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HO
O
HO
O
O
O
O
i
Pr
O
O
i
Pr
O
R = H
O
O
OB
R
OB
R
O
i
Pr
O
i
Pr
O
O
CF
3
H
H
H
H
NOE
R =
H
H
Me
CF
3
Me
s-trans, anti
s-cis, anti
Figure 8D.7.
Ph
O
Et
Cl
Cl
O
Cl
Sn
O
Cl
Et
O
Ph
Figure 8D.8.
8D.2.8.
endo
- Preference
The
endo
-preference is the key issue in the D-A reaction, where the
endo
- transition
structure can be stabilized by the secondary orbital interaction. Acrylate, crotonate, and
cinnamate derivatives generally provide the
endo
- products. Cyclohexadiene and acyclic
dienes provide the
endo
-products. However, the D-A reaction of α - substituted α , β -
unsaturated aldehydes with cyclopendadiene provides the
exo
- products (see Table
8D.3 ).
An
endo
/
exo
-selectivity is not obvious in an ene reaction, since the ene reaction does
not afford cyclic adducts. Few examples have been reported for the
endo
- preference in
the ene reaction of butadiene with maleic anhydride. However, both the
endo
- and
exo
-
transition structures in the ene reaction show a relatively small energy difference, in
contrast to the D-A reaction. Furthermore, our theoretical studies show that the elec-
trostatic interaction of carbonyl oxygen and cationic central carbon of an ene component
in the Lewis acid-promoted carbonyl-ene reaction is obvious rather than the secondary
orbital interaction [16] .
