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

684 NITRONATES
was reduced to form a non-natural amino acid derivative (464a), as an equimolar
mixture of two diastereoisomers.
By contrast, L-phenylalanine methyl ester does not react with BENA generated
from 1-nitropropane (R’ = H) due apparently to low nucleophilicity of the amino
group. However, the N,C -coupling reaction of the ester of this amino acid with
another internal BENA (R ′ =CO2Me) proceeds rather readily but is characterized
by extremely low diastereoselectivity. Probably, the last N,C-coupling does not
occur via an intermediate α-nitroso alkene but by a classical Michael addition to
BENA MeCH=C(CO2Me)N(OSi)2 as to Michael substrate.
N,C-Coupling reactions of BENAs with derivatives of NH-acids. As mentioned
above (see Scheme 3.226), NH -acids catalyze the rearrangement of BENAs (501,
502), which is the most general process that hinders both the synthesis of BENAs
and their desired transformations. Hence, silyl derivatives of NH -acids should
be used in N,C-coupling reactions with BENAs.
N,C -Coupling reaction of BENA with trimethylsilyl azide is the key step of a
very convenient and versatile procedure for the synthesis of otherwise difÞcultly
accessible α-azido oximes (524, 525) (Scheme 3.248).
Both terminal and internal BENAs (434) are readily subjected to this transformation
to give the α-azido oximes (465) in very high yields. The improved
procedure allows one to prepare compounds (465) virtually without by-products.
The use of a large excess of silyl azide and the presence of small additives of
triethylamine (5%) are of principal importance.
As can be seen in Scheme 3.249, α-azido oximes (465) can be involved
with advantage in the selective reduction of the azido group in the presence
of the oximino fragment and also in the selective or nonselective reduction of
the oximino fragment (525).
R 1
R 1
NO 2
431
R 2
N(OSiMe3)2
R 2
Me 3SiN3 exc.
CH2Cl2; Et3N cat; 20°C
R 1
NOSiMe3
R 1
N
OH
465
80%–95%
434
466
R1 = H, Me, (CH2)2CO2Me, CH2OSiMe3 (OH), Ph, Bn, CO2Me, CO2Et; R2 = H, Me.
R 2
N3
Scheme 3.248
MeOH
85%–90%
R 2
N 3