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Chapter 25 801
Copyright © 2017 Pearson Education, Inc.
b.
The label is lost from sample B when mevalonic pyrophosphate loses CO
2
to form isopentenyl pyro-
phosphate, so none of the carbons will be labeled in citronellal.
CH
3
CCH
2
CH
2
OPP
i
OH
CH
2
CO
−
O
14
CH
3
CCH
2
CH
2
OPP
i
CH
2
C
H
14
O
+
CO
2
c.
Because the methyl groups are equivalent in the carbocation that is formed as an intermediate when
isopentenyl pyrophosphate is converted to dimethylallyl pyrophosphate, either of the methyl groups
can be labeled in dimethylallyl pyrophosphate. This means that either of the two methyl groups can be
labeled in geranyl pyrophosphate and in citronellal.
CH
3
CCH
2
CH
2
OPP
i
OH
CH
2
CO
−
CH
3
CCH
2
CH
2
OPP
i
CH
2
mevalonyl pyrophosphate
isopentenyl pyrophosphate
H
+
CHCH
2
OPP
i
C
CH
3
CH
3
H
O
14
14
14
+
CHCH
2
OPP
i
CH
3
C
CH
3
14
+
CHCH
2
OPP
i
CH
3
C
CH
3
14
H
+
dimethylallyl pyrophosphate
+
CHCH
2
CH
2
C
CH
3
C
14
+
+
geranyl pyrophosphate
CH
3
CHCH
2
OPP
i
CH
3
14
CHCH
2
CH
2
C
CH
3
C
14
CH
3
CHCH
2
OPP
i
CH
3
14
C
H
O
14
14
C
H
O
14
14
citronellal
CH
3
CCH
2
CH
2
OPP
i
CH
2
14
39.
H
O
O
O
+
CH
2
O