Preparation and antimalarial activity of a novel class of carbohydrate-derived, fused thiochromans

Article abstract of DOI:10.1016/j.ejmech.2014.09.060

A novel class of fused thiochroman derivatives has been prepared by an efficient and versatile synthetic procedure involving nucleophilic displacement of the side-chain iodo substituent in 2-deoxy-2-C-iodomethyl glucosides by thiophenolate ions, and subsequent intramolecular C-glycoside formation. A range of aromatic substituents is tolerated, and the subsequent facile selective oxidation of the sulfur to the sulfoxide or sulfone level expands the range and molecular diversity of the series of compounds. A selection of the sulfoxide and sulfone derivatives bearing lipophilic substituents on the aromatic portion were found to have antimalarial activities in the low micromolar range.

Full text of DOI:10.1016/j.ejmech.2014.09.060

European Journal of Medicinal Chemistry 87 (2014) 197e202
Contents lists available at ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Original article
Preparation and antimalarial activity of a novel class of carbohydrate-
derived, fused thiochromans
,
Henok H. Kinfe a *, Paseka T. Moshapo a, Felix L. Makolo a, David W. Gammon b,
Martin Ehlers c, Carsten Schmuck c
a Department of Chemistry, University of Johannesburg, Auckland Park 2006, South Africa
b Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
c
€
Institute of Organic Chemistry, University of Duisburg-Essen, Universitatstrasse 7, 45141 Essen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel class of fused thiochroman derivatives has been prepared by an efficient and versatile synthetic
procedure involving nucleophilic displacement of the side-chain iodo substituent in 2-deoxy-2-C-
iodomethyl glucosides by thiophenolate ions, and subsequent intramolecular C-glycoside formation. A
range of aromatic substituents is tolerated, and the subsequent facile selective oxidation of the sulfur to
the sulfoxide or sulfone level expands the range and molecular diversity of the series of compounds. A
selection of the sulfoxide and sulfone derivatives bearing lipophilic substituents on the aromatic portion
were found to have antimalarial activities in the low micromolar range.
Received 30 May 2014
Received in revised form
23 August 2014
Accepted 17 September 2014
Available online 19 September 2014
Keywords:
Antimalarial
Thiochromans
© 2014 Elsevier Masson SAS. All rights reserved.
Chloroquine-sensitive
Chloroquine-resistant
1. Introduction
were curative at dose levels of 160e360 mg/kg against Plasmodium
berghei in mice [11]. Although these results were encouraging,
Thiochromans, 3,4-dihydro-2H-1-benzothiopyrans and their
derivatives, continue to attract significant interest from organic and
medicinal chemists due to their presence as key components in
biologically active compounds that have shown anti-cancer [1,2],
anti-HIV [3], anti-bacterial [4] and anti-fungal [5] activity, as well as
being employed in the treatment of depression [6], schizophrenia,
Parkinson's [7] and Alzheimer's diseases [8,9]. Moreover, the
enhanced biological activity exhibited by thiochromans in com-
parison to their corresponding oxygen analogues [7,10] and the
ease of derivatizing them into sulfoxides, sulfones, and Pummerer
products, thus generating libraries of compounds for structure ac-
tivity relationship studies, highlights their potential in medicinal
chemistry programmes. However, despite their wide spectrum of
biological and medicinal importance, the evaluation of thiochro-
man derivatives for their antimalarial activity has been overlooked.
According to our literature survey, the last antimalarial activity
study conducted on thiochroman derivatives was four decades ago.
In 1978 Razdan et al. from the company Arthur D. Little Inc. reported
that some derivatives of thiochromans were found to be active and
there have been no further reports on any studies conducted to
improve the efficacy and cytotoxicity (if any) of such thiochroman
derivatives in relation to their antimalarial activity.
The reliability, effectiveness, limited host toxicity as well as low
cost of chloroquine as an antimalarial drug in the past decades
might have discouraged research and development of alternative
antimalarials until the emergence of chloroquine-resistant malaria
strains [12,13]. Currently, artemisinin-based combination therapy is
World Health Organization's standard treatment against Plasmo-
dium falciparum, the most lethal species that causes malaria in
humans, in which the regimen uses a double or triple combination
therapy with the aim of either delaying or preventing the devel-
opment of drug resistance [14e16]. Although there are preventative
and curative measures that have been adopted and with no clinical
resistance having been reported against artemisinin, recent in-
dications from South-East Asia are increasingly pointing towards
tolerance which may eventually lead to resistance against this class
of drugs [17e19]. Malaria accounts for over half-a-million deaths
per annum worldwide and thus still constitutes a global health risk.
Spitzmüller and Mestres have recently highlighted that while there
is an urgent need to map the macromolecular drug target space in
P. falciparum, there is also a need to expand the range of chemo-
types with potential antimalarial activity [20]. Research in these
* Corresponding author.
E-mail address: hhkinfe@uj.ac.za (H.H. Kinfe).
http://dx.doi.org/10.1016/j.ejmech.2014.09.060
0223-5234/© 2014 Elsevier Masson SAS. All rights reserved.

198
H.H. Kinfe et al. / European Journal of Medicinal Chemistry 87 (2014) 197e202
directions will increase the chances of discovery and development
of new antimalarial drugs with novel mechanisms of action [21].
Our interest in the stereoselective synthesis of carbohydrate
based thiochroman derivatives and expansion of the range of
antimalarial chemotypes, prompted us to prepare novel thiochro-
man derivatives and evaluate their antimalarial activities. Herein
we report the in vitro antimalarial activity of thiochroman de-
rivatives 3e9, differing in their sulfur oxidation states, aromatic
substituents, anomeric configurations and nature of protecting
groups.
was confirmed by a combination of common techniques such as IR,
NMR and HRMS (Scheme 2) [24].
Oxidation of sulfides 3 with excess OXONE® produced sulfones
7. The acetylated thiochroman 7g was further hydrolysed to afford
the triol 8. Portions of 7(aee) were then epimerized upon treat-
ment with NaH to provide the b-C-glycosides 9(aee) in excellent
yields following our recently reported protocol (Scheme 3) [25].
2.2. Antimalarial activity of synthetic thiochromans
After the successful synthesis of the thiochroman derivatives,
these were evaluated for their activity against the chloroquine-
sensitive 3D7 and chloroquine-resistant FCR3 strains of the ma-
laria parasite, P. falciparum, by measuring parasite survival using a
parasite lactate dehydrogenase (pLDH) assay. Compounds that
exhibited parasite survival rates of 0e15% at a single concentration
2. Results and discussion
2.1. Chemistry: preparation of thiochromans
Thiochroman derivatives (3e4) were synthesized starting with
an
a,
b-anomeric mixture of 2-C-iodomethyl-glucosyl acetates 1 as
of 10 mM were screened further for doseeresponse to determine
outlined in Scheme 1 following our recently reported protocol
[22,23]. Treatment of the anomeric mixture 1 with a range of aryl
sodium thiolates that were each freshly prepared in DMF yielded
sulfides (2) which then underwent Lewis acid catalysed Friedele-
the IC50 values and the results are summarized in Table 1 (refer to
the supporting information for detailed data).
The structural parameters explored for SAR correlation were
oxidation state of the sulfur (sulfide, sulfoxide and sulfone), nature
and position of the substituent on the aromatic ring of the thio-
chroman motif, stereochemistry of the sulfoxide group, anomeric
configuration of the sugar moiety (and consequent geometry of the
ring fusion), nature of protecting group and hydrophilicity.
The lack of high potency and activity of thiochromans 3aef and
thiochroman derivatives 5a, 6a, and 7a, in comparison to the other
analogues, indicates the requirement for sulfur in higher oxidation
states and the presence of a substituent on the aromatic ring of the
thiochroman moiety to impart activity/potency. In agreement with
the literature report, the sulfone functional group imparted higher
antimalarial activity than the corresponding sulfides (compare
3aef vs 7bef and the epimerized sulfones in Table 1) [26]. With
regard to the sulfoxides, there is only one instance of significant
difference in the activity of the diastereomeric sulfoxides, viz. the
case of 5b and 6b, with 5b essentially inactive against strain 3D7,
Crafts alkylation to give the a-C-glycosides (3e4) as single isomers.
Efforts to debenzylate thiochromans 3 using traditional hydro-
genolysis using H2 and catalytic Pd/C to obtain hydrophilic thio-
chroman derivatives were unsuccessful; however, acetylated
thiochroman 3g could be readily hydrolysed under basic conditions
to afford the more hydrophilic derivative 4 (Scheme 1).
Oxidation of thiochromans 3(aef) in the presence of cerium
ammonium nitrate, wet silica gel and catalytic amounts of potas-
sium bromide formed a diastereomeric 2:1 mixture of sulfoxides
without over-oxidation to sulfones. The diastereomers were sepa-
rated by column chromatography to afford 5(aef) and 6(aef). Ef-
forts to identify the stereochemistry of each sulfoxide by growing
crystals of the respective compounds with a view to obtain single
crystal X-ray diffraction were unsuccessful and the configurations
are tentatively assigned. The chemical structure of the sulfoxides
OR1
OR1
OR1
O
R1O
O
O
R1O
R1O
R1O
R1O
i
ii
R1O
OAc
OAc
S
S
I
R3
R2
1a R1 = Bn
1b R1 = Ac
R3
R2
2a R1 = Bn, R2 = R3 = H (83%)
2b R1 = Bn, R2 = CH3, R3 = H (87%)
2c
3a R1 = Bn, R2 = R3 = H (69%)
R
1 = Bn, R2 = CH3, R3 = H (71%)
3b
R
R
1 = Bn, R2 = OCH3, R3 = H *
3c
1
R = Bn, R2 = OCH3, R3 = H (70%)
1 = Bn, R2 = C(CH3)3, R3 = H (93%)
3d R1 = Bn, R2 = C(CH3)3, R3 = H (69%)
3e R1 = Bn, R2 = H, R3 = CH3 (73%)
2d
2e R1 = Bn, R2 = H, R3 = CH3 (87%)
S
S
2f
2-naphthyl (
) (86%)
3f
2-naphthyl (
) (68%)
2g R1 = Ac,R2 = C(CH3)3, R3 = H (91%)
3g R1 = Ac, R2 = C(CH3)3, R3 = H (75%)
R1 = H, R2 = C(CH3)3, R3 = H (77%)
iii
4
aReagents and conditions: i) 60% NaH, Aryl thiol, DMF, rt, 5 min; ii) BF3·Et2O, DCM, 0 °C,
5 min; iii) K2CO3, MeOH, rt, 3 h. *Sulfide 2c was unstable on standing and thus it was
immediately cyclized without purification.
Scheme 1. Diastereoselective synthesis of thiochroman derivatives 3-4a.

H.H. Kinfe et al. / European Journal of Medicinal Chemistry 87 (2014) 197e202
199
OBn
OBn
OBn
O
O
O
BnO
BnO
BnO
BnO
BnO
BnO
+
-O
S+
S
S+
-O
R3
3(a-f)
R2
R3
5(a-f)
R2
R3
6(a-f)
R2
5a R2 = R3 = H (40%)
R
R
2 = R3 = H (27%)
6a
6b
2
R
R
= CH3, R3 = H (44%)
5b
5c
2 = CH3, R3 = H (24%)
6c R2 = OCH3, R3 = H (23%)
6d R2 = C(CH3)3, R3 = H (33%)
6e
2 = OCH3, R3 = H (49%)
5d R2 = C(CH3)3, R3 = H (39%)
5e R2 = H, R3 = CH3 (43%)
5f 2-naphthyl (26%)
R
2 = H, R3 = CH3 (28%)
6f
2-naphthyl (18%)
bReagents and conditions: CAN, KBr, 50% wet silica gel, CH3CN:DCM (8:2), rt, 30 min.
Scheme 2. Oxidation of thiochroman derivatives 3 to their diastereomeric sulfoxides 5 and 6b.
cReagents and conditions: i) Wet Al2O3, OXONE®, DCM, rt, 12 h; ii) K2CO3, MeOH, rt, 3h;
iii) NaH, DMF, rt, 15 min.
Scheme 3. Base-promoted epimerization of a-C-glycosides 7(aef) to b
-C-glycosides 9(aee)c.
while 6b has activity comparable to the other sulfoxides. Moreover,
the activity of the sulfones was dependent on the anomeric
configuration of the sugar moiety (or ring-fusion geometry), with a
1,2-cis isomer found to be more potent than the corresponding 1,2-
trans isomer (compare 7c vs 9c, 7d vs 9d, 7e vs 9e and 7f vs 9f). The
structural feature with the highest impact on the potency is a bulky,
lipophilic tert-butyl substituent at the para-position of the aromatic
ring in both the sulfoxide and sulfone variants of the thiochroman,
Almost all the compounds tested against the FCR3 strain
exhibited activity with the exception of 3d, 6a and 9f. In contrast to
the activity profile against 3D7, activity against FCR3 could not be
correlated with structural features. However, as for the activity
against the 3D7 strain, the sulfoxides and sulfones having a lipo-
philic tert-butyl group exhibited the highest antimalarial potency.
To assess the effect of the protecting group on the sugar moiety
and water solubility of the analogues, acetate protected analogues
3g and 7g as well as the water soluble analogues 4 and 8 were
assayed for their antimalarial activity against both the 3D7 and
FCR3 strains. However, both sets of compounds were found to be
inactive against both strains of the P. falciparum confirming the
importance of the balance between lipophilicity and polarity.
Finally, to investigate the selectivity of all of the active com-
pounds, they were assayed for cytotoxicity towards human fetal
lung fibroblast (WI-38) cell line and the results are summarized in
Table 1. All tested compounds except 6c were found to be non-toxic.
as is evident from the lowest recorded IC50 values (<0.4 mM for 5d
and 7d) in the entire series. A similar observation was noted by
Bachi et al. where a free hydroxyl group was protected to reduce
polarity and resulted in a higher potency [26]. Thus, fine tuning the
balance between polarity and lipophilicity could be a crucial factor
in the antimalarial activity of the test compounds.
Unlike the activity against the 3D7 strains, the oxidation state of
sulfur was not crucial for activity against the FCR3 strain: thio-
chromans 3bef did not show any activity against 3D7 but the same
compounds (with the exception of 3d) exhibited activity against
FCR3 (Entries 2e6 in Table 1). This also suggests the strain speci-
ficity of the tested compounds.

200
H.H. Kinfe et al. / European Journal of Medicinal Chemistry 87 (2014) 197e202
Table 1
represented class of compounds with antimalarial activity, suggests
they are worthy of further study. These observations therefore
provide a platform for preparation and evaluation of modified
sulfoxide and sulfone derivatives bearing alternative and perhaps
bulkier lipophilic substituents. Since thiochromans have rarely
been reported as having antimalarial activity, an investigation on
the possible mode of action and bioassay of the active compounds
in combination with known antimalarials is underway and the
results will be reported in due course.
In vitro antimalarial activities of the thiochroman derivatives against chloroquine-
sensitive (3D7) and chloroquine-resistant (FCR3) Plasmodium falciparum strains,
and their cytotoxicities.
Entry S-oxidation Tested compound
3D7 IC50 FCR3 IC50 Cytotoxicity
M) M) IC50 M)
(
m
(
m
(m
1
2
3
4
5
Sulfide
3a R1 ¼ Bn,
R2 ¼ R3 ¼ H
3.46
2.05
3.37
1.73
>100
2.26
>100
>100
>100
n.d.a
3b R1 ¼ Bn, R2 ¼ CH3, >100
R3 ¼ H
3c R1 ¼ Bn, R2 ¼ OCH3, >100
R3 ¼ H
4. Experimental protocols
3d R1 ¼ Bn,
>100
R2 ¼ C(CH3)3, R3 ¼ H
3e R1 ¼ Bn, R2 ¼ H,
R3 ¼ CH3
4.1. General methods
>100
>100
6
7
3f 2-naphthyl
>100
>100
7.11
>100
>100
n.d.
All the solvents used were freshly distilled. Dichloromethane
was distilled over phosphorous pentoxide in a condenser fitted
with a drying tube containing calcium chloride. Other solvents
were dried by appropriate techniques. The 2-C-iodomethyl-gluco-
syl acetates 1, sulfides 2 and 3, sulfoxides 5 and 6 and sulfones 7 and
9 were synthesized according to literature methods previously
reported and their experimental data were in agreement with the
literature [22e25]. The synthesis of the acetate analogues (2g, 3g, 4,
7g) and the triol 8 is discussed below. All reagents were purchased
from Sigma Aldrich. All reactions were monitored by thin layer
chromatography (TLC) on aluminum-backed Merck silica gel 60
F254 plates using an ascending technique. The plates were visual-
ized by spraying with a 1:1 solution of 5% p-anisaldehyde in ethanol
and 10% sulfuric acid in ethanol baking at 150 ꢀC. Gravity column
chromatography was done on Merck silica gel 60 (70e230 mesh).
Melting points were determined using a Reichert-Jung Thermovar
hot-stage microscope and are uncorrected. Optical rotations were
determined on a PerkineElmer 141 polarimeter in chloroform so-
lutions at 25 ꢀC. The concentration c refers to g/100 mL. Infrared
spectra were recorded using Tensor 27 Bruker and PerkineElmer
FT-IR spectrum BX. All proton nuclear magnetic resonance (1H
NMR) spectra were recorded as deuteriochloroform solutions using
tetramethylsilane as an internal standard on a Bruker Ultrashield
(400 MHz) spectrometer. Carbon-13 nuclear magnetic resonance
3g R1 ¼ Ac,
R2 ¼ C(CH3)3, R3 ¼ H
4 R1 ¼ H,
8
>100
>100
>100
n.d.
R2 ¼ C(CH3)3, R3 ¼ H
9
Sulfoxide 1 5a R2 ¼ R3 ¼ H
10.34
4.11
2.62
0.33
>100
>100
>100
>100
10
11
12
5b R2 ¼ CH3, R3 ¼ H >100
5c R2 ¼ OCH3, R3 ¼ H 2.75
5d R2 ¼ C(CH3)3,
R3 ¼ H
0.30
13
14
15
16
17
18
5e R2 ¼ H, R3 ¼ CH3 4.13
2.76
0.52
>100
2.91
2.07
0.33
>100
>100
n.d.
>100
8.76
5f 2-naphthyl
3.55
Sulfoxide 2 6a R2 ¼ R3 ¼ H
>100
6b R2 ¼ CH3, R3 ¼ H 2.57
6c R2 ¼ OCH3, R3 ¼ H 2.21
6d R2 ¼ C(CH3)3,
R3 ¼ H
1.33
>100
19
20
21
6e R2 ¼ H, R3 ¼ CH3 6.29
4.34
2.17
3.95
>100
>100
>100
6f 2-naphthyl
7a R1 ¼ Bn,
R2 ¼ R3 ¼ H
2.55
Sulfone
>100
22
23
24
25
7b R1 ¼ Bn, R2 ¼ CH3, 2.32
R3 ¼ H
1.93
1.70
0.28
1.62
>100
>100
>100
>100
7c R1 ¼ Bn, R2 ¼ OCH3, 1.80
R3 ¼ H
7d R1 ¼ Bn,
0.39
R2 ¼ C(CH3)3, R3 ¼ H
7e R1 ¼ Bn, R2 ¼ H,
R3 ¼ CH3
2.34
(
13C NMR) spectra were recorded on the same instrument at
26
27
7f 2-naphthyl
2.09
3.83
>100
7g R1 ¼ Ac,
>100
>100
n.d.
100 MHz using tetramethylsilane as an internal standard. All
chemical shifts are reported in ppm. Anomeric ratios are calculated
from the 1H NMR spectroscopy of the crude product. Mass spec-
trometers were recorded on a Walters API Quattro Micro spec-
trometer at the University of Stellenbosch, South Africa.
R2 ¼ C(CH3)3, R3 ¼ H
8 R1 ¼ H,
28
>100
>100
n.d.
R2 ¼ C(CH3)3, R3 ¼ H
29
30
31
32
Epimerized 9a R1 ¼ R2 ¼ H
4.14
3.21
1.69
1.95
0.70
>100
>100
>100
>100
sulfone
9b R1 ¼ CH3, R2 ¼ H 2.34
9c R1 ¼ OCH3, R2 ¼ H 2.50
9d R1 ¼ C(CH3)3,
R2 ¼ H
>100
4.1.1. 1,3,4,6-tetra-O-acetyl-2-deoxy-2-C-(4-tert-butylphenyl)thio-
methyl-D-glucopyranosyl (2g)
33
34
35
9e R1 ¼ H, R2 ¼ CH3 3.40
2.14
>100
0.064
>100
n.d.
9f 2-naphthyl
Chloroquine
>100
To a solution of 4-tert-butylbenzenethiol (219 mL 1.27 mmol) in
0.016
DMF (10 mL), sodium hydride (60% dispersion on oil, 50.8 mg,
1.62 mmol) was added and the mixture was vigorously stirred at
room temperature for 5e10 min under nitrogen. A solution of
iodoacetate 1b (500 mg, 1.06 mmol) in DMF (2 mL) was then added
and after 5 min stirring of the reaction mixture, methanol (3 mL)
was added dropwise and the resulting clear solution was concen-
trated in vacuo. The solution was directly submitted to silica gel
chromatography (ethyl acetate/hexane, 2:8) to give the corre-
a
n.d. ¼ not determined.
3. Conclusion
Thirty four carbohydrate-fused thiochromans and their respec-
tive sulfoxide and sulfone derivatives were synthesised and eval-
uated for their antimalarial activity against both the chloroquine-
sensitive 3D7 and chloroquine-resistant FCR3 strains. Sulfoxide
and sulfone derivatives of the carbohydrate based thiochromans
possessing a bulky and lipophilic tert-butyl group at the para-po-
sition of the aromatic ring of the thiochroman moiety exhibited the
sponding sulfide 2g: 495 mg, 91% yield; 1:0.2
mixture, colourless syrup; IR (neat cmꢁ1) 2961, 1743, 1430, 1210,
1008, 821; 1H NMR (400 MHz, CDCl3)
: major anomer: 7.39e7.18
a:b anomeric
d
(m, 4H, Ar), 6.39 (d, J ¼ 3.2 Hz,1H, H-1), 5.25 (t, J ¼ 10.2 Hz,1H, H-3),
4.99 (t, J ¼ 9.6 Hz, 1H, H-4), 4.36 (dd, J ¼ 4.4 Hz and 12.8 Hz, 1H, H-
6a), 3.96 (m, 2H, H-6b and H-5), 3.02 (dd, J ¼ 3.8 Hz and 13.6 Hz, 1H,
H-7a), 2.63 (dd, J ¼ 10.8 Hz and 13.6 Hz,1H, H-7b), 2.40e2.25 (m,1H,
highest potency, with IC50 values in the range of 0.3e0.4 mM against
both strains. Although these fall short of the nanomolar activity of
chloroquine (6.77 nMe9.32 nM), their moderate to good activity,
low cytotoxicity and the fact that they represent a hitherto under-
H-2), 2.15e1.95 (m, 12H, 4ꢂ eOCOCH3), 1.28 (s, 9H, eC(CH3)3); 13
C
NMR (100 MHz, CDCl3d: 170.7 (eOCOCH3), 170.6 (eOCOCH3), 170.1

H.H. Kinfe et al. / European Journal of Medicinal Chemistry 87 (2014) 197e202
201
(eOCOCH3), 168.6 (eOCOCH3), 150.2 (Ar), 130.0 (Ar),126.3 (Ar), 91.2
(C-1), 71.6 (C-3), 69.5 (C-5), 68.7 (C-4), 61.7 (C-6), 42.8 (C-2), 34.5
(eC(CH3)3), 31.2 (C-7), 29.7 (eC(CH3)3), 20.8 (eOCOCH3), 20.7
(C-2), 35.3 (eC(CH3)3), 31.7 (eC(CH3)3), 27.1 (C-7). HRMS (ES þ -
TOF) m/z: [MþH]þ Calcd for C17H25O4S 325.1474; Found: 325.1487.
(eOCOCH3), 20.6 (eOCOCH3); 1H NMR (400 MHz, CDCl3)
d
: Minor
anomer: 5.62 (d, J ¼ 8.8 Hz, 1H, H-1), 5.34 (t, J ¼ 10.2 Hz, 1H, H-3),
1.26 (s, 9H, eC(CH3)3); 13C NMR (100 MHz, CDCl3)
: 169.9
4.1.4. 2-(Acetoxymethyl)-9-tert-butyl-2,3,4,4a,5,10b-hexahydro-S-
S-dioxothiochromeno[4,3-b]pyran-3,4-diyl diacetate (7g)
d
Sulfide 3g (126 mg, 0.280 mmol) was added to a vigorously
(eOCOCH3), 169.7 (eOCOCH3), 168.3 (eOCOCH3), 131.0 (Ar), 130.6
(Ar), 126.2 (Ar), 92.8 (C-1), 72.3 (C-3), 71.9 (C-5), 68.9 (C-4), 61.7 (C-
6), 44.8 (C-2), 31.6 (C-7), 20.8 (eOCOCH3), 20.6 (eOCOCH3). HRMS
(ES þ -TOF) m/z: [M þ NH4]þ Calcd for C25H38NO9S 528.2267;
Found: 528.2269.
stirring suspension of wet alumina (852 mg wetted with 90 mL of
water) and OXONE (1.4 g, 2.28 mmol) in DCM (5 mL). The reaction
mixture was stirred to completion at room temperature for 12 h.
The reaction mixture was then filtered to remove the adsorbent.
Evaporation of the solvent and flash chromatographic purification
on silica gel (ethyl acetate/hexane, 3:7) afforded the title sulfone:
103 mg, 76% yield; white solid, Mp 183e185 ꢀC; IR (neat cmꢁ1
)
4.1.2. 2-(Acetoxymethyl)-9-tert-butyl-2,3,4,4a,5,10b-
2955, 1745, 1218, 1035, 786; [
a
]
(c 0.1, CH3OH) þ26.5; 1H NMR
D
hexahydrothiochromeno[4,3-b]pyran-3,4-diyl diacetate (3g)
Sulfide 2g (100 mg, 0.19 mmol), was dissolved in dry dichloro-
methane (3 mL) under an atmosphere of nitrogen and stirred
together with 3 Å molecular sieves at room temperature for 1 h. The
mixture was cooled down to 0 ꢀC and then treated dropwise with
(400 MHz, CD3OD)
d
7.82 (d, J ¼ 8.4 Hz, 1H, Ar), 7.61e7.55 (m, 2H,
Ar), 5.33e5.23 (m, 2H, H-1, H-3), 4.83 (t, J ¼ 6.0 Hz, 1H, H-4), 4.73
(dd, J ¼ 8.0 Hz and 12.0 Hz, 1H, H-6a), 4.05 (dd, J ¼ 3.0 Hz and
12.2 Hz, 1H, H-6b), 4.06e4.00 (m, 1H, H-5), 3.77 (dd, J ¼ 8.6 Hz and
14.2 Hz, 1H, H-7a), 3.36 (dd, J ¼ 3.4 Hz and 14.2 Hz, 1H, H-7b),
3.10e3.00 (m, 1H, H-2), 2.12 (s, 3H, eOCOCH3), 2.09 (s, 3H,
eOCOCH3), 2.01 (s, 3H, eOCOCH3), 1.32 (s, 9H, (eC(CH3)3); 13C NMR
BF3$Et2O (414 mL, 0.783 mmol) of 48% BF3 solution in diethylether.
After stirring at this temperature for 5 min, Et3N (0.7 mL) was
added and the solids removed by filtration through a pad of Celite®
bed. The solution was then diluted with water (10 mL) and the
aqueous phase was extracted with dichloromethane (3 ꢂ 10 mL).
The combined organic phases were successively washed with
saturated aqueous NaHCO3 solution and brine, dried over MgSO4,
filtered, and evaporated. The residue was purified by column
chromatography on silica gel (ethyl acetate/hexane, 2:8) to yield
the corresponding thiochroman 3g: 66 mg, 75% yield; white solid,
Mp 215e217 ꢀC; IR (neat cmꢁ1) 2963, 1744, 1362, 1219, 1031, 831;
(100 MHz, CDCl3)d: 170.6 (eOCOCH3), 169.5 (eOCOCH3), 169.0
(eOCOCH3), 157.0 (Ar), 135.9 (Ar), 132.5 (Ar), 127.3 (Ar), 126.0 (Ar),
123.7 (Ar), 73.0 (C-5), 69.4 (C-3), 68.1 (C-4), 67.5 (C-1), 61.2 (C-6),
48.4 (C-7), 37.1 (C-2), 35.2 (eC(CH3)3), 30.9 (eC(CH3)3), 20.8
(eOCOCH3), 20.7 (eOCOCH3). HRMS (ES þ -TOF) m/z: [MþH]þ
Calcd for C23H31O9S 483.1689; Found: 483.1686.
4.1.5. 9-tert-Butyl-2-(hydroxymethyl)-2,3,4,4a,5,10b-hexahydro-S-
S-dioxothiochromeno[4,3-b]pyran-3,4-diol (8)
[
a]
(c 0.1, CHCl3) þ89.5; 1H NMR (400 MHz, CDCl3)
d: 7.55 (dd,
D
To a solution of thiochroman 7g (80.0 mg, 0.177 mmol) in
methanol (5 mL) was added anhydrous K2CO3 (2.45 mg,
0.0177 mmol) and the mixture was stirred at room temperature for
3 h upon which the reaction indicated disappearance of starting
material on TLC. The solvent was removed under reduced pressure
and the residue was submitted for purification by silica gel column
chromatography (100% ethyl acetate) to afford the thiochroman 8:
43 mg, 68% yield; white solid, Mp 218e220 ꢀC; IR (neat cmꢁ1) 3331,
J ¼ 0.4 Hz and 2.0 Hz,1H, Ar), 7.16 (ddd, J ¼ 0.5 Hz, 2.3 Hz and 8.2 Hz,
1H, Ar), 7.00 (d, J ¼ 8.4 Hz, 1H, Ar), 5.50 (dd, J ¼ 9.0 Hz, and 11.0 Hz,
1H, H-3), 5.17 (d, J ¼ 6.0 Hz, 1H, H-1), 5.04 (dd, 1H, J ¼ 9.2 Hz and
10.0 Hz, H-4), 4.26 (dd, J ¼ 2.9 Hz and 12.2 Hz, 1H, H-6a), 4.04 (dd,
J ¼ 2.0 Hz, and 12.0 Hz, 1H, H-6b), 3.75e3.63 (m, 1H, H-5), 3.33 (dd,
J ¼ 2.4 Hz and 13.6 Hz, 1H, H-7a), 2.84 (dd, J ¼ 4.4 Hz and 13.6 Hz,
1H, H-7b), 2.78e2.65 (m, 1H, H-2), 2.10 (s, 3H, eOCOCH3), 2.04 (s,
3H, eOCOCH3), 1.95 (s, 3H, eOCOCH3), 1.27 (s, 9H, eC(CH3)3); 13C
2922, 1600, 1350, 1112, 788; [
a
]
(c 0.1, CH3OH) þ51.0; 1H NMR
D
NMR (100 MHz, CDCl3)d: 170.7 (eOCOCH3), 170.2 (eOCOCH3),
(400 MHz, CD3OD)
d
: 7.76 (t, J ¼ 1.2 Hz, 1H, Ar), 7.67 (d, J ¼ 8.4 Hz,
170.0(eOCOCH3), 148.4 (Ar), 130.7 (Ar), 129.4 (Ar), 126.5 (Ar), 125.4
(Ar), 124.2 (Ar), 72.4 (C-1), 70.7 (C-4), 70.0 (C-3), 69.6 (C-5), 63.1 (C-
6), 36.7 (C-2), 34.4 (eC(CH3)3), 31.2 (eC(CH3)3), 25.9 (C-7), 20.8
(eOCOCH3), 20.7 (eOCOCH3). HRMS (ES þ -TOF) m/z: [M þ NH4]þ
Calcd for C23H34NO7S 468.2056; Found: 468.2054.
1H, Ar), 7.52 (ddd, J ¼ 0.8 Hz, 2.0 Hz and 8.4 Hz, 1H, Ar), 5.21 (d,
J ¼ 5.6 Hz, 1H, H-1), 3.88e3.73 (m, 3H, H-3, H-6a and H-7a),
3.68e3.55 (m, 2H, H-6b and H-7b), 3.28e3.12 (m, 2H, H-4 and H-5),
2.63e2.50 (m, 1H, H-2), 1.26 (s, 9H, eC(CH3)3); 13C NMR (100 MHz,
CD3OD)d: 158.1 (Ar), 138.0 (Ar), 135.8 (Ar), 127.3 (Ar), 125.9 (Ar),
124.4 (Ar), 77.3 (C-4), 73.4 (C-5), 72.1 (C-1), 71.1 (C-3), 63.2 (C-6),
50.6 (C-7), 42.6 (C-2), 36.3 (eC(CH3)3), 31.4 (eC(CH3)3). HRMS
(ES þ -TOF) m/z: [MþH]þ Calcd for C17H25O6S 357.1372; Found:
357.1363.
4.1.3. 9-tert-Butyl-2-(hydroxymethyl)-2,3,4,4a,5,10b-
hexahydrothiochromeno[4,3-b]pyran-3,4-diol (4)
To a solution of thiochroman 3g (92.0 mg, 0.18 mmol) in
methanol (5 mL) was added anhydrous K2CO3 (2.49 mg,
0.018 mmol) and the mixture was stirred at room temperature for
3 h upon which the reaction indicated disappearance of starting
material on TLC. The solvent was removed under reduced pressure
and the residue was submitted for purification by silica gel column
chromatography (100% ethyl acetate) to afford the thiochroman 4:
45 mg, 77% yield; white solid, Mp 222e224 ꢀC; IR (neat cmꢁ1) 3268,
4.2. In vitro antimalarial assay
The in vitro antimalarial activity of test samples against the 3D7
and FCR3 strains of the malaria parasite, P. falciparum, is measured
by parasite survival using parasite lactate dehydrogenase (pLDH)
assay. This enzymatic assay involves the parasite lactate dehydro-
genase, which is distinguishable from the host lactate dehydroge-
nase. Lactate dehydrogenase is an enzyme found in all the cells and
catalyses the formation of pyruvate from lactate reducing a co-
enzyme NAD (nicotinamide adenine dinucleotide) to NADH. In
parasites, the NAD analogue APAD (3-acetylpyridine adenine
nucleotide) is reduced to APADH and upon this reduction the yel-
low NBT/PES (nitro blue tetrazolium þ phenazine ethosulphate) is
converted to purple formazan crystals. The absorbance is read at
620 nm using a multiwell spectrophotometer (Infinite F500). The
2909, 1078, 869, 778; [
(400 MHz, CD3OD)
a
]
(c 0.1, CH3OH) þ105.5; 1H NMR
D
d
: 7.62 (dd, J ¼ 0.8 Hz and 2.0 Hz, 1H, Ar), 7.06
(ddd, J ¼ 0.7 Hz, 2.3 Hz and 8.4 Hz, 1H, Ar), 6.88 (d, J ¼ 8.0 Hz, 1H,
Ar), 4.99 (d, J ¼ 5.6 Hz, 1H, H-1), 3.80e3.71 (m, 2H, H-3, H-6a), 3.61
(dd, J ¼ 6.2 Hz and 11.8 Hz, 1H, H-6b), 3.34e3.22 (m, 2H, H-4, H-7a),
3.18e3.12 (m, 1H, H-5) 3.09 (dd, J ¼ 4.0 Hz and 13.2 Hz, 1H, H-7b),
2.31e2.20 (m, 1H, H-2), 1.21 (s, 9H, (eC(CH3)3); 13C NMR (100 MHz,
CD3OD)d: 148.9 (Ar), 132.5 (Ar), 132.3 (Ar), 126.8 (Ar), 125.8 (Ar),
125.6 (Ar), 76.0 (C-5), 73.5 (C-1 and C-4) 71.3 (C-3), 63.3 (C-6), 40.0

202
H.H. Kinfe et al. / European Journal of Medicinal Chemistry 87 (2014) 197e202
analogues, Bioorg. Med. Chem. Lett. 14 (2004) 3341e3343 (and references
cited therein).
formation of these crystals indicates the pLDH activity and there-
fore the survival of parasites.
[4] M.J. Brown, P.S. Carter, A.E. Fenwick, A.P. Fosberry, D.W. Hamprecht,
M.J. Hibbs, R.L. Jarvest, L. Mensah, P.H. Milner, P.J. O'Hanlon, A.J. Pope,
C.M. Richardson, A. West, D.R. Witty, The antimicrobial natural product
chuangxinmycin and some synthetic analogues are potent and selective in-
hibitors of bacterial tryptophanyl tRNA synthetase, Bioorg. Med. Chem. Lett.
12 (2002) 3171e3174.
The percentage survival of parasites is a measure of their
inhibitory activity against P. falciparum. This inhibitory activity is
determined by the IC50 and is measured by making 10 three-fold
serial dilutions of the test samples in triplicates in a transparent
96-well flat bottom plate (Netstar). The plate is put in an airtight
box, gassed and incubated for 48 h followed by developing with
NBT/PES reagent. The IC50 are expressed as the % parasite survival
relative to the control, calculated from fitted sigmoidal dos-
eeresponse curves. The doseeresponse curves were obtained by
plotting percentage parasite survival against the logarithm of the
concentration using the GraphPad Prism software package
(GraphPad software, Inc, California, USA). IC50s were calculated
graphically by interpolation from these curves. The Z'-factor for all
the tests were found to be between 0.5 and 1.0.
[5] T. Al Nakib, V. Bezjak, M.J. Meeganz, R. Chandy, Synthesis and antifungal ac-
tivity of some 3-benzylidenechroman-4-ones, 3-benzylidenethiochroman-4-
ones and 2-benzylidene-1-tetralones, Eur. J. Med. Chem. 25 (1990) 455e462.
€
[6] L.A. van Vliet, N. Rodenhuis, D. Dijkstra, H. Wikstrom, Synthesis and phar-
macological evaluation of thiopyran analogues of the dopamine D3 receptor-
selective agonist (4aR,10bR)-(þ)-trans-3,4,4a,10b-tetrahydro-4-n-propyl-
2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol (PD 128907), J. Med. Chem. 43
(2000) 2871e2882.
[7] Z. Du, C. Zhou, Y. Gao, Q. Ren, K. Zhang, H. Cheng, W. Wang, J. Wang, Expe-
ditious diastereoselective construction of
a thiochroman skeleton via a
cinchona alkaloid-derived catalyst, Org. Biomol. Chem. 10 (2012) 36e39 (and
references cited therein).
[8] M.L. Bolognesi, M. Bartolini, A. Cavalli, V. Andrisano, M. Rosini, A. Minarini,
C. Melchiorre, Design, synthesis and biological evaluation of conformationally
restricted rivastigmine analogues, J. Med. Chem. 47 (2004) 5945e5952.
[9] A. Seifert, R. Mahrwald, Stereoselective one-pot synthesis of highly differently
substituted thiochromans, Tetrahedron. Lett. 50 (2009) 6466e6468.
[10] L.W. Spruce, S.N. Rajadhyaksha, K.D. Berlin, J.B. Gale, E.T. Miranda, W.T. Ford,
E.C. Blossey, A.K. Verma, M.B. Hossain, D.V.D. Helm, T.R. Breitman, Hetero-
arotinoids. Synthesis, characterization, and biological activity in terms of an
assessment of these systems to inhibit the induction of ornithine decarbox-
ylase activity and to induce terminal differentiation of HL-60 cells, J. Med.
Chem. 30 (1987) 1474e1482.
4.3. In vitro cytotoxicity assay
The WI-38 cell line e normal Human Fetal Lung Fibroblast from
ECACC was routinely maintained as a monolayer cell culture at
37 ꢀC, 5% CO2, 95% air and 100% relative humidity in EMEM con-
taining 10% fetal bovine serum, 2 mM L-glutamine and 50 mg/ml
gentamicin. For screening experiment, the cells (21-50 passages)
were inoculated in a 96-well microtiter plates at plating densities of
10 000 cells/well and were incubated for 24 h. After 24 h the cells
were treated with the experimental drugs which were previously
dissolved in DMSO and diluted in medium to produce 5 concen-
trations. Cells without drug addition served as control. The blank
contains complete medium without cells. Parthenolide was used as
a standard. The plates were incubated for 48 h after addition of the
compounds. Viable cells were fixed to the bottom of each well with
cold 50% trichloroacetic acid, washed, dried and dyed by SRB. Un-
bound dye was removed and protein-bound dye was extracted with
10 mM Tris base for optical density determination at the wave-
length 540 nm using a multiwell spectrophotometer. Data analysis
was performed using GraphPad Prism software. 50% of cell growth
inhibition (IC50) was determined by non-linear regression.
[11] R.K. Razdan, R.J. Bruni, A.C. Mehta, K.K. Weinhardt, Z.B. Papanastassiou, A new
class of antimalarial drugs: derivatives of benzothiopyrans, J. Med. Chem. 21
(1978) 643e649.
[12] H. Weniger, Review of Side Effects and Toxicity of Chloroquine, WHO/MAL/
79.906, 1979, http://www.who.int/iris/handle/10665/65773.
[13] T.E. Wellems, C.V. Plowe, Chloroquine-resistant malaria, J. Infect. Dis. 184
(2001) 770e776.
[14] WHO's Good Procurement Practices for Artemisinin-based Antimalarial
Medicines, World Health Organization, March, 2010. ISBN 9789241598927,
http://www.who.int/malaria/publications/atoz/9789241598927/en/.
[15] G. Malenga, A. Palmer, S. Staedke, W. Kazadi, T. Mutabingwa, E. Ansah,
K.I. Barnes, C.J.M. Whitty, Antimalarial treatment with artemisinin combina-
tion therapy in Africa, Brit. Med. J. 331 (2005) 706e707.
[16] F. Nosten, N.J. White, Artemisinin-based combination treatment of falciparum
malaria, Am. J. Trop. Med. Hyg. 77 (2007) 181e192.
[17] A.M. Dondorp, F. Nosten, P. Yi, D. Das, A.P. Phyo, J. Tarning, K.M. Lwin, F. Ariey,
W. Hanpithakpong, S.J. Lee, P. Ringwald, K. Silamut, M. Imwong,
K. Chotivanich, P. Lim, T. Herdman, S.S. Yeung, P. Singhasivanon, N.P.J. Day,
N. Lindegardh, D. Socheat, N.J. White, Artemisinin resistance in Plasmodium
falciparum malaria, N. Engl. J. Med. 361 (2009) 455e467.
[18] H. Noedl, Y. Se, K. Schaecher, B.L. Smith, D. Socheat, M.M. Fukuda, Evidence of
artemisinin-resistant malaria in Western Cambodia, N. Engl. J. Med. 359
(2008) 2619e2620.
[19] A.P. Phyo, S. Nkhoma, K. Stepniewska, A.E. Ashley, S. Nair, R. McGready, C. ler
Moo, S. Al-Saai, A.M. Dondorp, K.M. Lwin, P. Singhasivanon, N.P.J. Day,
N.J. White, T.J.C. Anderson, F. Nosten, Emergence of artemisinin-resistant
malaria on the western border of Thailand: a longitudinal study, Lancet 379
(2012) 1960e1966.
[20] A. Spitzmüller, J. Mestres, Prediction of the P. falciparum target space relevant
to malaria drug discovery, PLoS Comput. Biol. 9 (2013) 1e14.
[21] P.J. Guerin, P. Olliaro, F. Nosten, P. Druilhe, R. Laxminarayan, F. Binka,
W.L. Kilama, N. Ford, N.J. White, Malaria: current status of control, diagnosis,
treatment, and a proposed agenda for research and development, Lancet.
Infect. Dis. 2 (2002) 564e573.
[22] H.H. Kinfe, P.T. Moshapo, F.L. Makolo, A. Muller, Facile diastereoselective
synthesis of carbohydrate-based thiochromans, J. Carbohydr. Chem. 32 (2013)
139e157.
Acknowledgements
The authors are grateful to: Research centre for synthesis and
catalysis of the Department of Chemistry, University of Johannes-
burg 275610 for financial support, and the Biosciences and
Biomedical Technologies research group of the CSIR for carrying out
the antimalarial tests.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.ejmech.2014.09.060.
[23] D.W. Gammon, H.H. Kinfe, D.E. De Vos, P.A. Jacobs, B.F. Sels, A new procedure
for highly regio- and stereoselective iodoacetoxylation of protected glycals
References
and a-1,2-cyclopropanated sugars, J. Carbohydr. Chem. 26 (2007) 141e157.
[24] H.H. Kinfe, F.M. Mebrahtu, F.L. Makolo, P.T. Moshapo, M.M. Manana, Stereo-
selective synthesis of thiochromenes via intramolecular tandem thio-Michael
[1] T. Yoneya, K. Taniguchi, R. Nakamura, T. Tsunenari, I. Ohizumi, Y. Kanbe,
K. Morikawa, S.I. Kaiho, H. Yamada-Okabe, Thiochroman derivative
CH4986399, a new nonsteroidal estrogen receptor down-regulator, is effec-
tive in breast cancer models, Anticancer Res. 30 (2010) 873e878.
[2] Y. Kanbe, M.H. Kim, M. Nishimoto, Y. Ohtake, T. Tsunenari, K. Taniguchi,
I. Ohizumi, S.I. Kaiho, Y. Nabuchi, S. Kawata, K. Morikawa, J.C. Jo, H.A. Kwon,
H.S. Lim, H.Y. Kim, Discovery of thiochroman derivatives bearing a carboxy-
containing side chain as orally active pure antiestrogens, Bioorg. Med.
Chem. Lett. 16 (2006) 4090e4094.
addition of in situ generated
(2014) 3111e3118.
a,b-unsaturated aldehydes, J. Org. Chem. 79
[25] H.H. Kinfe, F.L. Makolo, P.T. Moshapo, Z.H. Phasha, New carbohydrate-derived
S,S-dioxothiochroman derivatives and their unexpected easy epimerization at
the anomeric position, J. Carbohydr. Chem. 32 (2013) 193e204.
[26] M.D. Bachi, E.E. Korshin, P. Ploypradith, J.N. Cumming, S. Xie, T.A. Shapiro,
G.H. Posner, Synthesis and in vitro antimalarial activity of sulfone endoper-
oxides, Bioorg. Med. Chem. Lett. 8 (1998) 903e908.
[3] P. Xia, Z.J. Yin, Y. Chen, Q. Zhang, B. Zhang, Y. Xia, Z.Y. Yang, N. Kilgore, C. Wild,
S.L. Morris-Natschke, K.H. Lee, Anti-AIDS agents. Part 58: synthesis and anti-
HIV activity of 1-thia-di-O-(ꢁ)-camphanoyl-(þ)-cis-khellactone (1-thia-DCK)