Isorosthornins A-C, new ent-kaurane diterpenoids from Isodon rosthornii
Rui ZHAN
0
1
Xue DU
1
Jia SU
1
Xiao-Nian LI
1
Wei-Guang WANG
1
Cheng-Qin LIANG
1
Jian-Hong YANG
1
Yan LI
1
Jian-Xin PU
1
Han-Dong SUN
1
0
Graduate University of Chinese Academy of Sciences
,
Beijing 100049, China
1
State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences
, Kunming 650201,
China
Three new ent-kauranoids, isorosthornins A-C (1-3), and a new natural product, dihydroponicidin (4), together with five known ones were isolated from the aerial parts of Isodon rosthornii. The structures were determined by means of extensive spectroscopic analysis. All diterpenoids isolated were evaluated for their cytotoxicity against HL-60, SMMC-7721, A-549, MCF-7, and SW480 cell lines, and compounds 5 and 7 showed significant inhibitory effects on all cell lines.
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The genus Isodon, which includes about 150 species, is an
important genus of the family Lamiaceae. It is widely spread
and abundant with diterpenoids which were reported to exhibit
antibacterial, anti-inflammatory, and anti-cancer activities.1
Over the past 30 years, more than 500 diterpenoids were
isolated and characterized from over 50 Isodon species.2
Isodon rosthornii (Diels) Hara, a perennial herb, is mainly
distributed in Sichuan and Guizhou Provinces of China.3 It has
been used to treat rheumatism and sore throat in folk tradition.3
Previous phytochemical investigations of this herb led to the
discovery of four new ent-kauranoids, rosthornins AD, with
antibacterial activity.4 Due to the secondary metabolites of the
genus Isodon often differ when grown in different ecological
environments, and I. rosthornii indigenous to Qionglai City of
Sichuan had not been studied on the secondary metabolites
before, we explored it with the intent of finding novel and
bioactive substances. This investigation led to the isolation of
three new ent-kaurane diterpenoids, isorothornins AC,
dihydroponicidin (4), along with five known ones. This work
will describe the isolation, structure determination, and
cytotoxic evaluation of these compounds.
Results and Discussion
The air-dried and powdered aerial parts of I. rosthornii was
extracted by 70% aqueous acetone, and then partitioned
between EtOAc and H2O. The EtOAc solubles were subjected
to silica gel, MCI CHP-20 gel, Sephadex LH-20, Lichroprep
*To whom correspondence should be addressed. E-mail:
;
RP-18 gel column chromatographies (CC), and semipreparative
HPLC to afford three new ent-kaurane diterpenoids,
isorothornins AC (13), a new natural product, dihydroponicidin
(4)5, and five known compounds. The known compounds were
identified as ponicidin (5)5, macrocalin B (6)6, xerophilusin B
(7)6, rabdonervosin A (8)7, rabdonervosin B (9)8. The structures
of the known compounds were determined by comparing
spectroscopic data with literature values.
Compound 1 was isolated as white amorphous powder. The
molecular formula was determined as C20H28O6 by HRESIMS
(quasi-molecular ion peak at m/z 387.1792 [M + Na]+),
indicating of seven degrees of unsaturation. The IR spectra of
1 showed absorptions at 3500, 1629 cm1, indicating the
existence of OH and C=C groups. The 13C NMR and DEPT
spectra showed typical signals of a
7,20:14,20-diepoxy-entkaurane: three methines (C-5, C-9, C-13); three quaternary
carbon (C-4, C-8, C-10); characteristic signals of a hemiketal
quaternary carbon C-7 (C 101.3) and two oxymethines C-20
(C 96.9) and C-14 (C 70.7) (Table 1). In the 13C NMR and
DEPT spectra, another three oxymethines and two olefinic
(one was a quaternarycarbon, the other was a methlyene)
carbons were observed. Detail analyses of 1H-1H COSY
5.54 (s); 5.28 (s)
1.37 (d, 13.5)
(Fig. 1), HSQC, HMBC (Fig. 1) spectra helped furnish the
planar structure of compound 1. HMBC correlations from
H20 (H 5.88) to C-7 (C 101.3) and C-14 (C 70.7) further
verified 1 to be a 7,20:14,20-diepoxy-ent-kaurane. The
HMBC correlations from H-5 (H 1.62) and H-9 (H 3.03) to
C-1 (C 72.6), H-5 (H 1.62) to C-6 (C 72.7), H2-17 (H 5.28
and 5.54) to C-15 (C 76.5) led to the assignment of OH
groups at C-1, C-6, C-15, respectively. Therefore, the gross
structure of 1 was determined as
1,6,7,15-tetrahydroxy7,20:14,20-diepoxy-ent-kaur-16-ene.
Fig. 1. 1H-1H COSY, selected HMBC and ROESY
correlations of 1.
In ROESY spectra, H-1 correlated to H-5 and H-9, H-6 to
Me-19, H-15 to H-13 and H-14, revealed H-1, H-6, H-15 to
be -,-,-oriented, respectively (Fig. 1). Thus, compound 1
was determined to be 1,6,7,15-tetrahydroxy-7,20:14,20
-diepoxy-ent-kaur-16-ene.
Compounds 2 and 3 had the same molecular formula
C20H30O6 on the basis of HRESIMS and NMR data which
required six degrees of unsaturation. Comparison of NMR data
of 2 and 3 with those of the analogue, ponicidin (5)5, showed
that 2 and 3 were similar to ponicidin, and the only difference
was that the ,-unsaturated ketone in ponicidin was reduced
in both compounds 2 and 3 (Tables 1 and 2). 1H-1H COSY,
HSQC, HMBC spectra suggested the same p (...truncated)