Design, synthesis and antibacterial activity of novel pleuromutilin derivatives with 4H-pyran-4-one and pyridin-4-one substitution in the C-14 side chain

Article abstract of DOI:10.1016/j.cclet.2015.09.019

A series of novel pleuromutilin derivatives with 4H-pyran-4-one and pyridin-4-one substitution in the C-14 side chain have been designed and synthesized. In vitro antibacterial activity evaluation showed that most of the derivatives exhibited potent antib

Full text of DOI:10.1016/j.cclet.2015.09.019

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Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Design, synthesis and antibacterial activity of novel pleuromutilin
derivatives with 4H-pyran-4-one and pyridin-4-one substitution in
the C-14 side chain
Chen-Yu Ling a,1, Yun-Liang Tao b,1, Wen-Jing Chu a, Hui Wang c, Hai-Dong Wang b,
,
*
Yu-She Yang a,
*
a State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China
b School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China
c Department of Microbiology, China Pharmaceutical University, Nanjing 210009, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of novel pleuromutilin derivatives with 4H-pyran-4-one and pyridin-4-one substitution in the
C-14 side chain have been designed and synthesized. In vitro antibacterial activity evaluation showed
that most of the derivatives exhibited potent antibacterial activity against drug resistant Gram-positive
strains. Compounds 12a, 12d, and 28 are the most active derivatives in this series, displaying activity
comparable to that of retapamulin and BC-3781. As the metabolic stability of this series is not
satisfactory, further modifications are going on to improve their pharmacokinetic profile.
ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Received 10 March 2015
Received in revised form 15 June 2015
Accepted 15 September 2015
Available online xxx
Keywords:
Multi-drug resistant bacteria
Pleuromutilin
4H-Pyran-4-one
Antibacterial activity
1. Introduction
clinical use [6]. The distinct mode of action of pleuromutilin has
made it an attractive target in the development of novel antibiotics
The emergence and spread of multi-drug resistant bacteria have
become a serious threat to public health [1]. In particular, multi-
drug resistant Gram-positive bacteria including methicillin-resis-
tant Staphylococcus aureus (MRSA) [2], penicillin-resistant Strepto-
coccus pneumoniae (PRSP) [3], and vancomycin-resistant
Enterococci (VRE) [4] represent major concerns. There is, therefore,
an urgent need to identify and develop novel antibiotics with
modes of action that are distinct from those of established classes.
Pleuromutilin (1, Fig. 1), a diterpenoid natural product with a
fused 5–6–8 tricyclic skeleton, was first isolated in 1951 from two
basidiomycete species [5]. It displays modest in vitro antibacterial
activity against Gram-positive pathogens and mycoplasmas.
Further studies have shown that the pleuromutilin class of
antibiotics selectively inhibits bacterial protein synthesis by
specifically targeting the 50s subunit of the bacterial ribosome
and displays no cross-resistance with antibiotics currently in
for the treatment of multi-drug resistant bacterial infections.
Since the 1960s, numerous semisynthetic pleuromutilin analogs
have been prepared and evaluated, and it was recognized that
pleuromutilin derivatives containing a sulfide linkage in the C-14
side chain demonstrated superior antibacterial activity [7]. Of the
derivatives generated, tiamulin [8] (2, Fig. 1) and valnemulin [9] (3,
Fig. 1) were successfully developed as veterinary medicine. In 2007,
retapamulin [10] (4, Fig. 1), a novel pleuromutilin derivative, was
first approved forhuman use as a topical antimicrobial agent to treat
skin infections. Other pleuromutilin derivatives in clinical trials
include Nabriva Therapeutics’ BC-7013 [11] (5, Fig. 1) and BC-3781
[12] (6, Fig. 1). Especially, BC-3781 has completed phase II clinical
trial for the systemic treatment of acute bacterial skin and skin
structure infections (ABSSSI) and acquired Qualified Infectious
Disease Product (QIDP) as well as fast track status designation.
4H-Pyran-4-one and pyridin-4-one ring systems are very
attractive moieties in medicinal chemistry because of their
occurrence in a variety of bioactive compounds [13]. Existing
studies have proved their wide range of biological activities such as
antiproliferative, antibacterial, and antimalarial activity [14–
16]. Based on these interesting biological activity and previous
SAR of pleuromutilins, a series of novel thioether pleuromutilin
*
Corresponding authors.
E-mail addresses: whdoctor@163.com (H.-D. Wang), ysyang@simm.ac.cn
(Y.-S. Yang).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.cclet.2015.09.019
1001-8417/ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: C.-Y. Ling, et al., Design, synthesis and antibacterial activity of novel pleuromutilin derivatives with
4H-pyran-4-one and pyridin-4-one substitution in the C-14 side chain, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/
j.cclet.2015.09.019

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thioacyl groups in compounds 9a–9c revealed the corresponding
thiol anions that reacted with mutilin 14-tosyloxyacetate 10 under
basic condition to give compounds 11a–11c in 57%–68% yield.
Deprotection of the 4-methoxybenzyl group in compounds 11a–
11c by trifluoroacetic acid yielded compounds 12a–12c in 55%–
70% yield. Treatment of compound 12a with formaldehyde
solution furnished compound 12d. Mannich reaction of compound
12a with corresponding amines afforded compounds 12e–12i in
46%–59% yield.
As shown in Scheme 2, compound 16 was prepared from
compound 8d. Reaction of compound 8d with PMBCl and K2CO3 in
DMF at 70 8C afforded compound 13 in 68% yield and N-4-
methoxybenzylated byproduct was isolated in 12% yield. Com-
pound 13 was then converted to compound 16 by the same
procedures as described for compounds 12a–12c.
As shown in Scheme 3, the synthesis of compound 22 started
from maltol (compound 17), which was converted to compound 19
by TBDMS-protection and subsequent bromination. Nucleophilic
substitution of compound 19 with potassium thioacetate followed
by hydrolysis of the thioacyl group and reaction with 10 gave
compound 21. Deprotection of the TBDMS group in compound 21
afforded compound 22 in quantitative yield.
Fig. 1. Structures of pleuromutilin and its derivatives.
derivatives bearing 4H-pyran-4-one and pyridin-4-one moieties in
the C-14 side chain were designed, synthesized, and evaluated for
their antibacterial activity. Herein, we described the details of this
study.
The synthesis of compound 28 is illustrated in Scheme 4. The
primary alcohol of kojic acid (compound 7) was protected with the
tetrahydropyranyl group to give compound 23, which was treated
with formaldehyde solution to yield compound 24. Protection of
the C-5 hydroxyl group afforded compound 25 which was
converted to compound 28 following procedures similar to those
described for compounds 12a–12c.
As shown in Scheme 5, compound 31 was synthesized from
compound 29 by the same procedures as described for compounds
12a–12c.
2. Experimental
The synthesis of compounds 12a–12i is shown in Scheme
1. Commercially available kojic acid was protected with 4-
methoxybenzyl group at C-5 position to give compound 8a, which
was transformed to 8b–8d by reported method [17]. Mesylation of
the 2-hydroxymethyl group in compounds 8a–8c followed by
nucleophilic substitution with potassium thioacetate gave com-
pounds 9a–9c in 55%–62% yield over 2 steps. Hydrolysis of the
Scheme 1. Synthesis of compounds 12a–12i. Reagents and conditions: (a) PBMCl, K2CO3, DMF, 65 8C, 8 h; (b) (i) NH2OHÁHCl, CH3COONa, N-methyl pyrrolidone, 70 8C, 12 h;
(ii) PBMCl, K2CO3, DMF, 65 8C, 6 h; (c) CH3NH2/alcohol solution, 70 8C, 6 h; (d) 25% aqueous NH4OH, 60 8C, 12 h; (e) (i) MsCl, TEA, CH2Cl2, 0 8C–r.t., 12 h; (ii) KSAc, DMF, 40 8C,
2–4 h; (f) 5% aqueous NaOH, MeOH/CH2Cl2, r.t.; (g) CF3COOH, CH2Cl2, 0 8C–r.t., 4–6 h; (h) 37% aqueous CH2O, NaOH, MeOH, r.t. or 37% aqueous CH2O, R1R2NH, MeOH, r.t.
Please cite this article in press as: C.-Y. Ling, et al., Design, synthesis and antibacterial activity of novel pleuromutilin derivatives with
4H-pyran-4-one and pyridin-4-one substitution in the C-14 side chain, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/
j.cclet.2015.09.019

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3
Scheme 2. Synthesis of compound 16. Reagents and conditions: (a) PBMCl, K2CO3, DMF, 65 8C, 8 h; (b) (i) MsCl, TEA, CH2Cl2, 0 8C–r.t., 12 h; (ii) KSAc, DMF, 40 8C, 4 h; (c) 5%
aqueous NaOH, mutilin 14-tosyloxyacetate 10, MeOH/CH2Cl2, r.t.; (d) CF3COOH, CH2Cl2, 0 8C–r.t., 6 h.
Scheme 3. Synthesis of compound 22. Reagents and conditions: (a) TBDMSCl, imidazole, CH2Cl2, 0 8C–r.t., 6 h; (b) NBS, AIBN, CCl4, 60 8C, N2, 8 h; (c) KSAc, DMF, 40 8C, 5 h; (d)
mutilin 14-tosyloxyacetate 10, 5% aqueous NaOH, MeOH/CH2Cl2, r.t.; (e) TBAF/THF, 0 8C–r.t., 5 h.
Scheme 4. Synthesis of compound 28. Reagents and conditions: (a) 3,4-dihydro-2H-pyran, 0.1% PTSAÁH2O, CH2Cl2, r.t., 4 h; (b) 37% aqueous CH2O, NaOH, MeOH, r.t.; (c)
PBMCl, K2CO3, DMF, 65 8C, 8 h; (d) (i) MsCl, TEA, CH2Cl2, 0 8C–r.t., 12 h; (ii) KSAc, DMF, 40 8C, 4 h; (e) 5% aqueous NaOH, mutilin-14-tosyloxyacetate 10, MeOH/CH2Cl2, r.t.; (f)
CF3COOH, CH2Cl2, 0 8C–r.t., 6 h.
Scheme 5. Synthesis of compound 31. Reagents and conditions: (a) (i) MsCl, TEA, CH2Cl2, 0 8C–r.t., 12 h; (ii) KSAc, DMF, 35 8C, 4 h; (b) 5% aqueous NaOH, mutilin 14-
tosyloxyacetate 10, MeOH/CH2Cl2, r.t.
3. Results and discussion
inhibitory concentration (MIC) values were determined using
agar dilution method according to CLSI guidelines [18]. The
results were summarized in Table 1. It was interesting that
compound 12a exhibited very potent antibacterial activity
The in vitro antibacterial activities of the target compounds
were tested against clinically isolated Gram-positive strains
with retapamulin and BC-3781 as positive controls. The minimal
(MIC = 0.031–0.25
mg/mL), whereas compound 12b, bearing
Please cite this article in press as: C.-Y. Ling, et al., Design, synthesis and antibacterial activity of novel pleuromutilin derivatives with
4H-pyran-4-one and pyridin-4-one substitution in the C-14 side chain, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/
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Table 1
Minimum inhibitory concentration of pleuromutilin derivatives.
OH
O
R
H
O
O
Compd.
R
MIC range (
MSSA
m
g/mL)a
MRSA
0.063–0.25
MSSE
0.031–0.25
MRSE
0.063–0.125
PRSP
0.063–0.25
12a
0.063–0.25
O
HO
S
O
O
∗
12b
12c
>16
>16
>16
>16
>16
HO
HO
HO
S
N
∗
OH
2–4
1–4
2–4
2–4
1–4
2–4
O
S
N
O
∗
16
1–4
1–4
1–4
1–4
S
N
H
∗
12d
12e
0.063–0.5
0.063–0.125
0.125–0.5
0.125–0.25
0.25–0.5
O
HO
HO
S
O
O
∗
0.125–0.25
0.125–0.5
0.25–1
0.25–0.5
0.5
HO
N
S
∗
O
12f
12g
12h
0.5–2
0.25–1
0.5–1
0.5–2
0.25–1
0.25–1
0.25–0.5
0.25–1
0.5–1
0.5
0.5–2
0.5–2
0.5–1
O
O
HO
N
S
∗
0.25–1
0.25–1
O
HO
N
S
∗
O
O
HO
N
S
∗
HO
O
O
12i
22
0.5–2
0.5–2
0.5–1
0.25
0.5–1
0.5–1
HO
HO
N
S
∗
O
0.25–0.5
0.25–0.5
0.25–0.5
0.25–1
O
OH
S
O
∗
28
31
0.063–0.5
0.25–0.5
0.063–0.5
0.25–0.5
0.25–0.5
0.25–0.5
0.25
0.5
O
OH
HO
S
∗
O
0.125–0.5
0.25–0.5
O
O
MeO
S
∗
Please cite this article in press as: C.-Y. Ling, et al., Design, synthesis and antibacterial activity of novel pleuromutilin derivatives with
4H-pyran-4-one and pyridin-4-one substitution in the C-14 side chain, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/
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Table 1 (Continued )
Compd.
R
MIC range (
MSSA
m
g/mL)a
MRSA
MSSE
MRSE
PRSP
Retapamulin
BC-3781
0.125
0.125
0.125–0.25
0.125–0.5
0.063–0.125
0.125–0.25
0.125–0.25
0.063–0.125
0.125–0.25
0.125–0.25
a
Abbreviations are as follows: MSSA, methicillin-sensitive Staphylococcus aureus, 5 strains; MRSA, methicillin-resistant Staphylococcus aureus, 5 strains; MSSE, methicillin-
sensitive Staphylococcus epidermidis, 5 strains; MRSE, methicillin-resistant Staphylococcus epidermidis, 5 strains; PRSP, penicillin-resistant Streptococcus pneumoniae, 5 strains.
Table 2
Pharmacokinetic parameters of compounds 12a, 12d, 12e, 28 and BC-3781 by cassette dosing at 2 mg/kg in rats (i.v., n = 3).
Compd.
Cmax
AUC0–t
MRT
(h)
T1/2
CL
Vss
(ng/mL)
(h ng/mL)
(h)
(L/h/kg)
(L/kg)
12a
644 Æ 46
308 Æ 35
472 Æ 33
377 Æ 34
279 Æ 24
195 Æ 11
87 Æ 9
0.16 Æ 0.01
0.13 Æ 0.01
0.25 Æ 0.02
0.17 Æ 0.02
2.44 Æ 0.11
0.15 Æ0.01
0.11 Æ 0.01
0.24 Æ 0.05
0.27 Æ 0.14
2.51 Æ 0.27
10.3 Æ 0.6
23.2 Æ 2.3
11.7 Æ 1.0
17.9 Æ 1.5
6.3 Æ 0.5
1.7 Æ 0.2
2.9 Æ 0.5
2.9 Æ 0.3
3.0 Æ 0.4
15.5 Æ 1.8
12d
12e
170 Æ 13
112 Æ 9
296 Æ 23
28
BC-3781
All values are represented as mean Æ SD; AUC0–t, area under the serum concentration–time curve up to last sampling time; MRT, mean residence time; T1/2, elimination half-life;
Vss, volume of distribution at steady-state.
N-hydroxylpyridin-4-one moiety, almost lost activity. Replace-
ment of the N-hydroxyl group in compound 12b with a methyl
group or hydrogen atom restored some of the activity
(compounds 12c and 16 vs. compound 12b). This suggested
that 4H-pyran-4-one moiety is very important for maintaining
antibacterial activity. Further modifications were then
focused on compound 12a. Introduction of hydroxymethyl
or amino groups onto the pyranone ring of compound 12a
gave compounds 12d–12i, which displayed moderate to
good activity. Compound 22, the regioisomer of compound
and PRSP. Cassette-dosing experiments showed that theses novel
pleuromutilin derivatives underwent a rapid clearance after
intravenous administration. Further investigation and structural
modifications are needed to improve their PK profiles.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2015.09.019.
12a, showed
a 2–4-fold decreased antibacterial activity,
whereas compound 28 exhibited comparable activity to its
regioisomer, compound 12d. O-Methylation of the hydroxyl
group in the 4H-pyran-4-one moiety of compound 12a gave
compound 31 with 2–4-fold reduced activity, indicating the
hydroxyl group was essential for potent antibacterial activity.
Among all the derivatives, compounds 12a, 12d, and 28 are most
potent in this series, exhibiting antibacterial activity compara-
ble to or slightly better than retapamulin and BC-3781.
Compounds 12a, 12d, 12e and 28 were then selected for
pharmacokinetic study. Cassette-dosing experiments were per-
formed on these compounds with clinical investigational drug BC-
3781 included for comparison. Results of this study are summa-
rized in Table 2.
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Please cite this article in press as: C.-Y. Ling, et al., Design, synthesis and antibacterial activity of novel pleuromutilin derivatives with
4H-pyran-4-one and pyridin-4-one substitution in the C-14 side chain, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/
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4H-pyran-4-one and pyridin-4-one substitution in the C-14 side chain, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/
j.cclet.2015.09.019