Cyclopentanone ring-cleaved pleuromutilin derivatives

Article abstract of DOI:10.1016/j.ejmech.2006.07.018

Ring-cleaved pleuromutilin derivatives comprised of a [5.3.1] bicyclic core structure have been synthesized and evaluated in vitro as antibacterial agents. Four of the compounds described were found to have MICs ≤ 4 μg/mL against marker strains of Strepto

Full text of DOI:10.1016/j.ejmech.2006.07.018

European Journal of Medicinal Chemistry 42 (2007) 109e113
http://www.elsevier.com/locate/ejmech
Preliminary communication
Cyclopentanone ring-cleaved pleuromutilin derivatives
Dane M. Springer a, , Amy Bunker a,1, Bing-Yu Luh a, Margaret E. Sorenson a,
*
Jason T. Goodrich a, Joanne J. Bronson a, Kenneth DenBleyker b,
Thomas J. Dougherty b,2, Joan Fung-Tomc b
a Anti-infective Chemistry, Bristol-Myers Squibb Pharmaceutical Research Institute, 5 Research Parkway, P.O. Box 5100, Wallingford, CT 06492, USA
b Department of Microbiology, Bristol-Myers Squibb Pharmaceutical Research Institute, 5 Research Parkway, P.O. Box 5100, Wallingford, CT 06492, USA
Received 28 July 2005; received in revised form 12 July 2006; accepted 12 July 2006
Available online 6 December 2006
Abstract
Ring-cleaved pleuromutilin derivatives comprised of a [5.3.1] bicyclic core structure have been synthesized and evaluated in vitro as anti-
bacterial agents. Four of the compounds described were found to have MICs ꢀ 4 mg/mL against marker strains of Streptococcus pneumoniae and
Staphylococcus aureus.
Ó 2006 Elsevier Masson SAS. All rights reserved.
O
OH
3
H
11
12
OH
6
3
14
6
14
O
O
O
OH
OH
1
O
O
O
HO2C
As part of our program to discover a pleuromutilin (1) de-
rivative for use in human antibacterial therapy, we wished to
synthesize compounds containing novel core structures [1,2].
One of our approaches took advantage of chemistry developed
by Birch during structural studies of the antibiotic [4]. Birch
observed that autoxidation of diacetoxymutilin (2) using
potassium t-butoxide in t-butanol afforded the ring-cleaved
ketoacid 3 in good yield.
O
3
H
t-BuOK, t-BuOH
OAc
OAc
OAc
OAc
O2
(88%)
6
14
3
2
We decided to reproduce this ring-cleavage chemistry on
a more suitably functionalized C-14 glycolic ester derivative,
to more rapidly access biologically active pleuromutilin ana-
logues containing acyl groups at C-14 [5]. Our first attempt
utilized the bis-methoxy ethyl (MOM) ether 4 as the starting
material. Unfortunately, compound 4 was substantially
degraded upon exposure to the autoxidation conditions suc-
cessfully applied in the conversion of 2 to 3. We surmised
* Corresponding author. Chemical and Screening Sciences, Wyeth Research,
CN 8000, Princeton, NJ 08543-8000, USA.
E-mail address: springd@wyeth.com (D.M. Springer).
1
Present address: Pfizer Global Research and Development, 2800 Plymouth
Road, Ann Arbor, MI 48105, USA.
2
Present address: AIC Antibacterials Biology, Pfizer Inc., MS 8220-2358,
Eastern Point Road, Groton, CT 06340, USA.
0223-5234/$ - see front matter Ó 2006 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2006.07.018

110
D.M. Springer et al. / European Journal of Medicinal Chemistry 42 (2007) 109e113
HO2C
R1O2C
OCH2OCH3
R1O2C
OH
OH
OH
O
c, d
a
+
O
O
O
HO2C
O
O
O
OSiPh2t-Bu
R
SR
O
O
O
O
6
10a-f R1 = H
10g-i R1 = CH2CH3
11
7 R = OH; R1 = CH2CH3
8 R = Cl; R1 = CH2CH3
9 R = OH; R1 = H
b
d
Scheme 1. Reaction conditions: (a) AcCl, EtOH, 20 h rt; (b) MsCl, LiCl, pyr; (c) NaSR, DMF; (d) 1 N NaOH, EtOH, THF rt.
Et2NOC
R1
Et2NOC
d, b
OH
OR2
OH
e, f
O
O
O
O
O
O
SR
OR3
OH
15a-c
14
O
O
O
6
R1 = CO2H, R2 = MOM, R3 = SiPh2t-Bu
a, b
c, b
12 R1 = CONHPh, R2 = R3 = H
13 R1 = CONH2, R2 = R3 = H
Scheme 2. Reaction conditions: (a) PhNH2, DCC, HOBt, Et3N, DMF, CH3CN; (b) AcCl, MeOH, 20 h rt; (c) BOC2O, NH4HCO3, dioxane, pyr; (d) EDC, Et2NH,
CH2Cl2; (e) TsCl, pyr; (f) NaSR, DMF (or 2,6-dimethyl-4-mercaptopyridine, MeOH).
O
EtO2C
OH
O
OH
O
N
H
a, b
R1
O
O
NH2
S
NH2
S
17 R1 = CO2t-Bu
18 R1 = CO2H
16
c
O
O
Scheme 3. Reaction conditions: (a) 1 N NaOH, EtOH, THF rt.; (b) t-butylglycine hydrochloride, DCC, CH2Cl2, Et3N; (c) TFA, CH2Cl2.
HO2C
OCN
OCH2OCH3
OCH2OCH3
20 + 21
b
a
ROH
O
O
O
O
OSiPh2t-Bu
e
OSiPh2t-Bu
t-BuOH
22
c, d
O
O
19
6
RH2C
OCH2OCH3
OCH2OCH3
H2N
i, j
k, c, d
b
29
24
O
O
O
O
OSiPh2t-Bu
OSiPh2t-Bu
23 R = OH
25 R = OTosyl
26 R = H
27
28
f
O
O
g
c, h, d
Scheme 4. Reaction conditions: (a) DPPA, Et3N, toluene; (b) AcCl, EtOH, CHCl3; (c) TBAF, THF; (d) TFA, CH2Cl2; (e) BH3, DMS; (f) TsCl, Et3N; (g) NaBH4,
DMF; (h) i) TsCl, Et3N; ii) sodium 3-amino-benzenethiolate, DMF; (i) NaN3, DMF; (j) Ph3P, THF, H2O; (k) isoxazole-5-carbonyl chloride, Et3N.

D.M. Springer et al. / European Journal of Medicinal Chemistry 42 (2007) 109e113
111
Table 1
OH
R1
O
O
R2
O
Compound
R1
R2
MIC (mg/mL)a
S. pneumoniae
S. aureus
1
Pleuromutilin
CO2H
CO2H
0.5
8
0.5
8
9
10a
OH
SPh
>64
>128
HO
10b
CO2H
4
8
S
NH2
10c
10d
10e
CO2H
CO2H
CO2H
0.125
0.25
8
S
S
8
N
64
128
S
N
N
N
10f
CO2H
>64
>128
S
NH2
N
H
NH2
10g
CO2CH2CH3
1
1
S
S
S
10h
10i
CO2CH2CH3
CO2CH2CH3
1
8
1
8
N
N
12
13
14
CO2NHPh
CONH2
CON(CH2CH3)2
OH
OH
OH
>64
64
>128
128
>128
>64
NH2
15a
CON(CH2CH3)2
4
32
S
S
15b
15c
CON(CH2CH3)2
CON(CH2CH3)2
4
4
32
32
N
HO2C
S
S
NH2
17
CONHCH2COOt-Bu
0.5
4
(continued on next page)

112
D.M. Springer et al. / European Journal of Medicinal Chemistry 42 (2007) 109e113
Table 1 (continued)
Compound
R1
R2
MIC (mg/mL)a
S. pneumoniae
S. aureus
NH2
18
CONHCH2CO2H
2
8
S
20
21
22
24
NHCO2CH2Ph
NHCO2CH2CH3
NH2
OH
OH
OH
OH
0.25
0.25
16
16
>64
>64
>128
>128
CH2OH
NH2
27
CH3
8
2
16
32
S
O
N
29
OH
N
O
H
a
MICs were determined according to NCCLS. Strains reported: Streptococcus pneumoniae A9585 and Staphylococcus aureus A15090, for details, see Ref. [6].
that the decomposition of 4 was related to the presence of the
glycolic ester group, since the cleavage performed well with
diacetoxymutilin 2 and the corresponding bis-MOM mutilin.
We decided to block the C-14 glycol hydroxyl with a bulky
protecting group, with the hope to sterically shield this site
from undesired reactivity. The t-butyldiphenylsilyl group
served well in this regard; silyl ether 5 proved to be an excel-
lent substrate for the oxidative cleavage reaction affording
acid 6 in 57e70% yield.
obtained via mixed anhydride chemistry and deprotection.
Thioglycolate derivatives 15aec were synthesized from amide
14 via tosylation and thiolate displacement. Ester 16 (made
from thiolate displacement of chloroacetate 8) was hydrolyzed
to the corresponding acid and then converted to glycinamide
17 (Scheme 3). Trifluoroacetic acid cleaved the t-butyl group
of 17 to afford the free acid derivative 18.
Acid 6 served as the precursor to carbamates 20 and 21 via
intermediate isocyanate 19 (Scheme 4). Amine 22 was ob-
tained by trapping 19 with t-butanol, followed by desilylation
and removal of the BOC and MOM groups with TFA. The car-
boxyl group of 6 could be reduced to afford alcohol 23 which
was deprotected to yield triol 24. Alcohol 23 was also con-
verted to derivative 27 by reduction of the tosylate 25 followed
by desilylation, thiolate displacement, and final MOM cleav-
age. Tosylate 25 was converted via azide displacement and
subsequent reduction to amine 28. The amine was readily ac-
ylated, and the intermediates deprotected to produce deriva-
tives such as 29.
We observed that structureeactivity relationships in the na-
tive pleuromutilin series did not always track with the SAR of
the ring-cleaved series of compounds presented in Table 1. For
example, the thioaryl derivatives 10aee have widely ranging
activity. However, these thioaryl groups, when used as replace-
ments for the hydroxyl group of pleuromutilin in the natural
core system, produced very potent compounds of similar activ-
ity. While the activity of any specific compound was difficult
to predict using knowledge of pleuromutilin SAR, certain
trends within this new class of derivatives could be gleaned
from the data in Table 1. Carbamates such as 20 and 21
were more active than amides such as 14. Esters (represented
by 10gei) were generally more active than amides, but less
potent than the carbamates.
While these compounds were less potent than derivatives of
the natural ring system, some compounds possessed surprising
activity. Derivatives such as 10c, 10geh, 17, 20, and 21 may
serve as interesting leads in continuing efforts to develop an
antibacterial agent from the pleuromutilin class. Recent ad-
vances in understanding the nature of the pleuromutilin
HO2C
OCH2OCH3
OCH2OCH3
11
11
12
12
t-BuOK, t-BuOH
O2
6
6
3
O
14
14
O
O
6 R = SiPh2t-Bu
O
OR
OR
4 R = CH2OCH3
5 R = SiPh2t-Bu
O
O
With quick access to intermediate 6 we could beginto synthe-
size derivatives of the glycolate which was previously the C-14
site of the parent ring system. Acid-catalyzed ethanolysis of 6
served to deprotect both the MOM- and silyl-protected alcohols,
and simultaneously protects the acid group as an ester (Scheme
1). Ester 7 could be converted to chloroacetate 8, the precursor to
sulfide derivatives 10aei via displacement with thiolates, fol-
lowed by optional hydrolysis of the ethyl ester. The hydrolysis
step is capricious in this ring system due to the presence of the
keto group in the cyclooctane ring. The use of a slight excess
of hydroxide, or longer reaction times, generally favored in-
creased amounts of a by-product assigned structure 11. Acid
11 is formed by hydrolysis of the thioglycolate, and retroealdol
cleavage of the cyclooctane ring. The nascent aldehyde function
is then trapped by the free hydroxyl to render the lactol. Im-
proved yields of the desired targets were obtained when the
amount of hydroxide was limited to an equivalent, and the reac-
tion monitored for its completion. The same care had to be exer-
cised in the hydrolysis of ester 7 to yield acid 9.
Acid 6 could be converted to amides such as 12 and 14 us-
ing standard coupling agents followed by acidic methanolysis
of the hydroxyl protecting groups (Scheme 2). Amide 13 was

D.M. Springer et al. / European Journal of Medicinal Chemistry 42 (2007) 109e113
113
(mutilin) derived from basic hydrolysis of pleuromutilin, containing free
alcohols at C-14 and C-11, is inactive as an antibacterial agent.
[6] Antibacterial MICs were determined by the microbroth dilution method
according to the standard conditions recommended by the National Com-
mittee for Clinical Laboratory Standards (NCCLS). MIC assays utilized
MuellereHinton broth for S. aureus and 50% MuellereHinton medium
plus 50% Todd Hewitt medium for S. pneumoniae, a bacterial inoculum
of w5 Â 105 CFU/mL, and were incubated at 35 ꢁC for 24 h. The MIC
was defined as the lowest drug concentration that inhibited all visible bac-
terial growth.
ribosomal binding site [7] may one day be used to rationalize
the activity of the compounds reported here [8].
Acknowledgements
We thank the Analytical Chemistry Department for analyt-
ical data produced in support of this work.
References
[7] (a) F. Schlunzen, E. Pyetan, P. Fucini, A. Yonath, J.M. Harms, Mol. Micro-
biol. 54 (2004) 1287e1294;
(b) M. Pringle, J. Poehlsgaard, B. Vester, K.S. Long, Mol. Microbiol. 54
(2004) 1295e1306.
[1] D.M. Springer, J.T. Goodrich, S. Huang, Tetrahedron Lett. 43 (2002)
4857e4860.
[8] Note added in proof: Reviewers of this manuscript have suggested that it
would be useful to assay these compounds in a protein synthesis inhibi-
tion assay. Data from such experiments would be of interest in determin-
ing the mechanism of antibacterial action of these derivatives. However,
these data were not collected on the pleuromutilin derivatives reported
here, and it is not possible for us to obtain them in the near future since
the antibacterial efforts at BristoleMyers Squibb have been suspended.
[2] D.M. Springer, M. Sorenson, S. Huang, T.P. Connolly, J.J. Bronson,
J.A. Matson, R.L. Hanson, D.B. Brozozowski, T.L. LaPorte, R.N. Patel,
Bioorg. Med. Chem. Lett. 13 (2003) 1751.
[4] A.J. Birch, C.W. Holzapfel, R.W. Rickards, Tetrahedron(Suppl. 8, Part II)
(1966) 359e387.
[5] Pleuromutilin structureeactivity relationships clearly indicate that C-14
acyl derivatives are very active antibacterial agents. The compound