Chemistry Reference
In-Depth Information
5.2.2 The Phytochemistry of Qinghao and Other Natural
Products From Qinghao
After the discovery of qinghaosu (1) from qinghao (A. annua L.) in the early 1970s,
A. annua became one of the most extensively investigated plants thereafter. As the
intriguing chemotaxonomic marker in A. annua, qinghaosu (1) attracted intense
efforts initially devoted to the establishment of the highest content of qinghaosu
in A. annua and possibly other Artemisia plants. Hence, studies on the time course
of the levels of qinghaosu (1), its biosynthetic precursors, and the biosynthetically
related sesquiterpenes were conducted by several research groups around the
world.
31-35
According to the results, qinghaosu (1) is identified in all A. annua
plants from different geographical origins, whereas its content is varied drastically
with its growing area and stages of plant development. Qinghaosu (1) is present in
the leaves and flowers of A. annua in
0.01-1.1% of dry weight.
31,32,34
Production
of qinghaosu (1) from A. annua rarely exceeds 1.0% of the dry weight, with the
highest content just before flowering.
Apart from A. annua, qinghaosu (1) was detected in only one other Artemisia
species: Artemisia apiacea.
32
But the abundance was too low (0.08%) to justify
an isolation on a technical scale.
Since the discovery of qinghaosu, systematic phytochemical studies on A. annua
have been also conducted. Different A. annua materials including the leaves, stems/
flowers, roots, and seeds as well as the endophytes inside A. annua have been
employed for phytochemical investigations. Up to the time of this writing, more
than 150 natural products were reported to belong to different chemical structure
types. Herein, we try to give a summary of these secondary metabolites isolated
from A. annua to date.
5.2.2.1 Terpenoids from A. annua
As mentioned, qinghaosu chemically belongs to the cadinane sesquiterpene; there-
fore, the other sesquiterpene components in A. annua have been given preferential
attention. From indigenous A. annua L., continuous phytochemical studies by
Chinese researches in the early 1980s led to the excavation of another ten sesqui-
terpenes including deoxy-artemisinin (14),
36
artemisinin D (15),
37
artemisinin
F(16),
38
artemisinin E (17),
37
artemisinin A (18),
36,37
epoxyarteannuinic acid
(19),
39
artemisinic acid (20),
40,41
artemisinic acid methyl ester (21),
42
artemisinol
(22),
42
and arteannuin B (10).
36
Among them, arteannuin B (10) was reported in the
early 1970s.
28,29
They are all closely related to the amorphene series of sesquiter-
pene characterized by the presence of a cis-decalin skeleton with the isopropyl
group trans to the hydrogen on the ring juncture. From a biogenetic viewpoint, arte-
misinic acid (20) or its 11,13-dihydro analoge, dihydro-artemisinic acid (23), which
was isolated later from A. annua, are late precursors in the biogenesis of qinghaosu
(1).
43
The two compounds 20 and 23 were first reported by Chinese research-
ers,
40,43
and procedures for their isolation were also reported in the early 1980s
elsewhere.
44
In the late 1980s, another procedure for the isolation of artemisinic
acid (20) was described by Roth and Acton.
45
By 1991, 16 closely related
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