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3.0 Synthesis and Use of Cross-Conjugated Trienes
3.1 Transition Metal-Catalyzed Ene-type Cycloisomerization Reactions
The ene reaction is recognized as a thermal pericyclic reaction of an olefin containing an
allylic hydrogen (ene) and a multiple bond (X=Y, enophile), resulting in formation of a new C-C
and Y-H bond as shown in Scheme 3.1. 65 The reaction was first discovered by Alder in 1943,
and is often referred to as the Alder-ene reaction. 66 Since then, intramolecular variants have been
studied extensively. Depending on the placement of the ene component in the starting material,
there are three types of intramolecular ene reactions: type I – connected via the olefin terminal;
type II – connected via the central carbon; and type III – connected via the allylic terminal. 67
Scheme 3.1 Classification of ene reactions.
2
3
1
X
Y
H
ene enophile
X
Y
H
X=Y : C=C, C=O, C=S, O=O, N=O etc.
2
3
1
three types of intramolecular ene-reaction
H
H
H
∆
Type I
∆
Type II
∆
Type III
Thermal ene reactions of unactivated substrates generally require very high temperatures
between 300-500 °C to proceed, which has limited their direct application in synthesis. To
30
H
H
H