Synthesis of racemic and optically active forms of novel antimalarial agents, spirocyclopentanone-anthracene adducts, via tandem Michael addition-Dieckmann condensation reactions as the key steps

Article abstract of DOI:10.1016/j.tetasy.2012.03.006

Spirocyclopentanone-anthracene adducts, novel antimalarial agents, have been synthesized by employing the racemic and the optically active dimethyl itaconate-anthracene adducts in a reaction with piperine via tandem Michael addition-Dieckmann condensation reactions as the key steps. All adducts exhibited antimalarial activity with IC₅₀ values of 3.4-4.7 μg/mL, and importantly displayed non-cytotoxicity to vero cells.

Full text of DOI:10.1016/j.tetasy.2012.03.006

Tetrahedron: Asymmetry 23 (2012) 357–363
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Tetrahedron: Asymmetry
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Synthesis of racemic and optically active forms of novel antimalarial agents,
spirocyclopentanone–anthracene adducts, via tandem Michael
addition–Dieckmann condensation reactions as the key steps
Ruangrat Choommongkol a, Rattana Jongkol a, Samran Prabpai c, Palangpon Kongsaeree c,d
,
Puttinan Meepowpan a,b,
⇑
a Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Chiang Mai University, 239 Huay Kaew Road, Chiang Mai 50200, Thailand
b Material Science Research Center, Faculty of Science, Chiang Mai University, 239 Huay Kaew Road, Chiang Mai 50200, Thailand
c Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand
d Center for Excellence in Protein Structure and Function, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Spirocyclopentanone–anthracene adducts, novel antimalarial agents, have been synthesized by employ-
ing the racemic and the optically active dimethyl itaconate–anthracene adducts in a reaction with piper-
ine via tandem Michael addition–Dieckmann condensation reactions as the key steps. All adducts
Received 2 February 2012
Accepted 5 March 2012
Available online 11 April 2012
exhibited antimalarial activity with IC50 values of 3.4–4.7 lg/mL, and importantly displayed non-cytotox-
icity to vero cells.
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction
O
COOMe
MeOOC
O
O
N
Malaria is a serious problem that is spreading in tropical and
subtropical areas of the world.1 Approximately 3.3 billion people
were at risk of malaria in 2010.2 Malaria is an infectious disease
caused by the protozoa parasite of the genus Plasmodium and is
carried from person to person by the anopheles mosquitoes. The
parasites are of many types, but only five species of the genus Plas-
modium cause malaria in humans.2 The parasites that cause malar-
ia in humans are Plasmodium falciparum (malaria tropica);
Plasmodium vivax (malaria tertiana); Plasmodium ovale (malaria
quartana); Plasmodium malariae (malaria tertiana) and Plasmodium
knowlesi.2 The first two species (P. falciparum and P. vivax) cause
the most infections worldwide.2 P. falciparum is the agent of severe
and potentially fatal malaria because of antimalarial drug resis-
tance.3 Thus, the research and development of antimalarial drugs
is especially important in order to eradicate this infectious disease.
Piperine 1 (Fig. 1) is a major alkaloid compound in Piper ni-
grum.4 It displays a variety of pharmacological and other bioactiv-
ities such as antifungal,5 antidiarrheal,6 antiinflammatory,7
insecticidal,8 nematocidal activity,4a inhibition of life metabolism,9
immunomodulatory10, and antitumor activities.10 Recently, piper-
ine dimers such as chabamide11 (antiplasmodial) and dipiperamide
A, B, C12 (CYP3A4 inhibitor) were discovered.
1
2
Figure 1. Structures of piperine 1 and dimethyl itaconate–anthracene adduct 2.
Previously, the use of the dimethyl itaconate–anthracene ad-
duct13 ( )-2 (Fig. 1) in racemic form has been reported as a building
block in the syntheses of biologically active natural products,
including methylenomycin B,14 sarkomycin,15 deepoxy-4,5-dide-
hydromethylenomycin A,16 methylenomycin A methyl esters,16
diospyrol,17
a
-methylene-
nolactocin, nephrosterinic acid, and protolichesterinic acid, and
-methylene bis-
-butyrolactones18 such as cannadensolide, spo-
c
-butyrolactones18a,b such as methyle-
a
c
rothriolide, and xylobovide. Moreover, optically active methylene
lactones,19 such as methylenolactocin, nephrosterinic acid and pro-
tolichesterinic acid, can be prepared by employing tandem aldol–
lactonization reactions of the enantiomerically pure forms (+)-2
and (ꢀ)-2.
Herein we report the synthesis of the racemic and optically ac-
tive forms of a spirocyclopentanone–anthracene adduct via tan-
dem Michael addition–Dieckmann condensation reactions as the
key steps. The racemic and optically active forms of dimethyl itac-
onate–anthracene adducts 2 react with piperine 1 as a precursor
⇑
Corresponding author. Tel.: +66 53 94 3341 5x317; fax: +66 53 89 227.
E-mail addresses: pmeepowpan@gmail.com, puttinan@chiangmai.ac.th (P. Mee-
powpan).
0957-4166/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.03.006

358
R. Choommongkol et al. / Tetrahedron: Asymmetry 23 (2012) 357–363
O
O
O
O
O
N
N
O
COOMe
1
MeOOC
O
MeOOC
COOMe
11
11
MeOOC
11
Spirocyclopentanone-
anthracene adducts
( )-2, (+)-(11S)-2 and (-)-(11R)-2
tandem Michael addition-
Dieckmann condensation reactions
Scheme 1. Synthesis of spirocyclopentanone–anthracene adducts via tandem Michael addition–Dieckmann condensation reactions.
(Scheme 1). All products were subsequently tested for antimalarial
and cytotoxic activities.
of their analytical and spectroscopic data. In particular, the 1H
NMR spectrum of adduct ( )-5 showed three signals at the30 [d:
2.82 (H, d, J = 7.1 Hz)], 40 [d: 4.49 (H, m)], and 50 [d: 3.87 (H, d,
J = 9.8 Hz)] positions, indicating that the configuration of the H-30
between H-40 and H-40 between H-50 was cis- and trans-, respec-
tively. In a similar fashion, as for adduct ( )-6, the coupling con-
stants of H-30 between H-40 and H-40 between H-50 were also
established as cis- and trans-[30 (d: 2.35 (H, d, J = 7.1 Hz)), 40 (d:
3.93 (H, m)), and 50 (d: 4.00 (H, d, J = 11.3 Hz)]. The relative stereo-
chemistry of the spirocyclopentanone–anthracene adducts ( )-5
was confirmed by single-crystal X-ray diffraction analysis,20 as
shown in Figure 2. Therefore, the relative stereochemistries of
the adduct ( )-6 should be opposite to those of the adduct ( )-5.
The relative stereochemistries at the 30-, 40- and 50-position of
the adduct ( )-6 were confirmed by nOe difference experiments
(Fig. 3), and the nOe results on ( )-5 also showed stereochemistries
corresponding to the X-ray results. Therefore, the relative stereo-
chemistries of the adducts (ꢀ)-5 and (ꢀ)-6 from (+)-2, and the ad-
ducts (+)-5 and (+)-6 from (ꢀ)-2 had similar configurations to the
racemic adducts.
2. Results and discussion
Racemic dimethyl itaconate–anthracene adduct ( )-2 was syn-
thesized from dimethyl itaconate and anthracene via a [4+2]
Diels–Alder reaction in high yield.18a It was then hydrolyzed and
treated with (ꢀ)-menthol, followed by transmethylation to provide
enantiomerically pure (+)-2 and (ꢀ)-2, which have already been re-
ported on,19 as shown in Scheme 2.
Compound ( )-2 was initially reacted with LDA to generate the
corresponding ester enolate intermediate, which readily reacted
with piperine 1 in THF via a tandem Michael addition followed
by a Dieckmann condensation reaction. The reaction mixture was
quenched with 30% hydrochloric acid solution at 0 °C, and then
purified by flash column chromatography (silica gel, EtOAc/
CH2Cl2/hexane = 2.0:0.5:7.5 as an eluent) to give two diastereo-
meric spirocyclopentanone–anthracene adducts ( )-5 and ( )-6.
The racemic and enantiomerically pure forms of 2 were applied
to piperine 1 via tandem Michael addition–Dieckmann condensa-
tion reactions and the results are shown in Table 1.
The stereochemical course of the reaction can be explained by
considering the required transition state for the Michael addition
reaction (1,4-addition) and the Dieckmann condensation reaction
(ring closure). Dimethyl itaconate–anthracene adduct 2 was depro-
The relative configuration of the two spirocyclopentanone–
anthracene adducts ( )-5 and ( )-6 was determined on the basis
( )-2
i,ii
Me
Pri
COOMe
MeOOC
11
11
O
O
Me
Pri
O
O
(−)-(11S)-3
iii
(−)-(11R)-4
iii
COOMe
MeOOC 11
11
MeOOC
COOMe
(+)-(11S)-2
(−)-(11R)-2
Scheme 2. Separation of the optically active dimethyl itaconate–anthracene adducts (+)-(11S)-2 and (ꢀ)-(11R)-2. Reagents and conditions: (i) 1.3 equiv KOH, MeOH: H2O
(2:1), reflux 2 h, (97%); (ii) (a) 5.0 equiv SOCl2, DMF (cat.), N2, reflux 2 h, (b) 1.3 equiv (ꢀ)-(1R,2S,5R)-menthol, 1.3 equiv Et3N, benzene, reflux 2 h [(ꢀ)-(11S)-3, 35%; (ꢀ)-(11R)-
4, 35%]; (iii) excess anhydrous MeOH, H2SO4 (cat.), reflux 6 days [(+)-(11S)-2, 97%; (ꢀ)-(11R)-2, 89%].

R. Choommongkol et al. / Tetrahedron: Asymmetry 23 (2012) 357–363
359
Table 1
% Yield, % enantiomeric excess (ee) and the specific rotation [
a
] of the adducts 5 and 6
O
O
O
O
O
O
N
N
4' 5'
3'
4' 5'
3'
1) 1.2 equiv LDA, THF
-78 °C to 0 °C, 2 h
11
COOMe
MeOOC
MeOOC
O
MeOOC
O
R1
11
R2
11
R2
R1
R1
+
R2
2) 1.2 equiv piperine 1,
THF, 0 °C to rt, 3 days
3) 30% HCl
2
5
6
R1
R2
R2
R1
R1
R2
1
2
1
2
; (-)- : R1, R2
=
2
2
( )- : R , R
=
2
; (+)- : R , R
=
c
Entry
Starting materials
Adducts
Yielda (%)
eeb (%)
[a]
1
2
3
4
5
6
( )-2
( )-2
(+)-2
(+)-2
(ꢀ)-2
(ꢀ)-2
( )-5
( )-6
(ꢀ)-5
(ꢀ)-6
(+)-5
(+)-6
23
15
33
17
14
8
—
—
>99
>99
>99
>99
—
—
ꢀ18.5 (c 0.87, CHCl3)
ꢀ70.5 (c 0.58, CHCl3)
+17.5 (c 0.76, CHCl3)
+67.6 (c 0.60, CHCl3)
a
Isolated yield.
b
Enantiomeric excesses (ee) of all products were determined by 1H NMR using a chiral lanthanide shift reagent, tris[3-(heptafluoropropylhydroxymethylene)-d-
camphorato]praseodym(III), (Pr(hfc)3).
c
Specific rotation values of all products were measured at 24.4 °C and 589 nm.
O
O
O
H
N
H
4' 5'
3'
H
MeOOC
O
( )-5
( )-5
Figure 2. X-ray structure of the adduct ( )-5 (ORTEP plot).
tonated at the
a
-proton of the ester by LDA to give the enolate 7.
condensation reaction via a 5-exo-trig cyclization, led to the major
product 5. Conversely, piperine 1 was attacked with enolate-anion
7 on the Re-face through the chelated transition states 10 and 11.
This led to the formation of minor product 6 because of the large
steric repulsion between the vinylbenzo[1,3]dioxole being eclipsed
with the anthracene groups present in the chelated transition
state, as depicted in Scheme 3.
Next, piperine 1 was added and attacked with the enolate anion
7 on the Si-face to form the eight-membered ring chelated transi-
tion state 8. After the Michael addition reaction, transition state 9
was attained to give the favorable envelope-chair-like form,
whereupon all of the large substituents occupied the less sterically
demanding equatorial orientations, and the following Dieckmann

360
R. Choommongkol et al. / Tetrahedron: Asymmetry 23 (2012) 357–363
O
O
O
O
He
Hd
O
He
Hd
O
N
N
Hf
Hf
Hc
5'
Hc
Ha
4'
3'
He
4' 5'
3'
He
O
X MeOOC
MeOOC
Ha
O
no nOe
Hb
Hb
X
Hy
Hy
no nOe
Hx
Hx
5
6
( )-
( )-
Figure 3. The nOe difference results of the adducts ( )-5 and ( )-6.
N
O
Li
Si-face
H
Ar
MeOOC
H
O
R1
H
5
N
O
O
O
COOMe
Ar
Li
MeO
H
R2
OMe
H
Re-face
COOMe
O
H
R1
N
H
R2
Lio
Si-face
MeO
O
8
9
1
0 °C to rt, 3 days
7
O
R2
N
R1
Li
O
OMe
O
H
R1, R2
=
H
OMe
Li
Re-face
N
6
H
O
Ar
R1
H
O
O
COOMe
H
MeOOC
R2
and Ar =
H
R1
Ar
R2
10
11
Scheme 3. The proposed mechanism of Michael addition–Dieckmann condensation reactions of the enolate anion 7 with 1.
Table 2
falciparum (K1, multi-drug resistance strain). Mefloquine and
dihydroartemisinin (DHA) were used as positive standard con-
trols. The results of the antimalarial activity and cytotoxicity
against vero cells of the adducts 5 and 6 are summarized in
Table 2.
Adducts ( )-5 and ( )-6 exhibited antimalarial activity against
the parasite P. falciparum (K1, multi-drug resistance strain) with
IC50 values of 4.7 and 3.4 lg/mL, respectively (Table 2). Interest-
ingly, adducts ( )-5 and ( )-6 displayed non-cytotoxicity against
vero cells, as shown in Table 2. The differences in pharmacological
activity between the drug and the receptor site generally depend
on their ability to react selectively at an asymmetric center in
the biological system which is due to the influence of the steric fac-
tors, such as optical and geometric isomerism, conformational
isomerism, isosterism, and bioisosterism.22 We were therefore
interested in investigating the antimalarial activity of adducts 5
and 6 in enantiomerically pure forms, as outlined in Table 2. Ad-
ducts (ꢀ)-5, (ꢀ)-6, (+)-5, and (+)-6 displayed antimalarial activity
against the parasite P. falciparum (K1, multi-drug resistance strain)
Antimalarial activity and cytotoxicity of adducts 5 and 6
Compounds
IC50
Antimalarial activitya,b
(l
g/mL)
Cytotoxicitya,c(vero cells)
Piperine 1
( )-5
( )-6
(ꢀ)-5
(ꢀ)-6
(+)-5
Inactive
4.7
3.4
4.3
3.6
Non-cytotoxic
Non-cytotoxic
Non-cytotoxic
Non-cytotoxic
Non-cytotoxic
Non-cytotoxic
Non-cytotoxic
4.0
4.5
(+)-6
a
All biological activities resulted from the average of multiple (three)
determinations.
b
The IC50 values of the standard antimalarial compounds chloroquine diphos-
phate,23 mefloquine and dihydroartemisinin (DHA) were 0.16, 0.012 and 0.001
lg/
mL, respectively.
c
The IC50 value of the standard compound ellipticine was 0.976 lg/mL for the
vero cells.
The antimalarial activity of the synthetic compounds was
determined by means of a microculture radioisotope technique
based on the method described by Desjardins et al.21 The ad-
ducts were screened in vitro for activity against the parasite P.
with IC50 values of 4.3, 3.6, 4.0, and 4.5 lg/mL respectively, as
shown in Table 2. From the above results, it can be seen that all
optically active products showed non-specific antimalarial activity
against the parasite P. falciparum.

R. Choommongkol et al. / Tetrahedron: Asymmetry 23 (2012) 357–363
361
mixture was left to stir at room temperature for 3 days. The reac-
tion mixture was quenched with a saturated aqueous ammonium
chloride solution at 0 °C and the crude mixture was extracted sev-
eral times with CH2Cl2. The dichloromethane solution was washed
with H2O, a saturated NaCl solution, then dried over MgSO4, fil-
tered and evaporated to dryness. The crude product was purified
by flash column chromatography (silica gel) using EtOAc/CH2Cl2/
hexane = 2:0.5:7.5 as eluent to give the diastereoisomeric spirocy-
clopentanone–anthracene adducts, 40-((E)-2-(benzo[c][1,3]dioxol-
1-yl)vinyl)-30-methoxycarbonyl-50-(piperidine-1-carbonyl)cyclo
pentanone-20-spiro-11-9,10-dihydro-9,10-ethanoanthracenes ( )-
5 and ( )-6 in 23% (2.37 g), and 15% (1.55 g) respectively.
3. Conclusions
In conclusion, we have developed an enantioselective synthesis
for the racemic and enantiomerically pure forms of spirocyclopen-
tanone–anthracene adducts 5 and 6 using tandem Michael addi-
tion–Dieckmann condensation reactions as the key steps. These
compounds exhibited in vitro antimalarial activity, and impor-
tantly exhibited non-cytotoxicity against vero cells. Further studies
are underway in order to improve the activity of our novel class of
antimalarial agents.
4. Experimental
4.2.1. Compound ( )-5
4.1. General methods
White solid; mp: 205.9–207.6 °C (from CH2Cl2/hexane); Rf (15%
EtOAc/5%CH2Cl2/hexane) 0.50; IR (thin film): mmax 2850, 2930,
All reactions were carried out under a nitrogen or argon atmo-
sphere. Unless otherwise noted, materials were obtained from
commercial suppliers and used without further purification. Melt-
ing points were determined by using a Gallenkamp Electrothermal
apparatus and are uncorrected. The 1H and 13C NMR spectra were
recorded on Bruker DRX 500 MHz spectrometers and chemical
shifts were given in ppm downfield from tetramethylsilane
(TMS). All NMR spectra were measured in CDCl3 and chemical
shifts are reported as d-values in parts per million (ppm) relative
to residue CHCl3 as the internal reference (1H: d 7.26, 13C: d
77.00) and coupling constants (J values) are reported in Hertz
(Hz). Peak multiplicities are indicated as follows: s (singlet), d
(doublet), dd (doublet of doublets), and m (multiplet). Infrared
spectra were taken with a FT-IR model TENSER 27 (Bruker) spec-
trometer and absorption frequencies were reported in reciprocal
centimeters (cmꢀ1). Mass spectra (electrospray ionization mode,
ESI-MS) were measured on a micromass Q-TOF-2™ (Waters) spec-
trometer. Optical rotations were measured in CHCl3 and MeOH
with the Sodium D-line (589 nm) on a Jasco P-1030 digital polar-
imeter. Flash column chromatography was performed employing
Merck silica gel 60 and Merck silica gel 60H. Preparative thin layer
chromatography (PLC) plates were carried out using Merck silica
gel 60 PF254. Analytical thin layer chromatography was performed
with Merck silica gel 60 F254 aluminum plates. Solvents were dried
over CaH2 and distilled before use. Tetrahydrofuran (THF) was
freshly distilled from sodium and benzophenone ketyl under nitro-
gen. Diisopropylamine was distilled over CaH2 and stored under
nitrogen. n-Butyllithium was purchased from Fluka and Across as
solution in hexane and titrated periodically according to the 2,5-
dimethoxybenzyl alcohol method. Enantiomeric excess was deter-
mined by 1H NMR spectroscopy using the chiral lanthanide shift
reagent, tris[3-(heptafluoropropylhydroxymethylene)-d-camphor-
ato]praseodym(III), Pr(hfc)3.
1730, 1438, 1370, 1250, 1100 cmꢀ1 1H NMR (500 MHz, CDCl3): d
;
1.49–1.69 (m, 6H, CH2), 1.31, 2.24, 4.32 (ABX system, J = 12.7, 2.7,
2.5 Hz, 3H, CH2, ArCH), 2.82 (d, J = 7.1 Hz, 1H, CH3OOCCH), 3.44,
3.80, 3.92 (m, 4H, CH2NCH2), 3.55 (s, 3H, COOMe), 3.87 (d,
J = 9.8 Hz, 1H, COCHCON), 4.49 (m, 1H, C@CHCH), 4.86 (s, 1H,
ArCH), 5.80 (dd, J = 15.7, 8.1 Hz, 1H, C@CHCH), 5.96 (s, 2H, OCH2O),
6.54 (d, J = 15.7 Hz, 1H, ArCH@C), 6.73 (s, 2H, ArH-piperine), 6.84
(s, 1H, ArH-piperine), 7.02–7.45 (m, 8H, ArH-anthracene); 13C
NMR (125 MHz, CDCl3): d 24.6, 25.9, 26.6, 35.8, 42.0, 44.1, 44.3,
46.8, 47.6, 51.6, 53.3, 55.8, 60.6, 101.1, 105.5, 108.3, 121.8, 122.9,
123.7, 125.2, 125.4, 125.7, 125.8, 126.0, 126.6, 126.8, 132.8,
131.2, 138.7, 139.4, 143.5, 144.2, 147.4, 148.1, 165.4, 173.8,
207.4; HRMS (ESI) calcd for
612.2361, found 612.2362.
C
37H35NO6Na (M+Na)+: m/z
4.2.2. Compound ( )-6
White solid; mp: 223.4–225.9 °C (from CH2Cl2/hexane); Rf (15%
EtOAc/5%CH2Cl2/hexane) 0.36; IR (KBr-pellet): mmax 2850, 2930,
1730, 1460, 1370, 1250, 1100 cmꢀ1 1H NMR (500 MHz, CDCl3): d
;
1.40–1.71 (m, 6H, CH2), 1.90, 2.07, 4.37 (ABX system, J = 12.9, 2.9,
2.2 Hz, 3H, CH2, ArCH), 2.35 (d, J = 6.6 Hz, 1H, CH3OOCCH), 3.32,
3.54, 3.66 (m, 4H, CH2NCH2), 3.78 (s, 3H, COOMe), 3.93 (m, 1H,
C@CHCH), 4.00 (d, J = 11.3 Hz, 1H, COCHCON), 4.43 (s, 1H, ArCH),
5.72 (dd, J = 15.7, 7.4 Hz, 1H, C@CHCH), 5.94 (s, 2H, OCH2O), 6.33
(d, J = 15.7 Hz, 1H, ArCH@C), 6.65–6.82 (m, 3H, ArH-piperine),
6.90–7.44 (m, 8H, ArH-anthracene); 13C NMR (125 MHz, CDCl3): d
24.6, 25.6, 26.7, 41.1, 41.7, 43.6, 44.2, 47.5, 47.8, 51.5, 53.9, 57.9,
58.0, 101.1, 105.5, 108.3, 121.8, 122.6, 123.9, 124.2, 124.4, 125.5,
125.6, 125.9, 126.5, 126.9, 131.2, 132.4, 140.6, 141.4, 143.0,
143.5, 147.3, 148.0, 165.2, 174.0, 210.4; HRMS (ESI) calcd for
C
37H35NO6Na (M+Na)+: m/z 612.2361, found 612.2362.
4.3. Resolution to prepare optically active dimethyl itaconate–
anthracene adducts (+)-2 and (ꢀ)-2
4.2. General procedure for the synthesis of 40-((E)-2-(benzo[c]-
[1,3]dioxol-1-yl)vinyl)-30-methoxycarbonyl-50-(piperidine-1-
carbonyl)cyclopentanone-20-spiro-11-9,10-dihydro-9,10-ethano-
anthracenes ( )-5 and ( )-6
4.3.1. (ꢀ)-11-Carbomethoxy-11-((ꢀ)-menthoxyacetyl)-9,10-
dihydro-9,10-ethanoanthracenes (11S)-3 and (11R)-4
A solution of KOH (1.08 g, 19.4 mmol) in H2O (140 mL) was
added to a solution of ( )-dimethyl itaconate–anthracene adduct,
2 (5.01 g, 14.9 mmol), in MeOH (270 mL), and heated at reflux for
2 h. The cooled reaction mixture was diluted with water
(130 mL) and acidified to pH 2–3 by 10% HCl, then extracted with
CH2Cl2, dried over MgSO4, filtered, and evaporated to dryness. The
crude product was crystallized from CH2Cl2–hexane to give the
corresponding ( )-monoacid (4.65 g, 97%) as a white solid; mp:
To a 250 mL round-bottomed flask equipped with a magnetic
stirrer fitted with a three-way stopcock with a septum cap and
nitrogen inlet were added THF (50 mL) and dry diisopropylamine
(7.9 mL, 55.97 mmol) via syringes. The mixture was cooled down
to ꢀ78 °C, after which n-butyllithium (1.4 M in hexane, 33.0 mL,
46.64 mmol) was added and the mixture left to stir at 0 °C for
1 h. A solution of ( )-dimethyl itaconate–anthracene adduct ( )-2
(13.1 g, 38.9 mmol) in THF (50 mL) was introduced to the LDA
solution at ꢀ78 °C, then the mixture was stirred at 0 °C for 2 h. A
solution of piperine 1 (13.3094 g, 46.6 mmol) in THF (100 mL)
was added to the anion solution at ꢀ78 °C after which the reaction
207–209 °C.
A
mixture of the monoacid adduct (4.65 g,
14.4 mmol), thionyl chloride (5.3 mL, 72.1 mmol), and DMF (3
drops, as catalyst) was heated at reflux for 2 h after which the sol-
vent was removed under reduced pressure. A mixture of the acid
chloride obtained, benzene (160 mL), triethylamine (2.6 mL,

362
R. Choommongkol et al. / Tetrahedron: Asymmetry 23 (2012) 357–363
18.8 mmol), and (ꢀ)-menthol (2.93 g, 18.8 mmol) was heated at
reflux for 2 h, filtered through Celite 545, diluted with H2O, and ex-
tracted with CH2Cl2. The solution was washed with H2O, saturated
NaCl solution, dried over MgSO4, filtered and evaporated to dry-
ness. The crude product was purified by flash column chromatog-
raphy (silica gel) using EtOAc/acetone/hexane = 0.5:0.3:9.2 as
eluent to give two diastereoisomers, (ꢀ)-3 (2.33 g, 35%, >99% de)
Compound (+)-(30S,40S,50S,11S)-6 (8%): white solid, mp: 223.4–
24:4
225.9 °C (from CH2Cl2/hexane); ½
aꢁ
¼ þ67:6 (c 0.60, CHCl3)
589
whose IR, 1H NMR, 13C NMR, and mass spectroscopic data were
identical to those previously reported.
4.6. X-ray crystallographic analysis of adducts ( )-5
as a white solid; mp: 185–186 °C (from EtOAc/hexane); (lit.19
White crystals of ( )-5 were obtained in the mixture of CH2Cl2/
hexane by slow evaporation. X-ray diffraction data were measured
on a Bruker-Nonius kappaCCD diffractometer with graphite mono-
30
589
mp: 185–187 °C (from EtOAc/hexane)); ½
a
ꢁ
¼ ꢀ108:8 (c 1.10,
CHCl3); {lit.19
½
a 3D0
ꢁ
¼ ꢀ109:0 (c 1.29, CHCl3)}; and (ꢀ)-4 (2.32 g,
35%, >99% de) as a white solid; mp: 101–103 °C (from MeOH);
chromated Mo Ka radiation (k = 0.71073 Å) at 298(2) K. The struc-
30
589
(lit.19 mp: 101–103 °C (from MeOH)); ½
a
ꢁ
¼ ꢀ51:2 (c 1.10,
ture was solved by direct methods by SIR97,24 and refined with
CHCl3);}lit.19
½
a 3D0
ꢁ
¼ ꢀ51:4 (c 1.20, CHCl3)} whose IR, 1H, 13C
full-matrix least-squares calculations on F2 using SHELXL-97.25
NMR, and mass spectroscopic data were identical to those previ-
ously published.19
4.6.1. Crystal data of adduct 5
C
37H35NO6ꢂH2O, MW = 607.68, monoclinic, dimensions: 0.25 ꢃ
4.3.2. (11S)-11-Carbomethoxy-11-methoxyacetyl-9,10-dihydro-
9,10-ethanoanthracene (+)-2
0.15 ꢃ 0.10 mm3, D = 1.255 g/cm3, space group P21/c, Z = 4, a =
18.7483(9), b = 9.9445(2), c = 19.2932(9) Å, b = 116.5848(14)°, V =
3216.8(2) Å3, reflections collected/unique: 9565/4844 (Rint = 0.025),
To a solution of (ꢀ)-3 (2.01 g, 4.4 mmol) in excess anhydrous
MeOH (600 mL) was added concd H2SO4 (10 mL) and the mixture
was heated under reflux for 7 days. Next, H2O was added, and the
mixture neutralized with aqueous NaHCO3 solution, then ex-
tracted with CH2Cl2. The CH2Cl2 extract was washed with H2O,
dried over MgSO4, filtered, and evaporated to dryness. The crude
product was recrystallized from EtOAc/hexane to give optically
number of observation [>2r(I)] 3722, final R indices [I >2r(I)]:
R1 = 0.0785, wR2 = 0.2569.
4.7. Bioassay procedures
The in vitro antimalarial activity was evaluated against the par-
asite P. falciparum (K1, multi-drug resistance strain), which was
cultured continuously according to the method of Trager and Jen-
sen.26 Quantitative assessment of the antimalarial activity in vitro
was determined by means of the microculture radioisotope tech-
nique based on the method described by Desjardins et al.21 The
inhibitory concentration (IC50) represents the concentration that
causes 50% reduction in the parasite growth as indicated by the
in vitro uptake of [3H]-hypoxanthine by P. falciparum. The standard
compounds, mefloquine and dihydroartemisinin (DHA), exhibited
active adduct (+)-2 (1.43 g, 97% yield, >99% ee) as a white solid;
29
589
mp: 153–155 °C (from EtOAc/hexane); ½
aꢁ
¼ þ38:2 (c 1.01,
CHCl3); {lit.19
½
a 2D9
ꢁ
¼ þ38:7 (c 1.06, CHCl3)} whose IR, 1H, 13C
NMR, and mass spectroscopic data were identical to those previ-
ously published.19
4.3.3. (11R)-11-Carbomethoxy-11-methoxyacetyl-9,10-dihydro-
9,10-ethanoanthracene (ꢀ)-2
Under the same conditions, adduct (ꢀ)-4 provided (ꢀ)-2 (89%
yield, >99% ee) as white solid; mp: 154–155 °C (from EtOAc/hex-
IC50 values of 0.012 and 0.001 lg/mL, respectively, in this test sys-
29
589
ane); ½
aꢁ
¼ ꢀ38:8 (c 1.02, CHCl3); {lit.19
½
a 2D9
ꢁ
¼ ꢀ39:0 (c 1.18,
tem. The cytotoxicity of the purified compounds against vero cells
(African green monkey kidney) was evaluated employing the col-
orimetric method as described by Hunt et al.27 Ellipticine was used
as the reference substance, exhibiting the activity toward vero
cells, with IC50 of 0.976 /mL.
CHCl3)} whose IR, 1H, 13C NMR, and mass spectroscopic data were
identical to those previously published.19
4.4. Synthesis of enantiomerically pure 40-((E)-2-(benzo[c][1,3]-
dioxol-1-yl)vinyl)-30-methoxycarbonyl-50-(piperidine-1-carbonyl)
cyclopentanone-20-spiro-11-9,10-dihydro-9,10-ethanoanthracenes
(ꢀ)-5 and (ꢀ)-6 from (+)-2
Acknowledgments
This research was supported by a grant under the program Stra-
tegic Scholarships for Frontier Research Network for the Ph.D. Pro-
gram Thai Doctoral degree from the Commission on Higher
Education (CHE), Thailand (to R.C.). We also would like to thank
the Center for Innovation in Chemistry (PERCH-CIC) and the Na-
tional Research University Project under Thailand0s office of the
Higher Education Commission. In addition, we thank the Bioassay
Research Facility of the BIOTEC Center, NSTDA, for performing anti-
malarial and cytotoxicity tests, and also thank the Department of
Chemistry, Faculty of Science, Chiang Mai University for facilities
supporting this research. P.K. acknowledges Mahidol University
and the Thailand Research Fund (DBG545480015) for financial
support.
Compounds (ꢀ)-5 and (ꢀ)-6 were obtained when (+)-2 was em-
ployed in the above typical procedure. Compound (ꢀ)-
(30S,40S,50S,11R)-5 (33%): white solid; mp: 205.9–207.6 °C (from
24:4
589
CH2Cl2/hexane); ½
aꢁ
¼ ꢀ18:5 (c 0.87, CHCl3) whose IR, 1H
NMR, 13C NMR, and mass spectroscopic data were identical to
those previously reported.
Compound (ꢀ)-(40R,40R,50R,11R)-6 (17%): white solid, mp:
24:4
223.4–225.9 °C (from CH2Cl2/hexane);
½
aꢁ
¼ ꢀ70:5 (c 0.58,
589
CHCl3) whose IR, 1H NMR, 13C NMR, and mass spectroscopic data
were identical to those previously reported.
4.5. Synthesis of enantiomerically pure 40-((E)-2-(benzo[c][1,3]-
dioxol-1-yl)vinyl)-30-methoxycarbonyl-50-(piperidine-1-carbo-
nyl)cyclopentanone-20-spiro-11-9,10-dihydro-9,10-ethanoanth-
racenes (+)-5 and (+)-6 from (ꢀ)-2
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