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sesquiterpenes had been isolated from the aerial part of A. annua and briefly sum-
marized by Zaman and Sharma.
4
Four additional sesquiterpenes include the
b-epoxy isomer of arteannuin B (24),
46
6-epi-deoxyarteannuin B (25),
47,48
11,13-
dehydro-qinghaosu artemisitene (26),
49
and 6,7-dehydro-artemisinic acid (27).
47
Qinghaosu (1) can be classified as a cadinane sesquiterpene oxygenated at the
12-position; the other 15 cadinanae sesquiterpenes share this common structural fea-
ture (see Table 5-1, Nos. 1-16). A new sesquiterpenes called artemisinin G (28) was
purified from A. annua shortly after the Zaman and Sharma brief review by Chinese
scientists in 1992.
50
Because the authors found that compound 28 was a decompo-
sition product of qinghaosu by heating at 190
C for 10 min, or in refluxing xylene
for 22 h, the possibility that artemisinin G (28) could be an artifact during the iso-
lation procedure was eliminated through a heating experiment with qinghaosu that
mimicked the isolation process. A new member of the unusual cadinanolide series
of sesquiterpenes, annulide (29), was described in 1993 with its structure and
relative configuration determined by nuclear magnetic resonance (NMR) results,
although the sample was limitedly purified.
51
Meanwhile the closely related struc-
ture isoannulide (30) was also described in the same article with poor purity; the
complete and unambiguous NMR spectral assignments for isoannulide (30) were
presented in a later article by the same author.
52
Three other structural relatives
of artemisinin B (10), 31,
53
32,
54
and 6a-hydroxyisoannulide (33),
55
were recorded
in 1987, 1992, and 1994, respectively. Known as an acid hydrolysis product of arte-
misinin B (arteannuin B, 10) based on an earlier synthetic research, compound 33
was checked by thin-layer chromatography (TLC) analysis of the crude extract and
proved to be the endogenetic natural product from A. annua other than an artifact
during purification. A bisnor-sesquiterpene, norannuic acid (34) was reported in
1993,
56
and three new cadinane sesquitepenes (35-37) were isolated and reported
in 1994, both by Ahmad and Misra.
57
Compound 37 is of interest in that a
3-isobutyryl group was discovered for the first time in cadinane sesquiterpenes
from A. annua. Brown described compound 38 (a pair of isomers), 39 in 1994,
58
and Sy and Brown described another new cadinane sesquitepene (40) in addition
to a new eudesmane sesquiterpene (41) from the aerial parts of A. annua in
1998.
59
Compound 40 is unique in that it is oxygenated at the 7-position rather
than at the 12-position found in most other cadinanes isolated from this species.
Compound 41 is a 5a-hydroxyeudesmane incorporating an allylic tertiary hydro-
xide group evidenced by the results of HREIMS and 1D/2D-NMR, which is also
a 5-hydroxy derivative of trans-b-selinene previously identified from A. annua
growing in the United Kingdom.
51
Seven new sesquiterpenes, including a peroxy-
lactone arteannuin H (42) and arteannuin I - M (43-48), were isolated by the same
research group in 1998.
60
Meanwhile they also proposed that these compounds
were biogenetically related to dihydroartemisinic acid (23) via some intermediate
allylic hydroperoxide such as compound 49, which was eventually isolated as a nat-
ural product in 1999 and proved not to be an artifact of isolation.
61
A reinvestiga-
tion of A. annua gave a novel cadinane diol, arteannuin O (50); its structure was
established by two-dimensional (2D) NMR and x-ray crystallography.
62
Synthesis
of arteannuin O (50) from dihydro-epi-deoxyarteannuin B (22) led the authors to
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