Synthesis and antibacterial activity of a novel class of 4′-substituted 16-membered ring macrolides derived from tylosin

Article abstract of DOI:10.1021/jm034233n

Novel 4′-substituted 16-membered ring macrolides were synthesized by the cleavage of the mycarose sugar of tylosin and subsequent modification of 4′-hydroxyl group. This new class of macrolide antibiotics exhibited potent activity against some key erythro

Full text of DOI:10.1021/jm034233n

J . Med. Chem. 2004, 47, 2965-2968
2965
Letters
Syn th esis a n d An tiba cter ia l Activity of a
Novel Cla ss of 4′-Su bstitu ted
16-Mem ber ed Rin g Ma cr olid es Der ived
fr om Tylosin
Ly T. Phan, Tianying J ian, Zhigang Chen,
Yao-Ling Qiu, Zhaolin Wang,* Theresa Beach,
Alex Polemeropoulos, and Yat Sun Or
Enanta Pharmaceuticals, Inc., 500 Arsenal Street,
Watertown, Massachusetts 02472
Received November 11, 2003
Abstr a ct: Novel 4′-substituted 16-membered ring macrolides
were synthesized by the cleavage of the mycarose sugar of
tylosin and subsequent modification of 4′-hydroxyl group. This
new class of macrolide antibiotics exhibited potent activity
against some key erythromycin-resistant pathogens.
Macrolide antibiotics have been widely used to treat
bacterial infections for the past 50 years.1 They are
considered as the preferred therapeutic agents for the
treatment of upper and lower respiratory tract infec-
tions because of their safety and efficacy.2 Macrolides
exert their potent antibacterial effect by selectively
binding to the 50S subunit of the bacterial ribosome and
inhibit protein synthesis.3 As a large family of both
natural and semisynthetic antibiotics, macrolides are
classified according to the size of the lactone ring
consisting of 12-16 atoms. Erythromycin A (1) (Figure
1) and its derivatives, such as clarithromycin (2), which
have a 14-membered lactone ring, are among the most
commonly prescribed macrolides. However, erythromycin-
resistant pathogens have become increasingly prevalent
over the past decade because of extensive usage.4 The
two most common mechanisms of macrolide resistances
arise from ribosome methylation and efflux pumps
encoded by erm genes and mef genes, respectively. To
address the problem of bacterial resistance, a new
generation of 14-membered macrolides known as ke-
tolides, such as telithromycin (3)5 and cethromycin
(ABT-773),6 was developed.
The 16-membered ring macrolides, such as leucomy-
cins and tylosin (4) (Figure 1), for example, constitute
another important class of useful antibiotics of the
macrolide family.7 They have some advantages over the
14-membered erythromycin A derivatives such as gas-
trointestinal tolerance and activity against resistance-
expressing strains.8 During our efforts to develop novel
macrolides active against resistant bacteria, we became
interested in tylosin primarily because its activities are
not affected by efflux resistance. Although substantial
work to improve antibacterial activity has been carried
out on tylosin and its derivatives over many years, not
much success was observed with this subclass.9 Espe-
F igu r e 1. Structure of erythromycin A (1), clarithromycin (2),
telithromycin (3), and tylosin (4).
cially little research has been done to improve the acid
stability. We felt that removal of the acid labile my-
carose sugar from tylosin could be an innovative starting
point for further modifications to enhance its antibacte-
rial activity and acid stability. Herein we report the
synthesis of a series of novel 4′-substituted 16-mem-
bered ring macrolides and their activity against consti-
tutively resistant bacteria.
Our approach to 4′-substituted tylosin derivatives is
to remove the mycarose sugar and to modify the
4′-hydroxyl group. Scheme 1 outlines the synthesis of
the 4′-carbamate 6. The 2′-hydroxyl group of tylosin (4)
was first protected as an acetyl ester followed by
protetion of the aldehyde group with ethylene glycol and
selective cleavage of the mycarose sugar under acidic
conditions. Protection of the 4′′′-hydroxyl group on the
mycinose sugar with TBDPSCl generated 5 in 64% yield
over four steps. Subsequent reaction of 5 with phenethyl
isocyanate followed by the deprotections proceeded
smoothly to afford the desired product 4′-carbamate 6
in 43% yield over four steps.
The synthesis of an allyl ether is described in Scheme
2. Palladium-catalyzed reaction of compound 5 with
allyl-tert-butyl carbonate generated 4′-substituted allyl
ether 7 as the major product in 60% yield and a small
amount of the 3-position allylation compound.10 Depro-
tection of 7 gave the final product 9 in 61% yield over
three steps.
Introduction of a quinoline group6 to the allyl side
chain of 8 was achieved by utilizing Heck reaction.11
Under optimized conditions (Pd(OAc)2/P(o-Tol)3/Et3N/
* To whom correspondence should be addressed. Phone: 781-235-
0842. E-mail: zhaolinwang@comcast.net. Present address: 88 Kings-
bury Street, Wellesley, MA 02481.
10.1021/jm034233n CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/04/2004

2966 J ournal of Medicinal Chemistry, 2004, Vol. 47, No. 12
Letters
Sch em e 1a
Sch em e 3a
a
Conditions: (a) 3-bromoquinoline (2 equiv), Pd(OAc)2 (0.16
equiv), P(o-Tol)3 (0.31 equiv), Et3N (2.5 equiv), CH3CN, 80 °C, 16
h, 50%; (b) MeOH, room temp, 16 h.
a
Conditions: (a) Ac2O (5.3 equiv), acetone, room temp, 2 h,
3). The structure of 10a was confirmed by detailed NMR
analysis of the compound (see Supporting Information).
100%; (b) p-TsOH, ethylene glycol, benzene, reflux, 4 h; (c) Ch3CN,
room temp, 16 h; (d) TBDPSCl (1.1 equiv), imidazole (1.2 equiv),
DMF, room temp, 16 h, 64% in three steps; (e) PhCH2CH2-
NdCdO, Et3N, CH2Cl2, room temp, 16 h; (f) TBAF (5 equiv), THF,
room temp, 1 h; (g) 0.3 N HCl, CH3CN, room temp, 1 h; (h) MeOH,
room temp, 16h, 43% in four steps.
The 4′-substituted 16-membered macrolides were
tested against a panel of representative respiratory tract
pathogens together with erythromycin A and tylosin as
references. Various macrolide-resistant strains were
also included in order to evaluate the activities of this
novel series of macrolide to overcome bacterial resis-
tance. Staphylococcus aureus 25923 and Streptococcus
pneumoniae 49619 are erythromycin-susceptible strains.
Streptococcus pneumoniae 700904 and Streptococcus
pyogenes 2912 are macrolide-lincosamide-strepto-
gramin B (MLSB) resistant strains encoded by the erm
gene. Streptococcus pneumoniae 7701 and Streptococcus
pyogenes 1323 are efflux-resistant strains encoded by
the mef gene. Haemophilus influenzae 33929 is an
ampicillin-resistant strain. The in vitro antibacterial
activities are reported as minimum inhibitory concen-
trations (MICs) that were determined by the agar
dilution method as recommended by the NCCLS (Na-
tional Committee for Clinical Laboratory Standards).12
Sch em e 2a
The results in Table 1 show the antibacterial activity
of 4′-substituted tylosin derivatives with the reference
compounds (tylosin and erythromycin A). Erythromycin
A is active against the sensitive strains but weakly
active against the efflux-resistant strains and inactive
against the constitutively MLSB-resistant strains. Ty-
losin is active against the efflux-resistant strains but
not active against the erm-resistance strains.8
a
Conditions: (a) allyl-tert-butyl carbonate (3 equiv), Pd2(dba)3
(0.1 equiv), dppb (0.2 equiv), THF, reflux, 4 h, 60%; (b) TBAF (5
equiv), THF, room temp, 1 h; (c) 0.3 N HCl, CH3CN, room temp,
1 h; (d) MeOH, room temp, 16 h, 61% in three steps.
CH3CN), the reaction of 8 with 3-bromoquinoline fol-
lowed by deprotection in methanol gave cis-enol ether
10a as the major product together with the conjugated
quinolyl 10b as the minor product in a 8:1 ratio (Scheme
The carbamate compound 6 showed weak antibacte-
rial activity. However, the 4′-substituted ally ether
compound 9 exhibited improved activity compared to
Ta ble 1. In Vitro Antibacterial Activity of Selected Compounds (MIC, µg/mL)
strain
6
9
10a
0.5
0.0625
0.5
0.0625
16
0.0625
4
10b
0.5
0.0625
1
0.0625
8
0.0625
4
tylosin
Ery A
S. aureus 25923
Ery S
Ery S
2
2
1
2
0.25
64
0.25
>64
0.25
8
0.5
0.06
>64
8
>64
16
S. pneumoniae 49619
S. pneumoniae 700904
S. pneumoniae 7701
S. pyogenes 2912
S. pyogenes1323
H. influenzae 33929
0.5
>64
0.5
32
0.125
8
Ery R-erm
Ery R-mef
Ery R-erm
Ery R-mef
Amp R
>64
2
>64
0.25
16
4

Letters
J ournal of Medicinal Chemistry, 2004, Vol. 47, No. 12 2967
carbamate 6 against most of the pathogens tested (Table
1). Furthermore, compound 9 is slightly more active
than tylosin against erythromycin-resistant strains of
S. pyogenes 2912 and 1323 encoded by the erm and mef
genes, respectively. Compounds 10a and 10b showed
much improved overall antibacterial activities against
susceptible and resistant organisms, indicating that the
addition of a 3-quinolyl group to 9 has a significant effect
on its activity. Compared to erythromycin A, 10a and
10b exhibited excellent activity against erythromycin-
resistant strains. For example, 10a demonstrated 128-
fold greater activity than erythromycin against resistant
S. pneumoniae 700904 and S. pneumoniae 7701 strains.
In addition, 10b showed a 256-fold improved activity
against efflux-resistant strain of S. pyogenes 1323 than
erythromycin A.
some key erythromycin-resistant pathogens compared
to tylosin and erythromycin A. The 4′-substitution
presents promising opportunities for the development
of new macrolide antibiotics to overcome bacterial
resistance.
Ack n ow led gm en t. The authors thank Dr. Thoai
Lien for help in getting the HRMS data.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and spectral data for the new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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The recent high-resolution X-ray cocrystal structures
of the bacterial ribosome with the macrolides have
revealed their detailed interactions at the atomic level.13
These structures demonstrate that macrolides inhibit
bacterial protein synthesis by sterically blocking the
passage of the nascent polypeptides through the exit
tunnel of the ribosome. From the cocrystal structures
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tide. According to this structure, the mycarose group of
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dipeptide formation. In contrast, erythromycin A, which
has only one monosaccharide extension from the 5-posi-
tion, allows the synthesis of longer peptides up to four
amino acids. This structural information implies that
tylosin interferes more directly with the activity of the
PTC than erythromycin A. Therefore, replacing the acid
labile mycarose sugar of tylosin to make an even longer
extension at the 4′-position with aromatic substituents
should improve acid stability and may also increase its
binding interactions with the ribosome as well.
The most active compounds (10a and 10b) were
obtained when a 3-quinolyl group was attached to the
allyl ether at the 4′-position. We believe that the
improved antibacterial activity against MLSB-resistant
bacteria achieved by these novel macrolides compared
to tylosin is the result of possible interactions of the aryl
group with a secondary ribosomal binding site. The
heteroaromatic quinolyl group may have π-π stacking
and hydrophobic interactions with ribosome RNA bases
in the exit tunnel because these interactions are much
favored. In addition, the quinolyl group may reach into
the PTC and strongly prevent the formation of even the
first peptide bond. It is presumably these tight binding
interactions with the ribosome that overcome the bacte-
rial resistance.
The introduction of a versatile allyl group at the 4′-
position of tylosin paves the way for the further diverse
modifications of tylosin.15
Novel 4′-substituted 16-membered ring macrolides
were synthesized and evaluated for antibacterial activ-
ity. Compounds containing a quinolyl group at the 4′-
position were the most active against both erythromycin-
susceptible and erythromycin-resistant bacteria. Com-
pound 10a exhibited highly improved activities against

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J M034233N