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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 723
3.5.8. Selective Reduction of the Nitroso Acetal and Oximino Fragments in
Products of AN Silylation
In some cases, silylation of AN and their derivatives produces nitroso acetals
containing the N -siloxy fragment or cyclic ethers of oximes (predominantly
substituted 5,6-dihydro-4H-oxazines). To use these products in strategies for synthesis,
it is worthwhile to develop convenient procedures for selective reductions
of the above derivatives to the corresponding amines.
3.5.8.1. New Procedures for Reduction of Nitroso Acetals Containing the
N–OSiMe2Bu t Fragment Selective reduction of cyclic nitroso acetals was
brießy considered in Section 3.4.3.4.3. The latest data on catalytic hydrogenation
of cyclic and NOSiMe2Bu t -containing acyclic nitroso acetals (including
TBS – SiMe2But . (For R1 O
= CH2CO2Me) at H2; Ni/Ra, 50 atm 20˚C, no reaction)
N
R2 R1 Ph
75%
OH NHBoc
dr = 1:1.7
OTBS
597a,b
598a
a: R
NH
dr = 1:4
O
1 = H, R2 = Ph, dr = 1:3.3
b: R1 = H, R2 597a
H2;Ni/Ra, 10 atm 20˚C
MeOH,Boc2O, 3days
597b
H2;Ni/Ra, 10 atm 20˚C
MeOH, 3 days
OH
= CO2Me, dr = 1:4 598b
597d
Ph
EtO N OTBS
599a
R
CO 2Me
NH 4F + Boc2O
H2; Ni/Ra, 25 atm MeOH,
Boc2O, 24 h
84%
H2;Ni/Ra, 25 atm MeOH,
Boc2O, NH4F, 24 h
Ph
600
H2;Ni/Ra, 25 atm MeOH, 69%
Boc2O, NH4F, 24 h
Ph
CO2Me NHBoc
TBSO N OTBS
599b
HF + NH2Boc + Bu t OH + CO2
CO2Me
TBSO N R
OTBS
NHBoc
601a,b
602a,b
R = CH(Ph)CO2Me (a) (b) TBS – SiMe2But Yield:
a : 43%
b : 57%, dr 1:14 (for 601b dr 1:1.1)
(for 602b BocNH2 (33%) was obtained)
O
O
Scheme 3.284