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

578 NITRONATES
R′′O
R′
R′
R CH2CO2Me MeO2C
N OSi
R
O N OSi
R′
OH
Ni/Ra
H 2, Ni/Ra, NH 4F
O N OSiMe2Bu t
CO2Me
N(OEt)OSiMe2Bu t
R
NH2
H2; KF
R
R′
or
Boc2O NHBoc
MeOH/CF 3CO 2 H(cat)
MeOH/CF3CO2Hcat
R′
H 2
CO2Me
Scheme 3.162
MeO 2C
R′
OH
Ni/Ra
R
O N OMe
R
NHBoc
Si - SiMe 3; SiMe 2Bu t ; R′′ - Et; SiMe 2Bu t
O N OMe
70%
N(OEt)OMe
50%
Scheme 3.163
CO2Me
H 2, Ni/Ra
H2, Ni/Ra
Boc2O
HO
O
NHBoc
NH
CO2Me
In this reaction, the role of the ßuoride ion is apparently to generate a small
amount of HF, which transforms the sterically hindered N–OSi fragment into
the N–OH fragment. The latter undergoes hydrogenation more rapidly.
Hydrogenation of N-siloxynitroso acetals can also be facilitated by replacing
the N -siloxy group with the less hindered N -methoxy group (402) (Scheme
3.163).
This methodology remains to be optimized, but in the future it will be characterized
as a promising procedure for the use of nitroso acetals in organic synthesis.
A convenient procedure was developed for transformations of N-siloxyisoxazolidines
into functionalized β-hydroxycarbonyl compounds with aqueous titanium
trichloride (185, 189) (Scheme 3.164).
Therefore, acyclic and monocyclic nitroso acetals are convenient precursors
of functionalized β-hydroxy ketones and γ-amino alcohols (see Reference 403).
Catalytic hydrogenation of polycyclic fused nitroso acetals is a more complex
reaction sequence. This process was successfully used in the synthesis of natural
and biologically active compounds (see, e.g., Refs. 148, 367 and the review 99).