Synthesis and biological evaluation of epidithio-, epitetrathio-, and bis-(methylthio)diketopiperazines: Synthetic methodology, enantioselective total synthesis of epicoccin G, 8,8′-epi-ent-rostratin B, gliotoxin, gliotoxin G, emethallicin E, and haematocin and discovery of new antiviral and antimalarial agents

Article abstract of DOI:10.1021/ja308429f

An improved sulfenylation method for the preparation of epidithio-, epitetrathio-, and bis-(methylthio)diketopiperazines from diketopiperazines has been developed. Employing NaHMDS and related bases and elemental sulfur or bis[bis(trimethylsilyl)amino]trisulfide (23) in THF, the developed method was applied to the synthesis of a series of natural and designed molecules, including epicoccin G (1), 8,8′-epi-ent-rostratin B (2), gliotoxin (3), gliotoxin G (4), emethallicin E (5), and haematocin (6). Biological screening of selected synthesized compounds led to the discovery of a number of nanomolar antipoliovirus agents (i.e., 46, 2,2′-epi-46, and 61) and several low-micromolar anti-Plasmodium falciparum lead compounds (i.e., 46, 2,2′-epi-46, 58, 61, and 1).

Full text of DOI:10.1021/ja308429f

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Article
Synthesis and Biological Evaluation of Epidithio-, Epitetrathio-
and bis-(Methylthio)diketopiperazines. Synthetic Methodology,
Enanti-oselective Total Synthesis of Epicoccin G, 8,8#-epi-
ent-Rostratin B, Gliotoxin, Gliotoxin G, Emethallicin E and
Haematocin, and Discovery of New Antiviral and Antimalarial Agents
K. C. Nicolaou, Min Lu, Sotirios Totokotsopoulos, Philipp M Heretsch, Denis
Giguere, Ya-Ping Sun, David Sarlah, Thu Han Nguyen, Ian Coulter Wolf,
Donald F. Smee, Craig W Day, Selina Bopp, and Elizabeth A Winzeler
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Sep 2012
Downloaded from http://pubs.acs.org on September 30, 2012
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Page 1 of 15
Journal of the American Chemical Society
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Synthesis and Biological Evaluation of Epidithio-,
Epitetrathio- and bis-(Methylthio)diketopiperazines. Synthetic
Methodology, Enantioselective Total Synthesis of Epicoccin
G, 8,8ʹ-epi-ent-Rostratin B, Gliotoxin, Gliotoxin G, Emethal-
licin E and Haematocin, and Discovery of New Antiviral and
Antimalarial Agents
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K. C. Nicolaou*,† Min Lu,† Sotirios Totokotsopoulos,† Philipp Heretsch,† Denis Giguère,† Ya-Ping
Sun,† David Sarlah,† Thu Han Nguyen,† Ian Coulter Wolf,† Donald F. Smee,‡ Craig W. Day,‡ Selina
Bopp,§ Elizabeth A. Winzeler§
†Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North
Torrey Pines Road, La Jolla, California 92037, United States, and Department of Chemistry and Biochemistry, University of
California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States
‡Institute for Antiviral Research, Department of Animal, Dairy and Veterinary Sciences, Utah State University, Logan, Utah
84322, United States
§Department of Pediatrics, University of California, San Diego, School of Medicine, 9500 Gilman Drive 0741, La Jolla,
California 92093, United States
Natural Products, Total Synthesis, Anti Poliovirus Agents, Antimalarial Agents, Epidithiodiketopiperazines, bis-
(Methylthio)diketopiperazines
Supporting Information Placeholder
ABSTRACT: An improved sulfenylation method for the preparation of epidithio-, epitetrathio- and bis-
(methylthio)diketopiperazines from diketopiperazines has been developed. Employing NaHMDS and related bases and elemental
sulfur or bis[bis(trimethylsilyl)amino]trisulfide (23) in THF, the developed method was applied to the synthesis of a series of natu-
ral and designed molecules, including epicoccin G (1), 8,8ʹ-epi-ent-rostratin B (2), gliotoxin (3), gliotoxin G (4), emethallicin E (5)
and haematocin (6). Biological screening of selected synthesized compounds led to the discovery of a number of nanomolar anti
poliovirus agents (i.e. 46, 2,2ʹ-epi-46 and 61, Table 5) and several low micromolar anti Plasmodium falciparum lead compounds
(i.e. 46, 2,2ʹ-epi-46, 58, 61 and 1, Table 5).
In order to alleviate some of these deficiencies and facilitate
INTRODUCTION
biological investigations in this area, we recently initiated a
research program directed toward the development of im-
proved methods of sulfenylation of 2,5-diketopiperazines and
applied them to the total synthesis of natural and designed
epidithio-, epitetrathio- and bis-(methylthio)diketopiperazines.
In preliminary communications we already reported an im-
proved method for the sulfenylation of 2,5-diketopiperazines7
and the total synthesis8 of epicoccin G9 (1, Figure 1) and 8,8ʹ-
epi-ent-rostratin B10 (2, Figure 1). In this article we describe
further studies in this area that include enantioselective total
syntheses of gliotoxin11 (3, Figure 1), gliotoxin G12 (4, Figure
1), emethallicin E13 (5, Figure 1) and haematocin14 (6, Figure
1), as well as the monomeric unit (7, Figure 1) of aranotin15,16
(8, Figure 1). We also report our biological evaluation of a
number of selected synthesized compounds that led to the
discovery of potent anti poliovirus and anti Plasmodium falci-
parum agents.
2,5-Diketopiperazines are a ubiquitous class of compounds
of diverse molecular architectures and biological activities.1
Numerous have been discovered from natural sources while
many more have been synthesized in the laboratory for biolog-
ical investigations and drug discovery purposes.1 The 2,5-
diketopiperazine structural motif constitutes a unique scaffold
upon which three dimensional molecules, including chiral
ones, may be constructed,1,2 thereby providing a useful alterna-
tive to the planar structural motifs commonly found in drugs
and drug candidates, the latter being often far from ideal in
terms of pharmacological properties.3
Of particular interest are the naturally occurring epidithio-
diketopiperazines and
bis-(methylthio)diketopiperazines,
whose biological activities include antiviral, antibacterial,
antiallergic, antimalarial and cytotoxic properties.1,4 Despite
their promising biological profiles, however, these compounds
remain largely unexplored, primarily due to their natural scar-
city and the synthetic laboratory challenge they pose.5,6
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Sodeoka6f applied the use of 3,6-dihydroxydiketopiperazines
1
2
3
4
5
6
7
8
(15 and 16, respectively, Figure 2) and H2S to construct the
epidithiodiketopiperazine structural motifs (18, Figure 2) of
their synthetic targets, 11,11ʹ-dideoxyverticillin A and chae-
tocin, respectively. In 2010, Kim and Movassaghi described
the use of potassium trithiocarbonate (K2CS3) to generate an
epidithiodiketopiperazine moiety from monosilylated 3,6-
dihydroxydiketopiperazine intermediate (17→18, Figure 2) in
their elegant synthesis of chaetocins A and C and 12,12ʹ-
dideoxychetracin A.6g
9
Inspired by the U. Schmidt method20 of introducing sulfur
atoms into 2,5-diketopiperazines directly using S8 and NaNH2
in liq. NH3, we opted to employ S8 and NaHMDS or LiHMDS
as the base in THF. Our expectations included not only the
convenience of carrying out the sulfenylation reaction in an
organic solvent rather than liq. NH3, but also the possibility of
generating more well-defined sulfenylating species to effect
the desired reaction more efficiently and with stereocontrol. In
retrospect, we realized that the reaction of S8 with NaHMDS
had already been studied by M. Schmidt24a-c in the 1960s, a
study24a that we inadvertently missed in our preliminary com-
munications.7,8 Our investigations with this reaction are sum-
marized in Scheme 1. Thus, from the reaction of S8 (19) and
NaHMDS, we were able to isolate, chromatographically, and
characterize three reactive species: tetrasulfide 21 [40% yield,
1H NMR (CDCl3, 600 MHz): δ = 0.26 ppm; 13C NMR (CDCl3,
150 MHz): δ = 2.4 ppm; HRMS [M+H]+: calcd for
C12H36N2O2S4+H: 449.0911; found: 449.0908], pentasulfide
22 [5% yield, HRMS [M+H]+: calcd for C12H36N2O2S5+H:
512.0280; found: 512.0241], and trisulfide 23 [8% yield,
HRMS [M+H]+: calcd for C12H36N2O2S3+H: 417.1190; found:
417.1186]. Their formation, presumably through intermediate
20, may be explained as shown in Scheme 1 and it is con-
sistent with the observations of M. Schmidt et al.24 The pre-
dominance of the tetrasulfide 21 is most likely due to steric
shielding and charge repulsion during the second nucleophilic
attack by the (TMS)2Nˉ species on the sulfur chain (see 20,
Scheme 1). The reaction of the resulting mixture with 2,5-
diketopiperazines as exemplified with substrate 24 in the pres-
ence of excess base (NaHMDS) as shown in Scheme 2 is
consistent with the presence of these species, although only
epidi- and epitetrasulfides were isolated. Reduction of the
mixture (presumably containing additional sulfenylated spe-
cies, such as epitri- and epipentasulfides as well as open-chain
oligosulfides) with NaBH4 followed by oxidation of the result-
ing dithiolate 26 (aq. NH4Cl; then KI3) led to a good yield of
the epidithiodiketopiperazine 27 (69%). The same result was
obtained from the pure tetrasulfide 25 (obtained in 22% yield
from 24, see Scheme 2) upon reduction/oxidation (94%).
Reaction of dithiolate 26 with MeI furnished bis-
(methylthio)diketopiperazine 28 in 72% overall yield from 24.
In support of the proposed mechanism (Scheme 2), we found
that only mono-deuteration occurs upon quenching the initial-
ly formed species from substrate 24 and NaHMDS (2.2 equiv).
Also in support of the intramolecular nature of the second C-S
bond formation are the good yields of the epidithio- and
epitetrathiodiketopiperazine products observed.
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Figure 1. Selected naturally occurring epidithio-, epitetrathio- and
bis-(methylthio)diketopiperazines.
RESULTS AND DISCUSSION
Methodology Development. Recognizing the deficiencies
of the then available sulfenylation methods of 2,5-
diketopiperazines, we set as part of our goals the development
of improved sulfenylation methods to construct epidithio-
diketopiperazines and
bis-(methylthio)diketopiperazines.
Figure 2 depicts a number of selected sulfenylation methods of
2,5-diketopiperazines known at the outset of our investiga-
tions. Thus, as early as 1968, Trown,17 and subsequently Hash-
imoto18
(1987),
pioneered
the
use
of
3,6-
dibromodiketopiperazines (9) as substrates and KSAc as a
sulfur source to prepare epidithiodiketopiperazines (18). In
1971, Poisel and U. Schmidt19 introduced the use of sodium
tetrasulfide (Na2S4) as a source of sulfur to produce epidithio-
diketopiperazines (10→18, Figure 2), and in 1972 the classical
U. Schmidt method20 for the synthesis of these compounds
from 2,5-diketopiperazines employing sulfur (S8) and NaNH2
in liq. NH3 (11→18, Figure 2) was reported. In 1973, Kishi21
reported a method of masking 3,6-dithiodiketopiperazines
with anisaldehyde and then generating the desired epidithio-
diketopiperazines at a later stage (12→18, Figure 2), a tactic
that he elegantly applied to synthesize gliotoxin (3).6h,6i In
1975, Matsunari23 utilized 3,6-dimethoxydiketopiperazines as
substrates in conjunction with H2S as a source of sulfur to
prepare epidithiodiketopiperazines (13→18, Figure 2), where-
as in 2002 Overman and Sato24 employed the corresponding
bis-acetates and H2S in their quest of similar epidithiodiketop-
iperazines (14→18, Figure 2). In 2009, Movassaghi6e and
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Figure 2. Selected sulfenylation methods of 2,5-
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aReagents and conditions. NaHMDS (0.6 M in PhMe, 3.0
equiv), S8 (1.0 equiv), THF, 25 °C, 5 min, 21: 40%, 22: 5%, 23:
8%.
The generality and scope of the sulfenylation reaction was
explored with a variety of substrates. These explorations led to
a series of epidithiodiketopiperazines (Table 1) and bis-
(methylthio)diketopiperazines (Table 2). Thus, under the reac-
tion conditions shown in Table 1, 3,6-unsubstituted diketop-
iperazines such as 29 (entry 1) reacted to form epidithio-
diketopiperazines (i.e. 30, entry 1), albeit in modest yield
(40%), the latter observation being attributed to possible un-
hindered intermolecular reactions of the intermediate sulfur
species. This speculation is supported by the higher yields
observed with 3,6-mono- and 3,6-disubstituted substrates (e.g.
entries 2–5). The relatively low yield of epidithiodiketopipera-
zine 38 (entry 6) is most likely due to the steric congestion at
the sites of sulfenylation (i.e. positions 3 and 6). It is notable
that both syn (entries 4, 8, 10–12) and anti (entries 3 and 9)
3,6-disubstituted diketopiperazine systems enter the reaction
equally well. These include monocyclic (entries 3–7) and
polycyclic (entries 8–12) systems. The fact that sulfenylation
occurs from the same side of the molecule in both the syn and
the anti series provides support for the intramolecular nature
of the second C-S bond formation (see 24d→25, Scheme 2).
All epidithiodiketopiperazine products shown in Table 1 are
racemic as a consequence of the enolate intermediacy in these
reactions. Enantiopure compound 45 (entry 10) gave a mixture
of enantiopure diastereoisomers (ca. 1.4:1 dr) due to the addi-
tional chiral centers within the structure.
diketopiperazines.
Scheme 1. Reaction of Sulfur (S8) with NaHMDS
[NaN(TMS)2]a
Employment of the reaction conditions shown in Table 2 on
the indicated substrates led to the corresponding bis-
(methylthio)diketopiperazines. All products were isolated as
single racemic syn compounds with the exception of 55 (entry
6), which was formed as a mixture of enantiopure diastereoi-
somers (ca. 1.4:1 dr) due to the additional stereocenters within
the substrate. Again, the observation of only the syn product
provides support for the intramolecularity of the second sul-
fenylation step. The excellent stereoselectivity and good yields
obtained in this sulfenylation reaction and its epidithiodiketop-
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iperazine-forming counterpart (see Table 1) demonstrate the
superiority of this method in comparison to the traditional U.
Schmidt process that often leads to mixtures of the syn and
anti products in lower yields.
Page 4 of 15
1
2
3
Scheme 2. Sulfenylation of 2,5-Diketopiperazines with
[NaHMDS-S8]. Preparation of Epitetrathiodiketopipera-
zine 25, Epidithiodiketopiperazine 27 and bis-
(Methylthio)diketopiperazine 28a
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aReagents and conditions. a) NaHMDS (0.6 M in PhMe, 3.0
equiv), S8 (1.0 equiv), THF, 25 °C, 1 min; then 24 (1 M in THF,
1.0 equiv), 1 min; then NaHMDS (0.6 M in PhMe, 2.0 equiv), 25
°C, 30 min; b) NaBH4 (25 equiv), THF/MeOH (1:1), 0→25 °C,
45 min; c) NH4Cl aq. (1.0 M), 25 °C; d) KI3 aq. (1.4 M), 25 °C,
10 min, 69% over the four steps from 24; e) MeI (50 equiv), 25
°C, 15 h, 72% over the three steps from 24.
Table 1. Preparation of Epidithiodiketopiperazinesa
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aReactions were performed on 100 mg scale at 25 °C. bRacemic
products. As we shall see below, however, this is not always
the case, especially with more sensitive substrates (see Table
4).
c
1
2
mixture unless otherwise stated. Yield of isolated products after
chromatography. dca. 1.4:1 dr.
3
4
5
Table 3. Influence of the Base in the Sulfenylation of Se-
lected Epidithiodiketopiperazinesa
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7
8
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10
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25
26
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45
46
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57
58
59
60
b
aReactions were performed on 50 mg scale at 25 °C. Racemic
c
mixtures were obtained. Yield of isolated products after chroma-
tography.
Previously known24a bis[bis(trimethylsilyl)amino]trisulfide
(23, Scheme 3) was prepared and investigated for its suitabil-
ity as a sulfenylating agent of 2,5-diketopiperazines in the
presence of base. Thus, pure 23 reacted with diketopiperazine
24 in the presence of NaHMDS in THF at ambient tempera-
ture to produce a mixture of epidithiodiketopiperazine 27
(43%) and epitetrathiodiketopiperazine 25 (22%). A specula-
tive mechanism for the formation of these products is shown
in Scheme 3.25 Thus, the initially formed enolate 24b may
react with trisulfide 23 through path a (attack at terminal S) to
afford trisulfide intermediate 24e, which may then suffer in-
tramolecular attack by the second enolate (24f) to afford epidi-
thiodiketopiperazine 27 and (TMS)2NSNa (23b). The same
product (27) could be formed from enolate 24b and trisulfide
23 through path b (attack at the central S) via the intermediacy
of species 24g and 24h by intramolecular attack as shown in
the scheme. Alternatively, trisulfide intermediate 24f may
undergo different intramolecular collapse to generate, through
path c, epitrithiodiketopiperazine 24i,26 whose opening with
(TMS)2NSˉ as shown may form epitetrathiodiketopiperazine
25 via intermediate species 24j.
Table 2. Preparation of bis-(Methylthio)diketopiperazinesa
aReactions were performed on 100 mg scale at 25 °C. bRacemic
c
mixture unless otherwise stated. Yield of isolated products after
chromatography. dca. 1.4:1 dr.
The effect of the alkali metal in the base on the efficiency of
the reaction was then examined. Thus, KHMDS, NaHMDS
and LiHMDS were used in the sulfenylation protocol shown in
Table 1 using diketopiperazine substrates 24, 41 and 2-epi-43
to generate epidithiodiketopiperazines 27, 42 and 44, respec-
tively. As shown in Table 3, the results consistently point to
NaHMDS as the preferred base for this reaction, although all
three bases gave good yields of the epidithiodiketopiperazine
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Page 6 of 15
Scheme 3. Reaction of Diketopiperazine 24 with Scheme 4. Thus, epicoccin G (1) was disconnected retrosyn-
1
2
3
4
5
6
7
8
bis[bis(trimethylsilyl)amino]trisulfide [(TMS)2SSS(TMS)2]
and NaHMDS and Mechanistic Considerations. Direct
Formation of Epidithio- and Epitetrathiodiketopipera-
zinesa
thetically to its bis-unsaturated precursor 58 through a bis-
hydrogenation step. The latter intermediate was then traced to
bis-endoperoxide 60 through the rarely used Kornblum–
DeLaMare rearrangement,27 anticipating a regioselective rup-
ture of the endoperoxide moieties under basic conditions.
Steric control in the latter process was envisioned to furnish
the
desired
regioisomer
(58).
Through
sequence,
a
bis-
bis-
photooxygenation/bis-sulfenylation
endoperoxide 60 was traced back to bis-diene diketopiperazine
45 through the intermediacy of bis-diene 55. Similar retrosyn-
thetic analysis of 8,8ʹ-epi-ent-rostratin B (2) led to the same
precursor (45) as shown in Scheme 4. The latter was envi-
sioned to arise from L-N-Boc-tyrosine (62) via bicyclic inter-
mediate 6328 (see Scheme 4) through appropriate elaboration
and dimerization procedures.
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The synthesis of the bis-diene 45 from the known tyrosine-
derived hydroxy enone 6328 is shown in Scheme 5. Thus,
acetylation of 63 followed by treatment with Zn and AcOH in
MeOH at 65 °C and exposure to DBU led to the deoxygena-
tion product bicyclic enone 64 possessing the desired syn ring
junction (51% yield for the three steps). Luche reduction29 of
the latter (NaBH4, CeCl3) gave allylic alcohol 65 (possessing
the α configuration as expected on steric grounds; inconse-
quential) in 92% yield. In preparation for the pending cy-
clodimerization, key intermediate 65 was separately processed
with LiOH and TFA to afford coupling partners 66 (99%
yield, TFA salt) and 67 (99% yield), respectively. N-Boc car-
boxylic acid 67 and amine methyl ester TFA salt 66 were
coupled in the presence of BOP-Cl and Et3N to afford amide
68 in 86% yield. Treatment of the latter with TFA followed by
exposure to Et3N led to the formation of pentacyclic diketop-
iperazine 69 in 77% yield for the two steps. The desired bis-
dehydration of bis-allylic alcohol 69 was achieved through the
intermediacy of bis-allylic trifluoroacetate 70 formed by
treatment of the former with (CF3CO)2O in the presence of
Et3N and 4-DMAP (69% yield). The latter intermediate (70)
was smoothly converted to the targeted bis-diene 45 upon
exposure to catalytic amounts of Pd(PPh3)4 in the presence of
K2CO3 (90% yield).30
The advancement of bis-diene 45 to the desired sulfenylated
intermediates epidithiodiketopiperazine 46 and bis-
(methylthio)diketopiperazine 55 and their diastereoisomers is
summarized in Scheme 6. Thus, sulfenylation of 45 according
to the developed procedure [NaHMDS-S8] furnished a mixture
of oligosulfides (71) from which emerged epidithiodiketop-
aReagents and conditions. a) (TMS)2NSSSN(TMS)2 (4.0
equiv), NaHMDS (0.6 M in PhMe, 4.0 equiv), THF, 25 °C, 30
min, 25: 22%, 27: 43%.
iperazines
46
and
2,2ʹ-epi-46
and
bis-
(methylthio)diketopiperazines 55 and 2,2ʹ-epi-55 upon reduc-
tion/oxidation (NaBH4; KI3; 55% combined yield for 46 and
2,2ʹ-epi-46, ca. 1.4:1 dr) and reduction/methylation (NaBH4;
MeI; 58% overall yield for 55 and 2,2ʹ-epi-55, ca. 1.4:1 dr).
The stereochemical configurations of these chromatograph-
ically separated products were deciphered by NOESY correla-
tions as indicated in Scheme 6 (bottom).
Total Syntheses of Epicoccin G (1), 8,8ʹ-epi-ent-Rostratin B
(2), Gliotoxin (3), Gliotoxin G (4), Emethallicin E (5) and
Haematocin (6). Empowered with the improved sulfenylation
method7,8 we were able to synthesize a number of biologically
active sulfenylated diketopiperazine natural products8 (Figure
1), including the antiviral agent epicoccin G9 (1), the 8,8ʹ-epi-
ent-isomer (2) of the cytotoxic agent rostratin B,10 the antiviral
and antibiotic gliotoxin11 (3), and its epitetrathio counterpart
gliotoxin G12 (4), the immunosuppressant emethallicin E13 (5),
and the antifungal agent haematocin14 (6). The designed syn-
thetic strategies employed to construct these molecules are
exemplified with those depicted for epicoccin G [1, a bis-
(methylthio)diketopiperazine] and 8,8ʹ-epi-ent-rostratin B (2,
an epidithiodiketopiperazine), in retrosynthetic format, in
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Scheme 4. Retrosynthetic Analysis of Epicoccin G (1) and
8,8ʹ-epi-ent-Rostratin B (2)
Scheme 5. Synthesis of bis-Diene Diketopiperazine 45a
1
2
3
4
5
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60
aReagents and conditions. a) Ac2O (2.0 equiv), Et3N (3.0
equiv), 4-DMAP (0.2 equiv), CH2Cl2, 0→25 °C, 4 h; b) Zn (8.0
equiv), AcOH (2.0 equiv), MeOH, 65 °C, 30 min; c) DBU (5.0
equiv), PhMe, 65 °C, 3 h, 51% for the three steps; d) NaBH4 (1.1
equiv), CeCl3·7H2O (1.0 equiv), MeOH, –78→0 °C, 1 h, 92%; e)
TFA/CH2Cl2 (1:1), 0→25 °C, 30 min, 99%; f) aq. LiOH (1.0
M)/THF (4:1), 0→25 °C, 3 h, 99%; g) 66, 67 (1.0 equiv each),
BOP-Cl (1.1 equiv), Et3N (3.0 equiv), CH2Cl2, 0→25 °C, 15 h,
86%; h) TFA (32 equiv), CH2Cl2, 0→25 °C, 1.5 h; then Et3N (5.0
equiv), CH2Cl2, 0→25 °C, 15 h, 77% for the two steps; i)
(CF3CO)2O (4.0 equiv), Et3N (6.0 equiv), 4-DMAP (0.3 equiv),
MeCN, –40→25 °C, 1 h, 69%; j) Pd(PPh3)4 (0.1 equiv), K2CO3
(2.1 equiv), dioxane, 65 °C, 30 min, 90%.
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Scheme 6. Synthesis of Dithiodiketopiperazines 46 and
Scheme 7. Completion of the Total Synthesis of Epicoccin
G (1)a
1
2
3
4
2,2ʹ-epi-46, and bis-(Methylthio)diketopiperazines 55 and
2,2ʹ-epi-55a and Stereochemical Assignments of 55 and
2,2ʹ-epi-55 by NOESY Studies (Arrows Designate NOESY
Correlations)
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53
54
55
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58
59
60
aReagents and conditions. a) O2, TPP (0.02 equiv), CH2Cl2, –45
°C, 45 min; then DBU (10.0 equiv), –45→0 °C; 1 h, 52% from
55; b) H2, Pd(OH)2/C (20% w/w, 0.4 equiv), MeOH, 25 °C, 1 h,
86%.
Scheme 8. Completion of the Total Synthesis of 8,8ʹ-epi-
ent-Rostratin B (2)a
aReagents and conditions. a) NaHMDS (0.6 M in PhMe, 3.0
equiv), S8 (1.0 equiv), THF, 25 °C, 1 min; then 45 (1 M in THF,
1.0 equiv), 1 min; then NaHMDS (0.6 M in PhMe, 2.0 equiv), 25
°C, 30 min; b) NaBH4 (25 equiv), THF/MeOH (1:1), 0→25 °C,
45 min; then MeI (50 equiv), 25 °C, 15 h, 58% over the three
steps from 45 (55:2,2ʹ-epi-55 ca. 1.4:1 dr); c) NaBH4 (25 equiv),
THF/MeOH (1:1), 0→25 °C, 0.75 h; then KI3 aq. (1.4 M), 25 °C,
10 min, 55% over the three steps from 45 (46: 2,2ʹ-epi-46 ca.
1.4:1 dr); d) NaBH4 (25 equiv), THF/MeOH (1:1), 0→25 °C, 45
min; then MeI (50 equiv), 25 °C, 15 h, 65% from 46(55:2,2ʹ-epi-
55 ca. 1.4:1 dr).
aReagents and conditions. a) O2, TPP (0.02 equiv), CH2Cl2, 0
°C, 2 h; then Et3N (5.0 equiv), 0→25 °C, 3 h, 55% for the two
steps; b) [CuH(PPh3)]6 (10.0 equiv), benzene, 25 °C, 30 min; then
KI3 aq. (1.4 M), 25 °C, 10 min, 82%.
The correct diastereoisomers 55 and 2,2ʹ-epi-46 were elabo-
rated to the target molecules epicoccin G (1) and 8,8ʹ-epi-ent-
rostratin B (2) through similar pathways as shown in Schemes
7
and 8, respectively. Thus, reaction of bis-
(methylthio)diketopiperazine bis-diene 55 with singlet oxygen
(generated from triplet oxygen and UV light in the presence of
tetraphenylporphyrin sensitizer)31 in CH2Cl2 at –45 °C fur-
nished bis-endoperoxide 60, which was treated with DBU (–
45→0 °C) without isolation to afford bis-hydroxy enone 58 as
the major product (52% overall yield, Scheme 7). The latter
compound was subjected to catalytic hydrogenation [H2, 20%
Pd(OH)2/C] to give smoothly epicoccin G in 86% yield. Pro-
cessing epidithio bis-diene 2,2ʹ-epi-46 with singlet oxygen (0
°C) followed by treatment of the resulting bis-endoperoxide
(61) with Et3N (0→25 °C) furnished epidithio bis-hydroxy
enone 59 in 55% overall yield (Scheme 8). The sensitivity of
the epidithiodiketopiperazine structural motif within 59 dictat-
ed the use of Stryker’s reagent32 [CuH(PPh3)]6 (as opposed to
the hydrogenation conditions employed for the conversion of
58 to epicoccin G, Scheme 7) for the required reduction of the
olefinic bonds, followed by re-oxidation with KI3 to regenerate
the partially cleaved epidithio moiety, thereby furnishing 8,8ʹ-
epi-ent-rostratin B (2) in 82% overall yield.
As further demonstrations of the applicability of the present
improved sulfenylation method, we pursued the enantioselec-
tive total synthesis of gliotoxin (3) and gliotoxin G (4), as well
as emethallicin E (5) and haematocin (6) (Figure 1). The de-
vised synthetic strategy toward these target molecules envi-
sioned bicyclic hydroxy diene 78 (see Scheme 9) as a common
intermediate. This key building block was obtained in multi-
gram quantities from the tyrosin-derived hydroxy enone N-
Boc methyl ester 63 as shown in Scheme 9.33 Thus, Luche
reduction (NaBH4, CeCl3) of 6328 gave diol 72 stereoselective-
ly (99% yield), which was smoothly acetylated to afford hy-
droxy acetate 73 in 91% yield. The latter was converted to
hydroxy diene 74 through palladium-catalyzed elimination
[Pd(OAc)2 (cat.), PPh3 (cat.), Et3N, 86%]. Photooxygenation
of this diene (O2, TPP, hν, 73%) generated hydroxy endoper-
oxide 75, whose reduction with thiourea afforded triol 76 in
84% yield. Selective monosilylation of the latter (TIPSOTf,
96% yield) followed by engagement of the 1,2-diol system
into a thionocarbonate moiety [(im)2C=S, 90% yield] fur-
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nished intermediate 77. The latter was deoxygenated tempting to achieve certain transformations, including the
1
2
3
4
5
6
7
8
[P(OMe)3, 82% yield] and desilylated (aq. HCl, 98% yield) to
afford the desired building block hydroxy diene 78.
present sulfenylation.
Scheme 11 summarizes the enantioselective total syntheses
of emethallicin E (5) and haematocin (6) from common inter-
mediate 78. Thus, a three-step sequence involving replacement
of the Boc protective group with Alloc (TFA, 95% yield; then
AllocCl, 88% yield) followed by saponification of the methyl
ester group (LiOH) furnished hydroxy carboxylic acid 83 in
high yield. Coupling of building blocks 83 and 84 (obtained in
the first step of the above sequence 78→83) under the influ-
ence of BOP-Cl and DIPEA led to amide 85 in 83% yield over
the two steps. Pentacyclic bis-hydroxy diketopiperazine 86
was generated from amide 85 in 84% overall yield upon
cleavage of the Alloc protecting group [Pd2(dba)3 (cat.)] in the
presence of Et2NH. The structure of intermediate 86 was un-
ambiguously confirmed by X-ray crystallographic analysis
(see ORTEP, Scheme 11). Sulfenylation of bis-hydroxy
diketopiperazine 86 with [LiHMDS-S8] led to tetrasulfide 87
as the major product (46% yield, plus 43% recovered starting
material 86).
Scheme 9. Synthesis of Common Key Building Block 78a
9
10
11
12
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58
59
60
As in the case of the gliotoxins discussed above (Scheme
10), the standard [NaHMDS-S8] conditions failed to produce
the sulfenylated product from substrate 86 in satisfactory
yield, leading only to 10% yield of epitetrasulfide 87 (Scheme
11). This observation prompted a systematic investigation to
optimize the yield of this sulfenylation reaction varying the
HMDS base and the solvent. The results of this study, shown
in Table 4, revealed LiHMDS in THF as the optimum condi-
tions (entry 5/LiHMDS). The formation of the epitetrasulfide
as the predominant product in this case is also of interest. This
example underscores once again the importance of careful
optimization of conditions to achieve the best results in
diketopiperazine sulfenylation reactions. It is also noteworthy
that the use of bis[bis(trimethylsilyl)amino]trisulfide (23,
Scheme 3) as a sulfenylating reagent in the presence of
LiHMDS as a base proved less reactive than the corresponding
in situ generated species [LiHMDS-S8], leading to recovery of
80% of starting material (86) and no epidisulfide or
epitetrasulfide products. The use of NaHMDS or KHMDS and
trisulfide 23 led primarily to aromatization under the same
sulfenylation conditions.
The stereochemical configuration of the epitetrasulfide 87
was based on NMR spectroscopic studies and was confirmed
by the successful synthesis of the natural products 5 and 6.
Indeed, intermediate 87 served as a common precursor to
emethallicin E (5) and haematocin (6) as shown in Scheme 11.
Thus, bis-esterification of 87 with phenylacetic acid
(PhCH2COOH) in the presence of DCC and 4-DMAP gave
bis-phenylacetate 88 (71% yield, plus 26% recovered starting
material 87), whose reduction/oxidation (1,3-propane dithiol;
then O2) furnished the desired product emethallicin E (5) in
54% overall yield. Alternatively, bis-acetylation of 87 (AcOH,
DCC, 4-DMAP, 71% yield, plus 24% recovered starting mate-
rial 87) followed by reduction/methylation (NaBH4; then MeI)
of the resulting bis-acetate afforded haematocin (6) in 97%
overall yield. The use of the DCC/4-DMAP esterification
protocol instead of the more conventional acid anhydride or
chloride methods was dictated by the sensitivity of the sub-
strate (87) and products (88, 89) under the reaction conditions,
especially toward aromatization.
aReagents and conditions. a) NaBH4 (2.0 equiv), CeCl3·7H2O
(1.3 equiv), MeOH, –20→0 °C, 3 h, 99%; b) Ac2O (2.0 equiv),
Et3N (3.0 equiv), 4-DMAP (0.2 equiv), CH2Cl2, 0 °C, 1 h, 91%;
c) Pd(OAc)2 (0.02 equiv), PPh3 (0.1 equiv), Et3N (1.2 equiv),
PhMe, 25→110 °C, 3 h, 86%; d) O2, TPP (0.0036 equiv), CH2Cl2,
25 °C, 24 h, 73%; e) thiourea (2.0 equiv), MeOH, 25 °C, 2 h,
84%; f) TIPSOTf (1.1 equiv), Et3N (2.0 equiv), CH2Cl2, 0 °C, 30
min, 96%; g) (im)2C=S (1.2 equiv), PhMe, 110 °C, 3 h, 90%; h)
P(OMe)3, 111 °C, 12 h, 82%; i) HCl aq. (1.0 M), CH2Cl2/Et2O
(1:1), 0 °C, 10 min, 98%.
The enantioselective total synthesis of gliotoxin (3) and
gliotoxin G (4) from the common building block 78 is summa-
rized in Scheme 10. Thus, hydrolysis of the methyl ester with-
in 78 (LiOH) led to carboxylic acid 79 (99% yield), which was
coupled with L-serine derivative 8034 (HATU, HOAt, DIPEA)
to afford amide 81 in 88% yield. Removal of the Boc group
from the latter and exposure of the resulting amino ester to
Et3N furnished tricyclic diketopiperazine 82 (63% overall
yield), whose structure was proven beyond doubt through X-
ray crystallographic analysis (see ORTEP, Scheme 10). Sul-
fenylation of the latter required the use of S8 and LiHMDS,
conditions that furnished directly gliotoxin (3, 23% yield) and
gliotoxin G (4, 33% yield) (plus 6% recovered starting materi-
al 82). Interestingly, attempts to effect the sulfenylation of 82
with [NaHMDS-S8] failed to produce gliotoxin or gliotoxin G,
leading instead to aromatization of the cyclohexadiene ring
and decomposition. These results underscore the subtle differ-
ences in reactivity of the various alkali metal HMDS bases and
point to the importance of thorough experimentation in at-
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Scheme 10. Completion of the Enantioselective Total Syn-
Scheme 11. Completion of the Enantioselective Total Syn-
theses of Gliotoxin (3) and Gliotoxin G (4)a
theses of Emethallicin E (5) and Haematocin (6)a
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2
3
4
5
6
7
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45
46
47
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49
50
51
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53
54
55
56
57
58
59
60
aReagents and conditions. a) aq. LiOH (1.0 M)/THF (6:1),
0→25 °C, 5 h, 99%; b) 79 (1.0 equiv), 80 (2.0 equiv), HOAt (1.1
equiv), HATU (1.1 equiv), DIPEA (3.0 equiv), CH2Cl2, 0→25 °C,
15 h, 88%; c) TFA/CH2Cl2 (1:1), 0→25 °C, 3 h; d) Et3N (5.0
equiv), CH2Cl2, 0→25 °C, 15 h, 63% for the two steps; e)
LiHMDS (1.0 M in THF, 4.0 equiv), S8 (8.0 equiv), THF, 25 °C,
5 min; then 82 (0.06 M in THF, 1.0 equiv) 5 min; then LiHMDS
(1.0 M in THF, 4.0 equiv), 25 °C, 1.5 h, 3: 23%, 4: 33%, plus 6%
recovered starting material 82.
aReagents and conditions. a) TFA/CH2Cl2 (1:2.5), 25 °C, 4 h,
95%; b) AllocCl (1.7 equiv), NaHCO3 (10.0 equiv), dioxane/H2O
(1:1), 0→25 °C, 3 h, 88%; c) LiOH aq. (1.0 M)/THF (1:1), 0→25
°C, 5 h; d) 83, 84 (1.0 equiv each), BOP-Cl (1.1 equiv), DIPEA
(3.0 equiv), CH2Cl2, 0→25 °C, 15 h, 83% for the two steps; e)
Pd2(dba)3 (0.02 equiv), dbbp (0.05 equiv), THF/Et2NH (2:1), 25
°C, 2 h, 84%; f) LiHMDS (1.0 M in THF, 20 equiv), S8 (37
equiv), THF, 25 °C, 5 min; then 86 (0.06 M in THF/Et2O (9:1),
1.0 equiv), 5 min; then LiHMDS (1.0 M in THF, 20 equiv), 25
°C, 5 h, 46%, plus 43% recovered starting material 86; g)
PhCH2COOH (30 equiv), DCC (30 equiv), 4-DMAP (3.0 equiv),
0→25 °C, 15 h, 71%, plus 26% recovered starting material 87; h)
AcOH (30 equiv), DCC (30 equiv), 4-DMAP (3.0 equiv), 0→25
°C, 15 h, 71%, plus 24% recovered starting material 87; i) 1,3-
propane dithiol (90 equiv), Et3N (0.32 equiv), MeCN/CH2Cl2
(25:1), 25 °C; then concentrate; then O2, MeOH, 2 h, 25 °C, 54%
overall; j) NaBH4 (80 equiv), MeOH/py (1:1), 0 °C; then MeI
(485 equiv) 0→25 °C, 4 h; 97% overall.
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Table 4. Optimization Study of the Sulfenylation of
Diketopiperazine 86a
ment/extrusion of 96 (which undergoes disproportionation
with expulsion of oxygen to form TrocN=NTroc).36
Scheme 12. Attempted Synthesis of Oxepin 7 by Ring Ex-
pansion of a Diazo Epoxidea
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60
b
aReactions were performed on 5 mg scale at 25 °C. Yields of
product and recovered starting material (rsm) are isolated
yields after chromatography, <5% yield refers to no detectable
1
product or starting material as determined by H NMR spec-
troscopic analysis of the crude reaction mixture. cReverse
addition of preformed sulfenylation reagent to substrate and
base.
aReagents and conditions. a) TrocN=NTroc (1.2 equiv),
CH2Cl2, 41 °C, 4 h, 93%; b) TBAF (1.0 M in THF, 2.0 equiv),
THF, 0 °C, 30 min, 92%; c) 1,1,1-trifluoro acetone (62 equiv),
Na2EDTA, NaHCO3 aq. (15 equiv), oxone(R) (38 equiv), MeCN,
0→25 °C, 15 h; d) Zn (21 equiv), MeOH/ NH4Cl aq. (1.0 M)
(4:1), 25 °C, 2 h; then NH4OH aq. (15 M), CuCl2 aq. (1.0 M), 25
°C, 5 min, 76% for the two steps.
As part of a program directed toward the total synthesis of
aranotin (8) we attempted to construct its monomeric unit (7,
see Scheme 13) through diazo epoxide precursor 95 as shown
in Scheme 12.35 Thus, bicyclic diene system 90 (for its synthe-
sis see Scheme 9, 77→78, step h) was reacted with bis-
trichloroethylazodicarboxylate (TrocN=NTroc) to afford
Diels–Alder adduct 91 stereoselectively (steric control) and in
93% yield. Desilylation of the latter with TBAF gave hydroxy
derivative 92 (92% yield), whose treatment with me-
thyl(trifluoromethyl) dioxirane followed by sequential expo-
sure to Zn and CuCl2 in the presence of NH4OH afforded
diazo epoxide 93 as a single diastereoisomer (76% overall
yield). The stereochemical configuration of 93 was established
through X-ray crystallographic analysis (see ORTEP, Scheme
12). As expected, this epoxide did not enter the obligatory
rearrangement with loss of N2 by virtue of the syn arrangement
of the diazo and epoxide moieties that does not allow for the
proper orbital orientations.35a
Our inability to reach the anti diazo epoxide 95 (Scheme
12), whose rearrangement to oxepin 94 was anticipated to be
facile, prompted us to pursue the alternative pathway (Scheme
13) involving trichloroethyl nitrosoformate compound 96
(generated from TrocNHOH and NaIO4) as the dienophile.
The latter reacted with bicyclic diene 90 to give Diels–Alder
adduct 97 (88% yield) diastereo- and regioselectively (pre-
sumably due to steric control).36 The structure of 97 was as-
signed based on NMR spectroscopic analysis (COSY,
NOESY, HMBC, HSQC). This intermediate was epoxidized
with methyl(trifluoromethyl) dioxirane to afford directly ox-
epin system 7 (40% yield), presumably via the fleeting epox-
ide 98 through a retro-Diels–Alder/epoxide opening. Epoxide
98 apparently must be of the anti configuration with respect to
the N–O bridge, which allows for the facile rearrange-
Scheme 13. Synthesis of Oxepin (7) by Ring Expansion of
Nitroso Epoxide 98a
aReagents and conditions. a) TrocNHOH (2.5 equiv), NaIO4
(1.0 equiv), TBAI (1.0 equiv), CH2Cl2/H2O (4:1), 0→25 °C, 10
min, 88%; b) 1,1,1-trifluoro acetone (7.0 equiv), Na2EDTA, Na-
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HCO3 aq. (60 equiv), oxone(R) (20 equiv), MeCN/H2O (1:1), 0 °C,
15 h, 40%.
inhibitory, effective concentration) for some of these com-
1
2
3
4
5
6
7
8
pounds. Compounds 46 (SI = 41–70), 2,2ʹ-epi-46 (SI = 27–75)
and 61 (SI = 23–59) were the most impressive in this regard
(see Table 5). The anti poliovirus drug Pirodavir(R) (99, Table
5, entry 9), used as a control in this poliovirus assay, exhibited
EC50 = 1.58 µM, underscoring the significant activities of
KCN-19 (46), KCN-2,2ʹ-epi-19 (2,2ʹ-epi-46), and KCN-21
(61).
In the anti Plasmodium falciparum assays (carried out in the
laboratories of E. A. W. at TSRI), epidithiodiketopiperazines
46 (IC50 = 3.6 µM, Table 5, entry 2), 2,2ʹ-epi-46 (IC50 = 2.7
µM, entry 3), 59 (IC50 = 4.5 µM; not included in the table, for
structure see Scheme 4), 61 (IC50 = 2.5 µM, entry 7), and bis-
(methylthio)diketopiperazines 58 (IC50 = 1.2 µM, entry 6),
2,2ʹ-epi-58 (IC50 = 4.4 µM; not included in the table, for struc-
ture see Scheme 4), and epicoccin G (1, IC50 = 2.5 µM) proved
to be the most potent.
Biological Evaluation. Having synthesized various types of
epidithio- and bis-(methylthio)diketopiperazines we selected a
number of them for biological evaluation. Specifically, select-
ed compounds were tested against poliovirus and Plasmodium
falciparum.37 Table 5 summarizes the results of these biologi-
cal assays. Thus, in the anti poliovirus assays (carried out in
the laboratory of D. F. S. under the auspices of the National
Institute of Allergy and Infectious Diseases, NIAID), epidithi-
odiketopiperazines 46 (code number KCN-19), 2,2ʹ-epi-46
9
10
11
12
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60
(code
number
KCN-2,2ʹ-epi-19)
and
epidithio-bis-
endoperoxide-diketopiperazine 61 (code number KCN-21)
proved to be the most potent, exhibiting EC50 = 101–115 nM,
107–123 nM, and EC50 = 21.4 nM values, respectively, de-
pending on the assay (see Table 5, entries 2, 3 and 7). Table 5
also displays selectivity indices (SI = CC50/EC50 or EC90, with
CC50 = 50% cell-inhibitory, cytotoxic concentration deter-
mined in stationary cells and EC50/90 = 50%/90% poliovirus-
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Table 5. Biological Evaluation of Selected Compounds in Poliovirus and Plasmodium falciparum Assaysa
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2
3
4
5
6
7
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aAssays for entries 2, 3 and 7 were carried out as triplicates; mean and standard deviation are given. For experimental details of all assays,
see Supporting Information. bVisual assay. cNeutral red assay. dVirus yield reduction assay. eNot determined. fStandard anti poliovirus drug
used as a control.
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(1) (a) Bull, S. D.; Davies, S. G.; Parkin, R. M.; Sancho, F. S. J.
CONCLUSION
Chem. Soc., Perkin Trans. 1 1998, 2313–2319. (b) Ding, G.; Jiang, L.;
Guo, L.; Chen, X.; Zhang, H.; Che, Y. J. Nat. Prod. 2008, 71, 1861–
1865. (c) Kamei, H.; Oka, M.; Hamagishi, Y.; Tomita, K.; Komishi,
M.; Oki, T. J. Antibiot. 1990, 43, 1018–1020. (d) Huang, R.; Zhou,
X.; Xu, T.; Yang, X.; Liu, Y. Chem. Biodiv. 2010, 7, 2809–2829. (e)
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An improved method for the sulfenylation of 2,5-
diketopiperazines based on the use of alkali metal hexame-
thyldisilazide bases (i.e. NaHMDS, LiHMDS and KHMDS)
and sulfur (S8) in THF at 25 °C as a means to prepare epidi-
thio-, epitetrathio- and bis-(methylthio)diketopiperazines has
been developed. A second method involving the use of
bis[bis(trimethylsilyl)amino]trisulfide
[(TMS)2NSSSN(TMS)2] and NaHMDS for the direct prepara-
tion of epidithio- and epitetrathiodiketopiperazines has also
been developed.
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56
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Application of these methods led to the synthesis of an array
of sulfenylated diketopiperazine systems, including the natural
products epiccocin G (1), gliotoxin (3), gliotoxin G (4), eme-
thallicin E (5), haematocin (6) and the 8,8ʹ-epi-ent-isomer (2)
of rostratin B. With the exception of gliotoxin (3),6h these
accomplishments represent the first enantioselective total
syntheses of these natural products and their analogs and fea-
ture a number of novel synthetic strategies and reactions,
including the [2+2] photooxygenation and the rarely used
Kornblum–DeLaMare rearrangement.
Biological investigations of selected members of the synthe-
sized compound libraries led to the discovery of a number of
potent anti poliovirus agents (i.e. 46, 2,2ʹ-epi-46 and 61) and a
series of anti Plasmodium falciparum lead compounds (i.e. 46,
2,2ʹ-epi-46, 58, 61 and 1) that may facilitate biological inves-
tigations and drug discovery efforts in the antiviral and anti-
malarial areas, respectively.
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By blending total synthesis of natural products of biological
and medical interest with method development endeavors and
chemical biology studies, the work described herein exempli-
fies the modern paradigm of natural product synthesis and
underscores its relevance and importance to chemistry, biolo-
gy and medicine.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures and charac-
terization data for key compounds (pdf cif files). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
(10) Tan, R. X.; Jensen, P. R.; Williams, P. G.; Fenical, W. J. Nat.
Prod. 2004, 67, 1374–1382.
Corresponding Author
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kcn@scripps.edu
Funding Sources
K.C.N. acknowledges funding from the National Institute of
Health (USA) (grant nos. AI055475 and CA100101), as well as
fellowships from Fonds de Recherche Québec (to D. G.), Bristol-
Myers Squibb (to D. S.) and The Skaggs Research Institute. This
project was funded in part for D.F.S. from the Division of Micro-
biology and Infectious Diseases, National Institute of Allergy and
Infectious Diseases, National Institute of Health (USA), Depart-
ment of Health and Human Services, under Contract No.
HHSN272201100019I/HHSN27200001/B01., and for E.A.W. by
the National Institute of Health (USA) (grant no. R01 AI090141).
ACKNOWLEDGMENT
We thank Drs. D. H. Huang and L. Pasternack for NMR spectro-
scopic, Dr. G. Siuzdak for mass spectrometric, and Dr. R. K.
Chadha for X-ray crystallographic assistance.
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TOC
Synthesis and Biological Evaluation of Epidithio-, Epitetrathio- and bis-
(Methylthio)diketopiperazines. Synthetic Methodology, Enantioselective Total Synthesis of Epi-
coccin G, 8,8ʹ-epi-ent-Rostratin B, Gliotoxin, Gliotoxin G, Emethallicin E and Haematocin, and
Discovery of New Antiviral and Antimalarial Agents
K. C. Nicolaou, Min Lu, Sotirios Totokotsopoulos, Philipp Heretsch, Denis Giguère, Ya-Ping Sun, David Sarlah, Thu Han
Nguyen, Ian Coulter Wolf, Donald F. Smee, Craig W. Day, Selina Bopp, Elizabeth A. Winzeler
ACS Paragon Plus Environment