Sesquiterpenolides from Inula racemosa and their chemical transformations

Article abstract of DOI:10.14233/ajchem.2017.20254

Inula racemosa roots are rich source of sesquiterpene lactones. Soxhlet extraction of roots using chloroform followed by column chromatography on silver nitrate impregnated silica gel led to the isolation of two sesquiterpene lactones namely alantolactone

Full text of DOI:10.14233/ajchem.2017.20254

Asian Journal of Chemistry; Vol. 29, No. 3 (2017), 568-570
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2017.20254
Sesquiterpenolides from Inula racemosa and their Chemical Transformations
*
RAMANDEEP KAUR, K.K. CHAHAL and URVASHI BHARDWAJ
Department of Chemistry, Punjab Agricultural University Ludhiana-141 004, India
*
Corresponding author: E-mail: drkkchahal@pau.edu
Received: 18 August 2016; Accepted: 28 October 2016;
Published online: 30 December 2016;
AJC-18201
Inula racemosa roots are rich source of sesquiterpene lactones. Soxhlet extraction of roots using chloroform followed by column
chromatography on silver nitrate impregnated silica gel led to the isolation of two sesquiterpene lactones namely alantolactone and
isoalantolactone. These sesquiterpene lactones were subjected to chemical transformations using different reagents (Mg/methanol, CHBr
3
/
KOH, LiAl[OC(CH and NaBH ) to afford modifications on their α-methylene-γ-lactone moiety. The structures of isolated compounds
3
)
3
]
3
4
and their transformation products were elucidated using spectroscopic techniques.
Keywords: Inula racemosa, Sesquiterpene lactones, Alantolactone, Isoalantolactone.
1
3
(
in CDCl ) using TMS as an internal reference. FTIR, C NMR
3
INTRODUCTION
1
and H NMR spectroscopic analysis was recorded at Central
Instrumentation Laboratories (CIL), Panjab University,
Chandigarh. The chemical shifts are expressed in δ (ppm)
values.
Sesquiterpenelactones a class of chemical compounds
made up of three isoprene units, containing lactone ring are
the main compounds present in wide varieties of plants. Inula
racemosa commonly known as Pushkarmula is a perennial
herb with a range of medicinal properties [1]. It is distributed
all over East Asia, Europe and Africa and in India this herb
has its distribution from temperate to alpine belts of Kashmir
and Himachal Pradesh. The chemical profiling of the roots of
this herb showed the presence of eudesmanolide group of
sesquiterpenelactones. The different sesquiterpene lactones
present in this herb are: alantolactone, isoalantolactone, inunal,
alloalantolactone, isoalloalantolactone and inunolide etc. [2-
Soxhlet extraction of powdered roots of I. racemosa:
The roots (250 g) of I. racemosa dried, powdered and subjected
to Soxhlet extraction using chloroform (1 L) as the solvent.
The chloroform extract was distilled to yield yellow oil (5 g).
Preparation of silver nitrate impregnated silica gel:
To acetonitrile (20 mL) was added silver nitrate (20 g) and
silica gel (200 g) in small proportions and mixed thoroughly.
Excess of acetonitrile was evaporated and silica gel impreg-
nated with silver nitrate was filled in the brown bottle and kept
in oven for 24 h for activation. The impregnated silica gel was
cooled and used further.
5
] along with sitisterol, daucosterol, inunolide, aplotexene and
phenylacetonitrite [6].The nor-eudesmane-type sesquiter-
penoids were also reported [7]. The α,β-unsaturated γ-lactone
is the most common group in sesquiterpene lactones. The iso-
lation of two isomeric lactones-alantolactone and isoalanto-
lactone from roots of I. racemosa and their chemical trans-
formations on exocyclic methylene α,β-unsaturated γ-lactone
moiety are reported.
Isolation of pure compounds: Extract of two batches
10 g) was subjected to column chromatography using silver
(
nitrate impregnated silica gel. The column was eluted with
solvents of increasing polarity (hexane:dicholoromethane) and
alantolactone (1) having melting point 78 °C was obtained in
hexane fraction and isoalantolactone (2) having m.p. 108 °C
in 5 % hexane: dichloromethane. The structures of the com-
pounds were confirmed using spectroscopic data (Table-1).
General procedure for reaction of lactones (alantolactone
and isoalantolactone) with Mg/methanol: To dry methanol
(50 mL), alantolactone (0.5 g) and active Mg turnings (0.5 g)
were added with constant stirring under dry conditions. A mild
exothermic reaction was observed after 3 h with the evolution of
EXPERIMENTAL
Melting points reported were determined in open capillaries
and are uncorrected. FT-IR spectra were measured in CHCl
solution or nujol on a Perkin Elmer, Model RX-1 FT-IR spectr-
3
1
13
ophotometer. H NMR and C NMR spectra were recorded
with BrukerAC (400 MHz) or mentioned otherwise as solutions

Vol. 29, No. 3 (2017)
Sesquiterpenolides from Inula racemosa and their Chemical Transformations 569
TABLE-1
SPECTROSCOPIC DATA OF ALANTOLACTONE (1), ISOALANTOLACTONE (2) AND THEIR DERIVATIVES (3-8)
-
1
1
13
Structure
IR (cm )
H NMR (δ)
C NMR (δ)
1
5
O
1
9
1.09 (d, 3H, J = 7.6 Hz, C -H ), 1.19 (s, 39.46 (C ), 22.56 (C ), 37.56 (C ),
14
s
1
2
3
2
3
10
8
1
2
1
O
3H, C -H ), 3.57-3.61 (m, 1H, C -H),
32.86 (C ), 148.94 (C ), 118.83 (C ),
4 5 6
15
s
7
5
7
11
6
1745, 1665, 1462, 1375, 4.81-4.84 (m, 1H, C
8
-H), 5.16 (d, 1H, J
41.70 (C ), 76.45 (C ), 42.63 (C ),
7 8 9
4
1262, 890 and 810
= 4.12 Hz, C -H), 6.19 (d, 1H, J = 1.87
32.69 (C ), 139.84 (C ), 170.45
6
10
11
Hz, C -H), 5.63 (d, 1H, J = 1.72 Hz,
(C ), 121.63 (C ), 2 16.74 (C ),
13
12
13
14
3
1
4
C -H)
and 28.58 (C15).
1
3
(1)
1
5
0
.82 (s, 3H, C -H ), 2.95-3.01 (m,
15 s
O
1
9
36.81 (C ), 17.68 (C ), 40.52 (C ),
1 2 3
2
10
8
7
1H, C -H), 4.44 (d, 1H, J = 1.4 Hz,
7
1
11
2
O
148.98 (C ), 46.15 (C ), 22.69 (C ),
4
5
6
3
5
1760, 1648, 1470,
262, 890, 880 and
15
C
14–H) and 4.77 (d, 1H, J = 1.52
6
41.35 (C
7
), 76.85 (C
8
), 42.18 (C
),
9
4
1
Hz, C –H), 4.48-4.51 (m, 1H, C -
1
4
8
3
2.28 (C ), 142.22 (C ), 170.68
12 13 14
10
11
8
H), 5.59 (d, 1H, J = 1.52 Hz, C –
13
H) and 6.13 (d, 1H, J = 1.52 Hz,
(
C ), 120.08 (C ), 106.41 (C )and
1
3
1
4
2
7.48 (C ).
15
C –H).
1
3
(2)
1
1
.04 (d, 3H, J = 3.84 Hz, C -Hs),
14
O
38.75 (C ), 22.67 (C ), 37.89 (C ),
1' 2 3
.15 (s, 3H, C -Hs), 2.91- 2.95 (m,
1
5
O
32.90 (C
4
), 148.90 (C ), 119.96 (C ),
5 6
1
1
750, 1660, 1450, 1370, 1 H, C -H), 3.29 (s, 3 H, -OCH ),
7
3
4
3
2.85 (C ), 76.74 (C ), 42.00 (C ),
7
8
9
262, 1138, 1109, 890
3.57 (dd, J = 9.16 Hz, J = 3.57 Hz,
C13-H), 3.67 (dd, J = 9.16 Hz, J =
2.72 (C ), 50.43 (C ), 177.94
10
11
and 810
(
(
C ), 72.43 (C ), 16.87 (C ), 28.86
C ) and 59.21 (C ).
12 13 14
OCH3
5
.12 Hz, C13-H), 4.79-4.82 (m, 1H,
15
16
(3)
C -H), 5.10 (d, 1H, J = 3.64, C -H.
8
6
0
1
.80 (s, 3H, C -H ), 2.93-2.95 (m,
15 s
O
H, C -H), 3.46 (dd, J =10.12 Hz, J 37.21 (C
), 19.65 (C
), 40.52 (C ),
2 3
7
1
O
= 3.2Hz, C13-H), 3.62 (dd, J
149.24 (C ), 47.45 (C ), 23.62 (C ),
4
5
6
1
770, 1660, 1470,
=
4
4
4
10.12 Hz, J = 6.12 Hz, C13-H),
40.25 (C ), 76.25 (C ), 41.26 (C ),
7 8 9
1
380, 1140, 1110, 895
and 730
.47 (d, 1H, J = 1.4 Hz, C –H) and 33.75 (C ), 44.78 (C ), 170.59
1
4
10
11
.73 (d, 1H, J = 1.52 Hz, C –H),
(C ), 75.30 (C ), 106.41 (C )
12 13 14
1
4
OCH3
.45 (m, 1H, C -H), 3.27 (s, 3 H, -
27.48 (C ) and 59.30 (C ).
8
15 16
(
4)
5)
OCH ).
3
O
1.05 (D, 3H, J = 7.6, C14-Hs), 1.19
40.64 (C1'), 22.83 (C ), 38.10 (C ),
2 3
(
s, 3H, C -Hs), 1.92 and 2.18 (D,
32.95 (C ), 151.38 (C ), 115.29 (C ),
15
4
5
6
O
1
767, 1640, 890 and
88
1H each, J = 8 Hz, C -Hs), 3.16-
41.26 (C ), 76.79 (C ), 42.02 (C ),
7 8 9
1
3
Br
6
3.18 (m, 1H, C -H), 4.95-4.98 (m,
32.83 (C ), 42.65 (C ), 171.84
7
10 11
1
H, C -H), 4.91 (d, 1H, J = 3.52
(C ), 27.27 (C ), 16.80 (C ), 28.54
12 13 14
(C ) and 29.66 (C ).
15 16
8
Br
Hz, C -H).
6
(
O
41.23 (C1'), 22.64 (C
2
), 36.70 (C
5
3
),
6
0
.86 (s, 3H, C -Hs), 1.6-1.8 (m,
14
1
48.85 (C ), 46.43 (C ), 23.74 (C ),
4
O
Br
2H, C -Hs), 2.59-2.64 (m, 1H, C -
H), 4.80-4.82 (m, 1H, C
1
3
7
1
762, 1643, 890 and
50
42.77 (C ), 76.73 (C ), 42.09 (C ),
7 8 9
-H), 4.79
8
6
25.67 (C ), 42.68 (C ), 172.11
10 11
(
d, 1H, C -H, J = 1.28), 4.45 (d,
15
(
1
C ), 34.64 (C ), 17.85 (C ),
06.69 (C ) and 28.97 (C ).
15 16
12 13 14
Br
1H, C15-H, J = 1.32).
(
6)
7)
1
1
.24 (d, 3H, J = 7.12 Hz, C -Hs),
38.76 (C ), 23.07 (C ), 38.52 (C ),
1' 2 3
O
13
.26 (s, 3H, C -Hs), 1.14 (d, 1H, J
33.05 (C ), 150.61 (C ), 115.56 (C ),
1
5
4 5 6
O
1
728, 1599, 1456 and
90
= 7.64 Hz, C1 -Hs), 2.40-2.54 (m,
1H, C11-H), 3.02-3.07 (m, 1H, C7-
42.92 (C ), 76.98 (C ), 40.38 (C ),
7 8 9
4
8
32.91 (C ), 42.25 (C ), 179.33
10 11
Hs), 4.74-4.77 (m, 1H, C -H), 5.18
(C ), 10.73 (C ), 16.88 (C ) and
28.75 (C15).
8
12
13
14
(
d, 1H, J = 3.24, C -H).
(
6
O
40.50 (C1'), 22.69 (C
2
), 36.80 (C
5
),
3
0
.83 (s, 3H, C -Hs), 1.22 (d, 3H, J
15
1
4
2
48.96 (C ), 46.50 (C ), 34.82 (C ),
4
6
O
= 7.16, C -Hs), 2.79-2.82 (m, 1H,
C -H), 2.95-3.01 (m, 1H, C -H),
11 7
4
2
13
2.24 (C ), 76.84 (C ), 41.75 (C ),
7
8
9
1
756, 1640 and 889
7.47 (C ), 41.57 (C ), 179.46
10
11
.44-4.51 (m1H, C -H), 4.77 (bs,
8
(
C ), 9.29 (C ), 106.59 (C ) and
12 13 14
H, C1 -Hs).
4
1
7.79 (C )
15
(8)
hydrogen gas. Unreacted metal was then separated and the
mixture was added to dilute HCl (150 mL) to get clear solution.
The pH was adjusted around 8-9 by adding ammonia solution.
The reaction mixture was then diluted with water and extracted
with dichloromethane (4 × 20 mL). The organic layer was
dried (anhydrous sodium sulfate) and evaporation of solvent
under vacuum afforded a mixture (0.44 g) which on chroma-
tography over silica gel yielded pure compound 13-methoxy-
dihydroalantolactone (3, 0.34 g) with m.p. 82 °C. Isoalanto-
lactonewas subjected to reaction using same method to yield
13-methoxydihydroisoalantolactone (4, 1.6 g) with m.p.
91 °C.

570 Kaur et al.
Asian J. Chem.
General procedure for reaction of lactones (alanto-
lactone and isoalantolactone) with CHBr : CHBr (15 mL)
(7, 1.8 g). Isoalantolactone give dihydroisoalantolactone (8,
1.9 g) under similar reaction conditions.
3
3
was treated with aqueous KOH (10 mL, 50 %) and phase transfer
catalyst (TEBAC,100 mg), stirred and treated with alanto-
lactone (2,1.0 g). Stirring was continued at room temperature
for 5 h. The reaction mixture was diluted with water and extrac-
ted with ethyl acetate (3× 50 mL) and dried over anhydrous
sodium sulfate. Solvent was removed under vacuum. The crys-
talline pure compound (5, 0.9 g) was obtained having m.p.
RESULTS AND DISCUSSION
The isolation of two isomeric lactones-alantolactone and
isoalantolactone was carried out using silver nitrate impreg-
nated silica gel and the chemical modifications were carried
out with alantolactone and isoalantolactone. Reactions were
carried out using different reagents such as Mg/methanol,
5
8 °C. Isoalantolactone on reaction with bromoform (CHBr )
3
under similar reaction conditions afforded a product (6, 0.8 g)
with m.p. 149 °C.
CHBr
3
/KOH in presence of phase transfer catalyst-TEBAC
and NaBH
(triethylbutylammonium chloride), LiAl[OC(CH
3
)
3 3
]
4
.
General procedure for reaction of lactones (alanto-
The products obtained on reaction with Mg/methanol are 13-
methoxydihydro derivatives (3 and 4) formed via Michael
lactone and isoalantolactone) with NaBH
4
: Alantolactone
(1, 1 g) was dissolved in methanol (20.0 mL) in round bottomed
addition of CH
3
OH in the presence of base (Mg). The reaction
flask. To this, sodium borohydride (1:2 molar ratio) was added.
The reaction mixture was stirred for 25 min. The extent of
completion of the reaction was monitored (TLC).After comp-
letion of reaction few drops of acetic acid were added and the
reaction mixture was extracted with water and then with
dichloromethane (3 × 50 mL). The organic layer was dried
with CHBr /KOH in the presence of phase transfer catalyst
resulted into dibromocarbene derivatives (5 and 6) of alanto-
lactone and isoalantolactone. The use of strong but bulky reducing
3
agent LiAl[OC(CH
3
)
3
] resulted into selective reduction of
3
conjugated double bond yielding dihydroderivatives (7 and
8) of alantolactone and isoalantolactone, respectively. These
products were similar to those obtained with sodium boro-
hydride but the yield was more although the amount of reagent
(anhydrous sodium sulfate) and excess of solvent was distilled
off to get pure crystals of compound, which was identified as
dihydroalantolactone (7, 0.7 g) with m.p. 85 °C. When similar
reaction was carried out using isoalantolactone dihydro-
isoalantolactone (8, 0.8 g) was obtained as a product with m.p.
used was less in case of LiAl[OC(CH
3
) ] . The spectroscopic
3
3
data of all the compounds is presented in Table-1.
1
72 °C.
General procedure for reaction of lactones (alanto-
lactone and isoalantolactone) with LiAl[OC(CH
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