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

230 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
Bn
+ −
N O
R 1
a: R
303
1 = H
c: R1 = Me
d: R1 = Et
k: R1 = iPr
N
H
N
H
A1
O
CO2Me
H
N
A4
L-Pro-L-Leu
N
H
+
O
R 2 R 3
A1-A7
b: R 2 = CF 3, R 3 = Ph
c: R 2 = R 3 = CO2Et
CO 2H Me
O
A6
H
N
L-Pro-L-Gly
Me
CO2H
Bn
Bn
O Me
N
Me
N
H
A2
N
H
Scheme 2.120
N
O
R 1
OH
R2
R 3
i: R
304
1 = H, R2 = CF3, R3 = Ph
s: R1 = Me, R2 = R3 = CO2Et
t: R1 = Et, R2 = R3 = CO2Et
u: R1 = iPr, R2 = R3 = CO2Et Me
O
A5
H
N
L-Pro-L-Phe
N
H
+ −
O
A7
Ph
H
N
N
H
A3
CO2H
L-Pro-L-Ala
Me
CO2H
CO2H
the structure of the starting aldonitrone, the reaction can give either dimers with
two conjugated nitrone groups (route A), or dimers with coupled nitrone and
imine groups (route B) (Scheme 2.123) (550).
Metalated cyclic aldo-nitrones are characterized by high reactivity toward electrophilic
reagents. Reactions with aldehydes and ketones afford satisfactory yields
of α-hydroxymethyl substituted derivatives of nitrones (551). The reactions were
also carried out with a number of aliphatic, aromatic, and hetero-aromatic aldehydes
and ketones (Schemes 2.124 and 2.125).
With a terminal double bond to the nitrone group in the α-hydroxymethyl
derivative (309), a typical 1,3-dipolar cycloaddition of nitrones gives compound