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

R 1
R
R 2
NO 2
+
R 5
R 3
42 43
R 6
R 4
LA
R2 R1 R
O
R
R 5
R 6
N O
O
N O
Scheme 3.42
SYNTHESIS OF NITRONATES 465
+ _
LA
R4 R3 R1 R2 if R 6 = R 4 = H
+
R
O
R 1 R 2
N _
O
R
O
LA
B B′
R 5
R 3
+
N O
R 5
R 3
R 1 R 2
R 5
24 35
R 6
R3 R4 (42) without substituent R 1 are used) (112). In this case C-alkylation occurs,
viz., cyclobutane derivatives (44) are obtained as by-products (Scheme 3.43)
(114).
The formation of nitrocyclobutanes (44) is attributed to the contribution of
the structure B ′′′ to the reactivity of stabilized zwitterionic intermediate B. The
contribution of C-alkylation decreases due to steric hindrance caused by the
presence of substituents at the α-position of the initial nitroalkene (42).
The reactions of nitroalkenes (42) with various enols (43b) (vinyl ethers,
silyl, and acyl enolates, ketene acetals) have been studied in most detail (110,
111, 125–154). As a rule, these reactions proceed smoothly to give the corresponding
nitronates (35f) in yields from high to moderate. As in the reactions
with enamines, the formation of compounds (44b) is attributed to the ambident
character of the anionic centers in zwitterionic intermediates analogous to those
shown in Scheme 3.43.
However, the reactions performed under drastic conditions sometimes produce
the corresponding nitrocyclobutanes (44b) instead of the target cyclic nitronates
(35) (see Scheme 3.44).
Terminal oleÞns (43c) (Scheme 3.45, Eq. 1) containing two substituents with
opposite electronic effects at a single carbon atom can react stereoselectively with
heterodienes (42) in a head-to-head fashion in the absence of LA (123, 155).
Denmark and coworkers demonstrated (156) that allenes (e.g., (43 d)) can
selectively react at one of two bonds with conjugated nitro oleÞns (42a)