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Antiproliferative activity of arborescidine alkaloids and derivatives
Leonardo S. Santosa,*, Cristina Theodulozb, Ronaldo A. Pillic, Jaime Rodriguezb
aLaboratory of Asymmetric Synthesis, Chemistry Institute of Natural Resources, Universidad de Talca, Av. Lircay s/n, Talca, P.O. Box 747, Chile
bFacultad de Ciencias de la Salud, Universidad de Talca, Talca, P.O. Box 747, Chile
cInstitute of Chemistry, Unicamp, State University of Campinas, Campinas, SP 13083-970, Brazil
a r t i c l e i n f o
Received 20 January 2009
Received in revised form
18 March 2009
Accepted 2 April 2009
Available online 14 April 2009
a b s t r a c t
Current issues in cancer research involve searching for novel anticancer compounds that can be used to
regulate the cell cycle and lead to more effective treatments of tumors. In this study, it was hypothesized
that possessing a cyclic alkaloid similar to harmine, arborescidines can disrupt the proliferative state of
cancer cells and block the activity of topoisomerases. The antiproliferative activity of arborescidines A–C
and their derivatives was evaluated in vitro against four human tumor cell lines: gastric adenocarcinoma,
lung cancer, bladder carcinoma and leukemia. Assuming the mechanism of action by topoisomerase II
binding model, the compounds possessing the greatest activity had nonpolar side-chain into hydro-
phobic binding region on the DNA/topo II complex.
? 2009 Elsevier Masson SAS. All rights reserved.
Current issues in cancer research involve searching for novel
anticancer compounds that can be used to regulate the cell cycle
and lead to more effective treatments of tumors. In vitro studies
have shown the ability of many b-carbolines to retain the cell cycle
progression of tumor cells [1,2]. Although no information is
available concerning the biological effects of chiral arborescidines
, it was hypothesized that possessing a cyclic alkaloid similar to
harmine, arborescidine A can disrupt the proliferative state of
cancer cells and block the activity of topoisomerases . The aim
of this work was to assess the in vitro antiproliferative activity of
arborescidines A–C and their derivatives against four human
tumor cell lines: gastric adenocarcinoma, lung cancer, bladder
carcinoma and leukemia. Human normal lung fibroblasts were
used as controls.
Structurally, the arborescidines comprise a tetracyclic frame-
work featuring a common b-carboline core. Although demon-
strated as a useful synthetic method, this asymmetric reduction
remains to be fully explored in the arena of total synthesis of
alkaloid natural products [4–8].
The basic framework of arborescidine alkaloids consists of
a tetracyclic moiety and was demonstrated previously that an
alternative to their synthesis to be the Noyori asymmetric
hydrogen-transfer reaction as a key step. For the present work, an
efficient synthesis of 6-bromotryptamine (4a), which was a key
precursor to b-carbolines 7 and 8 (Scheme 1), was required.
Schumaker and Davidson methodology showed to be the most
efficient for the synthesis of 4a. The precursor to tryptamine 4a is
6-bromoindole, which was easily prepared by a modified Batcho–
Leimgruber indole synthesis . Thus, reaction of 6-bromoindole
with 1-(dimethylamino)-2-nitroethylene (DMANE)  in the
presence of TFA afforded 3-[(E)-2-nitroethenyl]-6-bromoindole in
96% yield. Reduction with in situ generated borane proceeded
smoothly to provide 4a in 73% yield.
Then, arborescidine A (1a) and desbromoarborescidine A (1b)
were obtained following the sequence depicted in Scheme 1. Thus,
compounds 4a,b and glutaric anhydride in CH2Cl2 at room
temperature formed the corresponding amide carboxylic acids,
which upon treatment with SOCl2/MeOH afforded the corre-
sponding methylestersinyields around 83% (twosteps).Treatment
producing imines 7a,b in yields of 86%. b-Carboline imines 7a,b
were hydrogenated by the Noyori method using preformed (S,S)-
TsDPEN–Ru(II) complex in DMF and a HCO2H–Et3N mixture, which
* Corresponding author. Tel.: þ56 71 201575; fax: þ56 71 200448.
E-mail address: email@example.com (L.S. Santos).
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European Journal of Medicinal Chemistry 44 (2009) 3810–3815
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afforded lactams (R)-9a,b in 89% (96% ee) and 91% (>95% ee) yields,
respectively for 9a and 9b. The enantiomeric excesses were deter-
mined by HPLC analysis using a ChiralPack OD column. The (S)-9a,b
wereobtained bychanging the (S,S)-TsDPEN for (R,R)-TsDPEN chiral
ligand of ruthenium catalyst giving similar yields and ee%. Reduc-
tion of lactams 9a,b using Brown’s procedure with AlH3[11,12]
afforded (þ)-1a in 78% and (þ)-1b  in 89% yields, respectively.
Next, treatment of 4a,b with 5-hexenoic acid  in the pres-
ence of EDC/HOBt  gave amides 8a,b in quantitative yields
(Scheme 1). Bischler–Napieralsky reaction in MeCNaffordedimines
8a,b in 90% and 88% yields, respectively from 4a,b. Noyori asym-
metric hydrogenation reaction of imines 8a,b in DMF afforded the
respective amines 10a (96% yield) and 10b (95% yield). Free amines
10a,b were converted to the corresponding methyl carbamates
11a,b. Then, dihydroxylation of carbamates 11a,b employing OsO4
and NMO gave 12a,b in 88% and 89% yields, respectively. The
obtained diols were treated with NaIO4to give unstable aldehydes
13a,b in excellent yields (90%). Treatment of crude aldehydes
13a,b with aqueous trifluoracetic acid in THF afforded a >20:1
mixture of trans/cis-14a,b (90–95% yields). Finally, after reduction
of 14a,b with AlH3, amines 2a,b were obtained in 92–96% yields.
Then, conversion of 2a,b to arborescidine B and desbromoarbor-
escidine B required dehydration of the alcohols by using the
Burgess reagent in benzene  that gave 3a in 84% and 3b in 86%
The enantioselective syntheses of desbromoarborescidine A
(1b), desbromoarborescidine B (2b), and desbromoarborescidine C
(3b) were obtained via routes that proceeded in highyields and few
steps. Arborescidines wereobtained followingpreviouslydescribed
procedure in five steps and 50% overall yield (1a), eight steps and
61% overall yield (2a), and nine steps and 51% overall yield (3a),
respectively, from 6-bromotryptamine. The syntheses feature the
use of the Noyori catalytic asymmetric hydrogen-transfer reaction
to introduce chirality in dihydro-b-carbolines as 4. On the basis of
an ample precedent from Noyori’s work, the reduction produces
dihydro-b-carbolines, and ultimately products, possessing the R or
S absolute configuration, depending on Noyori’s catalyst employed.
The synthetic arborescidines and derivatives displayed optical
rotations that were in accordance with those of the natural
products, thereby supporting the S configuration for natural
Thus, the activity of these synthesized compounds and inter-
mediates to in vitro antiproliferative activity against four human
tumor cell lines were analyzed as described below.
3. Antiproliferative activity assay
Human cell lines were purchased from ATCC (VA, USA). They
included normal MRC-5 lung fibroblasts (CCL-171), AGS gastric
adenocarcinoma (CRL-1739), HL-60 leukemia cells (CCL-240), lung
cancer (SK-MES-1) and J82 bladder carcinoma (HTB-1). Cells were
plated at a density of 50,000 cells/mL in 96 well plates. One day
after seeding cells were treated with medium containing the
compounds at concentrations ranging from 0 up to 100 mM for 3
days and finally the MTT reduction assay was carried out .
Untreated cells were used as controls. Etoposide (inhibitor of
topoisomerase II) was used as reference compound. Normal human
lung fibroblasts were used in order to assess the selectivity of the
compounds against cancer cells. Results are expressed as IC50
4a, X = Br
4b, X = H
7a, R = (CH2)3CO2Me
7b, R = (CH2)3CO2Me
8a, R = (CH2)3CH=CH2
8b, R = (CH2)3CH=CH2
9a, X = Br
9b, X = H
1a, X = Br
1b, X = H
10a, R = Br
10b, R = H
2a, X = Br
2b, X = H
3a, X = Br
3b, X = H
Scheme 1. Reagents and conditions: (a) 4a/4b, glutaric anhydride, CH2Cl2, rt, 10 min. (b) SOCl2, MeOH, 0?C to rt, 3 h. (c) 4a/4b, EDC, HOBt, 5-hexenoic acid, CH2Cl2, rt, 12 h. (d)
POCl3, benzene, reflux, 2–3 h. (e) (R,R)-TsDPEN–Ru(II) complex (or (S,S)-TsDPEN–Ru(II) complex), Et3N:HCO2H (5:2, v/v), DMF, rt, 12 h. (f) (R,R)-TsDPEN–Ru(II) complex (or (S,S)-
TsDPEN–Ru(II) complex), Et3N:HCO2H (5:2, v/v), MeCN, rt,12 h. (g) MeOCOCl, Et3N, CH2Cl2, 0?C to rt,12 h. (h) OsO4/t-BuOH, NMO, THF–H2O, 0?C to rt,10 h. (i) NaIO4, THF:H2O (1:2).
(j) TFA, THF, 1 h (>20:1 trans/cis). (k) AlH3, THF, rt, 15 min. (l) MeO2CNSO2NEt3, benzene, 8 h.
L.S. Santos et al. / European Journal of Medicinal Chemistry 44 (2009) 3810–38153811
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values. All tested compounds were synthesized as described above
or according to previous work in good yields  (Fig. 1).
4. Results and discussion
Approaches to the construction of this heterocyclic target
system by other groups have included the diastereoselective
vinylogous Mannich reaction , Fischer indole synthesis ,
Bischler–Napieralski reactions [3,20,21], and the asymmetric Pic-
tet–Spengler reaction . Using our approach in the target
synthesis of complexindole
analogues, we undertook the synthesis of a simple indole alkaloid,
(desbromoarborescidine A), the main constituent of Dracontome-
lum mangiferum B1 [23,24].
alkaloids andtheir synthetic
All the compounds tested possess the S and R absolute stereo-
chemistry, and no significant differences in the IC50among R and S
compounds were observed. There was no significative improve-
ment in the IC50when natural arborescidines (S absolute stereo-
chemistry) and opposite configuration were tested. The better
results obtained for gastric, fibroblast and lung cells werepresented
by amine compound 10a (IC50y8.8, 18.1 and 12.7 mM, respec-
tively); for bladder was showed bycarbamate 14b (IC50y18.1 mM);
and for leukemia arborescidine A, 1a (IC50y34.5 mM). Arbor-
escidine alkaloids are known to be active against oral carcinoma
(KB, 100% of activity at 30 mM). The most active compound in that
screening was Arborescidine D with an IC50of 9.0 mM against KB
cells . In our approach testing different cell lines the best result
was obtained with bromo compound 10a. Most of the compounds
did not show antiproliferative activity towards leukemia cells
at concentrations up to 100 mM, however product 1a displayed
Fig. 1. Arborescidine alkaloids (1–3) and b-carboline derivatives tested towards cytotoxicity to human lung fibroblasts and different human tumor cell lines.
β β-carboline template
Scheme 2. Topoisomerase II binding model : the compounds possessing the greatest activity (10a and 14b) incorporate either nonpolar side-chain into hydrophobic binding
region on the DNA/topo II complex (Y/Z) that might enhance activity against topo II.
L.S. Santos et al. / European Journal of Medicinal Chemistry 44 (2009) 3810–38153812
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a selective inhibition against this cell line. At 100 mM compounds
12a, 3a and 14b were active against two or more cancer cell lines
but were not toxic towards normal fibroblasts. Chbani  has
reported a moderate activity of the brominated indole alkaloid
arborescidine D against the growth of KB human oral carcinoma
cells with an IC50value of 3 mg/mL.
Assuming the mechanism of action by topoisomerase II binding
model , the compounds possessing the greatest activity (14b and
10a, IC50from 16.8 to 8.8 mM) incorporate either nonpolar side-
chain into hydrophobic binding region on the DNA/topo II complex
(Y/Z) that might enhance activity against topo II when occupied by
a compound as depicted in Scheme 2. Derivatives substituted with
NH functionalities in the pendant group domain (W) as well as Br
(X) in the pharmocophore showed to be more potent toxins. Data
from other derivatives further support this hypothesis.
The three parent compounds showed antiproliferative activity
values ranging from 30 up to >100 mM, arborescidine C being the
most active product. Assuming the mechanism of action by top-
oisomerase II binding model [1,2], the compounds possessing the
greatest activity (14b and 10a, IC50from 16.8 to 8.8 mM) incorporate
either nonpolar side-chain into hydrophobic binding region on the
DNA/topo II complex (Y/Z) that enhances activity against topo II
when occupied by a compound. Derivatives substituted with NH
functionalities in the pendant group domain (W) as well as Br (X) in
the pharmocophore are more potent toxins. Data from other
derivatives further support this hypothesis. Studies are ongoing in
order to obtain more selective and active alkaloid derivatives as
well as to corroborate their mechanism(s) of action (Table 1).
6. Experimental protocols
6.1.1. General methods
Purchased chemical reagents were used without further puri-
fication. THF was freshly distilled from sodium benzophenone ketyl
under nitrogen prior to use. Dichloromethane was distilled under
CaH2prior to use. DMF was pre-dried by P2O5, then collected from
CaH2prior to use. Methanol was freshly distilled from magnesium
prior to use. Solvents for extraction and column chromatography
were distilled prior to use. Merck silica gel (230–400 mesh) was
used as stationary phase for flash chromatography. Melting points:
m.p. (uncorrected) were determined using an Electrothermal 9100
apparatus. IR spectroscopy: FT-IR Nicolet Nexus 470 equipment and
KCl cell. Other known compounds used in this research were
purchased from standard chemical suppliers and were dried and/or
performed using silica gel Merck 230–400 mesh. TLC analyses were
performed with silica gel plates Merck using iodine, KMnO4and
UV-lamp for visualization. Mass spectrometry experiments were
performed on a high-resolution high accuracy hybrid double
spectrometer (QTof, Micromass UK). The temperature of the
nebulizer was 50?C. The ESI source and the mass spectrometer
were operated in the positive-ion mode. The cone and extractor
potential were set to 40 and 10 V, respectively. NMR spectroscopy:
NMR spectra were recorded on Bruker (400 MHz for
100 MHz for13C) with TMS as internal.
126.96.36.199. N1-[2-(1H-3-Indolyl)ethyl]-5-hexenamide. To a solution of
tryptamine (1.19 mmol) and 5-hexenoic acid (0.136 g,1.19 mmol) in
CH2Cl2(12.0 mL) at 0?C were added HOBt (0.177 g,1.31 mmol) and
EDC (0.251 g,1.31 mmol). The reaction mixture was stirred at room
temperature for 10 h, then washed with 5% aqueous HCl
(3?15.0 mL), 5% aqueous NaHCO3(20.0 mL), H2O (20.0 mL), and
brine (20.0 mL), and dried (Na2SO4). Purification by flash chroma-
tography (CH3Cl/MeOH, 10%, Rf¼0.43) afforded amidoalkene in
99% yield as a brown oil. FT-IR (KBr film) cm?1: 3045, 3284, 3077,
2973, 2929, 2859, 1648, 1537, 1546, 1434, 1340, 1253, 1228, 1099,
914, 742.1H NMR (400 MHz, CDCl3) d: 1.68 (2H, quint, J 7.8), 2.03
(2H, q, J 7.2), 2.10 (2H, t, J 7.8), 2.96 (2H, t, J 6.7), 3.58 (2H, q, J 6.1),
4.94 (1H, dd, J 10.2, 3.0), 4.96 (1H, dd, J 17.6, 3.0), 5.68 (1H, br s), 5.73
(1H, ddt, J 17.6, 10.2, 6.1), 6.99 (1H, d, J 1.5), 7.10 (1H, dt, J 7.8, 0.7),
7.19 (1H, dt, J 7.8, 0.7), 7.35 (1H, d, J 8.7), 7.58 (1H, d, J 8.7), 8.48 (1H,
br s, NH).13C NMR (100 MHz, CDCl3) d: 24.7, 25.3, 33.0, 35.9, 39.7,
11.3, 112.7, 115.2, 118.6, 119.3, 122.0, 122.1, 127.3, 136.4, 137.8, 173.0.
188.8.131.52. 1-(4-Pentenyl)-4,9-dihydro-3H-b-carboline (8b). A solution
of N1-[2-(1H-3-indolyl)ethyl]-5-hexenamide (1.65 g, 4.94 mmol)
and 3.21 mL of POCl3in 123 mL of dry MeCN was heated to reflux
for 3 h, cooled to room temperature, and then concentrated. The
Cytotoxic activity (IC50, mM) of synthesized compounds (table shows S-configuration data) towards human lung fibroblasts (MRC-5), human gastric adenocarcinoma (AGS),
human lung cancer (SK-MES-1), human bladder carcinoma (J82) and human leukemia (HL-60) cells. Both (S)- and (R)-derivative compounds were assayed without statistically
differences in IC50.
Compound Lung fibroblastsGastric adenocarcinomaBladder carcinoma Lung cancerLeukemia
Values represent the means of three experiments in quadruplicate.
L.S. Santos et al. / European Journal of Medicinal Chemistry 44 (2009) 3810–38153813
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resulting orange viscous oil was purified by flash chromatography
(CH3Cl/MeOH,10%, Rf¼0.71) to afford a yellow oil in 86% yield. The
spectroscopic properties of the product were in accordance with
imine 8b. FT-IR (KBr film) cm?1: 3070, 2935, 2877, 2836, 2768,
2730, 1639, 1602, 1546, 1444, 1373, 1319, 1250, 1218, 998, 914, 746.
1H NMR (400 MHz, CDCl3)d: 1.83 (2H, quint, J 7.7), 2.09 (2H, q, J 7.7),
2.76 (2H, t, J 8.9), 2.88 (2H, t, J 8.5), 3.88 (2H, t, J 8.2), 4.89 (1H, dd, J
12.0,1.6), 4.94 (1H, dd, J 15.7, 3.1), 5.73 (1H, ddt, J 15.7,12.0, 8.5), 7.12
(1H, dt, J 7.1, 0.7), 7.24 (1H, dt, J 7.1, 0.7), 7.40 (1H, d, J 8.3), 7.59 (1H,
d, J 8.3),10.20 (1H, br s, NH).13C NMR (100 MHz, CDCl3) d: 19.3, 26.1,
33.2, 34.6, 47.4 112.2, 115.0, 117.1, 119.9, 124.5, 125.0, 125.3, 128.6,
137.2,137.8,162.2. HRMS, ESI(þ)-MS: m/z calcd. for [C16H18N2þH]þ
239.1548, found 239.1542.
184.108.40.206. 1-(4-Pentenyl)-2,3,4,9-tetrahydro-1H-b-carboline (10b). The
preformed catalyst solution was added to a mixture of imine 8b
(2.37 mmol) in 24 mL of DMF, followed by a mixture of HCO2H–
Et3N (5:2 v/v, 1.22 mL) at room temperature. After the resulting
solution was stirred at room temperature for 12 h, the DMF was
distilled off under a high vacuum, and the crude purified by flash
chromatography (CHCl3/MeOH, 10%, Rf¼0.52) to afford in 95% the
amine 10b as a brown solid. (R)-10b, [a]D ?25 (c¼1, CHCl3);
(S)-10b, [a]Dþ23 (c¼1, CHCl3). FT-IR (KBr film) cm?1: 3409, 3218,
3062, 2929, 2844, 2744, 1641, 1562, 1452, 1343, 1317, 1288, 1155,
1108, 1002, 909, 744.1H NMR (400 MHz, CDCl3) d: 1.52–1.70 (3H,
m), 1.84–1.91 (1H, m), 2.09–2.19 (2H, m), 2.68–2.80 (1H, m), 2.75
(1H, dq, J 8.0,1.9), 3.03 (1H, ddd, J 15.5, 8.0, 5.5), 3.34 (1H, dt, J 14.5,
4.5), 4.07 (1H, br s), 4.98 (1H, br d, J 10.2), 5.03 (1H, dd, J 17.1, 1.6),
5.80 (1H, ddt, J 17.1, 10.2, 6.7), 7.09 (1H, dt, J 7.6, 0.7), 7.14 (1H, dt, J
7.6, 0.7), 7.30 (1H, d, J 7.8), 7,47 (1H, d, J 7.8), 7.84 (1H, br s, NH).13C
NMR (100 MHz, CDCl3) d: 22.6, 25.0, 33.7, 34.3, 42.5, 52.5, 109.0,
110.7, 115.0, 118.0, 119.3, 121.5, 127.5, 135.6, 136.1, 138.3. HRMS,
ESI(þ)-MS: m/z calcd. for [C16H20N2þH]þ241.1705, found 241.1701.
220.127.116.11. Methyl 1-(4-pentenyl)-2,3,4,9-tetrahydro-1H-b-carboline-2-
carboxylate (11b). To a cold solution of amine 10b (5.67 mmol) and
triethylamine (0.861 g, 8.50 mmol) in dry CH2Cl2(94.0 mL) kept at
0?C was added dropwise a solution of methyl chloroformate
(1.07 g, 11.3 mmol) in CH2Cl2 (10 mL). After 1 h, the reaction
mixture was diluted with water (60.0 mL), followed by saturated
aqueous NH4Cl solution (100 mL) and extracted with CH2Cl2. The
organic layers were washed with saturated aq NaHCO3solution
(100 mL) and water (100 mL) and dried. The solvent was removed
abrown solid in 99%. (R)-11b, [a]D?2.0 (c¼1, CHCl3); (S)-11b, [a]D
þ3.0 (c¼1, CHCl3).1H NMR (400 MHz, d6-DMSO) d: 1.44–1.55 (2H,
m), 1.73–1.81 (1H, m), 1.87–1.94 (1H, m), 2.10 (2H, quint, J 7.6),
2.61–2.67 (2H, m), 3.15 (2H, s), 3.15–3.20 (1H, m), 3.65 (3H, s), 4.35
(1H, br t d, J 12.6), 4.95 (1H, d, J 8.2), 5.17 (1H, br d, J 4.4), 5.20 (1H,
dt, J 17.2,15.0), 5.83 (1H, ddt, J 17.2, 8.2, 5.5), 6.95 (1H, dt, J 7.6, 0.7),
7.04 (1H, dt, J 7.8, 0.7), 7.29 (1H, d, J 8.1), 7.36 (1H, d, J 8.1), 10.69
(1H, br s, NH).13C NMR (100 MHz, d6-DMSO), d: 20.6, 24.8, 32.7,
33.3, 37.7, 50.9, 52.1, 106.2, 110.7, 114.5, 117.3, 118.2, 120.5, 126.2,
134.6, 135.7, 138.2, 155.3. HRMS, ESI(þ)-MS: m/z calcd. for
[C18H22N2O2þH]þ299.1760, found 299.1762.
line (5b). To a solution of methyl 1-(4-pentenyl)-2,3,4,9-tet-
rahydro-1H-b-carboline-2-carboxylate (11b, 0.502 mmol) in
dry THF (6.0 mL) was added a solution of AlH3 in THF
(1.55 M, 1.94 mL, 3.01 mmol) at room temperature. After
10 min, the reaction was quenched with saturated aq sodium
sulphate solution and filtered. The solids were washed with
CH2Cl2(200 mL), dried with Na2SO4, and evaporated in vacuo.
Purification by chromatography eluting with EtOAc/Et3N (5%)
afforded a white solid in 92% yield. (R)-5b, [a]D ?1.2 (c ¼1,
CHCl3); (S)-5b, [a]Dþ1.5 (c ¼1, CHCl3). FT-IR (film, KBr) cm?1:
3411, 3301, 3059, 2938, 2839, 2779, 1690, 1646, 1464, 1376,
1321, 1299, 1178, 903, 749.1H NMR (400 MHz, CDCl3) d: 1.28–
1.34 (1H, m), 1.41–1.49 (1H, m), 1.57–1.66 (1H, m), 1.70–1.79
(1H, m),1.95 (2H, q, J 7.0), 2.35 (3H, s), 2.55–2.71 (3H, m), 3.00–
3.08 (1H, m), 3.37 (1H, t, J 5.2), 4.84 (1H, dt, J 10.2, 0.8), 4.89 (1H,
dd, J 17.1, 1.7), 5.66 (1H, ddt, J 17.1, 10.2, 6.6), 6.97 (1H, dt, J 6.8,
0.9), 7.01 (1H, dt, J 6.8, 0.9), 7.15 (1H, d, J 7.5), 7.36 (1H, d, J 7.5),
7.96 (1H, br s, NH).13C NMR (100 MHz, CDCl3) d: 18.9, 24.3, 32.0,
33.8, 41.7, 49.5, 59.7, 107.8, 110.6, 114.6, 117.8, 119.0, 121.0, 127.1,
[C17H22N2þH]þ255.1861, found 255.1855.
18.104.22.168. Methyl 1-(4,5-dihydroxipentil)-2,3,4,9-tetrahydro-1H-b-car-
boline-2-carboxylate (12b). Osmium tetroxide (67.0 mL of a freshly
prepared 0.039 M solution in t-BuOH) was added to a solution of
carboxylate (11b, 0.775 mmol) and N-methylmorpholine N-oxide
(0.256 mL, 50% v/v in water) in a 9:1 THF–H2O solution (9.70 mL) at
0?C. After 12 h at room temperature, the mixture was treated with
Florisil (0.350 g) and NaHSO3(0.111 g), stirred for 1 h, filtered, and
concentrated. The residue was diluted with EtOAc, and the organic
layer was washed with 5% H3PO4and brine, dried, and concen-
trated. Purification by flash chromatography gave a mixture of diols
12b in 89% yield.1H NMR (400 MHz, d6-DMSO, 353 K) d: 1.18–1.54
(4H, m), 1.78–1.88 (2H, m), 2.64–2.68 (2H, m), 3.18–3.21 (1H, m),
3.28 (1H, d, J 5.5), 3.43–3.46 (1H, m), 3.66 (3H, s), 4.26 (1H, dd, J
12.3, 4.4), 5.16 (1H, dd, J 8.9, 4.4), 6.95 (1H, t, J 7.3), 7.03 (1H, t, J 7.3),
7.30 (1H, d, J 8.0), 7.36 (1H, d, J 8.0), 10.57 (1H, br s).13C NMR
(100 MHz, d6-DMSO, 353 K) d: 20.5, 21.4, 32.9, 34.0, 37.7, 51.0, 51.8,
65.6, 70.8, 106.1, 110.6, 117.1, 118.0, 120.3, 126.1, 134.7, 135.7, 155.4.
HRMS, ESI(þ)-MS: m/z calcd. for [C18H24N2O4þH]þ333.1814,
line-2-carboxylate (13b). A solution of 12b (0.584 mmol) in 58.4 mL
of THF–H2O (1:2) was treated at 0?C with a solution of 0.131 g
(0.613 mmol) of sodium metaperiodate (NaIO4) in 6 mL of water.
After the resulting solution was stirred for 1 h at 0?C, the reaction
mixture was diluted with H2O and extracted with CHCl3. The CHCl3
extracts were washed with brine, dried, and concentrated in vacuo
to give aldehyde 13b as a colorless solid in 90% yield. (R)-13b, [a]D
?5.1 (c¼1, CHCl3); (S)-13b, [a]Dþ6.0 (c¼1, CHCl3). FT-IR (film, KBr)
cm?1: 3371, 3054, 3010, 2949, 2848, 2723, 1701, 1675, 1471, 1448,
1409, 1228, 1112, 1018, 744.1H NMR (400 MHz, CDCl3) d: 1.55–1.93
(4H, m), 2.39–2.52 (2H, m), 2.71 (1H, dd, J 15.3, 3.5), 2,84 (1H, br d, J
5.0), 3.19 (1H, br q, J 9.7), 3.76–3.79 (3H, br s), 4.35/4.52 (1H, br d, J
9.4), 5.20/5.35 (1H, br s), 7.10 (1H, dt, J 7.2,1.2), 7.15 (1H, dt, J 7.2,1.2),
7.29 (1H, d, J 7.9), 7.47 (1H, d, J 7.9), 9.66/9.74 (1H, br s).13C NMR
m/z calcd. for [C17H20N2O3þH]þ301.1552, found 301.1548.
(0.256 mmol) in dry THF (4.3 mL) was added a solution of AlH3in
THF (1.55 M, 0.330 mL, 0.512 mmol) at room temperature. After
10 min, the reaction was quenched with saturated aq sodium
sulphate solution and filtered. The solids were washed with CH2Cl2
(50 mL), and the filtrate was dried with Na2SO4, evaporated, and
concentrated in vacuo. Purification of the residue by column
chromatography afforded a white solid in 92% yield, which was
characterized as desbromoarborescidine C. (R)-2b, [a]D?3.7 (c¼1,
CHCl3); (S)-2b, [a]Dþ3.3 (c¼1, CHCl3).1H NMR (500 MHz, CDCl3) d:
C(2b). Toa solutionof
L.S. Santos et al. / European Journal of Medicinal Chemistry 44 (2009) 3810–38153814
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1.58–1.75 (4H, m), 1.85–1.92 (2H, m), 2.27 (1H, q, J 13.2), 2.41 (1H,
dd, J 11.5, 2.1), 2.81 (3H, s), 2.93–2.98 (1H, m), 3.05–3.07 (1H, m),
3.14–3.20 (1H, m), 3.38–3.42 (1H, m), 6.28 (1H, d, J 3.4), 7.16 (1H, t,
J 6.0), 7.27 (1H, t, J 6.0), 7.40 (1H, d, J 6.6), 7.48 (1H, d, J 6.6). HRMS,
ESI(þ)-MS: m/z calcd. for [C16H20N2OþH]þ
22.214.171.124. DesbromoarborescidineB(3b). Asolutionof2b(0.0896 mmol)
and Burgess reagent (0.043 g, 0.179 mmol) in dry benzene (9.0 mL)
was heated to reflux for 8 h under nitrogen atmosphere. The reaction
solution was cooled, diluted with EtOAc (9.0 mL), washed with brine
(4? 9.0 mL), dried, and evaporated. Column chromatography of the
residue over silica gel gave 3bas colorlessoil in 86% yield. (R)-3b, [a]D
?68 (c¼1, CHCl3); (S)-3b, [a]Dþ61 (c¼1, CHCl3). FT-IR (KBr film)
1316,1222,1052, 997, 898, 744.1H NMR (500 MHz, CDCl3) d: 1.89 (H,
dq, J 10.2, 3.6), 2.37 (1H, br d, J 13.0), 2.43 (1H, br d, J 13.0), 2.51–2.60
dd, J 5.1,1.7), 3.43 (1H, d, J 10.1), 5.06 (1H, q, J 4.2), 6.93 (1H, dt, J 10.0),
NMR (125 MHz, CDCl3) d: 20.6, 28.0, 30.0, 42.4, 52.8, 62.5,108.1,109.1,
109.2, 110.0, 118.2, 120.2, 121.8, 122.0, 127.2, 136.1. HRMS, ESI(þ)-MS:
m/z calcd. for [C16H18N2þH]þ239.1548, found 239.1552.
L.S.S. thanks FONDECYT (1085308) for financial support of
research activity. J.R.C. and C.T. also thank FONDECYT (1060841).
R.A.P. thanks the Brazilian research foundations FAPESP and CNPq
for financial assistance. IFS (F/4195-1), Organisation for the
Prohibition of Chemical Weapons, and Programa de Investigacio ´n
en Productos Bioactivos-UTalca are also acknowledged for support
of research activity.
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