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

REACTIVITY OF NITRONATES 561
Both of these processes are regioselective, and are different from intermolecular
[3 + 2]-cycloadditions of nitronates in that the terminal carbon atom of the C,C
double bond is bound to the oxygen atom of the nitronate fragment regardless of
the degree of its substitution.
The stereoselectivity of the INOC process substantially differs from that of the
ISOC process, the stereoselectivity of the ISOC process being generally much
higher. Evidently, this is due to a considerable differentiation of the transition
states. It gives to different stereoisomers due to higher hindrance of the nitronate
dipole compared to the linear nitrile oxide dipole.
It does not help to discuss the inßuence of substituents in nitro substrate (187)
on the stereoselectivity and stereodirection of [3 + 2]-cycloaddition in the general
case, because this effect is determined by the sum of different factors and, hence,
it depends on the number and nature of the above mentioned substituents.
At the same time, the stereoselectivity and stereodirection of the ISOC process
substantially depend on the tether length (Scheme 3.141).
If the tether consists of three atoms (-RCH–X-(·)n- (n = 1)), the stereoselectivity
of the process is very high, and trans isomer 188a prevails among the reaction
products, regardless of the composition of the tether, due to the endo approach of
the tether to the Z isomer of the nitronate. An increase in the length of the tether
to four atoms (n = 2) leads to a sharp decrease in selectivity of cycloaddition,
and cis isomer (188b) prevails among the reaction products. Evidently, the latter
isomer is formed as a result of the exo approach of the tether to the nitronate.
Me3SiO
Entry
N
O R′ R′′
R
N
R
N
X
( )n
X
( ) n
O
+
X
( )n
O
R
H R′
R′′
H R′
R′′
189 188a (trans) 188b (cis)
R R’ R” X n dr a/b Ref.
1 C6H5 H CH2199:1 199
2 4-MeO-C6H4 H CH2199:1 199
3 i-Pr H H CH2199:1 199
4 C6H5 Me Me CH2 1 99:1 199
5 C6H5 H CH221:2.6 199
6 i-Pr H S 1 99:1 a
388
7 C6H5 H S 1 99:1b 388
8 4-MeO-C6H4 H C(CO2Me)2 1 99:1 c
H
H
H
H
H
H
200
9 C6H5 H H S 2 5:3 388
a b
INOC – 1:1; INOC – 3:2;
c
INOC – 1:5.
Scheme 3.141