Chemistry Reference
In-Depth Information
O
PP
= diphosphate
O
P
cation formation via
loss of leaving group
electrophilic addition
giving tertiary cation
O
P
O
O
O
O
PP
O
PP
O
PP
O
O
isopentenyl
diphosphate (IPP)
H
dimethylallyl diphosphate
DMAPP
- H
+
O
PP
monoterpenes
(C
10
)
O
PP
resonance-stabilized
allylic cation
geranyl diphosphate
(GPP)
Dimethylallyl diphosphate is responsible for generating the carbocation. Loss of diphosphate as the leaving
group produces a resonance-stabilized allylic cation. An intermolecular electrophilic addition follows, with
isopentenyl diphosphate as the source of
π
electrons. Addition to the cationic species takes place at the terminal
carbon that is sterically less congested - at first glance, we appear to be invoking the less favourable resonance
form, but an alternative addition through the double bond onto the tertiary cation could be drawn. At this stage,
it is important to appreciate that these reactions are enzyme controlled, so that we can have two different species
reacting in a highly specific manner. The carbocation product is the more stable tertiary carbocation, as we might
predict, and this loses a proton to form the more-substituted alkene product,
geranyl diphosphate
.
electrophilic addition
giving tertiary cation
O
PP
O
PP
O
PP
H
geranyl diphosphate
IPP
resonance-stabilized
allylic cation
- H
+
sesquiterpenes
(C
15
)
O
PP
farnesyl diphosphate
(FPP)
An exactly analogous process can then occur, in which geranyl diphosphate provides the allylic cation, and a
further molecule of isopentenyl diphosphate adds on, giving
farnesyl diphosphate
; this can subsequently yield
geranylgeranyl diphosphate
.
- H
+
IP
P
allylic
cation
O
PP
O
PP
farnesyl diphosphate
geranylgeranyl diphosphate
(GGPP)
diterpenes
(C
20
)
The compounds geranyl diphosphate, farnesyl diphosphate, and geranylgeranyl diphosphate are biochemical
precursors of monoterpenes, sesquiterpenes, and diterpenes respectively, and virtually all subsequent modifications
of these precursors involve initial formation of an allylic cation through loss of diphosphate as the leaving group.
The formation of cyclic terpenoids involves intramolecular electrophilic addition, and this can be exemplified
by the following monoterpene structures, again with all reactions being enzyme controlled.
