidently mixtures of tautomers. It should be noted that
the nitrogen atom in the imide form is sterically unhindered and sufÞciently basic
to provoke deprotonation of silylated AN
DPSU is a rather weak silylating agent, which has found use for silylation
of the hydroxy group. The advantage of this reagent is that it does not give
by-products of silylation in solution because N,N ′ -diphenylurea is insoluble in
most organic solvents and can be almost completely separated by Þltration.
Silylation of AN with BSA and BSTFA produces as by-products N -trimethylsilylacetamide
(bp 70 ◦ C/35 mm Hg) and N -trimethylsilyltrißuoroacetamide (bp
60 ◦ C/35 mm Hg), respectively. Owing to high volatility typical of ßuorinated
hydrocarbons, distillation of BSTFA from the reaction mixture seems to be more
favorable, although their high casts should also be taken into account.
Silylation with N -trimethylsilylamides is generally performed at room or
higher temperature (but not higher than 100 ◦ C). More drastic heating leads to
O