Novel pleuromutilin derivatives as antibacterial agents: Synthesis, biological evaluation and molecular docking studies

Article abstract of DOI:10.1016/j.bmcl.2012.08.021

Owing to the increasingly serious problems caused by multidrug resistance in community-acquired infection pathogens, it has become an urgent need to develop new classes of antibiotics for overcoming the resistance. In this paper, we describe the design an

Full text of DOI:10.1016/j.bmcl.2012.08.021

Bioorganic & Medicinal Chemistry Letters 22 (2012) 6166–6172
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel pleuromutilin derivatives as antibacterial agents: Synthesis,
biological evaluation and molecular docking studies
Xinyang Wang a, Yong Ling a, , Hui Wang b, Jianghe Yu b, Junming Tang c, Heng Zheng d, Xi Zhao a,
⇑
Donggeng Wang a, Guangtong Chen a, Wenqian Qiu a, Jinhua Tao a
a Department of Pharmacy, School of Medicine, Nantong University, Nantong 226001, PR China
b Department of Microbiology, College of Life Science and Technology, China Pharmaceutical University, Nanjing 210009, PR China
c National Chemical Experimental Teaching Demonstration Center, College of Chemistry and Environmental Science, Henan Normal University, Xinxiang 453007, PR China
d Department of Bioinformatics, College of Life Science and Technology, China Pharmaceutical University, Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Owing to the increasingly serious problems caused by multidrug resistance in community-acquired
infection pathogens, it has become an urgent need to develop new classes of antibiotics for overcoming
the resistance. In this paper, we describe the design and synthesis of novel pleuromutilin derivatives con-
taining the (2-aminothiazol-4-yl)-4-methyl group, as well as their in vitro antibacterial activities against
Gram-positive clinical bacteria. Most of the tested compounds displayed strong antibacterial activities
against these methicillin-susceptible and methicillin-resistant bacteria. Particularly noteworthy com-
Received 3 May 2012
Revised 29 July 2012
Accepted 2 August 2012
Available online 10 August 2012
Keywords:
pound 15 and its derivative 16e, both showed potent antibacterial properties (0.0625–0.5 lg/mL) that
Pleuromutilin derivatives
Antibacterial activity
2-Amino-1,3-thiazole
Resistant Gram-positive pathogens
are superior to amoxicillin and tiamulin. Molecular docking studies suggested that the amino thiazole
ring on the side chains of the pleuromutilin derivatives can in general be accommodated near the mutilin
core in the binding pocket, and thus play an important role in the activity of the whole molecule. The
findings reported herein may provide a new insight into the design of novel pleuromutilin derivatives
for human clinical use.
Ó 2012 Elsevier Ltd. All rights reserved.
As a result of the increasing use or abuse of antibacterial agents
for all kinds of infectious diseases on mankind, many drug-resis-
tant pathogens have appeared in recent years. Especially, the
methicillin-resistant Staphylococcus aureus (MRSA) and penicillin-
resistant Streptococcus pneumoniae (PRSP), have become serious
problems in community settings and intensive care units (ICU) in
the hospitals.1 Therefore, developing novel classes of antibiotics
against these resistant bacterial strains has become an urgent issue
for scientists. The naturally occurring diterpene pleuromutilin can
inhibit the growth of a variety of Gram-positive bacteria and myco-
plasma, via a new action mechanism in that it selectively inhibits
bacterial protein synthesis through interaction with 50s subunits
of prokaryotic ribosomes, but has no effect on eukaryotic protein
synthesis and does not bind to a mammalian ribosome,2 and con-
sequently, has no cross-resistance to marketed antibacterial
agents. With the aim to develop new antibacterial agents, we
decided to use pleuromutilin as the lead compound to design
and synthesize a new class of pleuromutilins and evaluate their
antibacterial bioactivities.
passeckerianus), two pleuromutilin derivatives,3 tiamulin 2 and
valunemulin 3, have been successfully developed as therapeutic
agents for veterinary use in markets until now. Tiamulin, devel-
oped by Sandoz in 1974, is a prophylactic and therapeutic agent
for swine dysentery, and shows activity (MIC<1 lg/mL) against
anaerobic bacteria, intestinal spirochetes and Mycoplasma spp.4
Valnemulin was originally approved in 1999 in the European Un-
ion (EU) for the prevention and treatment of swine dysentery
caused by Brachyspira hyodysenteriae, and enzootic pneumonia
caused by Mycoplasma hyopneumoniae.5 The MIC value (0.03–
2 lg/mL) of valnemulin on B. pilosicoli is about two times lower
than that of tiamulin, and about 128 times lower than that of lin-
comycin (see Fig. 1).6,7
From then on, several other semisynthetic compounds 4–8 have
also been designed for human use. Azamulin 4, initially designed
for this purpose, has entered into Phase I clinical studies on volun-
teers,8,9 but due to its low solubility in water and a short half-life
in vivo, no further progress was made for this compound.7 Reta-
pamulin 5 is the first antibacterial agent of the pleuromutilin class,
which shows excellent in vitro antibacterial activity and was ap-
proved in April 2007 by FDA for the topical treatment of the impe-
tigo and secondarily infected traumatic lesions of skin infection,
mainly caused by the Gram-positive bacterial S. aureus
Since pleuromutilin 1 was first isolated in 1952 from two
basidiomycete species (e.g., Pleurotus mutilus and Pleurotus
⇑
Corresponding author. Tel./fax: +86 513 85051892.
E-mail address: Lyyy111@sina.com (Y. Ling).
(MIC = 0.12 lg/mL) and Streptococcus pyogenes (MIC = 0.016
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.08.021

X. Wang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6166–6172
6167
CH2
O
CH3
R
OH
O
H3C
H3C
CH3
O
NH2
H
N
3,
6,
S
N
2,
OH
N
R=
1,
4,
S
O
N
S
N
N
NH2
5,
S
N
N
MeHN
S
N
N
H
O
N
N
N
N
N
O
N
N
N
8,
7,
9,
S
O
MeHN
S
N
N
H2N
S
NH2
OH
10,
HO
Figure 1. Important pleuromutilin derivatives.
l
g/mL).10 Furthermore, to overcome strong hydrophobic problem,
side chains, C-14 extensions, pointing toward the P-site, which en-
hances the interactions with 23S rRNA. This observation led us to
speculate that modifying the side chain with the 2-aminothiazole
ring moiety may improve their antibacterial activities. With these
ideas in mind, we designed and synthesized a new pleuromutilin
derivative with 2-aminothiazole side arm (15) as our lead com-
pound, along with a novel class of target compounds with different
2-(substituted amino) acetyl amide group (Fig. 3, 16a–g).
Yoshimi H. et al designed and synthesized the water-soluble new
pleuromutilin derivatives 6 and 7 with amino purine group, which
not only showed excellent in vitro antibacterial activity against
MRSA, PRSP, VRE, S. pyogenes, and M. catarrhalis but also potent
in vivo efficacy (MIC = 0.016 l
g/mL).11,12
In recent years, several other pleuromutilins derivatives BC-
3205 (8) , BC-3781 (9) and BC-7013 (10) have already entered
Phase I clinical studies in volunteers, respectively. Especially BC-
3781 9 has become the second pleuromutilin derivative that en-
tered Phase II clinical studies, demonstrating excellent antimicro-
bial activity against relevant bacteria, including MRSA (see
Fig. 1).13–15 It is very important for BC-3781 to be well tolerated
in patients and to keep a proper plasma concentration level, indi-
cating a therapeutic potential for the treatment of skin infections
and bacterial lung, including community-acquired bacterial pneu-
monia (CABP). Nevertheless, there have been no pleuromutilin
derivatives for human oral use on the market so far to the best of
our knowledge.
To develop new pleuromutilin derivatives for human use, we
were inspired by the fact that many natural products isolated from
marine sources bearing a thiazole ring, for example, Leucamide A
1116 and myxothiazol 12,17 can exhibit various types of bioactivi-
ties, including antibacterial, anticancer, and antivirus activities.
Meanwhile, some of the new cephalosporins with a thiazole ring
moiety (e.g., cefpiromes 13 and claforan 14) have exhibited higher
bioactivities than the prototype compounds. Thus, we speculated
that the pleuromutilin derivatives containing a thiazole group
may demonstrate somewhat enhanced antibacterial activity as
compared to pleuromutilin (see Fig. 2). Also, we noted the interac-
tion detail demonstrated by the crystal structure of the Deinococcus
radio-durans 50S ribosomal subunit in complex with tiamulin and
valnemulin.18,19 In this crystal structure, besides that the ribosomal
peptidyl transferase center (PTC) with their tricyclic mutilin core
located inside a tight pocket confined by nucleotide residues at
the A-site of 50S ribosomal subunit, it is very important that their
The synthesis of the lead compound 15 and the target com-
pounds 16a–g is shown in Scheme 1. Pleuromutilin (1) was reacted
with 4-toluene sulfonyl chloride (TsCl) in ether using 30% aqueous
NaOH as the catalyst, to yield 14-deoxy-14-P-tosyloxyacetoxymu-
tilin 17, which was further treated with potassium iodide in ace-
tone to give the iodide 18 in good yield. The key intermediate 15
was obtained by the nucleophilic attack of 2-aminothiazol-4-
yl)methanethiol on 18 in the presence of 30% NaOH in tetrahydro-
furan (THF).20 Further treatment of the lead compound 15 with
2-chloroacetyl chloride in the presence of potassium carbonate
gave the intermediate 19, which was directly treated with various
amines in the presence of KI and potassium carbonate, to afford the
corresponding target compounds 16a–g in yields of 46–58%,
respectively. The target products were purified by column chroma-
tography, and their structures were characterized by IR, 1H-NMR,
21
13C-NMR, MS, and elemental analysis.
The antibacterial properties of the novel pleuromutilin deriva-
tives 16a–g were assessed in vitro by the conventional agar dilu-
tion method, using test agar against a panel of selected Gram-
positive clinical bacteria (Table 1). The minimal inhibitory concen-
trations (MIC, lg/mL) were determined in comparison with amox-
icillin and tiamulin as positive controls. An overnight culture of
bacteria in tryptosoy broth was diluted to about 105 cells/mL with
the same broth, and inoculated with an inoculating device onto
agar containing serial two-fold dilutions of the tested compounds.
Organisms were incubated at 37 °C for 18–20 h.
The in vitro antibacterial activities of the new pleuromutilin
derivatives 15 and 16a–g prepared above against both Gram-positive

6168
X. Wang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6166–6172
O
O
N
H
O
N
OCH3
N
S
N
S
N
OCH3
HN
O
N
S
H3CO2C
O
NH
N
O
11, Leucamide A
O
12, Myxothiazol
O
S
S
H
H
H2N
H
H
S
S
HN
N
N
H2N
N
N
H
N
N
O
N
N
O
O
O
O
O
CH3
O
O
ONa
O
14, Claforan
13, Cefpirome
Figure 2. Some bioactive compounds containing thiazolyl group.
R1
N
CH2
CH2
CH3
H2N
S
NH
R2
CH3
N
O
N
O
O
OH
CH3
OH
CH3
S
S
S
O
O
H3C
H3C
H3C
H3C
O
O
15. The new designed lead compound
16a-g. The target compounds
Figure 3. Novel pleuromutilin derivatives synthesized in the present work.
organisms, including methicillin-susceptible Staphylococcus aureus
(MSSA), meticillin resistant Staphylococcus aureus (MRSA), methi-
cillin-susceptible Staphylococcus epidermidis (MSSE), and methicil-
lin-resistant Staphylococcus epidermidis (MRSE) are listed in
Table 1. For comparison, the MIC values of amoxicillin and tiamulin
are also listed.
The data in Table 1 showed that most of the tested compounds
displayed superior or similar antibacterial activities to those of
amoxicillin and tiamulin. In particular, the key intermediate com-
pound 15 with the (2-aminothiazol-4-yl)methyl thioether group at
C-14 position was found 4-8-fold more potent than amoxicillin and
tiamulin against most of the selected Gram-positive organisms. For
examples of the activity data to the four MRSAs, amoxicillin
An examination of the structure–activity relationship (SAR) for
compounds with different 2-(substituted amino) acetyl amide
group on the thiazole ring 16a–g revealed significant differences
in their antibacterial activities. Firstly, the activities of compounds
16a–c were found to decrease with the increasing steric bulkiness
in the substituent amino group (bioactivities: 16a > 16b > 16c).
Especially, in case of 16c, bulky n-butyl group led to a significant
loss in antibacterial activity. This observation suggests that the
C-14 extensions of the compound with a bulky substituent group
pointing toward the P-site may not be favorable for its bioactivi-
ties. Secondly, the bioactivity of the compound with a secondary
amino substituent group 16f–g was a little lower than that of their
counterparts with a tertiary amino group 16a–e. It is very interest-
ing to note that compound 16e having a piperidine group showed
the best activity against all the Gram-positive organisms. These re-
sults revealed that the pleuromutilin derivatives with the thiazole
group in the side chain at C-14 position can demonstrate very
promising bioactivities against Gram-positive organisms, and
therefore, will provide a new insight into the design of novel anti-
bacterial drugs.
MIC = 16, 0.125, 16, and 32
lg/mL, tiamulin MIC = 2, 0.5, 2, and
2
l
g/mL, compound 15 MIC = 0.25, 0.0625, 0.5, and 0.5 l
g/mL,
respectively. These data indicate that pleuromutilin derivatives
with thiazole group on the side chain at C-14 position can indeed
demonstrate higher antibacterial activity against some organisms
than their prototype compounds. Especially, compound 16e having
piperidinyl groups not only showed a similar activity as compared
with compound 15 against MRSA, but also demonstrated the best
To explore the antibacterial mechanism of these novel pleurom-
utilin derivatives, two tested compounds with excellent bioactivi-
ties, 15 and 16e, and two representative compounds tiamulin and
6, were chosen for molecular docking studies. The X-ray crystal
structure of ribosome–Tiamulin complex (PDB code: 1XBP) with
a resolution of 3.50 Å was applied to build the starting model of
activity (MIC = 0.0625ꢀ2
which is almost 2-fold more potent than compound 15, 8 ꢀ 16-fold
g/mL), and tiamu-
g/mL) against the same infections patho-
lg/mL) against MRSE, MSSE and MSSA,
more potent than amoxicillin (MIC = 0.125 ꢀ 8
l
lin (MIC = 0.0625 ꢀ 8
l
gens, respectively.

X. Wang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6166–6172
6169
CH
2CH3
CH
2CH3
OH
O
OH
O
HO
H3C
SO2O
b
a
14
14
O
O
CH3
CH3
H3C
H3C
H3C
H3C
O
O
17
1
CH
2CH3
CH
2 CH3
S
OH
O
O
OH
c
H2N
S
d
I
14
14
N
O
O
H3C
H3C
CH3
CH3
H3C
H3C
O
O
15
18
R1
N
28
CH
27
20
19
2 CH3
CH2
NH
R2
S
O
26
CH318
OH
N
24
O
21
O
HN
S
e
OH
CH317
S
N
14
S
22
23
12
O
13
14
5
11
10
9
O
25
H3C
H3C
CH3
O
15
H3C
4
Cl
7
1
O
6
16
8
2
H3C
3
O
19
16a-f
N
N
= N
=
16e.
16f.
R1
R1
R2
R2
16a.
16b.
=
=
N(CH2CH3)2
N(CH2CH2CH3)2
R1
R1
R2
R2
N
N
N
HN
N
N
= HN
16g.
R1
R2
=N(CH2CH2CH2CH3)2
= N
R1
R1
R2
R2
16c.
16d.
OCH3
O
Scheme 1. Reaction conditions and reagents: (a) TsCl, 30% NaOH, ether, reflux, 2hr, 60%; (b) KI, acetone, reflux, 3 h, 87%; c) (2-aminothiazol-4-yl)methanethiol, 10% NaOH,
THF, 0 °C to r.t., 4 h, 72%; d) 2-chloroacetyl chloride, K2CO3, CH2Cl2, 0 °C to r.t., 1 h, 92%; (e) R1R2NH, K2CO3, THF, 40 °C, 4–8 h, 53%–65%.
Table 1
Antibacterial activity (MIC,
lg/mL) of pleuromutilin derivatives (16a–g) in vitro
Strainsb
MICa
(lg/mL)
Amoxicillin
Tiamulin
15
16a
16b
16c
16d
16e
16f
16g
MSSA-cpu0211
MSSA-cpu0587
MSSA-cpu1044
MSSA-cpu1344
MRSA-cpu0446
MRSA-cpu0887
MRSA-cpu1098
MRSA-cpu3256
MSSE-cpu0315
MSSE-cpu0347
MSSE-cpu2089
MSSE-cpu1561
MRSE-cpu2623
MRSE-cpu3889
MRSE-cpu3569
MRSE-cpu2736
4
2
1
1
2
8
8
2
2
0.5
2
2
0.0625
0.5
0.5
0.0625
1
1
0.5
2
2
0.125
0.25
0.0625
0.5
1
8
1
0.5
0.5
0.5
1
0.5
0.5
1
4
4
4
16
8
32
8
8
8
8
4
0.125
0.125
0.25
0.25
0.25
0.5
0.25
0.25
2
0.25
0.125
0.5
0.5
0.25
0.5
0.125
2
0.125
0.0625
0.5
0.0625
0.25
0.25
0.0625
0.0625
0.125
0.0625
0.0625
0.0625
0.0625
0.0625
0.25
0.25
0.5
0.5
0.5
0.5
0.25
0.5
1
0.5
0.25
8
0.25
1
0.25
0.25
0.5
0.5
1
0.25
0.5
0.25
0.5
2
0.25
0.25
0.25
0.25
0.25
2
4
0.125
4
8
0.125
8
0.125
0.125
8
1
0.5
0.5
4
0.5
0.25
1
0.5
1
0.5
0.5
0.5
1
8
0.125
0.125
0.125
0.125
0.125
0.125
0.125
0.125
0.5
0.5
4
0.5
2
16
4
16
8
8
8
0.25
0.0625
0.25
0.25
4
2
0.5
1
0.125
16
0.5
a
Minimal inhibitory concentrations (MIC): the lowest concentration of compound that inhibits visible growth of the organism.
b
All of strains were isolated from the clinical bacteria in the Nanjing Gulou hospital and reserved in Department of Microbiology, College of Life Science and Technology,
China Pharmaceutical University. Clinical strains were identified by the API bacteria analysis system.
50S ribosomal subunit from Deinococcus radiodurans. The active
sites were calculated with the ‘site finder’ function of MOE.
Compound 15 was found to form two strong hydrogen bonds
with ribosome in the docking studies, as shown in Figure 4. Specif-
ically, the hydroxyl group of 15 (on C11) formed one H-bond with
the oxygen atom of G2484 (OH/O distance: 1.42 Å), and the NH2
group on the thiazole ring of 15 formed the second H-bond with
the hydroxyl group of residue C2565 (NH/O distance: 1.57 Å).
Moreover, there was also a stacking interaction between thia-
zole ring of 15 and the purine ring of residue A2045, and ring to
ring distance was 4.27 Å. The binding free energy of 15 with ribo-
some was calculated to be À12.51 kcal/mol.
p–p

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X. Wang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6166–6172
Figure 4. Docking models of compound 15 in the active site of the 50S ribosomal subunit.
Figure 5. Docking models of compound 16e in the active site of the 50S ribosomal subunit.
Compound 16e with the best antibacterial activity also demon-
compound 15 and 16e had only an p–p stacking interaction with
strated a binding mode similar to that of 15, as shown in Figure 5.
In this case, there were three hydrogen bonds observed, through
the interaction of 16e with C-2431 (OH /O distance: 2.37 Å),
G2044 (NH2/O distance: 2.89 Å), and C2046 (NH/O distance: 2.33
residue A2045 respectively besides the hydrogen bonds interac-
tions with residues (as seen in Fig. 4ꢀ5). The binding free energy
of 6 with ribosome was equal to À18.97 kcal/mol and seemed to
testify the conclusion that the
p–p interactions between the het-
Å) respectively. A prominent
p
–
p
stacking interaction between
erocyclic moiety in the side chain of the compounds and nucleotide
residues of ribosome may improve further its anti-bioactivity.
In a word, all these results also indicated that the side chains of
the pleuromutilin derivatives played an important role in fine-tun-
ing the physical–chemical properties of the molecule, and impor-
tantly, amino thiazole ring in general can be adopted near the
mutilin core in the binding pocket and used to further improve
the binding affinity.
the thiazole ring of 16e and the purine ring of A2045 with ring
to ring distance of 4.61 Å was also found in docking mode. Com-
pared to 15, compound 16e had an advantage on the number of
hydrogen bonds formed with ribosome. Moreover, the higher bind-
ing free energy of 16e with ribosome (À14.82 kcal/mol) was lower
than that of 15. This may explain why the antibacterial activity of
16e was better than that of 15.
We also performed the docking studies of the benchmark com-
pound tiamulin and 6 with ribosome. From the docking result, both
tiamulin and 6 exhibited a docking mode similar to that of 15 or
16e in general (Figs. 6 and 7). An important difference was that
In summary, via the introduction of (2-aminothiazol-4-yl)-
methyl thioether group to the C-14 position of pleuromutilin, we
have designed and synthesized new classes of pleuromutilin deriv-
atives with different 2-(substituted amino) acetyl amide group on
the thiazole ring in the side chain, and evaluated their antibacterial
activities in vitro against Gram-positive clinical bacteria. Most of
these compounds were found to display strong antibacterial activ-
ities against the methicillin-susceptible and methicillin-resistant
bacteria. Particularly noteworthy are compound 15 and its deriva-
tive 16e, both showed potent antibacterial properties (0.0625–
no
p–p stacking interaction between tiamulin and ribosome was
found, and only two hydrogen bonds with G2484 (OH/H distance:
1.32 Å) and U2564 (NH/O distance: 2.00Å) were formed respec-
tively. This may be the reason that tiamulin had the weaker anti-
bacterial activity than that of the other three compounds 6, 15,
16e, although its binding free energy was up to À13.49 kcal/mol.
On the contrary, there were three
p
–p
stacking interactions
0.5 lg/mL) that are superior to those of amoxicillin or tiamulin.
between the purine rings of 6 and A2045, and their ring to ring
The related activity data have been primarily rationalized, to some
distances were 4.07 Å, 4.40 Å and 4.13 Å respectively, while
extent, by docking studies using MOE. These results may provide

X. Wang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6166–6172
6171
Figure 6. Docking models of tiamulin in the active site of the 50S ribosomal subunit.
Figure 7. Docking models of compound 6 in the active site of the 50S ribosomal subunit.
9. Berner, H.; Fischer, G.; Hildebrandt, J. F.; Laber, G.; Vyplel, H. Curr. Chemother.
Immunother. 1982, 1, 345.
new insights into the design of novel pleuromutilin derivatives,
and lay the basis for further studying on the promising antibiotics
for human clinical use.
10. Daum, R. S.; Kar, S.; Kirkpatrick, P. Nat. Rev. Drug Discov. 2007, 6, 865.
11. Hirokawa, Y.; Kinoshita, H.; Tanaka, T.; Nakamura, T.; Fujimoto, K.; Kashimoto,
S.; Kojima, T.; Kato, S. Bioorg. Med. Chem. Lett. 2008, 18, 3556.
12. Hirokawa, Y.; Kinoshita, H.; Tanaka, T.; Nakamura, T.; Fujimoto, K.; Kashimoto,
S.; Kojima, T.; Kato, S. Bioorg. Med. Chem. Lett. 2009, 19, 175.
13. Sader, S.; Biedenbach, D.J.; Ivezic Schoenfeld Z. Abstract F1–2105, Presented at
the 50th Interscience Conference on Antimicrobial Agents and Chemotherapy,
Boston, MA, September 12–15, 2010.
14. Craig, W. A.; Andes, D.; Ivezic, Z. Abstract F1-2108, Presented at the 50th
Interscience Conference on Antimicrobial Agents and Chemotherapy, Boston,
MA, September 12-15, 2010.
15. Prince, W. T.; Wicha, W. W.; Strickmann, D.B. Abstract 906, Presented at the
20th European Congress of Clinical Microbiology and Infectious Diseases
Vienna, Austria, April 10-13, 2010.
Acknowledgments
We gratefully acknowledge financial support by the Natural sci-
ence funds for colleges and universities in Jiangsu Province
(No.08KJD350003), the Natural Science Foundation of Jiangsu
Province in China (Grant No. BK2011389), and Natural Science Re-
search funds of Nantong University (No.07Z086); and also thank
for the project funded by the Priority Academic Programs Develop-
ment of Jiangsu Higher Education Institutions (PAPD).
16. Kehraus, S.; Konig, G. M.; Wright, A. D.; Leucamide, A. J. Org. Chem. 2002, 67,
4989.
17. Gerth, K. J. Antibiot. 1980, 33, 1474.
18. Poulsen, S. M.; Karlsson, M.; Johansson, L. B.; Vester, B. Mol. Microbiol. 2001, 41,
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20. The synthesis of the lead compound 15: To a solution of 2-aminothiazol-4-
yl)methanethiol (0.33 g, 2.24 mmol) in 2 mL 10% aqueous NaOH solution,
compound 18 (1.00 g, 2.04 mmol) in 25 mL THF was added dropwise. The
reaction mixture was stirred in the ice bath for 1 h. After the reaction was
completed, the solvent was removed under reduced pressure. The crude
residue was dissolved in ethyl acetate, and the resulting solution was washed
sequentially with water and brine, dried over MgSO4, filtered and
concentrated, before being purified by column chromatography on silica gel
(PE/EtOAc = 5:1–1:1) to yield 15 as a white solid (0.75 g, 72%). M.p.: 67–70 °C.
Analytical data for 15: IR (KBr, cmÀ1): 3436.30, 3353.02, 2984.70, 2856.58,

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X. Wang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6166–6172
1716.37, 1617.08, 1524.02, 1280.78, 685.05; 1H NMR (CDCl3, 400 MHz): d 6.50
reduced pressure. The crude residue was dissolved in chloroform and washed
with saturated NaCl solution. The organic layer was dried with MgSO4, filtered,
concentrated, and purified by column chromatography (PE/EtOAc = 2:1–1:4) to
yield 16a–g.
(1H, dd, J1 = 11.2, J2 = 17.6, H19, 6.33 (1H, s, H25), 5.77 (1H, d, J = 8, H14), 5.37
(1H, d, J = 11.2, H20), 5.23 (1H, d, J = 17.2, H20), 3.67 (2H, s, H23), 3.08 (2H, s,
H22), 3.38 (1H, m, H11), 2.38 ꢀ 2.05 (5H, m, H2, H4, H10, H13), 1.83 ꢀ 1.31
(11H, m, H1, H6, H7, H8, H15, H13,11-OH), 1.19 (3H, m, H, H18), 0.89 (3H, d,
J = 6.8, H17), 0.74 (3H, d, J = 6.8, H16); 13CNMR (CDCl3, 100 MHz): d 217.12
(C3), 168.82 (C21), 168.18 (C26), 147.13 (C24), 139.10 (C19), 117.27 (C20),
105.89 (C25), 74.82 (C11), 69.16 (C14), 58.20 (C4), 45.46 (C22), 44.76 (C9), 43.9
(C13), 41.73 (C12), 36.78 (C5), 36.00 (C6), 34.48 (C10), 32.80 (C2), 31.77 (C23),
30.42 (C8), 26.84 (C7), 26.39 (C18), 24.85 (C1), 16.90 (C16), 14.99 (C15), 11.58
(C17); ESI-MS (m/z): 507 [M+H]+; Anal. Calcd for C26H38N2O4S2: C, 61.63; H,
7.56; N, 5.53; Found: C, 61.41; H, 7.81; N, 5.46.
Analytical data for selected compounds 16e: yield 65%, white foam solid, IR (KBr,
cmÀ1): 2936.22, 2861.18, 1732.36, 1529.90, 1453.83, 1420.38, 1374.36,
1327.54, 1279.07, 1154.30, 1115.30, 1017.60; 1H NMR (CDCl3, 400 MHz): d
10.42 (1H, br, NH), 6.79 (1H, s, H25), 6.51 (1H, dd, J1 = 10.8 Hz, J2 = 17.2 Hz,
H19), 5.79 (1H, d, J = 8.4 Hz, C14), 5.39 (1H, d, J = 10.8 Hz, C20), 5.24 (1H, d,
J = 17.6 Hz, C20), 3.83 (2H, s, H23), 3.38 (1H, m, H11), 3.18 (2H, s, H22), 3.06
(2H, s, H28), 2.52 (4H, br, 2 Â NCH2), 2.39 ꢀ 2.05 (5H, m, H2, H4, H10, H13),
1.80 ꢀ 1.19 (20H, m, H1, H6, H7, H8, H15, H13,11-OH, H18, 3 Â CH2), 0.89 (3H,
d, J = 6.8 Hz, H17), 0.75 (3H, d, J = 6.8 Hz, H16); 13C NMR (CDCl3, 100 MHz): d
217.10 (C3), 168.64 (C21), 157.83 (C27), 146.17 (C26), 139.08 (C24), 117.30
(C19), 111.32 (C20), 107.03 (C25), 74.61 (C11), 69.19 (C14), 61.89 (CH2NCH2),
58.17 (C4), 56.68 (C28), 45.44 (C22), 45.38 (C9), 43.89 (C13), 41.72 (C12), 36.78
(C5), 35.99 (C6), 34.46 (C10), 32.74 (C2), 31.65 (C23), 30.41 (C8), 26.84 (C7),
26.36 (C18), 25.85 (2 Â NCH2CH2), 24.84 (C1), 23.42 (NCH2CH2CH2), 16.91
(C16), 14.94 (C15), 11.56 (C17); ESI-MS (m/z): 632 [M+H]+; Anal. Calcd for
21. General procedure for the synthesis of the target compounds 16a–g: To a mixture
of compound 15 (1.00 g, 1.98 mmol) and K2CO3 (0.41 g, 2.96 mmol) in dry
CH2Cl2 (20 mL), a solution of ClCH2COCl (0.33 g, 2.96 mmol) in dry CH2Cl2
(5 mL) was added drop wise at 0 °C. The reaction mixture was stirred for 2 h.
After completion of the reaction, the solution was washed with water for three
times, and the organic layer was dried over Na2SO4, filtered, concentrated, and
purified by column chromatography (PE/EtOAc = 3:1) to yield 19. A solution of
compound 19 (1.72 mmol), R1R2NH (8.60 mmol), and K2CO3 (5.16 mmol) in
20 mL THF was stirred at 40 °C for 4–8 h. The solvent was removed under
C
33H49N3O5S2: C, 62.73; H, 7.82; N, 6.65; Found: C, 62.67; H, 7.80; N, 6.58.