Borrelidin analogues with antimalarial activity: Design, synthesis and biological evaluation against Plasmodium falciparum parasites

Article abstract of DOI:10.1016/j.bmcl.2013.02.075

Borrelidin, a structurally unique 18-membered macrolide, was found to express antimalarial activity against drug-resistant Plasmodium falciparum malaria parasites, with IC₅₀ value of 0.93 ng/mL. However, it also displays strong cytotoxicity against human diploid embryonic MRC-5 cells. To investigate the issue of the cytotoxicity of borrelidin, borrelidin-based analogues were synthesized and their anti-Plasmodium properties were evaluated. In this communication, we report that a novel borrelidin analogue, bearing the CH₂SPh moiety via a triazole linkage, was found to retain a potent antimalarial activity, against drug-sensitive and drug-resistant parasite strains, but possess only weak cytotoxicity against human cells.

Full text of DOI:10.1016/j.bmcl.2013.02.075

Bioorganic & Medicinal Chemistry Letters 23 (2013) 2302–2305
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Borrelidin analogues with antimalarial activity: Design, synthesis
and biological evaluation against Plasmodium falciparum parasites
Akihiro Sugawara, Toshiaki Tanaka, Tomoyasu Hirose, Aki Ishiyama, Masato Iwatsuki, Yoko Takahashi,
⇑
⇑
¯
Kazuhiko Otoguro, Satoshi Omura , Toshiaki Sunazuka
The Kitasato Institute and Kitasato Institute for Life Sciences and Graduate School of Infection Control Sciences, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo,
108-8641, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Borrelidin, a structurally unique 18-membered macrolide, was found to express antimalarial activity
against drug-resistant Plasmodium falciparum malaria parasites, with IC50 value of 0.93 ng/mL. However,
it also displays strong cytotoxicity against human diploid embryonic MRC-5 cells. To investigate the issue
of the cytotoxicity of borrelidin, borrelidin-based analogues were synthesized and their anti-Plasmodium
properties were evaluated. In this communication, we report that a novel borrelidin analogue, bearing the
CH2SPh moiety via a triazole linkage, was found to retain a potent antimalarial activity, against drug-sen-
sitive and drug-resistant parasite strains, but possess only weak cytotoxicity against human cells.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 22 January 2013
Revised 12 February 2013
Accepted 14 February 2013
Available online 26 February 2013
Keywords:
Borrelidin
Antimalarial
Malaria
Plasmodium falciparum
Structure–activity relationships
Malaria, caused by Plasmodium species, such as Plasmodium fal-
ciparum, Plasmodium vivax, Plasmodium malariae, and Plasmodium
ovale, has historically been a major threat to public health world-
wide. According to the World Malaria Report 2011,1 malaria
caused an estimated 655,000 deaths in 2010, primarily among un-
der 5-year-old children in sub-Saharan Africa. Artemisinins, used
for centuries in China for anti-fever treatment, have relatively re-
cent been used as anti-malarial drugs. Artemisinin-based combina-
tion therapies are now recommended by the World Health
Organization for the treatment of malaria. However, malarial par-
asites have been used against them and reports of artemisinin-
resistant parasites are continuing to spread. Therefore develop-
ment of new anti-malarial agents, especially those with novel a
mode of action, is both an urgent and continuing need.
In 1949, Berger et al. reported the isolation of borrelidin (1, Fig. 1),
a structurally unique 18-membered macrolide, from a culture broth
of Streptomyces rochei,2 of which the planar structure was elucidated
by Keller-Schierlein et al. in 1967,3 and the absolute configuration
was determined by X-ray crystallography of a chiral solvent.4
Importantly, 1 expresses a variety of biological properties, such
as antiangiogenesis activity,5a,b antibacterial activity,2 antiviral
activity,6 insecticidal and herbicidal activity.7 Additionally, in
2003, as depicted in Table 1, we have found that 1 exhibited more
potent antimalarial activity against both the drug-resistant K1
strain8 and the drug-sensitive FCR3 strain of Plasmodium falcipa-
rum (IC50 = 0.93 ng/mL against the K1 strain, IC50 = 0.88 ng/mL
against the FCR3 strain). This was better than the clinically used
antimalarials, artemisinin (IC50 = 6 ng/mL against K1 strain), and
chloroquine (IC50 = 184 ng/mL against K1 strain).9–11 Antimalarial
activity against both the K1 and FCR3 strains of 1 was almost the
same. However, the cytotoxicity9 of 1 against human dipoid
embryonic cell line MRC-5 was high, 201 ng/mL, significantly
higher than that of artemisinin (45,170 ng/mL). To evaluate the
combined antimalarial activity and cytotoxicities, a Selectivity
index (SI) (cytotoxicity (IC50 for the MCR-5 cells)/antimalarial
activity (IC50 for the K1 or FCR3 strains)) was created. Conse-
quently, the SI for K1 strain of 1 (SI = 216) was considerably lower
than that of artemisinin (SI = 7528). In addition, our research group
reported that 1 showed stage-specific growth inhibition, most
⇑
Corresponding authors.
E-mail addresses: omuras@insti.kitasato-u.ac.jp (S. Omura), sunazuka@lisci.ki-
¯
Figure 1. Structure of borrelidin (1).
tasato-u.ac.jp (T. Sunazuka).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.02.075

A. Sugawara et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2302–2305
2303
Table 1
Biological evaluation of 1–4, artemisinin and chloroquine
Compd
IC50 (ng/mL)
Selectivity indexa (SI)
MCR-5/K1 MCR-5/FCR3
Antimalarial activity
Cytotoxicityb (MCR-5)
K1 strainc
FCR3 straind
1
2
3
4
0.93
107
10
130
6
0.88
120
10
130
6
201
55,700
14,600
13
45,170
18,572
216
522
1460
102
7528
101
228
464
1460
102
7528
1238
Artemisinine
Chloroquinee
184
15
a
b
c
Selectivity index = cytotoxicity (ng/mL)/IC50 (ng/mL).
Cytotoxicity (ng/mL), human dipioid embryonic cell line MRC-5.
K1 strain, drug-resistant Plasmodium falciparum.
FCR3, drug-sensitive; P. falciparum.
d
e
Drugs commonly used to treat malaria.
strongly in the trophozoite stage.12 However, the detailed mode of
action at molecular level has not been revealed yet.
MRC-5. The biological evaluation revealed that the diacetylated
analogue (2) exhibited approximately 100-fold lower antimalarial
activity against K1 strain than that of 1 (IC50 = 107 ng/mL, com-
pound 2 as opposed to IC50=0.93 ng/mL) (Table 1). Likewise, the
C12,13-14,15-tetrahydro derivative (4) exhibited a decreased IC50
against K1 (IC50 = 130 ng/mL) together with strong cytotoxicity
(IC50 = 13 ng/mL). Although the methyl ester (3) showed approxi-
mately 14-fold weaker IC50 against K1 (IC50 = 10 ng/mL), the SI
for K1 (SI = 1460) was approximately sevenfold better than that
of natural 1 (SI = 216), suggesting that modifying the carboxyl
group at C22 position to various functional groups could produce
a more effective and usable molecule which possesses strong anti-
parasitic activity and weak cytotoxicity (Table 1). Based on these
findings, our strategy focused on modification of the carboxyl
group at C22 position to the ester-, amide-, and thioester-type
analogues.
Given these biological activities, since 1 has promising antima-
larial activity against P. falciparum, even the chloroquine-resistant
K1 strain, it would be desirable to develop borrelidin-based ana-
logues possessing weak cytotoxicity against human cells. To the
best of our knowledge, no literature describing the chemical syn-
thesis of borrelidin analogues has been reported to date.
Our efforts have been concentrated on not only reducing cyto-
toxicity, while retaining potent antimalarial activity, but also on
revealing structure–activity and/or –cytotoxicity maps. Herein
we report the design and synthesis of borrelidin analogues as well
as observations on some structure–activity and/or –cytotoxicity
relationships.
At the outset, our efforts focused on making brief SAR maps by
analogue synthesis (1) 3,11-diacetylation, (2) methylation of the
carboxylic acid at C22 position and (3) reduction of the double
bonds at C12–15 position. As outlined in Scheme 1 and (1) treat-
ment of 1 with Ac2O, Et3N, DMAP in DCM provided the correspond-
ing diacetylated borrelidin (2) in 58% yield, (2) methylation with
TMSCHN2 afforded the corresponding methyl ester borrelidin (3)
in 82% yield, and (3) hydrogenation of 1 provided C12–15-tetrahy-
dro borrelidin (4) as a mixture of diastereomers at C12 position in
74% yield (Scheme 1).
From our synthetic strategy, the ester analogues (5a (R3 = OEt),
5b (R3 = On-Pr), 5c (R3 = OBn), 5d (R3 = Oallyl), 5e (R3 = Opropar-
gyl), 5f (R3 = O(CH2)2NMe2), 5g (R3 = O-morpholylethyl)) were pre-
pared by the following procedures, method (A) alcohols (EtOH, n-
PrOH, BnOH), p-TsOH, reflux for 5a–5c, or (B) alcohols, PyBOP,
Et3N, DMAP, DCM for 5d–5g. To introduce the amino acid moiety
(Scheme 2), treatment of 1 with chloroisobutyl formate, Et3N fol-
lowed by addition of several amino acids (Gly, Ala, Ser, Asn, Ile,
We tested analogues 2–4 for in vitro activity against P. falcipa-
rum strains K1 (drug-resistant) and FCR3 (drug-sensitive), as well
as for cytotoxicity against the human diploid embryonic cell line
D
-Ala, and Tyr) provided the corresponding amide analogues 6a–
6g (6a (R3 = Gly); 85%, 6b (R3 = Ala); 82%, 6c (R3 = Ser); 70%, 6d
(R3 = Asn); 44%, 6e (R3 = Ile); 84%, 6f (R3 =
D-Ala); 70%, 6g
(R3 = Tyr); 70%). Whereas, thioester analogues (7a (R3 = SEt), 7b
(R3 = Sn-Pr), 7c (R3 = Si-Pr), 7d (R3 = Sn-Bu), 7e (R3 = SPh)) were
Scheme 2. Reagents and conditions: For synthesis of ester analogues (5a–5g); (a)
alcohols (EtOH, n-PrOH, BnOH), p-TsOH, reflux (5a;85%, 5b; 51%, 5c; 29%); (b)
alcohols, PyBOP, Et3N, DMAP, DCM (5d; 91%, 5e; 81%, 5f; 83%, 5g; 73%), for
synthesis of amide analogues (6a–6g); (c) chloroisobutylformate, Et3N, THF, then
amino acids, rt (6a; 85%, 6b; 82%, 6c; 70%, 6d; 44%, 6e; 84%, 6f; 70%, 6g; 70%), for
synthesis of thioester analogues (7a–7e); (d) thiols, PyBOP, Et3N, DMAP, DCM, rt
(7a; 88%, 7b; 59%, 7c; 93%, 7d; 89%, 7e; 96%).
Scheme 1. Reagents and conditions: (a) Ac2O, Et3N, DMAP, DCM, rt, 58%; (b)
TMSCHN2, PhH/MeOH (v/v 10/1), rt, 82%; (c) H2, Pd/C, EtOH, rt, 74%.

2304
A. Sugawara et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2302–2305
ial activity levels against the K1 strain compared with 1. Based on
theses results, we expected that the terminal functionalized alkyl
group might bestow more potent antimalarial activity. Moreover,
the use of click chemistry13 using a propargyl analogue (5e), would
rapidly lead to such similar alkyl analogues through a heteroatom
linkage. Click chemistry is a modular synthetic approach to assem-
ble a small molecule via heteroatom linkage and widely utilized in
various research fields.14 In fact, our research group demonstrated
that the use of click chemistry of complex natural products can
lead to creation of several useful molecules.15 To introduce various
functional groups via a triazole moiety, copper catalyzed triazole
formation, one component of click chemistry, of 5e with various
azides (PhSCH2N3, 1-azidoadamantane, 4-azidobenzoic acid, 3-azi-
dopyridine, 3-azidoquinoline) under copper (0) turning, t-BuOH/
H2O (v/v 1/1), aq CuSO4 condition afforded the corresponding
anti-triazole derivatives 8a (R4 = CH2SPh), 8b (R4 = 1-adamantyl),
8c (R4 = p-CO2HPh), 8d (R4 = 3-pyridyl), 8e (R4 = 3-quinolyl) in 96,
100, 33, 98, and 90% yields, respectively16 (Scheme 3).
Scheme 3. Reagents and conditions: Synthesis of triazole analogues (8a–8e). (a) R4-
N3 (PhSCH2N3, 1-azidoadamantane, 4-azidobenzoic acid, 3-azidopyridine, 3-azido-
quinoline), copper (0) turning, t-BuOH/H2O (v/v 1/1), aq CuSO4, rt (8a; 96%, 8b;
quant., 8c; 33%, 8d; 98%, 8e; 90%).
obtained with the both the use of various thiols (EtSH, n-PrSH, i-
PrSH, n-BuSH, and PhSH) and PyBOP in 59–96% yields.
None of the amide analogues (6a–6g) and the thioester ana-
logues (7a–7e) were found to exhibit superior antimalarial activity
against the K1 strain than 1 (Table 2). While, in terms of the ester
analogues (5a–5g), the terminal functionalized alkyl group ana-
logues (5f–5g) were found to increase and/or maintain antimalar-
Pleasingly, of all triazole analogues (8a–8e, Table 3), a triazole
analogue 8a, introducing the CH2SPh moiety via a triazole linkage,
was found to exhibit more potent antimalarial activity (IC50 for
Table 2
Biological evaluation of borrelidin derivatives analogues (5a-5g, 6a-6g, and 7a-7e)
Compd
R3
IC50 (ng/mL)
Selectivity indexa (SI)
MRC-5/K1 MRC-5/FCR3
Antimalarial activity
K1 strainc FCR3 straind
Cytototoxicityb (MRC-5)
1
OH
OEt
On-Pr
OBn
Oallyl
Opropargyl
O(CH2)2NMe2
O-Morpholyl-ethyl
Gly
Ala
Ser
Asn
Ile
0.93
67.8
90.4
24.6
77.7
2.6
0.31
0.62
21
0.88
23.3
78.8
20.5
39.2
5
0.24
0.43
31
201
216
323
202
165
59.6
462
581
228
939
232
198
118
240
750
604
1900
4192
>500
>124
5330
22,400
5a
5b
5c
5d
5e
5f
5g
6a
6b
6c
6d
6e
6f
21,900
18,200
4050
4630
1200
180
260
419
58,900
54,500
>1,000,000
>1,000,000
64,000
16,100
2850
6750
>714
>172
3200
16,500
80
13
140
580
18
200
810
12
10
7
D-Ala
6g
7a
7b
7c
7d
7e
Tyr
SEt
Sn-Pr
Si-Pr
Sn-Bu
SPh
163
8
167
59
163
107
170
10
280
72
170
160
33,400
12,700
3400
10,200
2000
206
1570
20
172
12
197
1340
12
142
12
980
9
6
a
b
c
Selectivity index = cytotoxicity (IC50; ng/mL)/antimalarial activity (IC50; ng/mL).
Cytotoxicity (ng/mL), human dipioid embryonic cell line MRC-5.
K1 strain, drug-resistant Plasmodium falciparum.
d
FCR3, drug-sensitive; P. falciparum.
Table 3
Biological evaluation of borrelidin analogues (8a–8e)
Compd
R4
IC50 (ng/mL)
Selectivity indexa (SI)
Antimalarial activity
K1 strainc FCR3 straind
Cytototoxicityb (MRC-5)
MRC-5/K1
MRC-5/FCR3
1
—
0.93
0.031
70
37.8
2.9
0.88
0.85
168
48.5
4.8
201
16,800
2700
10,100
410
216
541,935
39
267
143
15,600
228
19,765
16
209
86
8a
8b
8c
8d
8e
CH2SPh
1-Adamantyl
p-CO2HPh
3-Pyridyl
3-Quinolyl
0.067
0.079
1050
13,400
a
b
c
Selectivity index = cytotoxicity (IC50; ng/mL)/antimalarial activity (IC50; ng/mL).
Cytotoxicity (ng/mL), human dipioid embryonic cell line MRC-5.
K1 strain, drug-resistant Plasmodium falciparum.
d
FCR3, drug-sensitive; P. falciparum.

A. Sugawara et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2302–2305
2305
10. In vitro activities against P. falciparum strains K1 and FCR3 and cytotoxicity
against human diploid embryonic cell line MCR-5 were measured, as described
previous literature, see, Ref. 8.
K1 = 0.031 ng/mL) with significantly reduced the cytotoxicity
(16,800 ng/mL). Therefore 8a indicates a better in vitro profile
(SI = 541,935) than that of artemisinin (SI = 7528). Although intro-
duction of the 3-quinoline moiety (8e) also led to dramatically in-
creased antimalarial activity (IC50 for K1 = 0.067 ng/mL),
irrespective of parasite strain, none of the others triazole analogues
showed a better SI than that of 8a (Table 3).
In conclusion, we designed and synthesized a comprehensive
group of borrelidin-analogues in order to try and find a compound
which displayed equivalent or higher antimalarial activity than the
parent compound but that displayed a markedly lower cytotoxicity
in human cells. As a result, via click chemistry, we developed a no-
vel borrelidin analogue 8a, containing a CH2SPh moiety via a tria-
zole linkage, which displays potent antimalarial activity and weak
cytotoxicity. In vivo testing of the promising candidates is now
underway in our laboratory.
11. Iwatsuki, M.; Takada, S.; Mori, M.; Ishiyama, A.; Namatame, M.; Nishihara-
¯
Tsukashima, A.; Nonaka, K.; Masuma, R.; Otoguro, K.; Shiomi, K.; Omura, S. J.
Antibiot. 2011, 64, 183.
12. Ishiyama, A.; Iwatsuki, M.; Namatame, M.; Nishihara-Tsukashima, A.;
¯
Sunazuka, T.; Takahashi, Y.; Omura, S.; Otoguro, K. J. Antibiot. 2011, 64, 381.
13. Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004.
14. (a) Kolb, H. C.; Sharpless, K. B. Drug Discov. Today 2003, 8, 1128; (b) Moses, J. E.;
Moorhouse, A. D. Chem. Soc. Rev. 2007, 36, 1249.
15. (a) Hirose, T.; Sunazuka, T.; Noguchi, Y.; Yamaguchi, Y.; Hanaki, H.; Sharpless,
¯
K. B.; Omura, S. Heterocycles 2006, 69, 55; (b) Sugawara, A.; Sunazuka, T.;
¯
Hirose, T.; Nagai, K.; Yamaguchi, Y.; Hanaki, H.; Sharpless, K. B.; Omura, S.
Bioorg. Med. Chem. Lett. 2007, 17, 6340; (c) Tsutsui, A.; Hirose, T.; Ishiyama, A.;
Iwatsuki, M.; Yokota, A.; Maruyama, H.; Matsui, H.; Otoguro, K.; Hanaki, H.;
¯
Omura, S.; Sunazuka, T. J. Antibiot. 2012. http://dx.doi.org/10.1038/ja.2012.113.
16. Experimental procedure and physico-chemical properties of the selected
compounds, 5e and 8a.
Compound 5e: To a solution of borrelidin (1) (40 mg, 82 mmol) in THF (0.5 mL)
was added Et3N (6 mL, 43 mmol), DMAP (1 mg, 8.2 mmol), and PyBop (15 mg,
29 mmol) at room temperature. After stirring at room temperature for 30 min,
2-propyn-1-ol (48 mL, 0.81 mmol) was added to the reaction. After stirring at
room temperature for 24 h, the reaction mixture was diluted with EtOAc
(20 mL). The mixture was then washed with 3% HCl aq (2 ꢀ 10 mL), sat.
NaHCO3 aq (2 ꢀ 10 mL), brine (2 ꢀ 10 mL). The resulting organic layer was
dried over Na2SO4 and concentrated in vacuo. The crude product was purified
by prep. TLC (hexanes/EtOAc = 3/2) to afford 5e (35 mg, 66 mmol) in 81% yield.
Acknowledgments
This work was supported by a Grant for the 21st Century COE
Program, together with funds from the Quality Assurance Frame-
work of Higher Education from The Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan and a Grant for All
Kitasato Project Study (AKPS). We thank Ms. Hitomi Sekiguchi
and Ms. Miyuki Namatame (Kitasato University) for their technical
assistance. We also thank Ms. Chikako Sakabe, Ms. Akiko Nakaga-
wa and Ms. Noriko Sato (Kitasato University) for various instru-
mental analyses.
½ ꢁ : ꢂ3:0 (c 2.00, CHCl3); IR (KBr) m
a 2D1 cmꢂ1: 3435, 3309, 2960, 2922, 2877,
2362, 2341, 2210, 1730, 1161, 754; 1H NMR (270 MHz, CDCl3, 7.26 ppm) d
(ppm) 6.82 (d, J = 11.3 Hz, 1H), 6.37 (dd, J = 14.3, 11.3 Hz, 1H), 6.18 (ddd,
J = 14.3, 9.7, 4.1 Hz, 1H), 4.92 (m, 1H), 4.74 (dd, J = 15.7, 2.4 Hz, 1H), 4.65 (dd,
J = 15.7, 2.4 Hz, 1H), 4.10 (m, 1H), 3.87 (m, 1H), 2.80–2.28 (complex m, 6H),
2.47 (t, J = 2.4 Hz, 1H), 2.17–1.57 (complex m, 9H), 1.43–0.90 (complex m, 6H),
1.05 (d, J = 6.5 Hz, 3H), 0.85 (d, J = 6.8 Hz, 3H), 0.84 (d, J = 6.2 Hz, 3H), 0.81 (d,
J = 6.5 Hz, 3H), 0.72 (m, 1H); 13C NMR (67.5 MHz, CDCl3, 77.0 ppm) d (ppm)
175.5, 172.4, 143.8, 138.1, 127.0, 118.3, 115.9, 77.5, 76.3, 75.1, 72.7, 70.2, 52.2,
48.0, 47.8, 45.2, 42.9, 38.0, 37.3, 35.6, 35.2, 34.9, 31.3, 29.6, 26.9, 26.1, 25.3,
20.1, 18.0, 17.5, 14.8; HRMS (FAB, m-NBA matrix) m/z: 550.3143 [M+Na]+,
calcd for C31H45NO6Na: 550.3145.
References and notes
Compound 8a: To a solution of 5e (8.7 mg, 16 mmol) in a mixture of t-BuOH/
H2O (v/v 1/1, 1.0 mL) was added cupper turning (100 mg), aq CuSO4 solution
(0.1 M), and azidomethyl phenyl sulfide (2.7 mg, 16 mmol) at room
temperature. The mixture was subsequently stirred at room temperature for
17 h, and diluted with EtOAc (20 mL). The mixture was washed with brine
(1 ꢀ 5 mL) and the resulting organic layer was dried over Na2SO4, and
concentrated in vacuo. The crude product was purified by prep-TLC (CHCl3/
1. World Health Organization World Malaria Report 2011; World Health
Organization: Geneva, 2011.
2. Berger, J.; Jampolsky, L. M.; Goldberg, M. W. Arch. Biochem. 1949, 22, 476.
3. Keller-Schierlein, W. Experientia 1966, 22, 355 [in German].
4. (a) Anderson, B. F.; Herlt, A. J.; Rickards, R. W.; Robertson, G. B. Aust. J. Chem.
1989, 42, 717; See also: (b) Kuo, M.-S.; Yurek, D. A.; Kloosterman, D. A. J.
Antibiot. 1989, 42, 1006.
5. (a) Wakabayashi, T.; Kageyama, R.; Naruse, N.; Tsukahara, N.; Funahashi, Y.;
Kitoh, K.; Watanabe, Y. J. Antibiot. 1997, 50, 671; (b) Kawamura, T.; Liu, D.;
Towle, D. L.; Kageyama, R.; Tsukahara, N.; Wakabayashi, T.; Littlefield, B. A. J.
Antibiot. 2003, 56, 709.
MeOH = 20/1) to provide 8a (11 mg, 16 mmol) in 96% yield. ½a D23
: ꢂ21:5 (c
ꢁ
2.00, CHCl3); IR (KBr)
m
cmꢂ1: 3453, 2985, 2923, 2873, 2211, 1727, 1162; 1H
NMR (270 MHz, CDCl3, 7.26 ppm) d (ppm) 7.62 (s, 1H), 7.30 (m, 5H), 6.76 (d,
J = 11.3 Hz, 1H), 6.37 (dd, J = 14.9, 11.3 Hz, 1H), 6.13 (ddd, J = 14.9, 9.7, 4.9 Hz,
1H), 5.60 (s, 2H), 5.20 (d, J = 12.7 Hz, 1H), 5.12 (d, J = 12.7 Hz, 1H), 4.90 (m, 1H),
4.10 (m, 1H), 3.82 (m, 1H), 2.75–2.25 (complex m, 6H), 1.97–1.61 (complex m,
9H), 1.40–0.88 (complex m, 6H), 1.04 (d, J = 6.5 Hz, 3H), 0.83 (d, J = 6.2 Hz, 3H),
0.82 (d, J = 7.5 Hz, 3H), 0.80 (d, J = 6.5 Hz, 3H), 0.73 (m, 1H); 13C NMR
(67.5 MHz, CDCl3, 77.0 ppm) d (ppm) 175.9, 172.4, 143.9, 143.1, 138.3, 132.2
(2C), 131.7, 129.5, 128.7, 126.9, 123.1, 118.3, 116.0, 77.2, 76.2, 73.0, 70.1, 57.7,
53.9, 48.1, 47.8, 45.2, 43.0, 38.9, 37.4, 35.7, 35.4, 35.1, 31.2, 29.7, 27.2, 26.3,
25.4, 20.1, 18.2, 17.3, 14.9; HRMS (FAB, m-NBA matrix) m/z: 715.3491 [M+Na]+,
calcd for C38H52N4O6SNa: 715.3505.
6. Dickinson, L.; Griffiths, A. J.; Mason, C. G.; Mills, R. F. Nature 1965, 206, 265.
7. Dorgrrloh, M.; Kretschmer, A.; Schmidt, R. R.; Steffens, R.; Zoebelein, G.; Tietjen,
K.; Roeben, W.; Stendel, W.; Salcher, O. DE Patent 3,607,287, 1988.
8. Drugs are defined as chloroquine, quinine, and pyrimethamine. See: Otoguro,
¯
K.; Kohana, A.; Manabe, C.; Ishiyama, A.; Ui, H.; Shiomi, K.; Yamada, H.; Omura,
S. J. Antibiot. 2001, 54, 658.
9. Otoguro, K.; Ui, H.; Ishiyama, A.; Kobayashi, M.; Togashi, H.; Takahashi, Y.;
¯
Masuma, R.; Tanaka, H.; Tomoda, H.; Yamada, H.; Omura, S. J. Antibiot. 2003, 56,
727.