Interesting transformations of naturally occurring cassane diterpenoids and bioevaluation of the products

Article abstract of DOI:10.1016/j.tetlet.2012.05.032

Two cassane diterpenoids, pulcharrin G (1) and 6β-cinnamoyl-7β- hydroxy-voucapen-5-α-ol (2), the constituents of Caesalpinia pulcherrima, were treated with BF₃·OEt₂ to furnish two olefinic products 3 and 4, respectively. The products were formed by elimination of water and migration of a methyl group from C-4 to C-5. The cytotoxic and antimicrobial activities of 3 and 4 were examined.

Full text of DOI:10.1016/j.tetlet.2012.05.032

Tetrahedron Letters 53 (2012) 3850–3852
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Interesting transformations of naturally occurring cassane diterpenoids
and bioevaluation of the products q
Chithaluri Sudhakar a, Parigi Raghavendar Reddy a, Tuniki Venugopal Raju b, Biswanath Das a,
⇑
a Organic Chemistry Division–I, Indian Institute of Chemical Technology (CSIR), Hyderabad 500007, India
b NMR Division, Indian Institute of Chemical Technology (CSIR), Hyderabad 500007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 February 2012
Revised 2 May 2012
Accepted 5 May 2012
Available online 19 May 2012
Two cassane diterpenoids, pulcharrin G (1) and 6b-cinnamoyl-7b-hydroxy-voucapen-5-a-ol (2), the
constituents of Caesalpinia pulcherrima, were treated with BF3ꢀOEt2 to furnish two olefinic products 3
and 4, respectively. The products were formed by elimination of water and migration of a methyl group
from C-4 to C-5. The cytotoxic and antimicrobial activities of 3 and 4 were examined.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Cassane diterpenoids
Chemical modification
BF3ꢀOEt2
Cytotoxicity
Antimicrobial activity
The plants of the genus Caesalpinia (Leguminosae) are evergreen
deciduous trees or shrubs grown in tropical and subtropical coun-
tries. The metabolites of the plants are mainly diterpenoids and
flavanoids.1 In continuation of our work2 on the chemical modifica-
tion of natural products we have considered two known cassane
diterpenoids, pulcharrin G (1) and 6b-cinnamoyl-7b-hydroxy-vou-
spectra) and the protons of Me-20 to C-4 (d 137.3 (for 3) and
136.9 (for 4) in the 13C NMR spectra). The stereo-configuration of
Me-20 was decided to be b from the NOESY experiment which
showed its intense correlation with Me-18 and weak correlation
with H-8 (Fig. 1).
The mechanism of the present conversion could be explained by
BF3-mediated water elimination with concomitant migration of
Me-20 from C-4 to C-5 (Scheme 2).
capen-5-a
-ol (2), the constituents of Caesalpinia pulcherrima3 for
their transformations using BF3ꢀOEt2. Herein we report the results
of these transformations and biological (cytotoxic and antimicro-
bial) activities of the products.
It is important to mention here that this transformation is
somewhat related to Westphalen-Lettre’ rearrangement of
Diterpenoids 1 and 2 were separately treated with BF3ꢀOEt2 in
CH2Cl2 at room temperature for 15 min and the products 3 and 4
were formed, respectively (Scheme 1).
cholestane-3b, 5a, 6b-triol diacetate with acetic anhydride in the
16
15
16
15
O
The structures of the products were established from their 1H
and 13C NMR spectral data (Table 1) assigned in assistance with
the help of 2D NMR (1H–1H COSY, NOESY, HSQC, and HMBC) and
DEPT experiments. The spectral data suggested that both the com-
pounds 3 and 4 contained a double bond at C-3 to C-4 and only one
methyl group at C-4 while another methyl group at C-5. The 1H
NMR showed that H-3 of 3 and 4 appeared at d 5.41 and 5.43,
respectively (Table 1). The methyl groups, Me-19 and Me-20 of 3
resonated at d 2.07 and 1.25, respectively while of 4 at d 1.98
and 1.15, respectively.
O
11
11
18
1
1
H
14
H
14
9
9
BF3.OEt2
2
3
2
17
17
H
8
8
H
DCM, r.t., 15 min
5
7
1
5
7
4
6
4
3
6
2
2
OR
OR
OH
20
1
OR
OR
20 19
19
3 (68%)
4 (76%)
1, 2
'
1
2
1
CO
1 & 3. R
R
H
=
,
The HMBC experiment revealed that H-3 was connected to the
5
'
=
=
2
7
'
carbon of Me-19 (d 22.7 (for 3) and 23.0 (for 4) in the 13C NMR
'
'
2
2
1
'
,
R
H
=
1
2 & 4.
R
CO
q
3
Part 61 in the series, ‘Synthetic studies on natural products’.
'
⇑
Corresponding author. Tel./fax: +91 40 27160512.
E-mail address: biswanathdas@yahoo.com (B. Das).
Scheme 1. Transformations of cassane diterpenoids.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.05.032

C. Sudhakar et al. / Tetrahedron Letters 53 (2012) 3850–3852
3851
Table 1
NMR spectral data of compounds 3 and 4a
Position
Compound 3
Compound 4
Multiplicity (J in Hz)
m
1H NMR
Multiplicity (J in Hz)
13C NMR
28.3
1H NMR
13C NMR
1
2
1.80(a)
1.41(b)
1.98
m
1.75(a)
1.46(b)
2.02(a)
1.96(b)
5.43
—
—
5.34
3.67
1.93
2.35
—
2.57
—
—
3.15
6.21
7.23
1.05
1.09
1.98
1.15
—
6.51
7.75
28.1
22.0
m
22.1
m
3
4
5
6
7
8
9
5.41
—
—
3.92
5.25
2.25
2.46
—
m
—
—
124.4
137.3
46.9
76.0
76.8
40.9
31.0
39.0
24.03
149.0
121.4
27.9
109.5
140.7
17.1
17.8
22.7
21.4
168.1
129.9
129.8
128.4
133.2
128.4
129.9
—
m
—
—
d(7.0)
t(7.0)
m
m
—
m
—
—
m
d(2.0)
d(2.0)
d(7.0)
s
124.8
136.9
46.2
79.4
70.8
43.3
31.1
39.7
23.9
149.1
121.7
27.4
109.8
140.5
16.9
17.8
23.0
21.1
168.1
118.1
145.5
134.2
129.0
128.1
130.5
128.1
129.0
—
d(7.0)
t(7.0)
m
m
—
m
—
—
m
10
11
12
13
14
15
16
17
18
19
20
10
2.61
—
—
2.92
6.15
7.24
0.98
1.02
2.07
1.25
—
d(2.0)
d(2.0)
d(7.0)
s
s
s
—
s
s
—
20
—
—
d(16.0)
d(16.0)
—
m
m
m
m
m
—
30
8.09
7.48
7.59
7.48
8.09
—
—
1.58
—
dd(8.0, 2.0)
t(8.0)
brt(8.0)
t(8.0)
dd(8.0, 2.0)
—
40
—
50
7.58–7.53
7.43–7.38
7.43–7.38
7.43–7.38
7.58–7.53
—
60
70
80
90
—
br s
—
—
—
—
OH-6
OH-7
1.6
br s
—
a
The spectra were run in CDCl3 at 600 MHz (1H NMR) and 150 MHz (13C NMR).
O
16
O
O
18
18
15
OR1
20
OR1
11
BF3.OEt2
20
18
8
1
9
9
6
H
14
2
OR2
3
OR2
2
17
5
7
8
H
19
H
OH
F3B
5
19
7
4
3
6
OR
20
1
19
OR
Scheme 2. Mechanism of the present conversion.
1
CO
'
2
1
,
R
R
H
R
'
=
5
=
2
'
Table 2
Cytotoxic activity of compounds 3 and 4
7
'
2
'
2
Test cell line
IC50 (50% inhibitory concentration in lM)
1
'
,
H
R
=
1
CO
=
Compound 3
Compound 4
Doxorubicin (Control)
3
'
A-549
MDA-MB-231
MCF-7
HeLa
Neuro-2a
10.42
35.09
12.18
11.63
—
16.49
13.39
6.86
10.58
7.98
1
<1
1
<1
1
)and NOE correlations(
) of 3, 4
HMBC(
Figure 1. 2D NMR studies of the products 3 and 4.
presence of H2SO4.4 In this rearrangement of cholestane diacetate a
dehydration product with a double bond at C-9 to C-10 and a
methyl group at C-5 (resulting from the migration of Me-18 from
C-10) was observed.
The present transformation involves the conversion from
trans-decaline to cis-decaline type conformation: The product with
trans-decaline type conformation is unfavorable possibly due to
strain generated from the proximity of Me-19 and Me-20 which
are at the opposite direction in the product with cis-decaline
conformation. Thus in this transformation the conformationally
stable lower energy product is formed. In case of substrates 1
and 2 there are two 1,3-diaxial interactions (Me-18 to Me-20 and
Me-18 to OR1-6) out of which Me-Me interaction is severe. In the
formation of products 3 and 4 this interaction is relived.
Compounds 3 and 4 were evaluated for in vitro cytotoxicity
against five cancerous cell lines; A- 549 (human alveolar adenocar-
cinoma), MDA-MB-231 (human breast adenocarcinoma), MCF-7

3852
C. Sudhakar et al. / Tetrahedron Letters 53 (2012) 3850–3852
Table 3
Antimicrobial activity of compounds 3 and 4
Test pathogen
Minimum inhibitory concentration (MIC lg/ml)
Compound 3
Compound 4
Neomycin (Control)
Staphylococcus aureus MTCC 96
Bacillus subtilis MTCC 121
S. aureus MLS 16 MTCC 2940
Micrococcus luteus MTCC 2470
Klebsiella planticola MTCC 530
Escherichia coli MTCC 739
—
9.37
9.37
—
18.75
—
18.75
18.75
18.75
18.75
18.75
18.75
18.75
4.68
18.75
—
9.37
4.68
—
4.68
18.75
Pseudomonas arruginosa MTCC 2453
(human breast adenocarcinoma) Hela (human cervical cancer), and
Neuro-2a (mouse neuroblastoma). Doxorubicin was used as the
positive control. The MTT assay (according to the method of Mos-
mann5) was utilized to evaluate the activity. IC50 value with each
cell line was determined after four individual observations (Table
2).
The results showed that compound 3 possessed significant
cytotoxicity against the first four cell lines while compound 4
against all of the five cell lines. The latter exhibited higher activity
against MCF-4 and Neuro-2a cell lines.
The antimicrobial activity of compounds 3 and 4 was also tested
against several bacterial organisms: Staphylococcus aureus (MTCC
96), Bacillus subtilis (MTCC 121), S. aureus MLS16 (MTCC 2940),
Micrococcus luteus (MTCC 2470), Klebsiella planticola (MTCC 530),
Escherichia coli (MTCC 739), and Pseudomonas arruginosa (MTCC
2453) Neomycin was used as the positive control and microtiter
broth dilution method was applied to determine the activity.6
The minimum inhibitory concentration (MIC) value with each
pathogen was evaluated after four individual observations (Table
3). The results indicated that the antimicrobial activity of both
the compounds 3 and 4 is promising. They showed highest activity
against E-coli. Compound 3 also showed significant activity against
B. subtilis.
References and notes
1. (a) Patel, A. D.; Freyer, A. J.; Webl, R. L.; Zuber, G.; Reichwein, R.; Bean, M. F.;
Faucettle, L.; Johnson, R. K. Tetrahedron 1977, 53, 1583; (b) McPherson, D. D.;
Cordell, G. A.; Soejarto, D. D.; Pizzuto, J. M.; Fong, H. H. S. Phytochemistry 1983,
27, 2835; (c) McPherson, D. D.; Che, T. T.; Cordell, G. A.; Soejarto, D. D.; Pezzuto,
J. M.; Fong, H. H. S. Phytochemistry 1986, 25, 167; (d) Che, C. T.; McPherson, D. D.;
Cordell, G. A.; Fong, H. H. S. J. Nat. Prod. 1986, 49, 561; (e) Ragasa, C. Y.; Hofilena,
J. G.; Rideout, J. A. J. Nat. Prod. 2002, 65, 1107; (f) Srinivas, K. V. N. S.; Rao, Y. K.;
Mahender, I.; Das, B.; Krishna, K. V. S. R.; Kishore, K. H.; Murthy, U. S. N.
Phytochemistry 2003, 63, 789; (g) Kalauni, S. K.; Awale, S.; Tezuka, Y.; Banskola,
A. H.; Luin, T. Z.; Kadota, S. Chem. Pharm. Bull. 2005, 53, 214.
2. (a) Das, B.; Madhusudhan, P. Tetrahedron Lett. 1998, 39, 9099; (b) Das, B.;
Venkataiah, B.; Kashinatham, A. Tetrahedron 1999, 55, 6585; (c) Ramesh, G.;
Harikishore, K.; Murthy, V. S. N. ARKIVOC 2003, IX, 126; (d) Das, B.; Krishnaiah,
M.; Venkateshwarlu, K.; Das, R. Nat. Prod. Commun. 2006, 1, 225.
3. (a) McPherson, D. D.; Che, C.-T.; Cordell, G. A.; Soejarto, D. D.; Pezzuto, J. M.;
Fong, H. H. S. Phytochemistry 1986, 25, 167; (b) Yodsaoue, O.; Karalai, C.;
Ponglimanont, C.; Tewtrakul, S.; Chantrapromma Tetrahedron 2011, 67, 6838.
4. (a) Westphalen, T. Berichte 1915, 48, 1064; (b) Lettré, H.; Muller, I. Berichte 1937,
1947, 70; (c) Rodig, O. R.; Brown, P.; Zaffaroni, P. J. Org. Chem. 1961, 26, 2431.
5. Mosmann, T. J. Immunol. Methods 1983, 65, 55.
6. (a) Amsterdam, D. In Antibiotics in Laboratory Medicine; Lomen, V., Ed., 4th ed.;
Williams and Wilkins: Baltimore MD, 1996; p 52 e111; (b) Das, B.; Shinde, D. B.;
Kanth, B. S.; Kamle, A.; Kumar, C. G. Eur. J. Med. Chem. 2011, 46, 3124.
7. General experimental procedure: BF3-OEt2 (0.5 mmol) was added dropwise to a
solution of compound (1 or 2) in dry CH2Cl2 (25 mL) under nitrogen atmosphere.
The mixture was stirred at room temperature and the reaction was monitored by
TLC. After completion, the reaction mixture was quenched with distilled water
(5 mL) and the mixture was extracted with CH2Cl2 (3 ꢁ 10 mL). The combined
organic portions were washed with water (2 ꢁ 10 mL), dried over anhydrous
Na2SO4, and concentrated under vacuum. The crude product was subjected to
column chromatography to obtain a pure compound (3 or 4). The following
compounds (amounts and eluting solvents are given) were obtained according to
the increasing order of polarity: compound 3 (20 mg (66%), hexane–EtOAc, 78:22),
compound 4 (23 mg (76%), hexane–EtOAc, 72:28). Compound 3: White solid, mp
In conclusion, we have carried out BF3-mediated interesting
conversions of two natural cassane diterpenoids to generate com-
pounds with impressive cytotoxic and antimicrobial properties.7
Acknowledgements
95–97 °C, yield 66%, ½a D25
ꢂ
+26.8 (c 0.2, CHCl3), IR: 3448, 1721, 1636, 1456,
1238 cmꢃ1 1H and 13C NMR: Table 1; HREIMS: m/z 443.2187 [M+Na]+ (Calcd for
;
The authors thank CSIR and UGC, New Delhi for financial assis-
tance. They are also thankful to NMR, Mass, and IR Divisions of IICT
for spectral recording.
C
27H32O4Na: m/z 443.2198). Compound 4: White solid, mp 169–171 °C, yield 76%,
+18.4(c0.2,CHCl3);IR:3446,1724,1459,1280 cmꢃ1;1Hand13CNMR:Table1;
HREIMS: m/z 469.2369 [M+Na]+ (Calcd for C29H34O4Na :m/z 469.2382).
½ ꢂ
a 2D5