Novel routes for the generation of structurally diverse labdane diterpenes from andrographolide

Article abstract of DOI:10.1016/j.tetlet.2004.04.142

Andrographolide 1, the major constituent of the Indian medicinal plant Andrographis paniculata (Acanthaceae) was converted into the key intermediate 4 by selective oxidative degradation of the C-12,13 olefin bond. The aldehyde functional group present in 4 has been utilized for synthesizing a number of structurally diverse labdane diterpenes. Synthesis and in vitro cytotoxic activity results of the compounds prepared are discussed.

Full text of DOI:10.1016/j.tetlet.2004.04.142

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 4883–4886
Novel routes for the generation of structurally
diverse labdane diterpenes from andrographolide
Srinivas Nanduri,a,* Vijay Kumar Nyavanandi,a Siva Sanjeeva Rao Thunuguntla,a
Mahendar Velisoju,a Sridevi Kasu,a Sriram Rajagopal,b R. Ajaya Kumar,b
R. Rajagopalanb and Javed Iqbala
aDiscovery Chemistry, Dr. Reddy’s Laboratories Ltd., Discovery Research, Bollaram Road, Miyapur, Hyderabad 500 049, India
bDiscovery Biology, Dr. Reddy’s Laboratories Ltd., Discovery Research, Bollaram Road, Miyapur, Hyderabad 500 049, India
Received 16 March 2004; revised 14 April 2004; accepted 22 April 2004
Available online 14 May 2004
Abstract—Andrographolide 1, the major constituent of the Indian medicinal plant Andrographis paniculata (Acanthaceae) was
converted into the key intermediate 4 by selective oxidative degradation of the C-12,13 olefin bond. The aldehyde functional group
present in 4 has been utilized for synthesizing a number of structurally diverse labdane diterpenes. Synthesis and in vitro cytotoxic
activity results of the compounds prepared are discussed.
Ó 2004 Elsevier Ltd. All rights reserved.
The utilization of natural products as a source of struc-
tural or functional templates for the design and synthesis
of a wide variety of novel molecules has been an impor-
tant aspect of new drug design. The isolation of a number
of biologically active di- and triterpenes such as podo-
lactones,1 quassinoids,2 labdane diterpenes3 and com-
pounds of mixed biosynthetic pathways viz., wiedendiol4
has greatly stimulated the development of efficient syn-
thetic methodologies for their preparation owing to their
presence in only small quantities in the natural sources.
Several groups have utilized the naturally occurring
sclareol,5;6 sclareolide,7;8 communic acids9;10 and ma-
nool11 as starting materials for these syntheses. Most of
the methods used involved side-chain degradations of
labdanes. In this study, we have developed an efficient
semi-synthetic procedure for the preparation of struc-
turally diverse labdanes from andrographolide 1, the
major constituent of the Indian medicinal plant And-
rographis paniculata (Acanthaceae).12
The method involved conversion of 1 into the key
intermediate 4, which served as a chiral intermediate
(Scheme 1). Initially, the hydroxyl groups at C-3 and 19
of 1 were protected as an isopropylidene to yield 2. Our
O
HO
O
O
HO
14
12
12
CHO
O
O
O
iv
13
17
3
i
ii, iii
O
3
15
O
14
HO
HO
O
O
19
O
O
1
3
2
4
Scheme 1. Reagents and conditions: (i) 2,2-dimethoxypropane/PPTS/benzene/reflux, 95%; (ii) Ac2O/TEA/DCM/rt, 95%; (iii) DMAP/DCM/rt, 80%;
(iv) aq KMnO4/THF, rt, 62%.
Keywords: Andrographolide; Andrographis paniculata; Labdane diterpenes.
DRL Publication No. 285-A
* Corresponding author. Tel.: +91-40-23045439; fax: +91-40-23045438; e-mail: nandurisrinivas@drreddys.com
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.04.142

4884
S. Nanduri et al. / Tetrahedron Letters 45 (2004) 4883–4886
efforts to oxidize the C-12,13 olefin bond present in 2
with KMnO4 to yield 4 did not give satisfactory results.
However, quantitative preparation of 413 could be
achieved by oxidation of 3. Compound 3 was synthe-
S
S
O
S
NH
O
O
O
NH
NH
HO
HO
HO
HO
HO
6 (68%)
i, ii
HO
7 (60%)
5 (65%)
R1
R2
NH2
NH
N
CHO
O
R1; R2 =H
R
R1 =H; R2=Cl 10 (60%)
8 (60%)
iii, ii
Scheme IV
1 =F; R2 =H 9 (65%)
HO
HO
O
O
4
O
Scheme III
O
18
iv, v, vi
O
X
R3
R2
R1
NH
N
NH2
X
N
N
C
R1 R2
OH Cl
R3
H
13
16
O
HO
HO
O
Cl
H
17
OH OMe OMe 14
O
Scheme 2. Reagents and conditions: (i) rhodanine/thiazolidinedione/oxindole, b-alanine/benzene/reflux; (ii) AcOH/H2O (7:3), rt; (iii) Wittig salts of
substituted benzyl bromides/NaH/benzene/rt; (iv) BrCH2CO2Et/Zn/benzene/reflux, (70%); (v) PDC, DCM/rt, (90%); (vi) guanidinium chloride/
NaOMe/MeOH/reflux, (60%).
X
N
R3
R2
OH
Cl
CHO
X
O
R3
R2
i
O
O
ii)
LDA / -78oC
O
X
N
C
R2
Cl
R3
H
O
O
15 (80%)
4
O
11 (60%)
OMe OMe 12 (80%)
ii
X
N
R3
R2
Cl
O
OH
HO
HO
HO
HO
X
N
C
R2
Cl
R3
H
16 (80%)
13 (75%)
OMe OMe 14 (75%)
Scheme 3. Reagents and conditions: (i) PDC/DCM, (ii) AcOH/H2O (7:3).

S. Nanduri et al. / Tetrahedron Letters 45 (2004) 4883–4886
4885
sized from 2 by elimination of the hydroxyl at C-14 as its
acetate.
acetals of 11 and 12 gave compounds 13 and 14,
respectively.
The key intermediate 4 obtained above was converted
into a number of labdane diterpenes by utilizing its
aldehyde functionality (Scheme 2). Knoevenagel con-
densation of 4 with rhodanine, thiazolidinedione and
oxindole followed by deprotection of the C-3,19-iso-
propylidene protection gave compounds 5, 6 and 7,
respectively. Compounds 8, 9 and 10 were prepared by
Wittig olefination of 4 with the ylides generated from
various benzyl bromides.
Compound 17, which has an aminopyrimidone moiety
on the side chain was prepared by Reformatsky reaction
of 4 with ethyl bromoacetate followed by PDC oxida-
tion and condensation with guanidine (Scheme 2). The
corresponding homologue 18 was prepared as shown in
Scheme 4.
All the labdane derivatives synthesized from 4 were
evaluated for their in vitro cytotoxic activity against
nine human cancer cell lines using the NCI standard
protocol14 and the GI50 values obtained are given in
Table 1. Among the Knoevenagel condensation prod-
ucts of 4,compound 7 exhibited moderate cytototoxic
activity against all the cell lines. Compound 6 was found
Reaction of 4 with different aryllithiums yielded 11 and
12 (Scheme 3). Compound 11 was oxidized to the 12-
keto derivative 15 and subsequent removal of isopro-
pylidene protection gave 16. Deprotection of the cyclic
O
O
H
OH
O
OEt
CHO
OEt
i
ii
iii
O
O
O
O
4
O
O
NH2
NH
O
O
N
OEt
O
iv
O
O
O
18
O
Scheme 4. Reagents and conditions: (i) ethyl acetoacetate/TiCl4/py, (70%); (ii) NaBH4/MeOH (95%); (iii) PDC/DCM, (95%); (iv) guanidine
hydrochloride/NaOMe/MeOH/reflux (60%).
Table 1. In vitro anticancer screening results of the labdanes synthesized
Compd No
Cytotoxicity (GI50 lM)a
Breast
MCF-7/ADR
Colon
HT29
Lung
Melanoma
Ovarian
Prostate
PC3
MCF-7
SW 620
H 522
UACC62
OVCAR8
DU145
5
>100
52
27
>100
72.7
24
>100
66
35
>100
55
24
41
31
65
40
24
ndb
4
27
0.78
ndb
17
84
100
18
20
14
18
50
42
60
42
41
6
ndb
29
7
36
19
8
14
10
14
19
20
20
16
11
3
9
4
13
17
18
2
7
18
10
13
14
16
17
18
27
65
19
14
14
8
>100
>100
>100
>100
52
>100
>100
>100
>100
>100
>100
90
51
ndb
ndb
ndb
ndb
ndb
>100
>100
39
42
37
6.2
37
29
26
43.2
>100
43.2
0.01
0.17
>100
0.2
20
90
34
>100
54
a Cytotoxicity GI50 values are the concentrations corresponding to 50% growth inhibition.
b nd––not done.

4886
S. Nanduri et al. / Tetrahedron Letters 45 (2004) 4883–4886
8. Oh, S.; Jeong, I. H.; Shin, W.-S.; Lee, S. Bioorg. Med.
Chem. Lett. 2003, 12, 13.
9. Barrero, A. F.; Simeon, A.; Jose, F. Q. M.; Herrador, M.
M.; Valdivia, M.; Jimenez, D. J. Org. Chem. 2002, 67,
2501.
10. Barrero, A. F.; Alvarez-Manzaneda, E. J.; Alvarez-Man-
zaneda, R.; Chahboun, R.; Meneses, R.; Cuerva, J. M.;
Aparicio, M.; Romera, J. L. Org. Lett. 2001, 3(5), 647.
11. Jose, V.; Juan, F.; Franklin, S.; Eleonora, T.; Randolph,
A. J. Nat. Prod. 2003, 66, 1623.
to exhibit potent activity against prostate cell line
(DU145). The Wittig olefination products 8, 9 and 10
showed good activity against all the cell lines listed (GI50
values 2–20 lM). Compounds 14 and 17 demonstrated
activity selectively against the DU145 cell line at 0.17
and 0.2 lM concentrations, respectively. Potent activity
against MCF-7/ADR cell line was observed for 18.
In summary, we have developed an efficient synthetic
methodology for the preparation of 4 from 1 by selective
oxidative degradation of the andrographolide side
chain. The aldehyde functional group on 4 has been
utilized for synthesizing a number of structurally diverse
labdane diterpenes, which have exhibited potent cyto-
toxic activity.
12. Rangaswamy, S.; Subba Rao, V. J. Sci. Ind. Res. 1951,
10B, 201.
13. Compound 4: 1H NMR (CDCl3, 400 MHz):
d
(ppm) ¼ 9.65 (s, 1H, CHO), 4.85 (s, 1H, H-13a), 4.44 (s,
1H, H-13b), 3.97 (d, 1H, H-15a, J ¼ 11:6 Hz), 3.50 (m,
1H, H-3), 3.19 (d,1H, H-15b, J ¼ 11:6 Hz), 2.57 (m, 1H),
2.45 (m, 2H), 2.35 (d, 1H), 2.10–1.95 (m, 2H), 1.85–1.70
(m, 2H), 1.65 (m, 1H), 1.40 (s, 3H), 1.38 (s, 3H), 1.35–1.25
(m, 3H), 1.22 (s, 3H), 0.92 (s, 3H). 13C NMR (50 MHz,
CDCl3) d (ppm) ¼ 202.4, 147.4, 108.6, 98.8, 76.3, 76.1,
63.6, 51.9, 50.2, 39.7, 37.6, 37.1, 34.3, 26.9, 25.9, 25.1, 24.8,
22.7, 16.2. Mass (CI) m=z: 307 (Mþ+1), 249, 232, 205. IR
mmax cmꢀ1 (neat): 3081, 2935, 2716, 1725, 1644, 1457, 1376,
1226, 1095, 996, 834, 757.
References and notes
1. Connolly, J. D.; Hill, R. A. In Dictionary of Terpenoids;
Chapman & Hall: London, UK, 1991; Vol. 2, pp 853–857.
2. (a) Polonsky, J. F. Chem. Org. Naturst. 1973, 30, 101; (b)
Polonsky, J. F. Chem. Org. Naturst. 1985, 47, 222.
3. Meenakshi, S.; Mahesh, P.; Sharma, R. P. Planta Med.
1999, 65, 2.
4. Coval, S. J.; Conover, M. A.; Mierzwa, R.; King, A.; Puar,
M. S.; Phife, D. W.; Pai, J. K.; Burrier, R. E.; Ahn, M. S.;
Boykow, G. C.; Patel, M.; Pomponi, S. A. Bioorg. Med.
Chem. Lett. 1995, 5, 605.
5. Barrero, A. F.; Alvarez-Manzaneda, E. J.; Rachid, C. C.;
Gonzalez, D. Synlett 2000, 11, 1561.
6. Barrero, A. F.; Alvarez-Manzaneda, E. J.; Rachid, C. C.
Tetrahedron 1998, 54, 5635.
7. Rosselli, S.; Bruno, M.; Pibiri, I.; Piozzi, F. Eur. J. Org.
Chem. 2002, 24, 4169.
14. Cell growth assay: Exponentially growing cells were
seeded (10,000 cells/well) in a 96-well plate. Cell culture
plates were incubated with different concentrations of test
compounds at 37 °C in a 5% CO2 incubator. After 48 h the
cells were fixed by adding ice-cold 50% trichloroacetic acid
(TCA), washed with distilled water and stained with SRB
solution. The plates were washed with 1% acetic acid, the
bound SRB stain was solubilized with 10 mM tris buffer
and the optical densities were read on a spectrophotomet-
ric plate reader at a wavelength of 515 nm. The percentage
growths were calculated and the GI50 values were inter-
polated from the growth curves.