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

584 NITRONATES
R′′Ο
R′′Ο
R
H
Regioselectivity (1):
Head-to-head:
Sterioselectivity (2):
Head-to-tail:
exo O
endo
R′′Ο
N R′
R,N - cis
R,N - trans
Facial selectivity (3):
Top
N
N
for exo
O
H
O
R
R′
R′
R′′Ο O
N
R
A
N
N
R′ R′′Ο
O
R
N
H
R′
R,R′ - trans
ΟR′′
O
ΟR′′
O
R
R
R
R′
R′
Bottom
R′′Ο O
N
R
R′′Ο
R
R′′Ο
O
N
O
R′
H
R,R′ - cis
Chart 3.16 Mechanistic interpretations for [3 + 2]cycloaddition of nitronates with oleÞns.
420, 421). It was demonstrated that the transition state of these processes exhibits
a weak zwitterionic character and that secondary orbital interactions facilitate the
endo approach.
However, the problems of cycloaddition of nitronates with dipolarophiles
require systematic studies. Unfortunately, only two studies dealing with these
problems have been reported. Let us consider them in more detail.
N
R′
R
R′