L. Wu et al. / Tetrahedron Letters 56 (2015) 229–232
231
Scheme 2. Biomimetic Synthesis of compounds 1 and 2.
properties, and the same molecular formula, suggested that 2 was
available phloroglucinol (3± and myrcene (9± as substrate
(Scheme 2±. The key intermediate isobutylidene syncarpic acid
(8± was arisen from phloroglucinol (3± through acetylation, meth-
0
an isomer of 1. The obvious difference of the chemical shift for C-1 ,
0
0
0
C-2 , and C-3 between 1 and 2 was observed. The signal for C-1
0
was upfield shifted from d
115.4 downfield shifted to d
upfield shifted to d
C
31.5 in 1 to d
C
24.1 in 2, for C-2 from
ylation, an acid induced retro-Claisen condensation and Knoevena-
0
13–15
d
C
C
119.3 and for C-3 from d
C
138.4
gel condensation.
Since 8 was prone to isomerization, it was
16
C
133.7. Above similarities and differences indi-
immediately reacted with the myrcene (9± in toluene at 110 °C.
cated that the isobutyl syncarpic acid conjugated with myrcene in
On the basis of electronic effect and steric effect, the C-7 in isobu-
0
different modes in 2. In the HMBC spectrum, H-4
H
a (d 2.49± had
tylidene syncarpic acid as electron-withdrawing, is prone to react
0
correlations with C-1 (d
C
208.9± and C-6 (d
C
68.8± in 2 instead of
2.21± had cross peaks with C-8 (d
H
2.23± in 1, which indicated that the
with the C-4 in myrcene as electron-donating. After separation
0
0
H-1
a
(d
H
2.48± in 1, H-1 (d
H
C
and purification, two target products were obtained with a ratio
of 6:1 and 35% yield successfully, whose structures were deter-
mined to be 1 and 2 by identical consistent t and UV in HPLC,
R
0
2
9.7± in 2 instead of H-4 (d
cyclohexene ring in 2 formed between d
more, this deduction was also confirmed by the key HMBC correla-
0 0 0 0
4 (3 ± 2 (1 ±
6(7±
and d
,
. Further-
HRESIMS, and 1D NMR. Compounds 10 and 11, two main by-prod-
ucts originated from the isomerization of 8, were also obtained in
about 45% yield, which lead to the low chemical yield of com-
pounds 1 and 2. In conclusion, our study provides strong evidence
for the proposed biosynthetic pathways of these two natural
products.
Compounds 1 and 2 were screened for inhibition of NO produc-
tion. MTT assay revealed no obvious cytotoxic effects (over 90% cell
survival± of cells treated with the compound at concentrations up
0
0
0
tions of H-5 (d
C-8 (d
and H-1 (d
H
1.98± with C-4 (d
29.7±, and H- H COSY correlation between H-2 (d
C
31.3±, H-1 (d
H
2.21, 1.98± with
1
1
0
C
H
5.35±
0
H
2.21, 1.98±. Thus, the planar structure of 2 was dem-
onstrated as shown in Figure 1, which also possessed a spiro-
5.5] undecene skeleton formed between C-6 and C-7 of isobutyl
[
0
0
syncarpic acid and C-4 and C-1 of myrcene moiety. The same
ROESY correlations with 1 and zero of optical rotation indicated
that 2 was also a racemic mixture of enantiomers and had the same
relative configuration as that of 1. Thus, the structure of 2 was
established as shown in Figure 1.
to 100
vated RAW 264.0 cells, with an IC50 value of 29.9 and 28.8
respectively, comparable to that of the positive control N-mono-
lM. Compounds 1 and 2 showed NO inhibition on LPS-acti-
lM,
The plausible biosynthetic pathway for 1 and 2 was depicted in
Scheme 1. According to the hypothesis, 1 and 2 were derived from
isobutylidene syncarpic acid and myrcene via a [4+2]-cycloaddi-
tion under the catalysis of enzyme or UV.8 In turn, isobutylidene
1
7
methyl-L-arginine at 40.45 lM. The result suggests that this kind
of skeleton may be a potential candidate for anti-inflammatory
agent.
9
,10
syncarpic acid could be generated from flavesone (b-triketones±
via reduction and dehydration. Notably, isobutylidene syncarpic
Acknowledgments
11
acid was prone to transformed into G
3
,
which was termed as
G-regulators exercise control over a wide range of physiological
functions in plants. In view of the fact that both 1 and 2 were iso-
lated as racemic mixture with the stereogenic center at C-7, the C–
C bond formation should be an enzyme-catalyzed without stere-
oselectivity process.
This research work was financially supported by the National
Natural Sciences Foundation of China (21272275, 81202899±, the
Youth Fund Project of Basic Research Program of Jiangsu Province
(Natural Science Foundation, BK2012351±, the Program for New
Century Excellent Talents in University (NCET-2013-1035±, the
Program for Changjiang Scholars and Innovative Research Team
in University (PCSIRT-IRT1193±, and the Project Funded by the Pri-
ority Academic Program Development of Jiangsu Higher Education
Institutions (PAPD±.
However, the different structures and ratios of 1 and 2 could be
attributed to the regioselectivity in [4+2]-cycloaddition. Isobuty-
lidene syncarpic acid possessed
a,b-unsaturated carbonyl moiety,
with open-chain carbon-dienes, are generally highly ortho–para
regioselective because the oxygen complexation increases the dif-
1
2
ference of LUMO coefficients of the alkene moiety. Myrcene bear-
Supplementary data
0
0
ing a EDG at C3 has its largest HOMO coefficient at C4 , while
isobutylidene syncarpic acid has the largest LUMO coefficient at
C7. Paring these two coefficients gives 1 as a major regioisomer.
To validate above biosynthetic hypothesis, a biomimetic syn-
thesis of compounds 1 and 2 was conducted with commercially
Supplementary data (detailed description of the experimental
procedures, copies of the HR-ESI-MS, 1D and 2D NMR spectra of
1