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acid (
1
; see route C in Fig. 3.3). The detour
via
C
6
-C
3
compounds, as
required for routes A and B, would thus be replaced by this shortcut from
a very early intermediate to gallic acid.
COOH
COOH
A
COOH
+
3 [OH]
- Ac
HO
OH
HO
OH
8
OH
OH
9
1
COOH
B
COOH
- Ac
+ [OH]
1
OH
OH
OH
OH
10
11
C
COOH
COOH
-2 [H]
1
HO
OH
O
OH
OH
OH
Fig. 3.3 Proposed biosynthetic pathways to gallic acid (
1
).
A
.
The route from cinnamic
acid (
8
)
via
3,4,5-trihydroxycinnamicacid (
9
) that undergoes β-oxidation to gallic acid.
B
.
Alternative route
via
cinnamate-derived caffeic acid (
10
) being degraded to
protocatechuic acid (
11
) and hydroxylated to yield gallic acid.
C
. Direct reduction of 5-
dehydroshikimic acid (
12
) via its
enol
-form (
13
) to gallic acid. Ac = acetyl residue. For
details, see text.
12
13
The latter concept is currently the most plausible scheme, although it
is still a matter of dispute whether this mechanism represents the
exclusive route to gallic acid. First conclusive results on the old enigma
of gallic acid biosynthesis were obtained by feeding [
13
C]glucose to the
fungus
Phycomyces blakesleeanus
and the dicotyledonous plant
Rhus
typhina
(staghorn sumac). NMR spectroscopy of the isotope distribution
of gallic acid and aromatic amino acids isolated from these species
