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

BuO
R 1
Me
Me
Me
Et
Et
Et
Me
*
**
OBz
O N
249c′
R 2
H
SiMe3 H
Me
OAc
H
H
H
not observed
O
R 1 O2C
R 3
regioselectivity 1/2,7
R 2
R 3
253a–g
H
H
SiMe3 H
H
OAc
CO2Me Olefin
253a
253b
253c
253d
253e
253f
253g
R 1 O2C
R 1 O2C
O
R2 R3 REACTIVITY OF NITRONATES 589
N O
OBz
OBu
O
N O
H
OBz
254a–g
255a–g
head to head head to tail
O
R3 R2 H
N O
H
OBz
254′a–g
Products
OBu
254a; 255a; 254′a; 255′a
254b; 255b; 254′b; 255′b
254c; 255c; 254′c; 255′c
254d; 255d; 254′d; 255′d
254e; 255e; 254′e; 255′e
254f; 255f; 254′f; 255′f
254g; 255g; 254′g; 255′g
Scheme 3.170
+
+
R 3
R 2
R 1 O2C
R 3
R 2
R 1 O 2C
Yield%
79
72
77
75
54
78
82
O
N O
H
OBz
255′a–g
d.r. 254/255/
254′/255′
7,6/ * /1/ *
2,4/1/ * / *
1/1,5/2,1/ *
1/1,6/ * / *
* /1/ ** /
**
1/ * /1/ *
OBu
OBu
the reactivity of ambident anions (426), is the best for prediction of the regioselectivity
of [3 + 2]-cycloadditions of 1,2-disubstituted oleÞns with nitronates.
However, the use of this interesting approach remains to be tested by experimental
methods and from a philosophical position. †
The authors (162) attempted to explain the stereochemical outcome of the
reactions (Schemes 3.169 and 3.170) in the terms used earlier (337), that is,
by steric factors, which destabilize the endo approach of a dipolarophile, and
the electronic effect (secondary orbital interactions), which is most typical for
electron-rich dipolarophiles and can slightly stabilize the endo approach of these
oleÞns.
† It remains unclear why the approach used for the description of ionic processes (426) should be
applied to the reaction, which is commonly considered as concerted. In particular, it is unclear how
the steric factors, which can play the decisive role in the description of concerted processes with
their sterically crowded transition states, are taken into account. Also, whether this explanation of
regioselectivity can be extended to other nitronates and monosubstituted oleÞns.