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

O
Ph
Et 2N
R
R 1
S
CHO
N
COOEt
R = Me; R'' = H
R, R' = −(CH=CH) 2-
CHO
+
BnNHOH . HCl
CH 2Cl 2
R'' R'''
Ph NHOH
70a, b
R'' = Me; R''' = H (a)
R'' = H; R''' = CH 2OH (b)
O
SYNTHESIS OF NITRONES 157
R 1
R
S
71a, b; 72a, b
Et2O, r.t.
R''
R'''
HC
N
MgSO4
N +
O− COOEt
Ph
a: R = Me; R' = H; R'' = Me; R''' = H
b: R, R' = -(CH=CH) 2-; R'' = H; R''' = CH 2OH
Scheme 2.25
Ph
Et2N
H
O
N
−
+
Bn
O
Ph
Et 2N
73 74 PEDC
Scheme 2.26
(74) was generated in quantitative yield by condensation of asymmetric cyclopropylcarbaldehyde
(73) with N -benzylhydroxylamine hydrochloride in CH2Cl2
(Scheme 2.26) (222, 223).
Condensation of 4-formyl-3-imidazoline (75) with tert-butylhydroxylamine in
aqueous-alcoholic solvent gives conjugated nitrone-imine (76) (224), whereas the
reaction of N -tert- butylhydroxylamine both with α-monobromomethyl nitrone
(77), and α,α-dibromomethylnitrone (78) generated dinitrone (79) (Scheme 2.27)
(225).
In order to study the mechanism of reverse Cope elimination reactions in the
condensation of pentenal and hexenal with N -methylhydroxylamine, it seemed
reasonable to synthesize unsaturated nitrones (226).
To generate chiral nitrones (81) and (82) the direct transformation of chiral
o-substituted cyanohydrins was used. Both of the approaches described in Reference
227 allow the production of nitrones in high yields and high optical purity
in a one-pot synthesis (Schemes 2.28 and 2.29).
H
Me
NH2