Synthesis and antibacterial activity of pyrroloaryl-substituted oxazolidinones

Article abstract of DOI:10.1016/j.bmcl.2003.08.031

A novel series of oxazolidinones containing a pyrroloaryl substituent was synthesized and screened against a representative panel of susceptible and resistant Gram-positive bacteria. Several members of this series were found to have antibacterial activity comparable to or better than linezolid.

Full text of DOI:10.1016/j.bmcl.2003.08.031

Bioorganic & Medicinal Chemistry Letters 13 (2003) 4173–4177
Synthesis and Antibacterial Activity of Pyrroloaryl-Substituted
Oxazolidinones
Steven D. Paget,* Barbara D. Foleno, Christine M. Boggs,y Raul M. Goldschmidt,
Dennis J. Hlasta,y Michele A. Weidner-Wells, Harvey M. Werblood, Ellyn Wira,
Karen Bush and Mark J. Macielag
Johnson & Johnson Pharmaceutical Research & Development, L.L.C., Route 202, PO Box 300, Raritan, NJ 08869, USA
Received 30 April 2003; accepted 7 August 2003
Abstract—Anovel series of oxazolidinones containing a pyrroloaryl substituent was synthesized and screened against a repre-
sentative panel of susceptible and resistant Gram-positive bacteria. Several members of this series were found to have antibacterial
activity comparable to or better than linezolid.
# 2003 Elsevier Ltd. All rights reserved.
Antimicrobial resistance among hospital-acquired
Gram-positive bacterial pathogens, including vancomy-
cin-resistant enterococci (VRE),1 has been increasing
over the last decade. Especially worrisome is the recent
identification of vancomycin-resistance in two clinical
isolates of Staphylococcus aureus,2 one of the most
common infectious agents in the hospital environment.
Because of the emergence of resistance among Gram-
positive cocci, vancomycin has become a less reliable
therapeutic option.
properties make this compound an effective anti-
bacterial agent: broad spectrum potency against resis-
tant and susceptible Gram-positive bacteria; high
bioavailability, which allows for a facile transition from
parenteral to oral dosage forms; and an acceptable
therapeutic index.3 Of note is the activity of linezolid
against vancomycin-resistant S. aureus clinical isolates.2
Nevertheless, clinical failures of linezolid due to resis-
tance in VRE,4 as well as documented safety concerns
following long term dosing,5 argue in favor of the con-
tinued development of new agents to treat multi-drug
resistant Gram-positive infections.
The introduction of linezolid has provided the health-
care community with a new weapon in the battle against
multi-drug resistant Gram-positive bacteria. Its mode of
action, inhibiting protein synthesis at an early phase of
translation by binding selectively to the 50S ribosomal
subunit,3 is distinct from earlier inhibitors of bacterial
protein synthesis. Linezolid has been approved for var-
ious indications in man including both hospital-
acquired and community-acquired pneumonia, skin
infections, including cases due to methicillin-resistant S.
aureus (MRSA), and infections associated with vanco-
mycin-resistant Enterococcus faecium (VREF), includ-
ing cases with bloodstream infection.4 Several key
We have implemented a research program to develop
second-generation oxazolidinones with a primary goal
of identifying compounds with increased potency
against resistant Gram-positive bacteria compared to
linezolid. To this end we have explored replacing the
morpholine ring of linezolid with a pyrroloaryl sub-
stituent (Fig. 1).6ꢀ11 Several pyrroloaryl-substituted
oxazolidinones (8b, 8c, 10, 11a, 11b, and 12) were iden-
tified with superior or comparable in vitro antibacterial
activity to linezolid against a representative panel of
*Corresponding author. Tel.: +1-908-704-5769; fax: +1-908-203-
8109; e-mail: spaget@prdus.jnj.com
yCurrent address: Johnson & Johnson Pharmaceutical Research &
Development, L.L.C., Welsh and McKean Roads, Spring House, PA
19477, USA.
Figure 1. Linezolid and pyrroloaryl oxazolidinone antimicrobial
structures.
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.08.031

4174
S. D. Paget et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4173–4177
Scheme 3. Reagents and conditions: (l) MeOCH(NMe2)2, 60 ꢁC; (m)
formamidine hydrochloride, DMF, 90 ꢁC.
Scheme 1. Reagents and conditions: (a) BH3–THF, THF; (b) unde-
cane, 180 ꢁC; (c) concd aq HCl, reflux.
Scheme 4. Reagents and conditions: (n) MnO2, CH2Cl2.
Scheme 2. Reagents and conditions: (d) 3,4-difluoronitrobenzene,
DIPEA, DMF, 60 ꢁC; (e) 10% Pd/C, HCOONH4, THF, MeOH; (f)
CbzCl, NaHCO3, H2O, acetone; (g) n-BuLi, THF then (R)-glycidyl
butyrate, ꢀ78 ꢁC to rt; (h) MsCl, Et3N, DMF; (i) NaN3, DMF, 60 ꢁC;
(j) H2, Pd/C, DMF; (k) Ac2O, pyr, DMF.
clinically relevant Gram-positive bacteria. The SAR of
this new series is discussed herein.
The synthetic routes to various pyrroloaryl-substituted
oxazolidinone analogues are outlined in Schemes 1–6.
Following literature procedures, isoindoline (2) was
produced by borane–THF reduction of phthalimide
(1).12 The two isomeric dihydropyrrolopyridines (4 and
5) were formed by an inverse electron demand Diels–
Alder reaction of acyclic precursor 3 with extrusion of
hydrogen cyanide,13 followed by acid hydrolysis of the
carbamate protecting groups (Scheme 1). Scheme 2
depicts the prototypical oxazolidinone synthesis.14 To
begin the sequence, SNAr reaction of 3,4-difluoronitro-
benzene with the desired amine (2, 4, or 5), followed by
transfer hydrogenation of the nitro group with palla-
dium on carbon and ammonium formate and carboxy-
benzylation of the resulting amine furnished the Cbz-
protected aniline 6. Nucleophilic attack of the lithium
anion of 6 on (R)-glycidyl butyrate provided oxazolidi-
none carbinol derivative 7. Following standard proce-
dures14 7 was converted to target 8 by the following
sequence: (1) mesylation, (2) sodium azide displacement,
(3) hydrogenation, and (4) acetylation.
Scheme 5. Reagents and conditions: (o) 1,2-aryldicarboxaldehydes,
acetic acid, acetonitrile; (p) arylanhydrides, acetonitrile, reflux.
Scheme 6. Reagents and conditions: (q) 2- or 3-pyridinecarbox-
aldehyde, NaBH4, MeOH; (r) acetic acid, NaBH4.
provided the oxo-heterocycles 14 and 15. The pyrro-
lidinone ring forms via cyclic imine formation and sub-
sequent 1,3-hydride shift.19 The dioxo-heterocycles (16)
were synthesized under standard conditions,20 via reac-
tion of aniline 13 with the corresponding anhydrides.
Access to acyclic analogues was achieved by sequential
reductive aminations (Scheme 6). Reaction of aniline
intermediate 13 with 2- or 3-pyridinecarboxaldehyde
in the presence of sodium borohydride followed by
treatment with acetic acid and sodium borohydride21
afforded 17.
Pyrrolopyrimidine derivative 10 was accessed by treat-
ment of the known compound 915 with a Bredereck-type
reagent followed by condensation with formamidine
(Scheme 3).16
Oxidation of the dihydropyrrolopyridines 8 and 10 with
manganese dioxide provided pyrroloaryl oxazolidinones
11 and 12, respectively (Scheme 4).17
Minimum inhibitory concentrations (MIC) were mea-
sured against a group of antibiotic susceptible and
resistant strains of Gram-positive bacteria with linezolid
as a standard. Aset of four strains was used as the
screening panel: S. aureus OC 4172 (Smith strain) is
methicillin-susceptible; S. aureus OC 2878 is methicillin-
Aseries of oxo- and dioxo-heterocycles were prepared
from the known aniline 13 (Scheme 5).18 Treatment of 13
with 1,2-aryldicarboxaldehydes under acidic conditions

S. D. Paget et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4173–4177
4175
resistant (MRSA); Enterococcus faecalis ATCC 29212
and E. faecium OC 3312 are susceptible and resistant to
vancomycin, respectively. Compounds were also tested
against S. aureus OC 4172 in the presence of 50%
mouse serum to gauge the extent of protein binding or
inactivation due to serum. Broth microdilution MIC
(lowest concentration of compound inhibiting visible
growth) determinations were performed according to
National Committee for Clinical Laboratory Standards
methods.22
Table 1. Antibacterial activity (MICa values in mg/mL) for pyrroloaryl-substituted oxazolidinones
Compd
Y
E. faecalis
VRE
2
MRSA
1
S. aureus broth
S. aureus 50% mouse serum
Linezolid
2
2
2
8a
4
4
1
4
16
8b
1
1
0.5
0.25
1
1
1
8c
0.5
2
0.5
0.5
1
1
10
1
2
11a
11b
12
1
0.5
1
0.5
0.5
1
0.5
1
1
1
2
1
0.5
8
1
2
14a
14b
15
8
4
4
16
8
16
32
8
4
16
8
8
8
8
16a
16b
16c
64
>128
>128
32
16
>128
>128
32
>128
>128
128
>128
>128
>128
>128
17a
17b
16
16
8
16
32
>128
>128
>128
>128
>128
aThe variance in the determination of MIC values is 2-fold such that an MIC difference of at least 4-fold is significant.

4176
S. D. Paget et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4173–4177
The initial compound screened, isoindoline analogue 8a,
was equipotent to linezolid versus the MRSAstrain and
2-fold less active against the other strains in the testing
panel (Table 1). The 4-fold increase in the MIC value in
the presence of serum observed for 8a, however, sug-
gested a high degree of protein binding or inactivation
by serum components, possibly due to the hydro-
phobicity of the molecule. Isoindoline 8a had an ED50
value (effective dose which protects 50% of mice from
death) greater than 80 mg/kg/day following oral
administration in a murine model of lethal systemic
infection due to S. aureus OC4172 (Smith).10 In con-
trast, linezolid had an oral ED50 of 8 mg/kg/day in this
model. The lack of oral activity was likely due to insuf-
ficient free drug concentration of 8a in the blood to
inhibit bacterial growth. Therefore, the next step con-
ceptually in the design of new analogues was to incor-
porate nitrogen atoms in the aryl ring of the substituent
to increase the overall polarity of the molecule.The het-
eroaryl compounds (8b, 8c, 10, 11a, 11b, and 12) in this
series exhibited antibacterial profiles comparable to or
better than linezolid. Also noteworthy was the atte-
nuated increase in MIC in the presence of serum upon
inclusion of nitrogen atoms in the aryl ring. In parti-
cular, the pyrrolo[3,4-b]pyridinyl analogue (8c) was 4-
fold more active than linezolid against all strains in the
testing panel, including MRSAand VRE. In addition,
8c was 2-fold more active than linezolid in the presence
of serum. Likewise, the antibacterial profile of the iso-
meric compound, 8b, was improved compared to line-
zolid with MIC values consistently 2-fold lower, both in
the presence and absence of serum. The diminished
effect of serum on the MIC values of 8b and 8c, com-
pared to 8a, translated into improved in vivo efficacy. In
particular, 8b and 8c had oral ED50 values of 13 and 11
mg/kg/day, respectively, in the murine model of sys-
temic infection due to S. aureus OC4172 (Smith), which
was nearly as efficacious as linezolid (oral ED50 of 8 mg/
kg/day). Additionally, MIC values for 8b and 8c against
four linezolid-resistant S. aureus clinical isolates23 were
4–64 mg/mL, with 8c generally 4-fold more active than
linezolid which had MIC values of 8–64 mg/mL.
bonyl group made little difference in the pyrrolopyridine
series, such that lactam isomer 14b had similar anti-
bacterial activity to isomer 15. In contrast to the parent
compounds, incorporation of nitrogen atoms in the aryl
ring of the lactam and imide derivatives offered no
advantage, with pyrrolopyridine isomers 14b and 15
having slightly less antibacterial activity than the iso-
indoline compound 14a. Furthermore, pyrrolopyridine
isomers 16b and 16c were inactive while the corre-
sponding isoindoline analogue 16a still retained some
antibacterial activity.
The pyrrole ring appeared to be important for optimal
activity. Acyclic compound 17a, in which the pyrrolo-
pyridine heterocycle was replaced with picolyl and ethyl
substituents, was much less potent (16- to 32-fold less
active than 8b, 8c, 11a, 11b). In addition, an isomeric
analogue, compound 17b, was inactive. This large differ-
ence in activity between the cyclic and conformationally
mobile analogues may be reflective of the relative degree
of bacterial cell penetration, or may be due to the reduced
entropic penalty upon binding of the conformationally
constrained compound to the bacterial ribosome.
In summary, replacement of the morpholine ring of
linezolid with a pyrroloaryl substituent has afforded a
potent series of oxazolidinones with in vitro (8b, 8c, 10,
11a, 11b, and 12) and in vivo (8b and 8c) activity com-
parable or superior to linezolid against Gram-positive
bacterial pathogens. Compound 8c exhibited the best in
vitro profile in our abbreviated testing panel with MIC
values of 0.25–0.5 mg/mL against staphylococci, includ-
ing MRSA, and 0.5 mg/mL against enterococci, includ-
ing VRE.
References and Notes
1. Linden, P. K. Drugs 2002, 62, 425.
2. Miller, D.; Urdaneta, V.; Weltman, A.; Park, S. MMWR
2002, 51, 902.
3. Clemett, D.; Markham, A. Drugs 2000, 59, 815.
4. Mutnick, A. H.; Biedenbach, D. J.; Turnidge, J. D.; Jones,
R. N. Diagn. Microbiol. Infect. Dis. 2002, 43, 65.
5. Green, S. L.; Maddox, J. C.; Huttenbach, E. D. JAMA
2001, 285, 1291.
6. Paget, S.; Hlasta, D. US 02/0161029 A1, 2002, CAN
137:337913.
7. Paget, S.; Hlasta, D. WO 01/42242 A1, 2001, CAN
135:46182.
8. Paget, S.; Boggs, C.; Foleno, B.; Goldschmidt, R.; Grant,
E.; Hilliard, J.; Hlasta, D.; Weidner-Wells, M.; Werblood, H.;
Wira, E.; Bush, K.; Macielag, M. 41st Interscience Conference
on Antimicrobial Agents and Chemotherapy, Chicago, IL, 2001;
F-1048.
Adding another ring nitrogen, as in the pyrrolo[3,4-
d]pyrimidinyl analogue 10, decreased activity generally
2-fold relative to 8b and 8c. Compounds 11a, 11b, and
12, obtained from oxidation of 8b, 8c, and 10 respec-
tively, were generally 2- to 4-fold more potent than
linezolid against both susceptible and resistant strains.
The antibacterial activities of compounds 11a and 12
were slightly improved relative to their parent com-
pounds, while compound 11b was equivalent to or 2-
fold less potent than parent 8c.
9. Foleno, B.; Goldschmidt, R.; Licata, L.; Montenegro, D.;
Paget, S.; Weidner-Wells, M.; Wira, E.; Macielag, M.; Bush,
K. 41st Interscience Conference on Antimicrobial Agents and
Chemotherapy, Chicago, IL, 2001; F-1049.
10. Hilliard, J.; Goldschmidt, R.; Fernandez, J.; Melton, J.;
Paget, S.; Weidner-Wells, M.; Macielag, M.; Bush, K. 41st
Interscience Conference on Antimicrobial Agents and Chemo-
therapy, Chicago, IL, 2001; F-1050.
Introduction of carbonyl groups in the pyrrole ring, as
in the lactam and imide derivatives 14a, 14b, 15, 16a,
16b and 16c, raised MIC values against both susceptible
and resistant strains. Adetrimental effect on anti-
bacterial activity resulted upon incorporation of each
carbonyl group. The isoindoline series exemplifies this
additive effect, where 8a is generally 2- to 4-fold more
active than lactam 14a, which is in turn 4- to 8-fold
more active than imide 16a. The position of the car-
11. Paget, S.; Boggs, C.; Foleno, B.; Goldschmidt, R.; Hil-
liard, J.; Melton, J.; Stryker, S.; Wira, E.; Bush, K.; Macielag,

S. D. Paget et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4173–4177
4177
M. 42nd Interscience Conference on Antimicrobial Agents and
Chemotherapy, San Diego, CA, 2002; F-1318.
12. Gawley, R. E.; Chemburkar, S. R.; Smith, A. L.; Ankle-
kar, T. V. J. Org. Chem. 1988, 53, 5381.
17. Ogawa, H.; Aoyama, T.; Shioiri, T. Heterocycles 1996, 42, 75.
18. Hutchinson, D. K. WO 96/23788, 1996, CAN 125:24827.
19. DoMinh, T.; Johnson, A. L.; Jones, J. E.; Senise, P. P., Jr.
J. Org. Chem. 1977, 42, 4217.
13. Petersen, U., Krebs, A., Schenke, T., Grohe, K., Bremm,
K.-D., Endermann, R., Metzger, K.-G., Zeiler, H.-J. US 94/
5371090, 1994, CAN 119:49377.
20. Harrington, P. M. Heterocycles 1993, 35, 683.
21. Marchini, P.; Liso, G.; Reho, A.; Liberatore, F.; Moracci,
F. M. J. Org. Chem. 1975, 40, 3453.
14. Brickner, S. J.; Hutchinson, D. K.; Barbachyn, M. R.;
Manninen, P. R.; Ulanowicz, D. A.; Garmon, S. A.; Grega,
K. C.; Hendges, S. K.; Toops, D. S.; Ford, C. W.; Zurenko,
G. E. J. Med. Chem. 1996, 39, 673.
22. National Committee for Clinical Laboratory Standards.
Methods for Dilution Antimicrobial Susceptibility Tests for
Bacteria that Grow Aerobically, 5th ed. Approved standard:
NCCLS Document M7-A5, 2000.
15. Hutchinson, D. K.; Barbachyn, M. R.; Taniguchi, M.;
Munesada, K.; Yamada, H.; Brickner, S. J. WO 96/13502,
1996, CAN 125:142706.
16. Brighty, K. E.; Lowe III, J. A.; McGuirk, P. R. U.S.
Patent 5,037,834, 1991, CAN 116:59210.
23. Isolates were received from the ‘Network on Anti-
microbial Resistance in Staphylococcus aureus’ (NARSA)
program, generously provided by National Institutes of
Health/National Institutes of Allergy and Infectious Diseases,
Bethesda, MD, USA.