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

O
O
N
R
+
R"CO
NO2 R′ N
42 43c
Ph
N O
O
•
O
+
43d
O
O
Me
NO 2
SiMe 2Ph
O
46a
N O
NO 2
R3
R
−20˚C, 2–12 h
O
R 2
SnCl4
−78˚C
O
O
R′
N O
Me
O
35g
N O
SYNTHESIS OF NITRONATES 467
R = Ph; R′ = Me; R′′ = Et or OMe
R = Me; R′ = H; R′′ = Et or OMe
COR′′
(1)
N
O
Ph SiMe 2Ph
35h
Scheme 3.45
NO2
45
O N O
R2=H, R3=NO2 or
R2= NO2, R3=H E
OAlk
N O O OAlk
O
Scheme 3.46
O
N
R OR'
46b
R
R'O
(2)
N O O OAlk
The above described approaches were used in the several studies (96, 137,
138, 140, 150, 159–163) to prepare a representative series of enantiometrically
pure six-membered cyclic nitronates (35).
To conclude this section, let us note an interesting study by Eaton and coworkers
(164), who examined the reactions of solutions of nitroacetylene (45) with
vinyl ethers and furan (Scheme 3.46); he reasonably suggested that very unstable
nitronates (46a) and (46b) are generated and that these intermediates undergo
unusual fragmentation resulting in generation of vinylated nitrile oxides, which
give [3 + 2]-cycloaddition products with nitroacetylene or vinyl ether.
3.2.2.3. Synthesis of Unusual Cyclic Nitronates
3.2.2.3.1. Synthesis of Four-membered Cyclic Nitronates Conjugated nitro
alkenes (42) (Scheme 3.47) can be considered as structural isomers of
O N