Catalytic asymmetric synthesis of antimalarial alkaloids febrifugine and isofebrifugine and their biological activity

Article abstract of DOI:10.1021/jo990877k

Antimalarial alkaloids febrifugine (1) and isofebrifugine (2) were efficiently synthesized from simple achiral starting materials on the basis of the catalytic asymmetric synthesis. The first key reaction was performed using the tin(II)-mediated catalytic asymmetric aldol protocol to afford chiral aldehyde 3 in high yield with high diastereo- and enantioselectivities. The second key step, a Mannich-type reaction, did not give satisfactory results according to the conventional methods. We then developed a novel aqueous Mannich-type three-component reaction of an aldehyde, an amine, and a vinyl ether using a Lewis acid-surfactant combined catalyst (LASC), and the key intermediates 16 and 17 were obtained in high yields. The final coupling reactions of bromoacetone 14 with 4- hydroxyquinazoline were carried out using basic conditions, and successive deprotection gave 1 and 2, respectively, without any isomerization. These- unambiguous total asymmetric syntheses revealed that the absolute configurations of febrifugine and isofebrifugine were not (2'S,3'R) and (2'R,3'R) as reported previously but (2'R,3'S) and (2'S,3'S), respectively (1' and 2'). Finally, antimalarial activities of the synthesized febrifugine and isofebrifugine, and their antipodes, were examined. It was revealed that the activities and selectivities of natural febrifugine and isofebrifugine were much higher than those of the antipodes.

Full text of DOI:10.1021/jo990877k

J . Org. Chem. 1999, 64, 6833-6841
6833
Ca ta lytic Asym m etr ic Syn th esis of An tim a la r ia l Alk a loid s
F ebr ifu gin e a n d Isofebr ifu gin e a n d Th eir Biologica l Activity
Shuj Kobayashi,*,† Masaharu Ueno,† Ritsu Suzuki,† Haruro Ishitani,† Hye-Sook Kim,‡ and
Yusuke Wataya‡
Graduate School of Pharmaceutical Sciences, The University of Tokyo, CREST, J apan Science and
Technology Corporation (J ST), Hongo, Bunkyo-ku, Tokyo 113-0033, J apan, and Faculty of
Pharmaceutical Sciences, Okayama University, Tsushima-naka, Okayama 700-8530, J apan
Received J une 1, 1999
Antimalarial alkaloids febrifugine (1) and isofebrifugine (2) were efficiently synthesized from simple
achiral starting materials on the basis of the catalytic asymmetric synthesis. The first key reaction
was performed using the tin(II)-mediated catalytic asymmetric aldol protocol to afford chiral
aldehyde 3 in high yield with high diastereo- and enantioselectivities. The second key step, a
Mannich-type reaction, did not give satisfactory results according to the conventional methods.
We then developed a novel aqueous Mannich-type three-component reaction of an aldehyde, an
amine, and a vinyl ether using a Lewis acid-surfactant combined catalyst (LASC), and the key
intermediates 16 and 17 were obtained in high yields. The final coupling reactions of bromoacetone
14 with 4-hydroxyquinazoline were carried out using basic conditions, and successive deprotection
gave 1 and 2, respectively, without any isomerization. These unambiguous total asymmetric
syntheses revealed that the absolute configurations of febrifugine and isofebrifugine were not
(2′S,3′R) and (2′R,3′R) as reported previously but (2′R,3′S) and (2′S,3′S), respectively (1′ and 2′).
Finally, antimalarial activities of the synthesized febrifugine and isofebrifugine, and their antipodes,
were examined. It was revealed that the activities and selectivities of natural febrifugine and
isofebrifugine were much higher than those of the antipodes.
Ch a r t 1. Rep or ted Str u ctu r e of F ebr ifu gin e a n d
Isofebr ifu gin e
In tr od u ction
Febrifugine (1) and isofebrifugine (2) (Chart 1), alka-
loids first found in the Chinese plant Dichroafebrifuga1
and later in the common hydrangea,2 have attracted
considerable attention due to their potentially powerful
antimalarial activity.3 Since natural sources are rather
limited, there has been strong demand on chemical
synthesis, especially asymmetric synthesis of naturally
occurring 1 and 2. Baker et al. performed the first
racemic synthesis in 1952,4 and in 1953, his group
reported enantiomer synthesis using a chiral intermedi-
ate that was obtained by optical resolution.5 Despite these
efforts, some obscurity regarding the structures of 1 and
2 including absolute configurations still remained. In
1962, Hill et al. reported the absolute configuration of 1
on the basis of a Baker’s synthetic intermediate;6 how-
ever, this was corrected by Barringer et al. in 1973 using
1H NMR techniques.7 This meant that the structure
proposed by Hill and previous researchers was erroneous.
Since then, although some unclear aspects, which were
mainly due to instability of 1 and 2,4-8 still remained,
the structure proposed by Barringer et al. has been
widely accepted and it was also used for the structure
determination of febrifugine- and isofebrifugine-related
compounds.8
† The University of Tokyo.
Our purpose is to show an unambiguous and efficient
asymmetric synthetic route to 1 and 2. In this paper, we
describe the asymmetric total synthesis of 1 and 2 using
catalytic enantioselective reactions as the key steps. We
also report here revision of the absolute configurations
of febrifugine and isofebrifugine, on the basis of the
unambiguous total synthesis, and the antimalarial ac-
tivities of 1 and 2 and their enantiomers (1′ and 2′).9
‡ Okayama University.
(1) (a) Koepfly, J . B.; Mead, J . F.; Brockman, J . A., J r. J . Am. Chem.
Soc. 1947, 69, 1837. (b) Koepfly, J . B.; Mead, J . F.; Brockman, J . A.,
J r. J . Am. Chem. Soc. 1948, 70, 1048.
(2) (a) Ablondi, F.; Gordon, S.; Morton, J ., II; Williams, J . H. J . Org.
Chem. 1952, 17, 14. (b) Kato, M.; Inaba, M.; Itahana, H.; Ohara, E.;
Nakamura, K.; Uesato, S.; Inouye, H.; Fujita, T. Shoyakugaku Zasshi
1990, 44, 288.
(3) (a) J ang, C. S.; Fu, F. Y.; Wang, C. Y.; Huang, K. C.; Lu, G.;
Thou, T. C. Science 1946, 103, 59. (b) Chou, T.-Q.; Fu, F. Y.; Kao, Y. S.
J . Am. Chem. Soc. 1948, 70, 1765. (c) Frederick, J r., A. K.; Spencer,
C. F.; Folkers, K. J . Am. Chem. Soc. 1948, 70, 2091. See also refs 1, 2,
and 25.
(4) Baker B. R.; Schaub, R. E.; McEvoy, F. J .; Williams, J . H. J .
Org. Chem. 1952, 17, 132.
(7) Barringer, D. F., J r.; Beakelhammer, G. B.; Wayne, R. S. J . Org.
Chem. 1973, 38, 1937.
(8) (a) Uesato, S.; Kuroda, Y.; Kato, M.; Fujiwara, Y.; Hase, Y.;
Fujita, T. Chem. Pharm. Bull. 1998, 46, 1. (b) Burgess, L. E.; Gross,
E. K. M.; J urka, J . Tetrahedron Lett. 1996, 37, 3255.
(9) A preliminary communication on the synthesis of febrifugine and
isofebrifugine: Kobayashi, S.; Ueno, M.; Suzuki, R.; Ishitani, H.
Tetrahedron Lett. 1999, 40, 2175.
(5) (a) Baker B. R.; McEvoy, F. J .; Schaub, R. E.; J oseph, J . P.;
Williams, J . H. J . Org. Chem. 1953, 18, 178. See also: (b) Baker B. R.;
Schaub, R. E.; McEvoy, F. J .; Williams, J . H. J . Org. Chem. 1953, 18,
153. (c) Baker B. R.; McEvoy, F. J . J . Org. Chem. 1955, 20, 136.
(6) Hill, R. K.; Edwards, A. G. Chem. Ind. 1962, 858.
10.1021/jo990877k CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/19/1999

6834 J . Org. Chem., Vol. 64, No. 18, 1999
Kobayashi et al.
Sch em e 1. Retr osyn th etic An a lysis of 1
Sch em e 3. Alter n a tive Syn th esis of Ald eh yd e 3
alcohol, which was converted to the key aldehyde (3-R)
under Swern oxidation conditions.13
Sch em e 2. Syn th esis of Key Ald eh yd e 3
To confirm the absolute configuration of 3-R, we also
prepared 3-R from D-glutamic acid according to Scheme
3. Lactone 8 was prepared from D-glutamic acid according
to the literature,14 and the hydroxyl group of 8 was
protected as its mono-p-methoxytrityl (MMTr) group.15
Reduction of 9 using LiBH4 to afford diol 10, whose
primary hydroxyl group was protected as its tert-butyl-
dimethylsilyl (TBS) ether, and then the secondary hy-
droxyl group was protected as its benzyl ether, respec-
tively. Deprotection of the MMTr group and successive
Swern oxidation gave chiral aldehyde 3-R. Its optical
rotation was consistent with that of 3-R prepared via the
catalytic asymmetric aldol reaction.
Syn th esis of Br om o Keton e 14 (Meth od 1). The
next key step is a Mannich-type reaction for the synthesis
of 12. The Mannich and related reactions provide a
fundamental and useful methodology for the synthesis
of â-amino ketones and esters.16 Although the classical
protocols include some severe side reactions, new modi-
fications using preformed iminium salts and imines have
improved the process.17 Some of these materials are,
however, unstable and difficult to isolate, and deamina-
tion of the products that occurs under the reaction
conditions still remains as a problem. The direct synthe-
sis of â-amino ketones from aldehydes, amines, and third
carbonyl compounds under mild conditions is desirable
from a synthetic point of view. On the basis of this
consideration, we have recently developed lanthanide-
catalyzed three-component reactions of aldehydes, amines,
and vinyl ethers in aqueous media,18 and this protocol
was successfully applied to the synthesis of 12 (Scheme
4). In the presence of 10 mol % of ytterbium triflate
(Yb(OTf)3), the three-component reaction of 3-R, 2-meth-
oxyaniline, and 2-methoxypropene was performed in
aqueous media (THF/H2O ) 9/1). The reaction proceeded
smoothly at 0-5 °C to afford the desired Mannich-type
adduct (12) in 92% yield (syn/anti ) 67/33). The diaster-
eomers were separated and the syn- and anti-adducts
Resu lts a n d Discu ssion
Syn th etic P la n . Our retrosynthetic analysis is shown
in Scheme 1. The final stage is coupling of 4-hydroxy-
quinazoline with bromoketone 14, which could be cyclized
from acyclic compound 12. Amino ketone 12 could be
prepared via the Mannich-type reaction using chiral
aldehyde 3-R. The chiral tin(II)-catalyzed asymmetric
aldol protocol could be convenient for the synthesis of 3-R.
Syn th esis of Ch ir a l Ald eh yd e 3-R. The aldehyde
3-R was prepared using the tin(II)-catalyzed asymmetric
aldol reaction (Scheme 2).10 In the presence of a chiral
tin(II) Lewis acid (20 mol %) derived from tin(II) triflate,
a
chiral diamine, and tin(II) oxide, 3-(tert-butyldi-
methysiloxy)propanal (4)11 reacted with 2-(benzyloxy)-1-
(trimethylsiloxy)-1-phenoxyethene (5) in propionitrile at
-78 °C to afford the corresponding aldol-type adduct (6)
in 70% yield with excellent diastereo- and enantioselec-
tivities (syn/anti ) 95/5, syn g 96% ee). The hydroxyl
group at the 3-position was removed via 2 steps,12 and
the resulting phenyl ester (7) was reduced to form an
(13) Huang, S. L.; Omura, K.; Swern, D. Synthesis 1978, 297.
(14) (a) Taniguchi, M.; Koga, K.; Yamada, S. Tetrahedron Lett. 1974,
30, 3547. (b) Ravid, U.; Silverstein, R.; Smith, L. R. Tetrahedron 1978,
34, 1449.
(15) (a) Khorana, H. G. Pure Appl. Chem. 1968, 17, 349. (b) Smith,
M.; Rammler, D. H.; Goldgerg, I. H.; Khorana, H. G. J . Am. Chem.
Soc. 1962, 84, 430.
(10) (a) Kobayashi, S.; Kawasuji, T. Synlett 1993, 911. (b) Kobayashi,
S.; Kawasuji, T.; Mori, N. Chem. Lett. 1994, 217. (c) Kobayashi, S.;
Fujishita, Y.; Mukaiyama, T. Chem. Lett. 1990, 1455. (d) Kobayashi,
S.; Horibe, M. J . Am. Chem. Soc. 1994, 116, 9805. (e) Kobayashi, S.;
Hayashi, T. J . Org. Chem. 1995, 60, 1098. (f) Kobayashi, S.; Horibe,
M. Chem. Eur. J . 1997, 3, 1472.
(11) The aldehyde was prepared via 2 steps from propane-1,3-diol.
(12) Rasmussen, J . R.; Slinnger. C. J .; Kordish. R. J .; Newman-
Evans, D. D. J . Org. Chem. 1981, 46, 4843.
(16) Blicke, F. F. Org. React. 1942, 1, 303.
(17) (a) Kleimman, E. F. In Comprehensive Organic Synthesis; Trost,
B. M., Ed.; Pergamon Press: Oxford, U.K., 1991; Chapter 2.3, Vol. 2.
(b) Arend, M.; Westermann, B.; Risch, N. Angew. Chem., Int. Ed. Engl.
1998, 37, 1044.
(18) Kobayashi, S.; Ishitani, H. J . Chem. Soc., Chem. Commun.
1995, 1379.

Antimalarial Alkaloids
J . Org. Chem., Vol. 64, No. 18, 1999 6835
Sch em e 4. Syn th esis of Br om o Keton e 14
Sch em e 5. A New Th r ee-Com p on en t Rea ction
Ta ble 1. Th r ee-Com p on en t Rea ction s in Wa ter
(12-syn and 12-a n ti) were used for the synthesis of
isofebrifugine (2) and febrifugine (1), respectively. The
anti-adduct (12-a n ti) was then treated with HF to
remove the TBS protecting group, and the following
bromination gave a spontaneously cyclized adduct whose
N-protected group (2-methoxyphenyl group) was removed
using cerium ammonium nitrate (CAN)19,20 to afford 13-
tr a n s. Several trials to perform direct bromination of 13
failed. Alternatively, piperidine 13-tr a n s was protected
as its N-Boc group and was treated with lithium hexa-
methyldisilazido (LHMDS) followed by trimethylsilyl
chloride (TMSCl). The resulting silyl enol ether was
oxidized and then brominated to give bromo ketone 14-
tr a n s. On the other hand, 12-syn was similarly con-
verted to piperidine 13-cis in a good yield. In this case,
direct bromination proceeded in a low yield to afford
bromo ketone 14-cis.5c Although the desired bromo
ketones 14-tr a n s and 14-cis were prepared according to
this scheme, the yields were not satisfactory. Despite
examination of several reaction conditions in these
schemes, the yields were not improved and we decided
to change the routes to bromo ketone 14.
reaction of 3-R, methoxyaniline, and 2-methoxy-3-(p-
methoxybenzyloxy)-1-propene (15) as shown in Scheme
5. The reaction was first tried in the presence of a
catalytic amount of Yb(OTf)3 in aqueous media (THF/H2O
) 9/1);18 however, only a trace amount of the desired
adduct was obtained.
At this stage, we were required to develop a novel
Mannich-type reaction. We have recently developed a
Lewis acid-surfactant-combined catalyst (LASC) that
forms efficient hydrophobic reaction fields and works
efficiently as a catalyst in water.21 It was found that a
new type of a three-component Mannich reaction of
aldehydes, amines, and vinyl ethers proceeded smoothly
in the presence of ytterbium(III) dodecyl sulfate (Yb(DS)3)
or copper(II) dodecyl sulfate (Cu(DS)2) in water. Several
examples are shown in Table 1. In all cases, the reactions
proceeded cleanly and the corresponding â-amino ketones
were obtained in good to high yields. When the same
combinations were performed in organic solvents, tetra-
hydroquinolines were produced as the main adducts.
Considerable amounts of deamination adducts were
isolated, when silyl enol ethers were used instead of vinyl
ethers in organic solvents. It is noted that the reactions
were successfully performed not in organic solvents or
water-organic solvents but in pure water and that use
of LASC and water as a catalyst and a solvent, respec-
tively, is crucial to obtain the desired products.
Syn th esis of Br om o Keton e 14 (Meth od 2). Since
the bromination of ketone 13 was found to be unexpect-
edly difficult, our next plan was to introduce bromine at
an earlier stage. We devised a new key three-component
(19) (a) Ishitani, H.; Ueno, M.; Kobayashi, S. J . Am. Chem. Soc.
1997, 119, 7153. (b) Kobayashi, S.; Ishitani, H.; Ueno, M. J . Am. Chem.
Soc. 1998, 120, 431.
(20) Kronenthal, D. R.; Han, C. Y.; Taylor, M. K. J . Org. Chem. 1982,
47, 2765.
(21) Kobayashi, S.; Wakabayashi, T. Tetrahedron Lett. 1998, 39,
5389.

6836 J . Org. Chem., Vol. 64, No. 18, 1999
Kobayashi et al.
Sch em e 6. Syn th esis of Br om o Keton e 14
(Meth od 2)
Sch em e 7. Cou p lin g w ith 4-Hyd r oxyqu in a zolin e
Ch a r t 2. Revised Str u ctu r e of F ebr ifu gin e a n d
Isofebr ifu gin e
Ta ble 2. An tim a la r ia l Activity a ga in st P . fa lcipa r u m in
Vitr o a n d Cytotoxicities a ga in st F M3A Cells
As the newly developed aqueous Mannich-type reac-
tions were developed, we continued the synthesis of the
alkaloids. In the presence of 10 mol % of Yb(DS)3, the
three-component reaction of 3-R, o-methoxyaniline, and
15 proceeded smoothly at 0 °C to afford the desired
â-amino ketone (16) in 95% yield. The TBSO group of 16
was then removed and brominated to give piperidine 17
(Scheme 6). The two diastereomers were separated at this
stage (17-cis and 17-tr a n s). After the p-methoxybenzyl
(PMB) group and the N-protecting o-methoxyphenyl
group of 17 were removed, tert-butoxycarbonyl (BOC)
protection of the secondary amine was performed selec-
tively, and the successive substitution to a bromo group
from the hydroxy group gave the desired bromo ketone
(14) in good yields (14-tr a n s and 14-cis).
EC50 (M)
compd
P. falciparuma
FM3A cellb
selectivityc
1
1′
2
(2.0 ( 0.3) × 10-7
(7.6 ( 0.4) × 10-11
(1.6 ( 0.2) × 10-7
(2.9 ( 0.3) × 10-10
(2.1 ( 0.1) × 10-5
(2.1 ( 0.1) × 10-7
(1.9 ( 0.1) × 10-5
(7.3 ( 1.4) × 10-7
105
2763
119
2′
2517
a
b
Chloroquine sensitive strain (FCR-3). Mouse mammary tu-
mor FM3A cells representing a model of host. c Selectivity ) mean
of EC50 value for FM3A cells/mean of EC50 for P. falciparum. The
data shown are the mean values (SD of the EC50 values from
independent triplicate measurements.
the structure of febrifugine (1) shown in Chart 1 was an
antipode of the natural product. We then repeated the
synthesis according to Scheme 6 using aldehyde 3-S. All
physical data of the synthetic sample, including the
optical rotation this time, were completely consistent with
those reported in the literature.
Similarly, both enantiomers of isofebrifugine (2) start-
ing from 17-cis and antipode were prepared (Schemes 6
and 7). It was shown that the optical rotation of the
synthetic sample from the antipode was consistent with
that reported in the literature. It is now concluded that
the structures of febrifugine and isofebrifugine were not
1 and 2 but 1′ and 2′ as shown in Chart 2.
Cou p lin g w ith 4-Hyd r oxyqu in a zolin e. The cou-
pling reaction of bromoacetone 14 with 4-hydroxyquinazo-
line was carried out using potassium hydroxide (KOH)22
to afford 19, whose protecting groups were successfully
removed using 6N HCl to afford 1 (Scheme 7). After
1
recrystallization from ethanol, it was found that its H
and 13C NMR spectra8a and melting point were com-
pletely consistent with those reported (mp 138-140 °C (lit.
mp 139-140,1b 137-138,2a 139-141 °C2b)), but the optical
rotation of synthetic 1 was negative while the reported
optical rotation was positive ([R]24 -28.0 (c ) 0.24,
Biologica l Activity. Antimalarial activities of syn-
thetic febrifugine, isofebrifugine, and their antipodes
were examined (Table 2). The EC50 values of synthetic
febrifugine 1′ and its antipode 1 against Plasmodium
falciparum are 7.6 × 10-11 and 2.0 × 10-7 M, and the
D
EtOH), lit.1b [R]25D +28 (c ) 0.5, EtOH)). This meant that
(22) Ranganathan, D.; Farrooqui, F. Tetrahedron Lett. 1984, 25,
5701.

Antimalarial Alkaloids
J . Org. Chem., Vol. 64, No. 18, 1999 6837
1
tone (8) was prepared from D-glutamic acid according to the
known protocol.14
EC50 values for 1′ was
/
of that of enantiomer 1.
2632
Moreover, the value for synthetic isofebrifugine 2′ and
its antipode 2 were 2.9 × 10-10 and 1.6 × 10-7 M, and
Plasmodium falciparum (ATCC 30932, FCR-3 strain) was
used in our study. P. falciparum was cultivated by a modifica-
tion of the method of Trager and J ensen29 using a 5%
hematocrit of type A human red blood cells suspended in RPMI
1640 medium (Gibco, NY) supplemented with inactivated 10%
type A. The plates were placed in a CO2-O2-N2 incubator (5%
CO2, 5% O2, and 90% N2 atmosphere) at 37 °C, and the
medium was changed daily until 5% parasitemia. Mouse
mammary tumor FM3A cells (wild-type, subclone F28-7) were
supplied by the J apanese Cancer Research Resources Bank
(J CRB). FM3A cells were maintained in suspension culture
at 37 °C in a 5% CO2 atmosphere in plastic bottles containing
ES medium (Nissui Pharmaceuticals, Tokyo, J apan) supple-
mented with 2% heat-inactivated fetal bovine serum (Gibco,
NY).30
1
the EC50 values for 2′ was /552 of that of enantiomer 2.
Excellent selectivities of the natural form 1′ and 2′ were
shown. Natural febrifugine and isofebrifugine obtained
from Dichroa febrifuga were reported as potent anti-
malarial agents1b,23 and the EC50 values of the compounds
for P. falciparum in vitro were approximately 10-10 and
10-9 M, respectively.24 These results also support that
the synthetic 1′ and 2′ correspond to natural febrifugine
and isofebrifugine. It is surprising to note that 1′ and 2′,
in contrast with their antipode 1 and 2, have potent
antimalarial activities, suggesting that the differences
in stereoisomeric structures play an important role for
development of a new antimalarial drug.
(2R,3S)-P h en yl 2-(Ben zyloxy)-5-(ter t-b u t yld im et h yl-
siloxy)-3-h yd r oxyp en ta n oa te (6). To a solution of tin(II)
triflate (0.16 mmol) and tin(II) oxide (0.16 mmol) in propio-
nitrile (2.0 mL) was added a solution of (R)-1-methyl-2-(5,6,7,8-
tetrahydro-1-naphthylaminomethyl)pyrrolidine (0.19 mmol) in
propionitrile (2.0 mL). The mixture was cooled to -78 °C, and
4 (0.77 mmol) in propionitrile (1.0 mL) and 5 (0.93 mmol) in
propionitrile (1.0 mL) were slowly added to the catalyst over
4 h. The reaction mixture was further stirred for 2 h at the
same temperature and then quenched with saturated aqueous
NaHCO3. The organic layer was separated, and the aqueous
layer was extracted with dichloromethane. The combined
organic layers were dried over Na2SO4, filtered, and evapo-
rated. The crude product was treated with THF-1 N HCl (20:
1) at 0 °C. After the usual workup, the crude aldol was
chromatographed on silica gel to afford 6 in 70% yield. The
Con clu sion a n d P er sp ective
In summary, catalytic asymmetric synthesis of febri-
fugine and isofebrifugine was performed using the tin(II)-
catalyzed asymmetric aldol reaction and lanthanide-
catalyzed aqueous Mannich-type three-component reaction
as the key steps. The novel Mannich-type reactions have
been developed using LASC (Lewis acid-surfactant-
combined catalyst) in water. These unambiguous total
syntheses revised the absolute configurations of febri-
fugine and isofebrifugine from (2′S,3′R) and (2′R,3′R) to
(2′R,3′S) and (2′S,3′S), respectively. In addition, anti-
malarial activities and selectivities of the synthesized
febrifugine and isofebrifugine were proved to be much
higher than those of the antipodes. Malaria is the most
prevalent disease in the world, and some 1.5-2.7 million
people die of malaria each year. Malaria risk of varying
degrees exists in 100 countries and territories, and it is
noted that over 40% of the world population live in areas
with malaria risk. The synthetic method reported here
would be applied to the preparation of many febrifugine
and isofebrifugine derivatives including combinatorial
synthetic techniques,25 and further investigations to
develop truly efficient antimalarial drugs are now in
progress in our laboratories.
1
diastereomer ratio was determined by H NMR analysis (syn/
anti ) 95/5). The enantiomeric excess of the syn adduct was
determined to be >96% ee by HPLC analysis using Daicel
Chiralcel AD (hexane/i-PrOH, 9:1, flow rate 1.0 mL/min, tR
)
8.2 min (2S, 3R), 15.6 min (2R,3S)). 6-syn : [R]25 +58.0 (c )
D
2.0, CHCl3); IR (neat) 3478, 2928, 1771, 1594, 1493, 1093 cm-1
;
1H NMR (CDCl3) δ 0.00 (s, 6H), 0.83 (s, 9H), 1.65-1.95 (m,
2H), 2.96 (d, 1H, J ) 5.6 Hz), 3.66-3.85 (m, 2H), 4.09 (d, 1H,
J ) 3.96 Hz), 4.26 (m, 1H), 4.52 (d, 1H, J ) 11.6 Hz), 4.85 (d,
1H, J ) 11.6 Hz), 7.04-7.35 (m, 10H); 13C NMR (CDCl3) δ
-5.5, 18.2, 25.8, 35.4, 60.8, 71.3, 72.9, 80.6, 121.3, 121.4, 126.0,
128.2, 128.3, 128.4, 128.5, 129.4, 136.9, 150.4, 169.6; HRMS
calcd for C24H34O5Si (M+) 430.2176, found 430.2169.
(R)-P h en yl 2-(Ben zyloxy)-5-(ter t-bu tyld im eth ylsiloxy)-
pen tan oate (7). Solid N,N′-thiocarbonyldiimidazole (8.1 mmol)
was added to a solution of 6 (2.7 mmol) in dry THF (18.0 mL).
The reaction mixture was heated under gentle refluxing
conditions for 1.5 h. After cooling, the solution was concen-
trated under reduced pressure and the product was purified
by flush chromatography on silica gel to afford the correspond-
ing thiocarbonate in 88% yield. The thiocarbonate (2.9 mmol)
in dry toluene (250 mL) was then added dropwise over 30 min
to a refluxing toluene solution (40.0 mL) of Bu3SnH (8.6 mmol).
The reaction mixture was heated under reflux for 6 h and then
cooled to room temperature. The mixture was concentrated
under reduced pressure and was chromatographed on silica
Exp er im en ta l Section
Gen er a l Meth od s. Melting points were uncorrected. Col-
umn chromatography was conducted on Silica gel 60 (Merck),
and preparative thin-layer chromatography was carried out
using Wakogel B-5F. Dichloromethane and acetonitrile were
distilled from P2O5 and then from CaH2 and dried over MS
4A. Toluene was distilled and dried over MS 4A. Sn(II)
triflate26 and Yb(III) triflate27 were prepared on the basis of
reported procedures. All silyl enol ethers were prepared
according to the modified House procedure.28 All chemical
compounds were purified on the basis of standard procedures;
especially, aldehydes, amines, and vinyl ether were purified
by distillation before use. (R)-γ-(Hydroxymethyl)-γ-butyrolac-
gel to afford 7 in 86% yield: [R]24 +14.3 (c ) 0.9, CHCl3); IR
D
(neat) 2954, 1739, 1593, 1493, 1254, 1093 cm-1
;
1H NMR
(CDCl3) δ 0.00 (s, 6H), 0.85 (s, 9H), 1.71 (m, 2H), 1.96 (m, 2H),
3.57-3.62 (m, 3H), 4.16 (m, 1H), 4.49 (d, 1H, J ) 11.6 Hz),
4.83 (d, 1H, J ) 11.6 Hz), 7.03-7.40 (m, 10H); 13C NMR
(CDCl3) δ -5.4, 18.2, 25.9, 28.4, 29.5, 62.4, 72.4, 77.7, 121.3,
121.5, 125.9, 128.0, 128.1, 128.4, 129.4, 137.3, 150.3, 171.3;
HRMS calcd for C24H34O4Si (M+) 414.2226, found 414.2230.
(R )-2-(Be n zyloxy)-5-(t er t -b u t yld im e t h ylsiloxy)p e n -
ta n a l (3-R). A solution of 7 (0.97 mmol) in dichloromethane
(5.0 mL) was cooled to -78 °C, and to this solution was added
(23) Wataya, Y.; Kim, H.-S. Unpublished data.
(24) Kobayashi, S. Chem. Soc. Rev. 1999, 28, 1.
(25) Kuehl, F. A., J r.; Spencer, C. F.; Folkers, K. J . Am. Chem. Soc.
1949, 71, 1048.
(26) (a) Batchelor, R. J .; Ruddick, J . N. R.; Sams, J . R.; Aubke, F.
Inorg. Chem. 1977, 16, 1414. (b) Mukaiyama, T.; Iwasawa, N.; Stevens,
R. W.; Haga, T. Tetrahedron 1984, 40, 1381.
(27) (a) Thom, K. F. U.S. Patent 3615169 1971; Chem. Abstr. 1972,
76, 5436a. (b) Forsberg, J . H.; Spaziano, V. T.; Balasubramanian, T.
M.; Liu, G. K.; Kinsley, S. A.; Duckworth, C. A.; Poteruca, J . J .; Brown,
P. S.; Miller, J . L. J . Org. Chem. 1987, 52, 1017.
(29) Trager, W.; J ensen, J . B. Science 1976, 193, 673.
(30) Yoshioka, A.; Tannaka, S.; Hiraoka, O.; Koyama, Y.; Hirota,
Y.; Ayusawa, D.; Seno, T.; Garrett, C.; Wataya, Y. J . Biol. Chem. 1987,
262, 8235.
(28) House, H. O.; Cuzba, L. J .; Gall, M.; Olmstead, H. D. J . Org.
Chem. 1969, 34, 2324.

6838 J . Org. Chem., Vol. 64, No. 18, 1999
Kobayashi et al.
DIBAL (0.98 M hexane solution, 3.0 mL) over 5 min. After
being stirred for 1 h, the reaction mixture was quenched with
MeOH at the same temperature. The resulting mixture was
warmed to room temperature, and 30% aqueous potassium
sodium tertarate solution was added to the mixture. After
being stirred for 1 h, the mixture was filtered through a Cerite
pad, the organic layer was separated, and the aqueous layer
was extracted with dichloromethane. The combined organic
layers were dried and concentrated under reduced pressure.
The crude product was purified by column chromatography
on silica gel to afford the corresponding alcohol in 75% yield.
A dichloromethane solution (1.0 mL) of oxalyl chloride (0.21
mmol) and DMSO (0.46 mmol) was cooled to -78 °C, and to
this solution was added the alcohol (0.096 mmol) in dichloro-
methane (0.50 mL); the mixture was then stirred for 30 min.
To this reaction mixture was added a dichloromethane solution
(0.50 mL) of triethylamine (0.48 mmol), and the mixture was
stirred at the same temperature for 5 min. The mixture was
then warmed to room temperature and further stirred for 30
min. Water was added to the mixture, the organic layer was
separated, and the aqueous layer was extracted with dichloro-
methane. The combined organic layers were dried and con-
centrated under reduced pressure. The crude product was
purified by column chromatography on silica gel to afford the
temperature until the reaction complete. After the usual work
up, the crude mixture was dissolved in ether (20.0 mL) and
the solution cooled to 0 °C. Concentrated formic acid was then
added in portionwise to the ether solution. The reaction was
monitored carefully, and as soon as the reaction was com-
pleted, water and ether were added to the acidic mixture. The
organic layer was separated and washed with water, saturated
aqueous NaHCO3, and brine. The crude product was purified
by silica gel column chromatography to afford (R)-11 in 64%
27
yield. (R)-11: [R]D -14.0 (c ) 1.2, CHCl3); IR (neat) 3435,
1
2938, 2856, 1471, 1254, 1094 cm-1; H NMR (CDCl3) δ 0.00
(s, 6H), 0.85 (s, 9H), 1.45-1.65 (m, 4H), 2.04 (s, 1H), 3.51 (m,
2H), 3.57 (m, 2H), 3.64 (m, 1H), 4.50 (d, 1H, J ) 11.6 Hz),
4.57 (d, 1H, J ) 11.6 Hz), 7.26 (m, 5H); 13C NMR (CDCl3) δ
-5.3, 18.3, 25.9, 27.0, 28.4, 63.0, 64.2, 71.4, 79.5, 127.7, 127.8,
128.4, 138.4. Anal. Calcd for C18H22O3S: C, 66.62; H, 9.94.
Found: C, 66.42; H, 10.15. (S)-11: [R]26 +14.4 (c ) 1.2,
D
CHCl3).
(R )-2-(Be n zyloxy)-5-(t er t -b u t yld im e t h ylsiloxy)p e n -
ta n a l (3-R). A dichloromethane solution (30.0 mL) of oxalyl
chloride (23.7 mmol) and DMSO (51.6 mmol) was cooled to
-78 °C. To this solution was added 11 (10.8 mmol) in
dichloromethane (12.0 mL), and the mixture was stirred for
30 min. Triethylamine (12.0 mL) was added to the reaction
mixture, and the mixture was stirred at the same temperature
for 5 min. The mixture was then warmed to room temperature
and further stirred for 30 min. After the usual workup, the
pure aldehyde was isolated by silica gel column chromatog-
corresponding aldehyde (3) in 97% yield. 3-R: [R]26 +42.5 (c
D
) 1.0, CHCl3); IR (neat) 1735, 1098 cm-1; H NMR (CDCl3) δ
1
0.00 (s, 6H), 0.85 (s, 9H), 1.57-1.78 (m, 4H), 3.57 (t, 2H, J )
6.2 Hz), 3.77 (m, 1H), 4.51 (d, 1H, J ) 11.6 Hz), 4.65 (d, 1H,
J ) 11.6 Hz), 7.28 (m, 5H), 9.62 (s, 1H); 13C NMR (CDCl3) δ
-5.4, 18.3, 25.9, 26.4, 27.9, 62.4, 72.4, 83.2, 128.0, 128.0, 128.5,
137.3, 203.6. Anal. Calcd for C18H30O3Si: C, 67.03; H, 9.38.
raphy in 97% yield: [R]26 +43.2 (c ) 1.0, CHCl3).
D
(4R,5R)-5-(Ben zyloxy)-8-(ter t-b u t yld im et h ylsiloxy)-
4-((2′-m eth oxyp h en yl)a m in o)-2-octa n on e (12-syn ) a n d
(4S, 5R)-5-(Ben zyloxy)-8-(ter t-bu tyld im eth ylsiloxy)-4-((2′-
m eth oxyp h en yl)a m in o)-2-octa n on e (12-a n ti). To a solu-
tion of aldehyde 3-R and o-anisidine (3.0 mmol, respectively)
in 2.5 mL of aqueous THF (9:1 THF/H2O) were added Yb(OTf)3
(0.30 mmol) and an aqueous THF solution (0.5 mL) of 2-meth-
oxypropene (15 mmol) successively at 0 °C. After the solution
was stirred for 45 h, saturated aqueous NaHCO3 was added
to quench the reaction. After the usual workup, the crude
product was purified by silica gel column chromatography to
afford 12 as a diastereomer mixture in 92% yield (syn/anti )
Found: C, 66.86; H, 9.34. 3-S: [R]26 -43.9 (c ) 1.2, CHCl3)
D
(R )-γ-(Mon om e t h oxyt r it yloxym e t h yl)-γ-b u t yr ola c-
ton e (9). The trytyl-type protected lactone ((R)-9) was obtained
from 8 quantitatively by the standard procedure:14 [R]26D -15.5
(c ) 1.0, CHCl3); IR (KBr) 1768, 1606, 1511, 1176 cm-1 1H
;
NMR (CDCl3) δ 1.99-2.07 (m, 1H), 2.18-2.28 (m, 1H), 2.45-
2.54 (m, 1H), 2.63-2.72 (m, 1H), 3.14 (dd, J ) 4.3, 10.4 Hz),
3.41 (dd, J ) 3.3, 10.4 Hz), 3.79 (s, 3H), 4.64 (m, 1H), 6.84 (d,
2H, J ) 8.1 Hz), 7.21 (d, 2H, J ) 8.1 Hz), 7.24-7.44 (m, 10H);
13C NMR (CDCl3) δ 24.2, 28.4, 55.2, 65.2, 79.1, 86.6, 113.2,
127.0, 127.9, 128.3, 130.3, 135.1 143.9, 144.0, 158.6, 177.5.
Anal. Calcd for C25H24O4: C, 77.03; H, 6.23. Found: C, 76.76;
H, 6.19.
67/33). 12-syn : [R]26 -2.0 (c ) 0.9, CHCl3); IR (neat) 2952,
D
1712, 1601, 1514, 1095 cm-1; 1H NMR (CDCl3) δ 0.00 (s, 6H),
0.86 (s, 9H), 1.49-1.70 (m, 4H), 1.95 (s, 3H), 2.69 (d, 2H, J )
6.6 Hz), 3.53 (m, 3H), 3.76 (s, 3H), 4.05 (s, 1H), 4.43 (d, 1H, J
) 11.6 Hz), 4.46 (d, 1H, J ) 11.6 Hz), 6.55-6.79 (m, 4H), 7.30
(m, 5H); 13C NMR (CDCl3) δ -5.3, 18.3, 25.9, 26.4, 27.9, 44.6,
50.1, 55.3, 62.8, 71.7, 79.2, 109.7, 116.3, 121.3, 128.0, 128.0,
128.5, 137.3, 138.4, 147.1, 208.1; HRMS calcd for C28H43NO4-
(R)-5-(ter t-Bu t yld im et h ylsiloxy)-1-(m on om et h oxyt r i-
tyloxy)-2-p en ta n ol (10). Lithium borohydride (67.0 mmol)
was suspended in dry THF (40.0 mL). A solution of 9 (22.3
mmol) in dry THF (20.0 mL) was added gently dropwise to a
suspension of lithium borohydride. Dry methanol (3.0 mL) was
then slowly added to complete the reaction, and after the
addition of methanol, the reaction mixture was cooled to 0 °C
and excess methanol was added to quench the reaction. Ether
and water were then added, the organic layer was separated,
and the aqueous layer was extracted with ether. The combined
organic layers were washed with water and brine and dried
over anhydrous MgSO4. The solvents were removed under
reduced pressure, and the resulting almost pure diol was
treated with (TBS)Cl and imidazole according to a usual
method. The crude product was purified by silica gel column
chromatography to afford 5-(tert-butyldimethylsiloxy)-1-(mono-
methoxytrityloxy)-2-pentanol ((R)-10) in 80% yield: [R]D27 -1.5
Si (M+) 485.2961, found 485.2964. 12-a n ti: [R]27 -9.8 (c )
D
0.7, CHCl3); IR (neat) 2952, 1714, 1602, 1512, 1095 cm-1; H
1
NMR (CDCl3) δ 0.00 (s, 6H), 0.85 (s, 9H), 1.49-1.71 (m, 4H),
2.08 (s, 3H), 2.68 (d, 2H, J ) 5.7 Hz), 3.54-3.61 (m, 3H), 3.76
(s, 3H), 4.02-4.04 (m, 1H), 4.41 (d, 1H, J ) 11.2 Hz), 4.55 (d,
1H, J ) 11.2 Hz), 6.60-6.84 (m, 4H), 7.27-7.32 (m, 5H); 13C
NMR (CDCl3) δ -5.3, 18.3, 25.9, 27.4, 28.7, 30.7, 43.7, 52.0,
55.3, 62.9, 72.6, 80.3, 109.7, 110.9, 116.8, 121.3, 127.6, 127.8,
128.3, 136.7, 138.5, 147.2, 207.9; HRMS calcd for C28H43NO4-
Si (M+) 485.2961, found 485.2976.
(2′S,3′R)-1-[2′-(3′-Ben zyloxy)p ip er id in o]-2-p r op a n on e
(13-tr a n s) a n d (2′R,3′R)-1-[2′-(3′-Ben zyloxy)p ip er id in o]-
2-p r op a n on e (13-cis). To a THF solution (10.0 mL) of
Mannich base 12-a n ti (2.76 mmol) was added 48% aqueous
HF. As soon as the reaction was completed, the reaction was
quenched with saturated aqueous NaHCO3, and the organic
layer was extracted with dichloromethane. The pure alcohol
was obtained by column chromatography on silica gel in
quantitative yield. The alcohol (2.7 mmol) was dissolved in
dichloromethane (15.0 mL), and to this solution were added
tetrabromomethane (5.3 mmol) and triphenylphosphine (5.3
mmol) in dichloromethane (10.0 mL) successively. After a few
minutes, water and dichloromethane were added. The organic
layer was separated, and the aqueous layer was extracted with
dichloromethane. The pure N-protected piperidine derivative
was obtained after column chromatography on silica gel in 96%
(c ) 1.1, CHCl3); IR (neat) 3432, 2926, 1210, 1252, 1088 cm-1
;
1H NMR (CDCl3) δ 0.00 (s, 6H), 0.84 (s, 9H), 1.42-1.59 (m,
4H), 2.76 (s, 1H), 3.02-3.11 (m, 2H), 3.57 (t, 2H, J ) 5.6 Hz),
3.75 (br s, 4H), 6.79 (d, 2H, J ) 8.8 Hz), 7.16-7.29 (m, 8H),
7.40 (d, 4H, J ) 7.8 Hz); 13C NMR (CDCl3) δ -5.4, 13.3, 25.9,
28.8, 30.4, 55.2, 63.3, 67.5, 70.7, 86.2, 113.1, 126.9, 127.8, 128.4,
130.3, 135.6, 144.4, 158.5. Anal. Calcd for C31H42O4Si: C, 73.47;
H, 8.35. Found: C, 73.38; H, 8.19.
(R )-2-(Be n zyloxy)-5-(t er t -b u t yld im e t h ylsiloxy)p e n -
ta n ol (11). To a suspension of NaH (60% dispersion in mineral
oil, 53.5 mmol) in dry THF (20.0 mL) was added a THF
solution (10.0 mL) of 10 dropwise at room temperature. After
the mixture was stirred for 30 min, benzyl bromide (3.0 mL)
was added at 0 °C. The reaction mixture was stirred at room

Antimalarial Alkaloids
J . Org. Chem., Vol. 64, No. 18, 1999 6839
yield. To a solution of the piperidine derivative (2.6 mmol) in
20.0 mL of aqueous acetonitrile (4:1 acetonitrile/water) was
added cerium(IV) ammonium nitrate (13.1 mmol) at 0 °C. After
the solution was stirried for 2 h, water and ethyl acetate were
added, and the aqueous layer was basified with potassium
carbonate. The insoluble materials were filtered through a
Cerite pad, and the aqueous layer was extracted with ethyl
acetate. The combined organic layers were dried over Na2SO4
and concentrated under reduced pressure. The crude product
was purified by silica gel column chromatography (1:10
methanol/chloroform as an elutant) to afford 13-tr a n s in 70%
yield: [R]26D -79.3 (c ) 1.3, CHCl3); IR (neat) 2933, 1709, 1455,
1357, 1099 cm-1; 1H NMR (CDCl3) δ 1.23-1.36 (m, 1H), 1.44-
1.56 (m, 1H), 2.07-2.28 (m, 1H), 2.11 (s, 3H), 2.45 (dd, 1H, J
) 17.6, 8.8 Hz), 2.61 (dt, 1H, J ) 2.7 12.0 Hz), 2.89-2.97 (m,
1H), 3.06 (dd, 1H, J ) 3.1, 17.9 Hz), 3.12 (m, 1H), 3.50 (br,
1H), 4.39 (d, 1H, J ) 11.5 Hz), 4.63 (d, 1H, J ) 11.5 Hz), 7.27-
7.37 (m, 5H); 13C NMR (CDCl3) δ 24.6, 29.8, 30.4, 45.6, 45.8,
57.3, 70.5, 77.9, 127.6, 127.7, 128.3, 138.3, 208.9; HRMS calcd
for C15H21NO2 (M+) 247.1572, found 247.1572.
at 0 °C. After being stirred for 1 h, the reaction was quenched
with saturated aqueous NH4Cl. The crude product was purified
by silica gel column chromatography to afford the correspond-
ing bromo ketone in 24% yield (2 steps): [R]D25 +22.5 (c ) 0.3,
CHCl3); IR (neat) 2929, 1731, 1685 cm-1; 1H NMR (DMSO-d6,
50 °C) δ 1.05-1.43 (m, 4H), 1.31 (s, 9H), 1.55-1.61 (m, 1H),
1.74-1.80 (m, 1H), 2.66 (dt, 1H, J ) 2.5, 13.2 Hz), 2.85 (dd,
1H, J ) 8.4, 16.0 Hz), 3.09 (dd, 1H, J ) 5.0, 16.0 Hz), 3.39 (m,
1H), 3.71 (dd, 1H, J ) 3.7, 13.2 Hz), 4.42 (d, 1H, J ) 12.1 Hz),
4.48 (d, 1H, J ) 12.1 Hz), 4.87 (m, 1H), 7.19-7.28 (m, 5H);
13C NMR (DMSO-d6, 50 °C) δ 23.9, 25.5, 28.4, 32.5, 46.4, 50.9,
70.1, 75.3, 79.6, 126.8, 127.9, 128.7, 138.8, 154.2, 194.3; HRMS
calcd for C20H28NO4Br (M+) 425.1202, found 425.1204.
LASC-Ca t a lyzed Ma n n ich -Typ e R ea ct ion in Wa t er .
Gen er a l P r oced u r e. To a solution of 0.030 mmol of LASC
(Yb(III) dodecyl sulfate or Cu(II) dodecyl sulfate) in water (1.8
mL) were added an aldehyde (0.30 mmol), an aniline derivative
(0.30 mmol), and a vinyl ether (0.45-0.90 mmol) at 0 °C. After
the reaction mixture was stirred for 18 h, saturated aqueous
NaHCO3 was added to quench the reaction. The aqueous layer
was saturated with sodium chloride and then extracted with
ethyl acetate. The combined organic layers were dried over
Na2SO4 and concentrated under reduced pressure. The crude
product was purified by silica gel column chromatography to
afford the corresponding â-amino ketone.
2-Methoxy-3-(p-methoxybenzyloxy)-1-propene (15) was pre-
pared from allyl alcohol according to standard procedures (p-
methoxybenzyl protection, bromomethoxylation, and elimina-
tion): IR (neat) 2939, 2841, 1667, 1611 cm-1; 1H NMR (CDCl3)
δ 3.59 (s, 3H), 3.79 (s, 3H), 3.92 (s, 2H), 4.10 (d, 1H, J ) 12.2
Hz), 4.19 (d, 1H, J ) 12.2 Hz), 4.48 (s, 2H), 6.87 (d, 1H, J )
8.4 Hz), 7.28 (d, 1H, J ) 8.4 Hz); 13C NMR (CDCl3) δ 55.3,
67.4, 70.2, 71.9, 83.4, 99.6, 129.5, 129.9, 159.2, 160.1; HRMS
calcd for C12H16O3 (M+) 208.1099, found 208.1104.
4-((2-Meth oxyp h en yl)a m in o)-4-p h en yl-2-bu ta n on e: IR
(neat) 3412, 2834, 1715, 1603, 1511, 1240, 1223 cm-1; 1H NMR
(CDCl3) δ 2.09 (s, 3H), 2.90 (dd, 1H, J ) 6.0, 15.0 Hz), 2.96
(dd, 1H, J ) 7.0, 15.0 Hz), 3.78-3.83 (m, 1H), 3.85 (s, 3H),
3.85 (s, 3H), 4.87 (dd, 1H. J ) 6.0, 7.0 Hz), 3.85 (s, 3H), 4.87
(dd, 1H, J ) 6.0, 7.0 Hz), 6.42-7.39 (m, 9H); 13C NMR (CDCl3)
δ 30.6, 51.7, 54.1, 55.5, 109.5, 111.3, 117.0, 121.1, 126.3, 127.3,
128.7, 136.6, 142.7, 147.0, 206.7; HRMS calcd for C17H19NO2
(M+) 269.1416, found 269.1404.
Similarly, 13-cis was prepared from 12-syn in 70% yield:
[R]26D -49.0 (c ) 0.4, CHCl3); IR (neat) 2930, 1713, 1602, 1455,
1
1095 cm-1; H NMR (CDCl3) δ 1.37-1.55 (m, 2H), 1.65-1.80
(m, 1H), 1.94 (br, 1H), 2.08 (s, 3H), 2.51-2.71 (m, 3H), 2.99
(d, 1H, J ) 12.7 Hz), 3.13-3.18 (m, 1H), 3.40 (t, 1H, J ) 2.0
Hz), 4.34 (d, 1H, J ) 11.9 Hz), 4.63 (d, 1H. J ) 11.9 Hz); 13C
NMR (CDCl3) δ 21.2, 26.9, 30.6, 45.7, 55.0, 70.3, 73.5, 127.5,
127.8, 128.2, 138.6, 208.1; HRMS calcd for C15H21NO2 (M+)
274.1572, found 247.1572.
(2′S,3′R)-1-[2′-(3′-(Ben zyloxy)-1′-(ter t-bu toxyca r bon yl)-
p ip er id in o)]-3-br om o-2-p r op a n on e (14-tr a n s). 13-tr a n s
(0.076 mmol) was treated with (Boc)2O in dichloromethane at
0 °C for 3 h. After the usual workup, the N-Boc piperidine
derivative was obtained. A solution of lithium hexamethyl-
disilazide (0.23 mmol) in dry THF (2.0 mL) was cooled to -78
°C, and to this solution was added the Boc-protected piperidine
derivative in THF (1.0 mL) slowly. After 1 h, (TMS)Cl (0.40
mmol) was added to the mixture, which was further stirred
for 1 h. The reaction mixture was diluted with 20.0 mL of
hexane at -78 °C and washed quickly with water, saturated
aqueous NaHCO3, and brine. The organic layer was dried over
MgSO4 and concentrated under reduced pressure. The crude
silyl enol ether was dissolved in dichloromethane (2.0 mL),
and to this solution was added MCPBA (0.15 mmol) in
dichloromethane (1.0 mL) at -78 °C. The reaction mixture was
warmed gradually to room temperature and quenched with
5% aqueous sodium sulfite. The crude mixture was then
treated with THF-1 N HCl (20:1) at 0 °C for 1 h. After the
usual workup, the R-hydroxy ketone was obtained in 32% yield,
and the starting material (the Boc-protected piperidine deriva-
tive) was recovered (39%, 52% conversion).
4-Cycloh exyl-4-((2-m et h oxyp h en yl)a m in o)-2-b u t a n -
1
on e: IR (neat) 3413, 1716, 1602, 1253, 1221 cm-1; H NMR
(CDCl3) δ 0.94-1.29 (m, 6H), 1.51-1.85 (m, 7H), 2.12 (s, 3H),
2.58 (m, 1H), 2.54-2.66 (m, 2H), 3.74-3.80 (m, 1H), 3.82 (s,
3H), 4.25 (br, 1H), 6.58-6.86 (m, 4H); 13C NMR (CDCl3) δ 26.3,
26.5, 29.2, 29.4, 30.6, 42.0, 46.0, 53.9, 55.4, 109.6, 110.1, 116.0,
121.3, 137.4, 146.7, 208.2; HRMS calcd for C17H25NO2 (M+)
275.1885, found 275.1866.
The R-hydroxy ketone (0.024 mmol) was dissolved in dichlo-
romethane (0.50 mL), and to this solution were added tetra-
bromomethane (0.050 mmol) and triphenylphosphine (0.050
mmol) in dichloromethane (0.50 mL) successively. After a few
minutes, water and dichloromethane were added. The organic
layer was separated, and the aqueous layer was extracted with
dichloromethane. The pure R-bromo ketone was obtained after
(S)-5-(Ben zyloxy)-4-((2-m eth oxyp h en yl)a m in o)-2-h ex-
a n on e: IR (neat) 3411, 1711, 1602, 1248, 1222 cm-1; 1H NMR
(CDCl3) δ 1.19-1.24 (m, 3H), 2.01-2.10 (s, 3H), 2.55-2.82 (m,
2H), 3.67-3.77 (m, 1H), 3.81 (s, 3H), 3.92-4.00 (m, 1H), 4.35-
4.69 (m, 2H), 4.37-4.59 (br, 1H), 6.55-6.80 (m, 4H), 7.22-
7.39 (m, 5H); 13C NMR (CDCl3) δ 15.7, 16.8, 30.6, 30.8, 43.7,
44.7, 52.6, 54.1, 55.4, 70.7, 71.2, 74.5, 109.7, 109.9, 111.0, 116.3,
116.8, 121.3, 127.6, 127.7, 127.9, 128.3, 128.4, 136.9, 137.0,
138.6, 149.6, 208.3; HRMS calcd for C20H25NO3 (M+) 327.1834,
found 327.1820.
1-(4-Meth oxyben zyloxy)-4-((2-m eth oxyp h en yl)a m in o)-
4-p h en yl-2-bu ta n on e: IR (neat) 3374, 1724, 1606, 1514,
1251, 1174 cm-1; 1H NMR (CDCl3) δ 1.08-1.35 (m, 3H), 2.63-
2.79 (m, 2H), 3.64-3.73 (m, 1H), 3.77-3.83 (m, 6H), 3.89-
3.95 (m, 2H), 4.06 (br, 1H), 4.40-4.41 (m, 2H), 4.44-4.73 (m,
3H), 6.65-7.35 (m, 13H); 13C NMR (CDCl3) δ 14.2, 15.7, 16.9,
39.4, 40.5, 52.7, 54.3, 55.2, 55.4, 60.4, 70.7, 71.2, 72.8, 72.9,
74.8, 75.1, 109.7, 109.8, 110.1, 111.2, 113.7, 113.8, 116.4, 116.9,
121.3, 127.6, 127.7, 127.8, 128.3, 128.4, 129.3, 129.4, 129.6,
136.8, 137.0, 138.5, 146.9, 159.4, 208.1, 208.2; HRMS calcd
for C25H27NO4 (M+) 405.1940, found 405.1916.
26
silica gel column chromatography in 70% yield: [R]D +28.4
(c ) 0.2, CHCl3); IR (neat) 2930, 1685 cm-1; 1H NMR (CDCl3)
δ 1.26-1.65 (m, 4H), 1.43 (s, 9H), 1.86-1.91 (m, 2H), 2.81-
2.86 (m, 3H), 3.43 (br, 1H), 4.01 (br, 1H), 4.52 (d, 1H, J ) 11.9
Hz), 4.70 (d, 1H, J ) 11.9 Hz), 4.98 (br, 1H), 7.25-7.36 (m,
5H); 13C NMR (CDCl3) δ 19.5, 24.7, 34.1, 39.8, 49.6, 70.1, 73.5,
76.8, 80.0, 127.4, 127.5, 128.3, 138.5, 155.3, 199.7; HRMS calcd
for C20H28NO4Br (M+) 425.1202, found 425.1195.
(2′R,3′S)-1-[2′-(3′-(Ben zyloxy)-1′-(ter t-bu toxyca r bon yl)-
p ip er id in o)]-3-br om o-2-p r op a n on e (14-cis). 13-cis (0.29
mmol) was treated with 0.20 mL of acetic acid and 0.20 mL of
30% HBr-AcOH. To the reaction mixture was added 15%
bromine in acetic acid (0.15 mL), and the resulting mixture
was stirred for 1 h. The excess reagents were removed under
the reduced pressure, and the residue was dissolved in
chloroform (2.0 mL). To this chloroform solution were added
saturated aqueous NaHCO3 (1.0 mL) and (Boc)2O (0.86 mmol)
(S)-5-(Ben zyloxy)-1-((4-m eth oxyben zyl)oxy)-4-(2-m eth -
oxyp h en yl)a m in o-2-h exa n on e: IR (neat) 3401, 1722, 1605,
1247 cm-1; 1H NMR (CDCl3) δ 2.94 (dd, 1H, J ) 6.0, 15.2 Hz),

6840 J . Org. Chem., Vol. 64, No. 18, 1999
Kobayashi et al.
3.14 (dd, 1H, J ) 7.6, 15.2 Hz), 3.80 (s, 3H), 3.84 (s, 3H), 3.91
(s, 2H), 4.41 (d, 1H. J ) 11.6 Hz), 4.48 (d, 1H. J ) 11.6 Hz)
4.40 (dd, 1H, J ) 6.0, 7.6 Hz) 5.01 (br, 1H), 6.59-7.40 (m, 13H);
13C NMR (CDCl3) δ 47.0, 54.0, 55.3, 55.5, 72.9, 75.1, 109.4,
111.4, 113.9, 117.0, 121.1, 126.3, 127.3, 128.5, 128.7, 129.0,
129.6, 142.5, 146.9, 159.5, 207.2; HRMS calcd for C28H33NO5
(M+) 463.2359, found 463.2352.
(R)-5-(Ben zyloxy)-8-(ter t-b u t yld im et h ylsiloxy)-1-((4-
m et h oxyb en zyl)oxy)-4-(2-m et h oxyp h en yl)a m in o-2-oc-
ta n on e (16): IR (neat) 3411, 1713, 1601, 1247 cm-1; 1H NMR
(CDCl3) δ 0.00 (m, 6H), 0.84-0.89 (m, 9H), 1.55-1.60 (m, 4H),
2.59-2.77 (m, 2H), 3.55 (m, 3H), 3.74-3.80 (m, 6H), 3.95 (s,
1H), 4.41-4.65 (m, 2H), 6.55-6.89 (m, 6H), 7.16-7.34 (m, 5H);
13C NMR (CDCl3) δ -5.4, 18.2, 25.9, 26.1, 27.3, 28.5, 29.1, 39.3,
40.4, 50.1, 52.2, 55.1, 55.2, 55.3, 62.8, 71.7, 72.5, 72.7, 72.8,
79.3, 80.2, 109.6, 109.7, 118.3, 121.0, 121.3, 127.5, 127.6, 128.0,
128.2, 128.3, 129.2, 129.5, 136.6, 136.7, 138.4, 146.8, 147.1,
159.3, 208.1; HRMS calcd for C36H51NO6Si (M+) 621.3486,
found 621.3485.
78.3, 112.5, 127.8, 128.5, 138.2, 209.7; HRMS calcd for C15H21
NO3 (M+) 263.1521, found 263.1542.
-
Similarly, 18-cis was prepared from 17-cis in 58% yield:
[R]D27 -21.5 (c ) 0.7, CHCl3); IR (neat) 3399, 2926, 1721, 1091
cm-1; 1H NMR (CDCl3) δ 1.25-1.79 (m, 4H), 2.02 (s, 1H), 2.17-
2.48 (m, 4H), 2.58-2.67 (m, 1H), 2.96-3.13 (m, 3H), 4.12-
4.25 (m, 1H), 4.38 (d, 1H, J ) 11.5 Hz), 4.64 (d, 1H, J ) 11.5
Hz), 7.23-7.37 (m, 5H); 13C NMR (CDCl3) δ 25.1, 29.8, 41.4,
45.8, 87.6, 68.7, 70.4, 78.4, 112.5, 127.8, 128.5, 138.2, 209.6;
HRMS calcd for C15H21NO3 (M+) 263.1521, found 263.1525.
(2′S,3′R)-1-[2′-(3′-(Ben zyloxy)-1′-(ter t-bu toxyca r bon yl)-
piper idin o)]-3-br om o-2-pr opan on e (14-tr a n s)an d (2′R,3′R)-
1-[2′-(3′-(Ben zyloxy)-1′-(ter t-bu toxycar bon yl)piper idin o)]-
3-br om o-2-p r op a n on e (14-cis). 18-tr a n s (0.26 mmol) was
treated with (Boc)2O in dichloromethane at 0 °C for 3 h. After
the usual workup, the N-Boc piperidine derivative was ob-
tained in 78% yield. The N-Boc piperidine derivative (0.20
mmol) was dissolved in dichloromethane (0.50 mL), and to this
solution were added tetrabromomethane (0.40 mmol) and
triphenylphosphine (0.40 mmol) in dichloromethane (2.0 mL)
successively. After a few minutes, water and dichloromethane
were added. The organic layer was separated, and the aqueous
layer was extracted with dichloromethane. The pure R-bromo
ketone (14-tr a n s) was obtained after column chromatography
on silica gel in 79% yield.
(2′S,3′R)-1-[2′-(3′-Ben zyloxy)p ip er id in o]-3-((4-m eth oxy-
ben zyl)oxy)-2-p r op a n on e (17-tr a n s) a n d (2′R,3′R)-1-[2′-
(3′-Ben zyloxy)p ip er id in o]-3-((4-m et h oxyb en zyl)oxy)-2-
p r op a n on e (17-cis). To a THF solution (4.0 mL) of the
Mannich base 16 (0.50 mmol) was added 48% aqueous HF.
After the reaction was completed, the reaction was quenched
with saturated aqueous NaHCO3, and the organic layer was
extracted with dichloromethane. The pure alcohol was ob-
tained by column chromatography on silica gel in quantitative
yield. The alcohol (0.50 mmol) was dissolved in dichloro-
methane (1.5 mL), and to this solution were added tetra-
bromomethane (1.0 mmol) and triphenylphosphine (1.0 mmol)
in dichloromethane (2.0 mL) successively. After a few minutes,
water and dichloromethane were added. The organic layer was
separated, and the aqueous layer was extracted with dichloro-
methane. The each diastereomer of the pure N-protected
piperidine derivative was obtained after column chromatog-
raphy on silica gel in 36% (cis) and 53% (trans) yield,
respectively. 17-tr a n s: [R]D25 -45.3 (c ) 1.9, CHCl3); IR (neat)
Similarly, 14-cis was prepared from 18-cis (58%, 76%
yields, respectively).
N-,O-P r otected febr ifu gin e (19-tr a n s, 19-cis). To an
ethanol solution (0.50 mL) of 14-tr a n s (0.15 mmol) were added
an ethanol solution (0.10 mL) of 4-hydroxyquinazoline (0.15
mmol) and potassium hydroxide (0.15 mmol) at room temper-
ature. The reaction mixture was stirred for 2 h and was
quenched with saturated aqueous NH4Cl. After the usual
workup, the crude product was purified by column chroma-
tography on silica gel (1:19 methanol/chloroform as an elutant)
to afford 19-tr a n s in 79% yield: [R]D27 +38.7 (c ) 0.3, CHCl3);
IR (neat) 2930, 1732, 1681, 1613 cm-1 1H NMR (CDCl3) δ
;
1.39-1.48 (m, 1H), 1.45 (s, 9H), 1.57-1.72 (m, 1H), 1.87-1.95
(m, 2H), 2.72 (dd, 1H, J ) 5.5, 14.3 Hz), 2.84 (dd, 1H, J ) 4.6,
14.2 Hz), 2.90 (br s, 1H), 3.49 (s, 1H), 3.98 (br s, 1H), 4.53 (d,
1H, J ) 12.2 Hz), 4.68 (d, 1H, J ) 12.2 Hz), 4.94 (br s, 1H),
4.98 (t, J ) 6.7 Hz), 7.25-7.36 (m, 5H), 7.50 (dd, 1H, J ) 1.5,
7.5 Hz), 7.73-7.79 (m, 2H), 7.93 (s, 1H), 8.26-8.28 (m, 1H);
13C NMR (CDCl3) δ 19.4, 24.5, 28.4, 41.0, 50.1, 53.8, 60.3, 70.3,
73.9, 80.2, 121.9, 126.7, 127.3, 127.5, 127.5, 127.6, 128.3,
134.4., 138.4, 146.6, 148.3, 155.7, 160.9, 200.0; HRMS calcd
for C28H33N3O5 (M+) 491.2420, found 491.2413.
1
2937, 1724, 1612, 1514, 1499, 1249 cm-1; H NMR (CDCl3) δ
1.52-1.98 (m, 4H), 2.50 (m, 2H), 2.83 (m, 1H), 3.11 (m, 1H),
3.40 (br, 1H), 3.69 (m, 1H), 3.79 (s, 3H), 3.81 (s, 3H), 3.89 (m,
1H), 4.24 (m. 2H), 4.26 (d, 1H, J ) 11.5 Hz), 4.29 (d, 1H, J )
11.5 Hz), 4.62 (m, 1H), 6.80-7.35 (m, 13H); 13C NMR (CDCl3)
δ 22.4, 27.0, 31.9, 38.8, 44.1, 49.5, 55.3, 56.5, 70.3, 72.8, 74.7,
111.6, 113.8, 120.7, 123.2, 124.2, 127.3, 127.7, 128.3, 129.5,
138.9, 139.8, 152.5, 154.2, 159.4, 207.2; HRMS calcd for C30H35
-
NO5 (M+) 489.2515, found 489.2508. 17-cis: [R]D +59.2 (c )
25
1.6, CHCl3); IR (neat) 2936, 1725, 1612, 1513, 1455, 1249 cm-1
;
Similarly, 19-cis was prepared from 14-cis in 76% yield:
1H NMR (CDCl3) δ 1.48-1.94 (m, 6H), 2.25-2.32 (m, 1H),
2.75-2.83 (m, 1H), 2.87-3.03 (m, 2H), 3.70-3.88 (m, 1H), 3.79
(s, 3H), 3.90 (s, 3H), 4.28 (m. 2H), 4.47 (d, 1H, J ) 11.9 Hz),
4.57 (d, 1H, J ) 11.9 Hz), 4.91 (m, 1H); 13C NMR (CDCl3) δ
24.0, 25.6, 31.8, 44.1, 55.0, 55.3, 55.6, 70.4, 72.7, 75.0, 75.9,
111.7, 113.8, 120.2, 123.1, 127.5, 127.9, 128.3, 129.5, 129.6,
138.6, 139.7, 152.4, 159.3, 207.6; HRMS calcd for C30H35NO5
(M+) 489.2515, found 489.2513.
[R]D27 -37.6 (c ) 1.2, CHCl3); IR (neat) 2934, 1732, 1680, 1612
1
cm-1; H NMR (DMSO-d6, 50 °C) δ 1.35-1.52 (m, 3H), 1.42
(s, 9H), 1.54-1.59 (m, 1H), 1.70 (d, 1H, J ) 13.4 Hz), 1.89-
1.93 (m, 1H), 3.07 (dd, 1H, J ) 5.5, 15.8 Hz), 3.47-3.50 (m,
1H), 3.82 (d, 1H, J ) 11.9 Hz), 4.54-4.58 (m, 2H), 5.03-5.05
(m, 3H), 7.34-7.41 (m, 5H), 7.62 (dd, 1H, J ) 1.2, 7.6 Hz),
7.75 (d, 1H, J ) 8.2 Hz), 7.90 (dd, 1H, J ) 1.5, 7.8 Hz), 8.03 (s
1H), 8.19 (dd, 1H, J ) 1.5, 7.8 Hz); 13C NMR (DMSO-d6, 50
°C) δ 23.99, 24.03, 25.4, 28.4, 36.2, 36.3, 54.6, 70.0, 75.2, 79.7,
121.8, 126.5, 127.7, 127.9, 128.7, 135.0, 138.8, 148.3, 148.4,
154.6, 160.4, 202.1; HRMS calcd for C28H33N3O5 (M+) 491.2420,
found 491.2406.
(2′S,3′R)-1-[2′-(3′-Ben zyloxy)p ip er id in o]-3-h yd r oxy-2-
p r op a n on e (18-tr a n s) a n d (2′R,3′R)-1-[2′-(3′-Ben zyloxy)-
p ip er id in o]-3-h yd r oxy-2-p r op a n on e (18-cis). To a solution
of 17-tr a n s (0.20 mmol) in 4.0 mL of aqueous acetonitrile (4:1
acetonitrile/water) was added cerium(IV) ammonium nitrate
(1.4 mmol) at 0 °C. After the solution was stirred for 4 h, water
and ethyl acetate were added, and the aqueous layer was
basified with potassium carbonate. The insoluble materials
were filtered through a Cerite pad, and the aqueous layer was
extracted with ethyl acetate. The combined organic layers were
dried over Na2SO4 and concentrated under reduced pressure.
The crude product was purified by silica gel column chroma-
tography (1:10 methanol/chloroform as an elutant) to afford
18-tr a n s in 73% yield: [R]D26 -41.4 (c ) 0.8, CHCl3); IR (neat)
F ebr ifu gin e (1). 19-tr a n s (0.026 mmol) was treated with
6 N HCl (1.0 mL), and the resulting mixture was heated under
reflux for 35 min. Aqueous Na2CO3 (20%) was added carefully
until the pH of the media changed to 9. The resulting mixture
was extracted with chloroform and dried over Na2SO4 and
Na2CO3. The crude product was purified by column chroma-
tography (silica gel) to afford febrifugine quantitaively: Mp
138-140 °C (lit. mp 139-140,1b 137-138,2a 139-141 °C2b);
[R]D25 -28.0 (c ) 0.1, EtOH) (lit.1b [R]25D +28 (c ) 0.5, EtOH));
IR (KBr) 2929, 2853, 1725, 1678, 1612 cm-1; 1H NMR (CDCl3)
δ 1.30-1.38 (m, 1H), 1.48-1.57 (m, 1H), 1.72-1.74 (m, 1H),
2.07-2.10 (m, 1H), 2.58 (dt, 1H, J ) 2.4, 12.2 Hz), 2.65 (dd,
1H, J ) 7.3, 15.8 Hz), 2.88 (dd, 1H, J ) 4.6, 7.7 Hz), 2.97 (d,
1H, J ) 12.2 Hz), 3.12 (dd, 1H, J ) 4.8, 15.8 Hz), 3.29 (m,
1H), 4.83 (d, 1H, J ) 17.4 Hz), 4.89 (d, 1H, J ) 17.4 Hz), 7.51
3413, 2934, 1719, 1095 cm-1 1H NMR (CDCl3) δ 1.20-1.77
;
(m, 4H), 2.24-2.63 (m, 2H), 2.88-2.98 (m, 2H), 3.14 (br, 1H),
3.90 (s, 1H), 4.06-4.23 (m, 2H), 4.11-4.18 (m, 2H), 4.36 (d,
1H, J ) 11.7 Hz), 4.63 (d, 1H, J ) 11.7 Hz), 7.23-7.36 (m,
5H); 13C NMR (CDCl3) δ 25.1, 29.8, 41.4, 45.8, 57.6, 68.5, 70.4,

Antimalarial Alkaloids
J . Org. Chem., Vol. 64, No. 18, 1999 6841
(dt, 1H, J ) 1.2, 8.1 Hz), 7.73 (d, 1H, J ) 7.6 Hz), 7.78 (dt,
1H, J ) 1.2, 8.1 Hz), 7.90 (s, 1H), 8.28 (dd, 1H, J ) 0.9, 7.9
Hz); 13C NMR (CDCl3) δ 25.6, 35.4, 44.0, 45.9, 54.8, 60.1, 72.2,
121.9, 126.8, 127.4, 127.6, 134.5, 146.3, 148.2, 161.0, 202.6;
HRMS calcd for C16H19N3O3 (M+) 301.1426, found 301.1422.
CO2-O2-N2 incubator (5% CO2, 5% O2, and 90% N2 atmo-
sphere). To evaluate the antimalarial activity of a compound,
we prepared thin blood films from each culture and stained
them with Giemsa (E. Merck, Darmstadt, Germany). A total
of 10 000 erythrocytes per one thin blood film were examined
by microscopy. All of the test compounds were assayed in
duplicate at each concentration. Drug-free control cultures
were run simultaneously. All data points represent the mean
of three experiments. The EC50 value refers to the concentra-
tion of the compound necessary to inhibit the increase in
parasite density at 72 h by 50% of control.
Toxicity a ga in st Ma m m a lia n Cell Lin e. FM3A cells grew
with a doubling time of about 12 h. Prior to exposure to drugs,
cell density was adjusted to 5 × 104 cells/mL. A cell suspension
of 995 µL was dispensed to the test plate, and compounds at
various concentrations suspended in ethanol (5.0 µL) were
added to individual wells of a multidish 24 well arrangement.
The plates were incubated at 37 °C in 5% CO2 atmosphere for
48 h. All the test compounds were assayed in duplicate at each
concentration. Cell numbers were measured using a microcell
counter CC-130 (Toa Medical Electric Co., Kobe, J apan). All
data points represent the mean of three experiments. The EC50
value refers to the concentration of the compound necessary
to inhibit the increase in cell density at 48 h by 50% control.
27
1′: [R]D +26.6 (c ) 0.1, EtOH)
Isofebrifugine was also obtained from 19-cis by the same
26
procedure in 46% yield. Isofebr ifu gin e (2): [R]D -115.6 (c
) 0.2, CHCl3) (lit.1b [R]25 +131 (c ) 0.35, CHCl3)); IR (neat)
D
2923, 2853, 1670, 1613 cm-1
;
1H NMR (CDCl3) δ 1.44-1.51
(m, 2H), 1.74-1.82 (m, 1H), 1.80 (d, 1H, J ) 13.2 Hz), 1.99-
2.08 (m, 1H), 2.01 (d, 1H, J ) 13.2 Hz), 2.46 (dt, 1H, J ) 1.7,
11.8 Hz), 2.92 (dd, 1H, J ) 2.7, 11.2 Hz), 3.22 (t, 1H, J ) 3.3
Hz), 3.81 (d, 1H, J ) 2.9 Hz), 4.08 (d, 1H, J ) 13.9 Hz), 4.40
(d, 1H, J ) 13.9 Hz), 7.41-7.45 (m, 1H), 7.63-7.72 (m, 2H),
8.23-8.25 (m, 2H); 13C NMR (CDCl3) δ 20.1, 26.8, 43.3, 44.5,
49.8, 55.7, 77.2, 105.4, 121.9, 126.9, 127.1, 127.5, 134.3, 148.0,
148.2, 161.4; HRMS calcd for C16H19N3O3 (M+) 301.1426, found
25
301.1418. 2′: [R]D +129.3 (c ) 0.2, CHCl3)
An tim a la r ia l Assa y: In vitr o An tim a la r ia l Activity of
F ebr ifu gin e, Isofebr ifu gin e, a n d Th eir En a n tiom er s: The
following procedures were used for assay of antimalarial
activity.31 Various concentrations of compounds in ethanol
were prepared. A 5 µL amount of each solution was added to
individual wells of a multidish 24 wells arrangement. Eryth-
rocytes with 0.3% parasitemia were added to each well
containing 995 µL of culture medium to give a final hematocrit
level of 3%. The plates were incubated at 37 °C for 72 h in a
Ack n ow led gm en t. This work was partially sup-
ported by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports, and Culture
of J apan.
(31) Kim, H.-S., Miyake, H.; Arai, M.; Wataya, Y. Parasitol. Int.
1998, 47, 59.
J O990877K