First stereoselective total synthesis and anticancer activity of new amide alkaloids of roots of pepper

Article abstract of DOI:10.1016/j.bmcl.2009.08.056

The first stereoselective total synthesis of new natural amide alkaloids 1-3 have been achieved from commercially available starting materials. Wittig olefination, Sharpless asymmetric dihydroxylation, epoxidation, a trans regioselective opening of 2,3-epoxy alcohol, Horner-Wadsworth-Emmons (HWE) olefination and amide coupling are the key steps. The amide alkaloids 1-3 are evaluated for their anticancer activity against colon (HT-29), breast (MCF-7) and lung (A-549) human cancer cell lines for the first time.

Full text of DOI:10.1016/j.bmcl.2009.08.056

Bioorganic & Medicinal Chemistry Letters 19 (2009) 5915–5918
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
First stereoselective total synthesis and anticancer activity of new amide
alkaloids of roots of pepper
c
Ch. Srinivas a, Ch. N. S. Sai Pavan Kumar a, B. China Raju b, V. Jayathirtha Rao a,d, , V. G. M. Naidu ,
*
S. Ramakrishna c,d, Prakash V. Diwan c,d,
*
a Organic Chemistry Division-II, Indian Institute of Chemical Technology, Hyderabad 500 607, India
b Organic Chemistry Division-I, Indian Institute of Chemical Technology, Hyderabad 500 607, India
c Pharmacology Division, Indian Institute of Chemical Technology, Hyderabad 500 607, India
d National Institute of Pharmaceutical Education and Research, Balanagar, Hyderabad 500037, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The first stereoselective total synthesis of new natural amide alkaloids 1–3 have been achieved from
commercially available starting materials. Wittig olefination, Sharpless asymmetric dihydroxylation,
epoxidation, a trans regioselective opening of 2,3-epoxy alcohol, Horner–Wadsworth–Emmons (HWE)
olefination and amide coupling are the key steps. The amide alkaloids 1–3 are evaluated for their antican-
cer activity against colon (HT-29), breast (MCF-7) and lung (A-549) human cancer cell lines for the first
time.
Received 9 June 2009
Revised 12 August 2009
Accepted 17 August 2009
Available online 20 August 2009
Keywords:
Ó 2009 Elsevier Ltd. All rights reserved.
Wittig olefination
Sharpless asymmetric dihydroxylation
Epoxidation
HWE olefination
Stereoselective synthesis
Amide alkaloids
Anticancer activity
Natural Products are biologically interesting molecules1 and play
an important role in modern drug discovery. Piperidine and pyrrol-
idine amide alkaloids2 are known to occur in Piper nigrum (Pipera-
ceae, black pepper) and possess a variety of pharmacological
activities such as CNS stimulant,3 analgesic4 and antipyretic.4 In
addition to these, other activities such as antifungal,5 anti-diarrho-
eal,6 anti-inflammatory,7 insecticidal,8 anti-tuberculosis and anti-
plasmodial,9 as well as inhibition of 5-lipoxygenase,10 cyclooxegen-
ase-110 and CYP3A411 are also reported. Recently Nikaido and co-
workers,12 isolated new amide alkaloids from roots of Piper nigrum
such as ( )-threo-1-(1-oxo-4,5-dihydroxy-2E-decaenyl)piperidine,
( )-threo-N-isobutyl-4,5-dihydroxy-2E-octaenamide, ( )-erythro-
1-(1-oxo-4,5-dihydroxy-2E-decaenyl)piperidine and established
their structure. Asymmetric molecules and their syntheses play sig-
nificant roles in obtaining enantiomerically pure or enriched com-
pounds of natural products, pharmaceuticals and agrochemicals.
In accordance of our continuing synthetic efforts13 towards the mol-
ecules with intriguing characteristics and usefulness, we embarked
on the synthesis of amide alkaloids (alkenamides) 1–3 (Fig. 1). The
structural similarities between these compounds 1–3 led us to de-
sign a common strategy.
The synthetic approach for natural amide alkaloids 1 and 2 ini-
tiated from commercially available butanal and hexanal as illus-
trated in Scheme 1. Aldehyde was subjected to Wittig
olefination14 to furnish the E-
a,b-unsaturated esters 5/6. The
unsaturated ester 5/6 on Sharpless dihydroxylation using AD-
mix-b15 provided the key intermediate, chiral dihydroxy ester 7/8
with an excellent enantio-selectivity. The compounds 7/8 were
converted to the corresponding acetonide esters 9/10 using 2,2-
dimethoxy propane in presence of p-TSA in high yields. Acetonide
ester 9/10 were then converted16 into corresponding aldehyde
11/12 using DIBAL-H at À78 °C and followed by two carbon
homologation with methyl-2-(diethoxyphosphoryl)acetate under
Horner–Wadsworth–Emmons (HWE) conditions17 to furnish the E-
a
,b-unsaturated ester 13/14. The obtained ester 13/14 was hydro-
lysed18 with LiOH to give corresponding carboxylic acid 15/17. The
acid 15 was coupled with piperidine using 1-hydroxybenzotriazole
(HOBt) and 1-(3-dimethylamino-propyl)-3-ethylcarbodiimide
hydrochloride (EDCÁHCl)19 to furnish corresponding amide 16
which on subsequent acetonide deprotection afforded (4R,5R)-2E-
4,5-dihydroxy-1-(piperidin-1-yl)dec-2-en-1-one (1) in 95% yield
* Corresponding authors. Tel.: +91 40 27160124; fax: +91 40 27160757 (V.J.R.).
E-mail address: jrao@iict.res.in (V. Jayathirtha Rao).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.08.056

5916
Ch. Srinivas et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5915–5918
OH
O
OH
O
OH
O
N
N
N
H
OH
OH
OH
2
3
1
Figure 1.
as a colorless viscous oil25 (Scheme 1). The same procedure of
hydrolysis and amide coupling with isobutyl amine adopted on
17 to get (4R,5R)-2E-4,5-dihydroxy-N-isobutyloct-2-enamide (2)
in 93% yield as a colorless viscous oil.25
lyzed with LiOH to afford carboxylic acid 28. The acid 28 was
coupled with piperidine using HOBt and EDCÁHCl to give amide
29, which on subsequent acetonide deprotection using p-TSA
afforded the new natural amide alkaloid (4S,5R)-2E-4,5-dihy-
Next we have chosen the synthesis of amide alkaloid 3 and is
outlined in the Scheme 2. We envisioned the two stereocentres
could be constructed by the Sharpless asymmetric epoxidation
on allylic alcohol and then trans regioselective ring opening of
the epoxide alcohol. The synthesis of the natural amide alkaloid
droxy-1-(piperidin-1-yl)dec-2-en-1-one (3) in 93% yield as a
white solid.25
The synthesized new amide alkaloid compounds (4R,5R)-2E-
4,5-dihydroxy-1-(piperidin-1-yl)dec-2-en-1-one (1), (4R,5R)-2E-
4,5-dihydroxy-N-isobutyloct-2-enamide (2) and (4S,5R)-2E-4,5-
dihydroxy-1-(piperidin-1-yl)dec-2-en-1-one (3) have been evalu-
ated for their in vitro anti cancer activity for the first time against
three cancer cell lines (Table 1) using MTT method.24 Compound 1
exhibits activity against both colon (HT-29) and lung cancer (A-
3 started from previously synthesized
a,b-unsaturated ester 6
which was converted to allylic alcohol 19 and subjected to Sharp-
less epoxidation20 (Scheme 2) to afford the epoxy alcohol 20. The
titanium assisted trans regioselective opening21 of the 2,3-epoxy
alcohol was accomplished with benzoic acid to get 21, which
was further converted into triol 22. Selective protection22 of the
primary hydroxyl group of 22 with tert-butyldimethylsilylchloride
gave silylether 23 and then the secondary hydroxyl groups were
protected with 2,2-dimethoxy propane in presence of p-TSA to af-
ford 24.
549) (IC50 values of 125
line, compound 2 exhibits activity against colon cancer (HT-29
with IC50 value of 181 g/mL) cell line and compound 3 exhibits
lg/mL and 135 lg/mL, respectively), cell
l
activity against lung cancer (A-549 with IC50 value of 182
lg/mL)
cell line.
In conclusion, the present letter describes an efficient first total
stereoselective synthesis of natural amide alkaloids 1–3 (roots of
pepper) in very good yields. Thus synthesized compounds are
new and evaluated for their in vitro anticancer activity in three cell
lines.
The compound 24 was deprotected using TBAF22 to give alco-
hol 25 followed by Swern oxidation23 to give the corresponding
aldehyde 26. HWE olefination was adopted to convert 26 to the
E-a,b-unsaturated ester 27. The obtained ester 27 was hydro-
O
OH O
a
CHO
b
OEt
n=1= 5; 90 %
n=3= 6; 95 %
OEt
n
n
n
OH
n = 1 or 3
n=1= 7; 91 %
n=3= 8; 90 %
c
O
O
O
O
O
O
O
e
d
H
OMe
OEt
n
n
n
O
O
n=1= 13; 83 %
n=3= 14; 85 %
n=1= 11; 73 %
n=3= 12; 75 %
n=1= 9; 97 %
n=3= 10; 95 %
O
O
O
O
N
f
H
g
14
h
N
OH
1
90 %
O
O
95 %
95 %
16
15
O
O
O
O
NH2
h
f
2
13
OH
N
93 %
g
93 %
H
O
17
O
18
96 %
Scheme 1. Reagents and conditions: (a) Ph3P@CHCOOEt, benzene, rt; (b) AD-mix-b, t-BuOH/H2O (1:1), 0 °C; (c) 2,2-DMP, p-TSA (cat.), CH2Cl2; (d) DIBAL-H, dry CH2Cl2,
À78 °C; (e) (H5C2O)2P(O)CH2COOMe, NaH, dry benzene, 0 °C–rt; (f) LiOH, THF/H2O (3:2), 45 °C; (g) HOBt, EDC, CH2Cl2, 0 °C–rt; (h) MeOH, p-TSA, rt.

Ch. Srinivas et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5915–5918
5917
O
a
OH
OEt
O
85 %
19
86 %
6
b
OBz
OH
O
c
d
OH
OH
OH
73 %
20
OH
95 %
OH
22
21
e
92 %
OH
O
O
h
f
g
OTBS
OTBS
OH
23
OH
95 %
95 %
O
24
25
O
O
93 %
O
O
O
O
i
j
OMe
O
OH
92 %
O
27
26
86 %
O
O
k
28
95 %
O
O
l
3
N
O
93 %
29
Scheme 2. Reagents and conditions: (a) DIBAL-H, dry CH2Cl2, 0 °C to rt; (b) Ti(OiPr)4, (+)-DET, cumene hydroperoxide, 4 Å MS, dry CH2Cl2, À22 °C; (c) PhCOOH, Ti(OiPr)4,
CH2Cl2, rt; (d) K2CO3, MeOH, 0 °C; (e) TBSCl, imidazole, dry DMF, 0 °C; (f) 2,2-DMP, p-TSA (cat.), CH2Cl2; (g) TBAF, THF, 0 °C; (h) (COCl)2, dry DMSO, Et3N, dry CH2Cl2, À78 °C; (i)
(H5C2O)2P(O)CH2COOMe, NaH, dry benzene, 0 °C–rt; (j) LiOH, THF/H2O (3:2), 45 °C; (k) HOBt, EDC, CH2Cl2, 0 °C to rt; (l) MeOH, p-TSA.
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Table 1
In vitro anti cancer activity of 1, 2 and 3 on cancer cell lines
2. (a) Parmar, V. S.; Jain, S. C.; Bisht, K. S.; Jain, R.; Taneja, P.; Jha, A.; Tyagi, O. D.;
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1617.
Cancer panel/cell line
IC50 (lg/mL)
1
2
3
Colon HT-29
Breast MCF-7
Lung A-549
125
NA
135
181
NA
NA
NA
NA
182
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Acknowledgments
The authors thank Director IICT for the constant support. Ch.S.
thank UGC; Ch.N.S.S.P.K. and V.G.M.N. thank CSIR, New Delhi for
fellowship.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2009.08.056.
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