Biology Reference
In-Depth Information
OH
HO
HO
HO
HO
HO
HO
OH
OH
OH
OH
HO
HO
HO
O
O
O
O
HO
HO
O
O
OH
O
O
OH
O
O
HO
HO
O
O
HO
O
2
(air)
O
O
O
O
O
OH
O
O
HO
HO
O
H
2
O, 60°C
O
O
O
O
HO
HO
OH
OH
HO
OH
HO
HO
OH
OH
HO
HO
HO
Fig. 9.16 Oxidative hemisynthesis of mongolicain A (
18
) form acutissimin A (
14
) in an
aqueous solution (isolated yield).
14
: acutissimin A
18
: mongolicain A (22%)
The construction of the characteristic
spiro
-linked dihydrofuran-
cyclopentenone motif of mongolicain A (
18
) is the result of a multi-step
oxidative process starting with dehydrogenation of the NHTP galloyl-
derived I-ring of the vescalagin part of acutissimin A (
14
), as depicted by
Tanaka in Fig. 4.19 of Chapter 4. In the presence of oxygen, autoxidation
can mediate this dehydrogenation into the α-hydroxy-
ortho
-quinone
41
,
with concomitant formation of hydrogen peroxide (Fig. 9.17). The
phenolic 7-OH group of the A-ring of the catechin-derived part of the
molecule would then attack this
ortho
-quinone in a 1,6-addition manner
to furnish
42
. Addition of water to its diketone tautomer
43
could give
rise to the hydrate
44
that can then undergo a ring contraction via a
benzylic acid-type rearrangement to furnish the carboxylic acid
45
.
Decarboxylation of its ketone tautomer
46
can lead to the enediol
47
,
which would require a final dehydrogenative oxidation into the
cyclopentane-1,2-dione
48
to lead to the thermodynamically more stable
cyclopentenone unit of mongolicain A (
18
).
This complex succession of events constitutes a plausible but
admittedly putative mechanistic description of the generation of
18
from
14
under autoxidation conditions, and one might wonder why the
galloyl-derived I-ring of acutissimin A (
14
) is the only pyrogallol unit
thus sucumbing to dehydrogenative oxidation.
