Synthesis, pH-dependent, and plasma stability of meropenem prodrugs for potential use against drug-resistant tuberculosis

Article abstract of DOI:10.1016/j.bmc.2013.05.024

Meropenem, a broad-spectrum parenteral β-lactam antibiotic, in combination with clavulanate has recently shown efficacy in patients with extensively drug-resistant tuberculosis. As a result of meropenem's short half-life and lack of oral bioavailability, the development of an oral therapy is warranted for TB treatment in underserved countries where chronic parenteral therapy is impractical. To improve the oral absorption of meropenem, several alkyloxycarbonyloxyalkyl ester prodrugs with increased lipophilicity were synthesized and their stability in physiological aqueous solutions and guinea pig as well as human plasma was evaluated. The stability of prodrugs in aqueous solution at pH 6.0 and 7.4 was significantly dependent on the ester promoiety with the major degradation product identified as the parent compound meropenem. However, in simulated gastrointestinal fluid (pH 1.2) the major degradation product identified was ring-opened meropenem with the promoiety still intact, suggesting the gastrointestinal environment may reduce the absorption of meropenem prodrugs in vivo unless administered as an enteric-coated formulation. Additionally, the stability of the most aqueous stable prodrugs in guinea pig or human plasma was short, implying a rapid release of parent meropenem.

Full text of DOI:10.1016/j.bmc.2013.05.024

Bioorganic & Medicinal Chemistry 21 (2013) 5605–5617
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Synthesis, pH-dependent, and plasma stability of meropenem
prodrugs for potential use against drug-resistant tuberculosis
Aaron M. Teitelbaum a, Anja Meissner a,b, Ryan A. Harding a, Christopher A. Wong b,
Courtney C. Aldrich b, , Rory P. Remmel a,
⇑
⇑
a Department of Medicinal Chemistry, University of Minnesota, Minneapolis, MN 55455, United States
b Center for Drug Design, University of Minnesota, Minneapolis, MN 55455, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 February 2013
Revised 4 May 2013
Accepted 15 May 2013
Available online 24 May 2013
Meropenem, a broad-spectrum parenteral b-lactam antibiotic, in combination with clavulanate has
recently shown efficacy in patients with extensively drug-resistant tuberculosis. As a result of merope-
nem’s short half-life and lack of oral bioavailability, the development of an oral therapy is warranted
for TB treatment in underserved countries where chronic parenteral therapy is impractical. To improve
the oral absorption of meropenem, several alkyloxycarbonyloxyalkyl ester prodrugs with increased lipo-
philicity were synthesized and their stability in physiological aqueous solutions and guinea pig as well as
human plasma was evaluated. The stability of prodrugs in aqueous solution at pH 6.0 and 7.4 was signif-
icantly dependent on the ester promoiety with the major degradation product identified as the parent
compound meropenem. However, in simulated gastrointestinal fluid (pH 1.2) the major degradation
product identified was ring-opened meropenem with the promoiety still intact, suggesting the gastroin-
testinal environment may reduce the absorption of meropenem prodrugs in vivo unless administered as
an enteric-coated formulation. Additionally, the stability of the most aqueous stable prodrugs in guinea
pig or human plasma was short, implying a rapid release of parent meropenem.
Keywords:
Meropenem
b-Lactam prodrugs
Aqueous stability
XDR-TB
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
therapy. As early as 1941, it was shown that mycobacteria are
resistant to penicillin,6 and a report from 1949 documented the
Mycobacterium tuberculosis (Mtb), the main bacterium that
causes the disease tuberculosis (TB), is estimated to latently infect
more than 2 billion people and resulted in 1.4 million deaths in
2011.1 TB is considered one of the most challenging bacterial infec-
tions to cure due to the ability of Mtb to switch its metabolism to a
non-replicating drug-tolerant state, which in turn necessitates pro-
longed antibiotic therapy.2 The standard treatment regimen devel-
oped by the British Medical Research Council uses a combination
therapy of isoniazid, rifampin, pyrazinamide, and ethambutol gi-
ven daily for 2 months followed by a 4–7 months continuation
phase of isoniazid and rifampin.3 Unfortunately, the emergence
of multidrug- and extensively drug resistant (MDR-, XDR-) TB is
undermining the great advances made in the 20th century to con-
trol TB.1 Emergence of resistance highlights the need for new TB
drugs with novel mechanisms of action that ideally target both
replicating and non-replicating mycobacteria.4,5
ability of Mtb to inactivate b-lactams.7 In the following decades,
the ineffectiveness of b-lactams for TB was largely attributed to
poor membrane penetration of the imposing outer mycobacterial
cell barrier.8 A series of studies culminating in a 2009 report by
Blanchard and co-workers shattered this long held dogma and
demonstrated: (1) the intrinsic resistance of Mtb toward b-lactams
results from a chromosomally-located extended-spectrum b-lacta-
mase (ESBL) encoded by the gene blaC, which hydrolyzes most b-
lactams at the diffusion controlled rate, (2) BlaC is irreversibly
inhibited by the b-lactamase inhibitor clavulanate, whereas other
approved b-lactamase inhibitors are slowly hydrolyzed, and most
importantly (3) inhibition of peptidoglycan synthesis is bacterici-
dal under non-replicating conditions.9–13 Based on their kinetic
studies, Blanchard and co-workers evaluated a combination of cla-
vulanate with a wide variety of b-lactams. Meropenem (a b-lactam
of the carbapenem class) was shown to exhibit the highest activity
with minimum inhibitory concentrations (MICs) ranging from
Although b-lactams are the most widely administered class of
antibiotics, they have never been systematically used in TB
0.03 to 1.25
lg/mL against 13 XDR-TB strains in the presence of
2.5
l
g/ml clavulanate.13 Meropenem’s activity is likely a result of
its good binding affinity to transpeptidases and endopeptidases
(the M. tuberculosis penicillin binding proteins) as well its slow rate
of inactivation by BlaC (has a kcat/KM value more than three orders
⇑
Corresponding authors. Tel.: +1 612 625 7956; fax: +1 612 626 5173 (C.C.A.);
tel.: +1 612 624 0472; fax: +1 612 624 0139 (R.P.R.).
E-mail addresses: aldri015@umn.edu (C.C. Aldrich), remme001@umn.edu
(R.P. Remmel).
0968-0896/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmc.2013.05.024

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A. M. Teitelbaum et al. / Bioorg. Med. Chem. 21 (2013) 5605–5617
of magnitude below the best substrates).12 These encouraging re-
sults were later confirmed in vivo in a murine TB model14 and in
the clinic, with several case reports describing the successful use
of this combination in patients with XDR-TB from the Russian Fed-
eration of Chechnya.15,16 Meropenem and clavulanate are both
FDA-approved drugs and are now available in less expensive gen-
eric forms (the patent for meropenem expired in 2010).
The major caveat to the treatment of TB with meropenem is the
requirement of multiple intravenous infusions due to its negligible
oral absorption and short 0.75–1-h half-life,17 which is impractical
for dosing of underserved populations where TB is most prevalent.
Meropenem is a highly polar zwitterionic molecule (log D <ꢀ2.5)18
and unstable in aqueous conditions and must therefore be given
intravenously within 3 h following aqueous reconstitution. Aque-
ous degradation products are derived from b-lactam hydrolysis
(major) and b-lactam dimerization (minor) via intermolecular at-
tack of the pyrrolidine nitrogen onto the b-lactam.19 Consequently,
new derivatives or formulations of meropenem are required to im-
prove stability, half-life, and oral bioavailability.
A standard approach to improve the oral absorption of b-lac-
tams is to synthesize an ester prodrug that increases the lipophil-
icity and thereby improves the absorption through the
gastrointestinal tract.20,21 Once absorbed into the bloodstream,
the ester prodrug is hydrolyzed by serum or tissue carboxyesteras-
es to release the parent drug. Prodrug strategies for b-lactams em-
ploy acyloxyalkyl-(I) and related alkyloxycarbonyloxyalkyl-(II)
esters (Fig. 1) where hydrolysis occurs at the remote carbonyl to af-
ford an intermediate acyl-hemiacetal-(III) that collapses with
expulsion of an aldehyde and release of the parent b-lactam-(IV).
The hydrolysis rates and stability can be tuned by introducing ste-
ric bulk at the acyl position (R2 of I) or carbonate position (R2 for II)
or the acetal linkage (R1) of I or II (Fig. 1). Simple esters of b-lac-
tams are ineffective since they enhance the hydrolytic lability of
the b-lactam carbonyl22 and/or are incompletely hydrolyzed as
found in cephalosporin and carbapenem b-lactams.20 Several acy-
loxyalkyl esters of carbapenems have been reported in an effort
to improve the bioavailability of the parent carbapenem.23 The
most advanced oral carbapenem in development is tebipenem piv-
oxil with bioavailabilities of 71%, 59%, and 35% in the mouse, dog,
and monkey, respectively (see V, Fig. 1 for the structure of the piv-
oxil ester).24 Meropenem bis-prodrugs with a pivoxil ester at-
tached to the carboxylate and various acyloxymethyl carbamates
attached to the pyrrolidine nitrogen were reported with bioavaila-
bilities ranging from 27.0% to 29.5% in the rat and 24.4–38.4% in
the beagle dog.18 Although pivoxil esters have been utilized as pro-
moieties to improve the lipophilicity of b-lactam antibiotics, the
released pivalic acid is a concern for chronic therapy such as TB.
Pivalic acid enters into the branched-chained fatty acid acyl carni-
tine pathway and forms pivaloyl CoA, which is excreted as pivaloyl
carnitine.25 Carnitine plays an essential role in fatty acid beta-oxi-
dation, energy metabolism, and mitochondrial function and its
depletion leads to serious side effects.26
In order to avoid generation of pivalic acid, we instead focused
on alkyloxycarbonyloxyalkyl (AOCO) esters, which upon hydrolysis
release an alcohol and carbon dioxide along with the parent b-lac-
tam and aldehyde linker. One successful AOCO prodrug (cefpodox-
ime proxetil-(VI), Fig. 1) releases innocuous isopropanol upon
prodrug cleavage. The extra methyl group at the acetal linkage of
the proxetil ester compensates for the decreased steric bulk of
the isopropoxy group (relative to the tert-butyl moiety in the re-
lated pivoxil prodrug). The primary drawback of the proxetil pro-
moiety is the introduction of an additional stereocenter at the
acetal linkage.
Herein we describe the synthesis and evaluation of a proxetil
ester and novel bicyclic (benzosuberyl, tetralyl, and indanyl) alky-
loxycarbonyloxyalkyl promoieties of meropenem in an effort to
add significant lipophilicity to the parent molecule and ultimately
improve oral absorption. We chose the bicyclo promoieties based
on the first broad-spectrum penicillin prodrug carbenicillin inda-
nyl (Geocillin) and the ability to release a soft non-toxic leaving
group such as 5-hydroxyindane or 1- or 2-benzosuberol. Upon es-
ter hydrolysis, a bicyclic alcohol is formed that is glucuronidated
and rapidly excreted from the blood.27 We also hypothesized that
the bulky bicyclic promoieties may obviate the methyl group on
the bridging acetal found in the proxetil ester.
2. Results and discussions
2.1. Chemistry
The synthesis of meropenem proxetil
2 was optimally
performed by reacting meropenem 1 with 1.1 equiv of freshly pre-
pared 1-iodoethyl isopropyl carbonate28 and 4 equiv of Cs2CO3 in
DMF at 0 °C for 30 min (Scheme 1). Under these conditions, a total
A
B
Figure 1. (A) Ester prodrugs strategies for b-lactams and mechanism of release. Carboxyesterase catalyzes cleavage at the remote carbonyl (highlighted in yellow), which
results in formation of an acyl hemiacetal intermediate that spontaneously breaks down to release the parent drug. (B) Representative acyloxyalkyl and
alkyloxycarbonyloxyalkyl (AOCO) promoieties found in clinically approved b-lactams.

A. M. Teitelbaum et al. / Bioorg. Med. Chem. 21 (2013) 5605–5617
5607
OH
OH
Me
H
Me
H
N
O
O
(a)
S
S
N
N
N
O
O
NH
2
NH
O
O
O
HO
1
O
O
O
Scheme 1. Reaction conditions: (a) 1-iodoethyl isopropyl carbonate, Cs2CO3, DMF, 0 °C, 30 min, 64%.
this improvement is based on the relatively large size of the
cesium atom compared to sodium or potassium. Because of its size,
the cesium coordinates to the carboxylate less efficiently and thus
causes an increased nucleophilicity of the carboxylate. Moreover,
under these conditions, the pyrrolidine nitrogen is not deproto-
nated and therefore not nucleophilic enough to attack the iodide
reagent.
The purification of 2 from the reaction mixture proved extre-
mely challenging owing to the poor stability of the compounds
(vide infra), but was ultimately achieved by evaporating the DMF
at 25 °C under reduced pressure followed by immediate silica gel
purification with 10% methanol–CH2Cl2 and 1% NH4OH as the mo-
bile phase to afford the compound as a white foam in 64% isolated
yield.
Table 1
Investigation of inorganic bases for the alkylation of meropenem (1) with 1-iodoethyl
isopropyl carbonate
Base
Equivalents
Reaction time (min)
% Conversion HPLC
K2CO3
1
2
4
8
4
2
4
120
120
45
45
60
41
51
62
72
67
81
98
Na2CO3
Cs2CO3
60
30
conversion of 98% was observed via HPLC analysis. The use of 1–
8 equiv of sodium or potassium carbonate as inorganic bases did
not produce the product at an acceptable conversion rate (41–
72%) compared with 4 equiv of Cs2CO3 (Table 1). We hypothesize
The synthesis of (R)- and (S)-1-indanyl-, 1-tetralyl-, and 1-ben-
zosuberyl- oxycarbonyloxymethyl meropenem prodrugs are out-
lined in Scheme 2. Enantioselective reduction of commercially
available ketones 3–5 employing either the (R) or (S)
Table 2
Aqueous stability of meropenem and prodrugs at pH 7.4, 6.0, and 1.2
OH
Me
H
O
S
N
N
N
R2
O
O
O
R1
O
O
O
R3
Compound
t1/2, mina OR (% remaining) at 120 minb
HEPES pH 7.4
(90%)
(87%)
<1
<1
1.4
1.5
28
27
64
MES pH 6.0
SGF pH 1.2
2, diasteromer-1, R1 = R2 = H; R3 = i-Pr
2, Diastereomer-2, R1 = R2 = H; R3 = i-Pr
17, R1 = R2 = H; R3 = (S)-1-Indanyl
20, R1 = R2 = H; R3 = (R)-1-Indanyl
18, R1 = R2 = H; R3 = (S)-1-Tetralyl
21, R1 = R2 = H; R3 = (R)-1-Tetralyl
19, R1 = R2 = H; R3 = (S)-1-Benzosuberyl
22, R1 = R2 = H; R3 = (R)-1-Benzosuberyl
23a, R1 = Me; R2 = H; R3 = (S)-1-Benzosuberyl
24a, R1 = H; R2 = Ac; R3 = (S)-1-Benzosuberyl
37, R1 = R2 = H; R3 = 2-Indanyl
(94%)
(92%)
n.d.c
n.d.
0.90
0.96
32
32
80
28
8.8
9.8
n.d.
n.d.
n.d.
n.d.
9.3
11
18
7.0
13
14
21
28
(70%)
(78%)
(79%)
(96%)
(93%)
(96%)
38, R1 = R2 = H; R3 = 2-Tetralyl
39, R1 = R2 = H; R3 = 2-Benzosuberyl
a
The hydrolysis of the prodrugs followed first-order degradation kinetics and the fractional rate of degradation (k) was determined
by plotting the log of prodrug peak area vs. time. Half-lives (min) were determined by dividing the ln 2 by k (fractional rate of
hydrolysis).
b
If a half-life was unable to be calculated, the% of prodrug remaining after a 120 min incubation is reported.
not determined.
c

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A. M. Teitelbaum et al. / Bioorg. Med. Chem. 21 (2013) 5605–5617
Scheme 2. Reaction conditions: (a) (+) or (ꢀ)-CBS, BH3ꢁSMe3, THF, 0 °C to rt, quant. yield; (b) for 12a: ClCO2CH(CH3)Cl, pyridine, CH2Cl2, rt, 30 min, 85%; for 9a,b–11a,b:
ClCO2CH2Cl, pyridine, CH2Cl2, rt, 30 min, 76–98%; (c) NaI, acetone, 40 °C, 5 h, 25% for 16a, 70–95% for 13a,b–15a,b; (d) for 17–22 and 23a:1, Cs2CO3, DMF, 0 °C, 30 min, 57–
84%; for 24a: (i) Ac2O, 1, DMF, 0 °C, 15 min; (ii) then add 16a, 30 min, 0 °C, 62%.
Scheme 3. Reaction conditions: (a) Refs. 29,30; (b) for 28: NaBH4, MeOH, rt, 97%; for 29–30: CBS, BH3ꢁSMe3, THF, 0 °C to rt, 68–89%; (c) ClCO2CH2Cl, pyridine, CH2Cl2, rt,
30 min, 70–94%; (d) NaI, acetone, 40 °C, 5 h, 62–70%; (e) 1, Cs2CO3, DMF, 0 °C, 30 min, 68–84%.
Corey-Bakshi-Shibata (CBS) catalyst and borane dimethylsulfide as
the reducing agent provided the corresponding alcohols 6a,b–8a,b
in enantiomeric ratios of 98:2 to 99:1 as determined by chiral
HPLC. Treatment of the enantiopure alcohols with chloromethyl-
or chloroethyl chloroformate afforded carbonates 9a,b–11a,b and
12a in excellent yields that were converted to the corresponding
iodo derivatives 13a,b–15a,b and 16a by a Finkelstein reaction. Fi-
nally, the carboxylate of meropenem was alkylated with 2.3 equiv
of the respective iodomethyl or iodoethyl carbonate 13a,b–15a,b
and 16a and 4 equiv of Cs2CO3 in DMF at 0 °C resulting in a 90–
95% conversion in 30 min. The reaction mixture was concentrated
and subsequently purified by the aforementioned procedure to af-
ford pure prodrugs 17–22 and 23a as white foams in moderate to
excellent yields. Compound 24a was prepared by alkylation of 15a
with N-acetyl meropenem, prepared in situ by treatment of
meropenem 1 with acetic anhydride in DMF at 0 °C.
The synthesis of the 2-indanyl-, 2-tetralyl-, and 2-benzosube-
ryl- oxycarbonyloxymethyl esters of meropenem 37–39 are pre-
sented in Scheme 3. The synthesis of 2-benzosuberone 27 was
accomplished in three steps from 1-tetralone 4 by following estab-
lished procedures described elsewhere.29,30 The enantioselective
reduction of 2-benzosuberone 27 and 2-tetralone 26 to the corre-
sponding (R)- and (S)-alcohols was attempted with the (R)- or
(S)-CBS catalyst and borane dimethyl sulfide as the reducing agent
by the aforementioned procedure. However, the reaction was not
enantioselective for 2-benzosuberone 27 and only the racemic
alcohol was produced. Reduction of 2-tetralone 26 was modestly
enantioselective providing an enantiomeric ratio of 7:3. The

A. M. Teitelbaum et al. / Bioorg. Med. Chem. 21 (2013) 5605–5617
5609
4
3
2
1
0
diastereomer was 9.8 and 8.8 min. To our surprise, the major deg-
radation product was identified as the b-lactam ring-opened
metabolite with the proxetil promoiety still intact, which was
identified by HRMS (LC-TOF, calcd for
C
23H37N3O9S (ESIꢀ):
530.2178, found: 530.2185 (error 1.3 ppm)). This supports the
assumption that the ester linkage is stable at acidic pH. However,
the promoiety does not protect the b-lactam bond from acidic
hydrolysis. Recently, the stability of meropenem (carbapenem),
cefotaxime (cephalosporin), ceftibuten (cephalosporin), and
faropenem (penem) were investigated in simulated gastric fluid,
pH 1.2 without enzymes.32 Meropenem was observed to possess
the shortest half-life compared to the other b-lactams showing
80% degradation in about 30 min. The amount of meropenem,
cefotaxime, ceftibuten, and faropenem remaining after 60 min in
simulated gastric fluid was 5%, 65%, 94%, and 70%, respectively.
Although the reason for the instability of meropenem was not
discussed, we hypothesize it stems from the increased torsional
and angle strain of the carbapenem scaffold compared with other
types of b-lactams.
The aqueous stability of the 1-bicyclo series of meropenem pro-
drugs was subsequently investigated. Interestingly, the stability of
prodrugs 17–22 decreases with ring size of the bicyclic promoiety
at both physiological and intestinal pH. The stereochemistry of the
alcohol substituent, however, has no impact on reactivity (Table 2).
This trend in stability might be an indication that the smaller ring
size means less steric bulk and thus less shielding of the proximate
functionals groups including the b-lactam from the acidic environ-
ment. Relative to the proxetil prodrug at all measured pH’s, the
1-bicyclo prodrugs were remarkably less stable as evidenced by
their short half-lives. Prodrugs with the 1-benzosuberyl promoiety
(19 and 22) were 20 and 37 times more stable to hydrolysis
compared with the 1-tetralyl (18 and 21) and 1-indanyl (17 and
20) containing prodrugs at physiological and intestinal pH, respec-
tively. Hydrolytical stability at physiologic versus intestinal pH,
however, differed only slightly for both derivatives. Consequently,
the major degradation products identified at pH 6 and 7.4 was the
expected parent compound meropenem as well as the correspond-
ing alcohol (Fig. 2). We suspected the lower stability of the 1-bicy-
clo series compared to 2 to be a result of the missing methyl group
and therefore less bulk at the acetal linkage. We thus hypothesized
that the addition of such a methyl group at the acetal linkage of
prodrug 19 (analogous to the proxetil ester 2) would decrease
the hydrolysis rate. The prodrug with the additional methyl group
23a improved the stability about 2-fold at all pH values, supporting
our former assumption. We also postulated that the secondary
Prodrug 19
1-Benzosuberol
Meropenem
0
50
100
time [min]
150
Figure 2. Aqueous stability of prodrug 19 at pH 7.4. The concentration of (d)
prodrug 19, the released meropenem (N), and 1-benzosuberol (j) are shown. The
concentration for each species is plotted as a function of incubation time. All
measurements were performed in triplicate and error bars represent the standard
deviation.
remarkable difference in enantioselectivity between the 1- and 2-
substituted ketones may be attributed to the difference in their
respective aryl ketone substituent sizes.31 With the alcohol at the
benzylic position (1-series), the substituents on either side of the
ketone differ decisively with a benzene ring on one side and an ali-
phatic chain on the other. In contrast, when the ketone is sur-
rounded by two neighboring methylene groups, as in the case of
the 2-series, the two substituents are relatively similar on a prox-
imate scale. Therefore, the chiral induction can be significantly re-
duced, which results in low to no selectivity. Since 2-indanol 28 is
an achiral molecule, no enantioselective reduction is required and
a simple NaBH4 reduction in MeOH of 2-indanone 25 afforded 2-
indanol 28. The chloromethyl and iodomethyl carbonate interme-
diates 31–36, and final prodrug molecules 37–39 were prepared
as previously described in Scheme 2. For both the 2-tetralyl 38
and 2-benzosuberyl 39, a mixture of diastereomers was isolated
in each case.
2.2. Prodrug aqueous stability
Aqueous stability was determined at pH 7.4, 6.0, and 1.2, which
simulates physiological, intestinal, and gastric pH, respectively. We
initially evaluated the aqueous stability of our first synthesized
prodrug, meropenem proxetil 2. At physiological pH 7.4, only
10% and 13% of each diastereomer was hydrolyzed to 1 in a period
of 2 h. At intestinal pH 6.0, similarly slow hydrolysis was apparent;
however, in simulated gastric fluid (pH 1.2), the half-life of each
O
O
NMe2
NMe2
OH
Me
H
OH
O
Me
OH2
NH
H
N
NH
S
H2O
N
S
O
O
O
O
O
O
O
O
O
O
O
19
OH
Me
H
N
HO
HO
O
S
N
O
NH
O
H2COH CO2
HO
1
Scheme 4.

5610
A. M. Teitelbaum et al. / Bioorg. Med. Chem. 21 (2013) 5605–5617
O
O
NMe2
NMe2
OH
O
OH
O
Me
Me
H
N
H
N
N
'
N
'
R
R
S
S
H
O
O
O
O
R
O
O
R
O
O
H2O
OH
O
O
O
O
NMe2
NMe2
OH
Me
Me
H
N
H
N
'
R
N
'
R
S
S
O
HO
HN
OH
OH
O
O
O
O
O
R
O
O
R
O
O
O
Scheme 5.
nitrogen in the pyrrolidine ring may attack the electrophilic car-
bonate or ester groups of the promoiety. Subsequently, the N-acet-
yl derivative 24a was synthesized; however, there was no
significant change in the hydrolysis rate (Tables 1 and 2). To fully
understand the mechanism of hydrolysis 19 was incubated at
37 °C for 4 h in 25 mM HEPES, pH 7.4 and the solution was evapo-
rated to dryness. The residue was dissolved in n-heptane and in-
jected onto a normal phase HPLC column using the conditions
described in Chiral HPLC method 3. To our surprise, the racemic
alcohol was identified demonstrating that hydrolysis takes place
via an SN1 solvolysis mechanism (Scheme 4) instead of an attack
on the carbonate or carboxylate functional groups. This process is
favored due to the attachment of the carbonate at the benzylic po-
sition of the bicyclic ring system, where a positive charge can be
Table 3
Guinea pig and human plasma stability of meropenem and selected prodrugs
OH
Me
H
O
S
N
N
O
NH
O
O
O
O
O
R3
stabilized through the p-system of the aryl ring.
Compound
t1/2 (min) Guinea pig
t1/2 (min) Human
As a result of the poor aqueous stability of the benzylic 1-series
of bicyclic prodrugs, we synthesized homobenzylic 2-substituted
analogues from the corresponding 2-ketones (Scheme 3). In that
case the alcohol would not be at the benzylic position any more,
which should prevent SN1 hydrolysis at the alcohol. Indeed,
prodrugs 37–39 showed substantially improved stability at physi-
ological and intestinal pH (70–96% remaining) compared with the
1-series (<1–28 min half-lives) due to their inability to form a
benzylic cation (Tables 1 and 2). Additionally, the 2-substituted
series of prodrugs were somewhat more stable (15%) at intestinal
pH relative to physiological pH. The stability was also investigated
in simulated gastric fluid to determine the rate of hydrolysis in the
stomach. In each case, half-lives ranged from 13 to 20 min and the
major degradation product identified by mass spectrometry was
the b-lactam ring-opened metabolite with the promoiety still
intact (scheme 5), similar to what was observed with meropenem
proxetil 2.
2, R3 = i-Pr (Diastereomer 1)
2, R3 = i-Pr (Diastereomer 2)
19, R3 = (S)-1-Benzosuberyl
22, R3 = (R)-1-Benzosuberyl
37, R3 = 2-Indanyl
6.5 1.5
108 12
11 1.0
44 1.8
1.4 0.23
1.3 0.26
0.9 0.10
0.9 0.51
1.7 0.05
1.9 0.08
34 3.3
4.3 0.74
8.1 0.38
23 0.87
38, R3 = 2-Tetralyl
39, R3 = 2-Benzosuberyl
related axetil promoieties.33 The half-lives of compounds 2, 19,
22, 37–39 range from 0.9 to 1.9 min, indicating poor plasma stabil-
ity (Table 3).
As human plasma generally contains less carboxylesterases
than rodent plasma, we determined the stability of those com-
pounds in human plasma as well. The half-lives were increased sig-
nificantly. While the hlaf-lives for 37 and 38 were still very short
with 4.3 and 8.1 min, the half-life for all benzosuberyl prodrugs
increased to 23–44 min with a difference of 10 min for the two
different diastereomers 19 and 22. This, again, can probably be
attributed to the different specificities for carboxylesterases. The
half-lives of the two proxetil diastereomers were very different
with 11 and 108 min. The orientation of the methyl group at the
acetal linkage, which adds to the steric bulk close to the active
center might be accountable for this remarkable difference. Kaku-
manu et al. found that the R isomer of cefpodoxime proxetil was
2.3. Plasma stability
The prodrugs with the longest stability in aqueous solution at
physiological and intestinal pH were further investigated for their
stability in guinea pig plasma. The diastereomers of meropenem
proxetil 2 had significantly different plasma stabilities, 7.1 and
1.4 min, that can be attributed to their different specificities for
carboxylesterases as previously described for the proxetil and

A. M. Teitelbaum et al. / Bioorg. Med. Chem. 21 (2013) 5605–5617
5611
metabolized more quickly than the S isomer in rat intestinal
homogenates, confirming stereoselective hydrolysis of proxetil
esters.34
High-resolution mass spectra were obtained on an Agilent TOF II
TOF/MS instrument equipped with an ESI probe.
4.2. General procedures
3. Conclusions
4.2.1. General procedures (A) for the enantioselective reduction
of ketones (3–5)
Several alkyloxycarbonyloxyalkyl prodrugs of meropenem (1)
were synthesized in an effort to improve the lipophilicity and oral
absorption of the parent carbapenem. Thereby, high yields were
obtained through established methods. Improved yields over con-
ventional methods for coupling meropenem with an alkyl iodide
were obtained with Cs2CO3. In addition, purification of the pro-
drugs was successfully achieved utilizing standard normal phase
silica gel and a CH2Cl2–MeOH–NH4OH solvent system. The aque-
ous stability of prodrugs 2, 17–24, and 37–39 at biologically rele-
vant pH 1.2 (stomach), 6.0 (intestinal), and 7.4 (blood) at 37 °C is
reported in Tables 1 and 2. The prodrugs containing the 1-ben-
zosuberyl, 1-tetralyl, and 1-indanyl-oxycarbonyloxymethyl pro-
moieties 17–24 were unstable in aqueous solution and resulted
in the formation of a racemic alcohol indicative of an SN1 solvol-
ysis mechanism. The prodrugs containing the 2-benzosuberyl, 2-
tetralyl, and 2-indanyl-oxycarbonyloxymethyl promoieties 37–39
were significantly more stable at physiological pH 7.4 and intesti-
nal pH 6.0. In simulated gastric fluid (pH 1.2), the 1-series 17–24
degraded quickly and formed the b-lactam ring-opened parent
molecule while the more stable 2-series 37–39 and 2 degraded
to form the ring-opened prodrug metabolite. The guinea pig plas-
ma stability of the most aqueous stable prodrugs was very short,
half-lives in human plasma were somewhat longer. These low
plasma stability implies a rapid esterase-catalyzed release of the
parent compound 1. It is reasonable to assume that esterases lo-
cated in both the intestine and liver may cleave the promoieties
as efficiently as the plasma esterases, which would affect the oral
absorption of the prodrugs. Overall, the work presented in this re-
search article describes the facile synthesis of meropenem pro-
drugs with increased lipophilicity; however, caution should be
exercised regarding the choice of promoiety for meropenem, and
experiments must be conducted to ensure the stability of the pro-
drug is acceptable for future in vitro and in vivo investigations.
The instability in stomach acid could be overcome with enteric-
coated formulations. Modification of the ionizable pyrrolidine
nitrogen (e.g., carbamate prodrugs) could be used to slow the pro-
duction of meropenem and this strategy has been explored
previously.18
To a solution of (R)-(+)-2-methyl-CBS-oxazaborolidine or (S)-
(ꢀ)-2-methyl-CBS-oxazaborolidine (0.2 equiv) in THF (1 mL/
0.21 mmol ketone) at 0 °C was added a 2.0 M solution of borane di-
methyl sulfide in THF (1.2 equiv). The mixture was stirred for
15 min then a solution of ketone (1.0 equiv) in THF (1 mL/
0.21 mmol ketone) was cannulated dropwise into the reaction
mixture. After stirring for 30 min, the reaction was quenched by
the addition of MeOH (1 mL/0.75 mmol of BH3ꢁSMe2), then concen-
trated under reduced pressure to afford colorless oils that solidified
overnight at ꢀ20 °C. The (R)-CBS reagent produced the (S)-alcohols
(6a–8a) and the (S)-CBS produced the (R)-alcohols (6b–8b).
4.2.2. General procedure (B) for the synthesis of
chloroalkylcarbonates (9a,b–11a,b, 12a, and 31–33)
To a solution of alcohols (6a,b–8a,b, and 28–30) (1.0 equiv) in
CH2Cl2 (0.1 M) at 23 °C was added pyridine (2 equiv) and the reac-
tion was stirred for 15 min. Next, chloromethyl or chloroethyl
chloroformate (2.0 equiv) was added dropwise and the resulting
solution was stirred for 30 min at 23 °C. The reaction mixture
was washed consecutively with H2O, 1 N aqueous HCl, and satu-
rated aqueous NaCl. The organic layer was dried (MgSO4), filtered,
and concentrated under reduced pressure to afford the crude prod-
uct, which was taken onto the next step without any further puri-
fication due to instability on silica gel.
4.2.3. General procedure (C) for the synthesis of
iodoalkylcarbonates (13a,b–15a,b, 16a, and 34–36)
To a solution of chloroalkylcarbonates (9a,b–11a,b and 12a)
(1.0 equiv) and 4 Å molecular sieves (0.5 g) in acetone (0.2 M)
was added sodium iodide (3.0 equiv). The solution was stirred for
5 h in the dark at 40 °C. Subsequently, the reaction mixture was fil-
tered and evaporated under reduced pressure to yield an orange
solid, which was dissolved in Et2O and washed consecutively with
10% Na2SO3, H2O, and saturated aqueous NaCl. The organic layer
was dried over MgSO4 and concentrated under reduced pressure
to afford the crude product. Purification was not performed due
to the instability of the molecules.
4. Experimental section
4.1. Chemistry
4.2.4. General procedure (D) for the synthesis of meropenem
prodrugs (17–22, 23a, 24a, and 37–39)
A solution of meropenem (100 mg, 1.0 equiv) and Cs2CO3
(367 mg, 1.04 mmol 4.0 equiv) in DMF (1.3 mL) was stirred at
0 °C for 10 min. Next, a solution of the crude iodoalkylcarbonate
(0.6 mmol, 2.3 equiv) in DMF (1.3 mL) was added dropwise to the
reaction mixture. After stirring for 20 min at 0 °C, the reaction mix-
ture was evaporated to dryness under reduced pressure. Purifica-
tion by flash chromatography (10% MeOH–CH2Cl2 with 1%
NH4OH) afforded the title compounds as white foams.
Meropenem was purchased from A.G Scientific (San Diego, CA)
and all other chemical reagents were purchased from Sigma–Al-
drich (St. Louis, MO). Flash chromatography was performed using
a Combiflash CompanionÒ equipped with flash column silica
cartridges with the indicated solvent system. Solvents for chroma-
tography (CH2Cl2, MeOH, EtOAc, hexanes, and NH4OH) were all
purchased from Fisher Scientific (Pittsburgh, PA). 1H and 13C
NMR spectra were obtained from a Varian 600 MHz spectrometer.
Proton chemical shifts (d) are reported in ppm from an internal
standard of chloroform (7.26) or dichloromethane (5.32). Carbon
chemical shifts are reported using an internal standard of residual
chloroform (77.23) or dichloromethane (54.00). Proton chemical
data are reported as follows: chemical shift, multiplicity
(s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet,
br = broad, ovlp = overlapping), coupling constant, and integration.
4.3. Chemical synthesis
4.3.1. (S)-1-Indanol (6a)
Synthesized from 1-indanone 3 (1.00 g, 7.57 mmol, 1.0 equiv)
using general procedure A. Purification by flash chromatography
(10% EtOAc–hexanes) afforded the title compound (1.01 g, quant.):
er = 99:1 (Chiral HPLC, method 1); Analytical data matched previ-
ously reported values.27

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4.3.2. (R)-1-Indanol (6b)
125.9, 126.0, 128.6, 129.5, 134.2, 135.6; HRMS (APCI+) calcd for
This compound was prepared similarly to 6a except (S)-(+)-2-
methyl-CBS-oxazaborolidine was utilized as the enantioselective
catalyst to afford the title compound (1.01 g, quant.): er = 99:1
(Chiral HPLC, method 1); Analytical data matched previously re-
ported values.27
C10H11 [MꢀOH]+ 131.0855, found: 131.0857 (error 1.5 ppm).
4.3.9. 2-Benzosuberol (30)
Synthesized from 2-benzosuberone 2729,30 following general
procedure A. Purification by flash chromatography (20% EtOAc–
Hexanes) afforded the title compound as a colorless oil that solid-
ified overnight (97 mg, 89%): Rf = 0.42 (20% EtOAc–Hexanes);
er = 1:1 (Chiral HPLC, method 3); 1H NMR (600 MHz, CDCl3) d
1.56 (m, 1H), 1.88 (m, 2H), 2.08 (s, 1H), 2.13 (m, 1H), 2.78 (m,
1H), 2.80 (m, 1H), 3.01 (d, J = 13.2 Hz, 1H), 3.09 (dd, J = 13.2,
9.0 Hz, 1H), 3.82 (t, J = 7.9 Hz, 1H), 7.12 (m, 1H), 7.16 (t,
J = 4.4 Hz, 2H), 7.19 (m, 1H); 13C NMR (150 MHz, CDCl3) d 24.4,
35.5, 40.6, 44.7, 69.3, 126.2, 126.6, 128.8, 130.5, 136.5, 143.4;
4.3.3. (S)-1-Tetralol (7a)
Synthesized from 1-tetralone 4 (1.00 g, 6.84 mmol, 1.0 equiv)
following general procedure A. Purification by flash chromatogra-
phy (10% EtOAc–hexanes) afforded the title compound (1.12 g,
quant.): er = 99:1 (Chiral HPLC, method 2); Analytical data
matched previously reported values.27 1H NMR (600 MHz, CDCl3)
d 1.80 (m, 1H), 1.93 (m, 1H), 2.00 (m, 2H), 2.74 (ddd, J = 16.8, 7.8,
6.0 Hz, 1H), 2.85 (dt, J = 16.8, 5.4 Hz, 1H), 4.80 (m, 1H), 7.12 (m,
1H), 7.22 (m, 2H), 7.45 (m, 1H).
HRMS (APCI+) calcd for
145.1013 (error 0.7 ppm).
C
11H13 [MꢀOH]+ 145.1012, found:
4.3.4. (R)-1-Tetralol (7b)
4.3.10. (S)-Indan-1-yl chloromethylcarbonate (9a)
This compound was prepared analogously to 7a, but (S)-(+)-2-
methyl-CBS oxazaborolidine was employed as the enantioselective
catalyst to afford the title compound (1.02 g, quant.): er = 98:2
(Chiral HPLC, method 2); Analytical data matched previously re-
ported values.27
The title compound was prepared from (S)-1-indanol 6a
(998 mg, 7.43 mmol, 1.0 equiv) according to general procedure B
to afford a yellow oil (1.6 g, 98%), which was directly taken onto
the next step without further purification due to instability on sil-
ica gel: Rf = 0.77 (20% EtOAc–Hexanes).
4.3.5. (S)-1-Benzosuberol (8a)
4.3.11. (R)-Indan-1-yl chloromethylcarbonate (9b)
This title compound was prepared analogously to 9a, but with
(R)-1-indanol 6b to afford a yellow oil (1.6 g, 97%).
Synthesized from 1-benzosuberone
5 (1.00 g, 6.24 mmol,
1.0 equiv) using general procedure A. Purification by flash chroma-
tography (10% EtOAc–hexanes) afforded the title compound
(974 mg, 96%) as a white solid: er = 98:2 (Chiral HPLC, method
3); Analytical data matched previously reported values.27 1H
NMR (600 MHz, CDCl3) d 1.48 (m, 1H), 1.80 (m, 3H), 1.96 (m,
1H), 2.06 (m, 1H), 2.72 (dd, J = 14.4, 10.8 Hz, 1H), 2.93 (dd,
J = 14.1, 8.8 Hz, 1H), 4.93 (d, J = 8.2 Hz, 1H), 7.10 (d, J = 7.6 Hz,
1H), 7.16 (t, J = 7.3 Hz, 1H), 7.21 (t, J = 7.3 Hz, 1H), 7.44 (d,
J = 7.6 Hz, 1H).
4.3.12. (S)-Tetral-1-yl chloromethylcarbonate (10a)
Synthesized from (S)-1-tetralol 7a (1.12 g, 7.6 mmol, 1.0 equiv)
according to general procedure B. Purification by flash chromatog-
raphy (1% EtOAc–hexanes) afforded the title compound (1.68 g,
92%) as a yellow oil: Rf = 0.58 (20% EtOAc–hexanes); 1H NMR
(600 MHz, CDCl3) d 1.82–1.89 (m, 1H), 1.96–2.07 (m, 2H), 2.14–
2.20 (m, 1H), 2.73–2.79 (m, 1H), 2.85–2.91 (m, 1H), 5.74 (d,
J = 6.0 Hz, 1H), 5.77 (d, J = 6.0 Hz, 1H), 5.92 (m, 1H), 7.14 (d,
J = 6.6 Hz, 1H), 7.18–7.20 (m, 1H), 7.24–7.29 (m, 1H), 7.36 (d,
J = 7.8 Hz, 1H); 13C NMR (150 MHZ, CDCl3) d 18.3, 28.7, 28.8,
72.2, 75.8, 126.2, 128.7, 129.2, 129.7, 132.9, 137.9, 153.2.
4.3.6. (R)-1-Benzosuberol (8b)
This compound was prepared analogously to 8a, but (S)-(ꢀ)-2-
methyl-CBS-oxazaborolidine was employed as the enantioselective
catalyst to afford the title compound (1.02 g, 99%) as a white solid:
er = 99:1 (Chiral HPLC, method 3); Analytical data matched previ-
ously reported values.27
4.3.13. (R)-Tetral-1-yl chloromethylcarbonate (10b)
The title compound was prepared analogously to 10a, but with
(R)-1-tetralol 7b to afford a yellow oil (1.4 g, 94%).
4.3.7. 2-Indanol (28)
To a solution of 2-indanone 25 (1.0 g, 7.5 mmol, 1.0 equiv) in
MeOH (150 mL) was added NaBH4 (500 mg, 12.2 mmol, 1.7 equiv)
portionwise. The reaction was stirred 10 min then diluted with
H2O (75 mL) and extracted with Et2O (2 ꢂ 100 mL). The organic
layer was dried (MgSO4), filtered, and concentrated under reduced
pressure to afford the crude product as a brown solid in quantita-
tive yield. The product was utilized in the following step without
further purification: Rf = 0.26 (20% EtOAc–Hexanes 1% Formic
Acid); 1H NMR (600 MHz, CDCl3) d 1.86 (br s, 1H), 2.91 (dd,
J = 16.4, 2.9 Hz, 2H), 3.21 (dd, J = 16.4, 5.9 Hz, 2H), 4.68 (m, 1H),
7.18 (dd, J = 5.4, 3.6 Hz, 2H), 7.25 (dd, J = 5.4, 3.6 Hz, 1H).
4.3.14. (S)-Benzosuber-1-yl chloromethylcarbonate (11a)
Synthesized from (S)-1-benzosuberol 8a (973 mg, 6.0 mmol,
1.0 equiv) according to general procedure B. Purification by flash
chromatography (1% EtOAc–hexanes) afforded the title compound
(1.43 g, 94%) as a colorless oil: Rf = 0.14 (1% EtOAc–hexanes); 1H
NMR (600 MHz, CDCl3) d 1.62–1.77 (m, 2H), 1.84–1.93 (m, 1H),
1.94–2.01 (m, 1H), 2.03–2.13 (m, 2H), 2.75 (dd, J = 10.2, 4.2 Hz,
1H), 3.01 (dd, J = 11.4, 1.8 Hz, 1H), 5.72 (d, J = 6.6 Hz, 1H), 5.79 (d,
J = 6.6 Hz, 1H), 5.87 (d, J = 8.4 Hz, 1H), 7.14 (t, J = 6.6 Hz, 1H),
7.18–7.23 (m, 1H), 7.32 (d, J = 6.6 Hz, 2H); 13C NMR (150 MHz,
CDCl3) d 26.9, 27.5, 33.1, 35.6, 72.2, 81.7, 126.1 (2C), 128.1,
129.9, 138.7, 141.4, 152.7.
4.3.8. 2-Tetralol (29)
Synthesized from 2-tetralone 26 (1.00 g, 6.84 mmol, 1.0 equiv)
following general procedure A. Purification by flash chromatogra-
phy (20% EtOAc–hexanes) afforded the title compound (1.0 g,
quant) as a colorless oil: Rf = 0.12 (20% EtOAc–hexanes); er = 7:3
(Chiral HPLC, method 2); 1H NMR (600 MHz, CDCl3) d 1.61 (m,
1H), 1.83 (dtd, J = 12.5, 9.3, 5.9 Hz, 1H), 2.07 (m, 1H), 2.78 (dd,
J = 16.1, 7.9 Hz, 1H), 2.85 (ddd, J = 16.2, 9.6, 6.6 Hz, 1H), 2.96 (dt,
J = 16.8, 6.0 Hz, 1H), 3.10 (dd, J = 16.1, 5.0 Hz, 1H), 4.17 (m, 1H),
7.10 (m, 4H); 13C NMR (150 MHz, CDCl3) d 27.0, 31.5, 38.4, 67.2,
4.3.15. (R)-Benzosuber-1-yl chloromethylcarbonate (11b)
The title compound was prepared analogously to 11a, with (R)-
1-benzosuberol 8b to afford a colorless oil (1.2 g, 76%).
4.3.16. (1S, 1S/R)-Benzosuber-1-yl chloroethylcarbonate (12a)
Synthesized from (S)-1-benzosuberol 8a (1.13 g, 6.99 mmol,
1.0 equiv) according to general procedure B employing chloroethyl
chloroformate as the alkylating agent. Purification by flash

A. M. Teitelbaum et al. / Bioorg. Med. Chem. 21 (2013) 5605–5617
5613
chromatography (10% EtOAc–hexanes) afforded the title com-
pound (1.60 g, 85%) as a 1:1 mixture of diastereomers at C-1 as a
colorless oil: Rf = 0.48 and 0.43 (10% EtOAc–hexanes); 1H NMR
(600 MHz, CDCl3) d 1.61 (m, 0.5H), 1.69 (m, 1H), 1.73 (m, 0.5H),
1.84 (d, J = 5.9 Hz, 1.5H), 1.86 (d, J = 5.9 Hz, 1.5H), 1.87 (m, 1H),
1.97 (m, 1.5H), 2.06 (m, 1.5H), 2.75 (m, 1H), 3.00 (m, 1H), 5.84
(d, J = 9.6 Hz, 0.5H), 5.86 (d, J = 9.6 Hz, 0.5H), 6.43 (q, J = 6.0 Hz,
0.5H), 6.47 (q, J = 6.0 Hz, 0.5H), 7.13 (m, 1H), 7.19 (m, 2H), 7.32
(m, 1H); 13C NMR (150 MHz, CDCl3) d 25.14, 25.18, 26.8, 27.1,
27.45, 27.55, 33.0, 33.3, 35.57, 35.58, 81.2, 81.5, 84.55, 84.60,
126.1, 126.2, 128.0, 128.1, 129.8, 129.9, 138.8, 138.9, 152.2, 152.3.
CDCl3) d 1.74–1.81 (m, 1H), 1.87–1.99 (m, 2H), 2.06–2.11 (m,
1H), 2.64–2.74 (m, 1H), 2.77–2.83 (m, 1H), 5.84 (br s, 1H), 5.87–
5.91 (m, 2H), 7.06 (d, J = 6.6 Hz, 1H), 7.10–7.14 (m, 1H), 7.16–
7.20 (m, 1H), 7.28 (d, J = 7.8 Hz, 1H); 13C NMR (150 MHz, CDCl3)
d 18.4, 28.7, 28.9, 34.1, 75.9, 126.2, 128.7, 129.2, 129.7, 132.9,
137.9, 153.0.
4.3.23. (R)-Tetral-1-yl iodomethylcarbonate (14b)
This compound was prepared analogously to 14a, but using 10b
as the substrate to afford a yellow oil (1.5 g, 70%).
4.3.24. (S)-Benzosuber-1-yl iodomethylcarbonate (15a)
4.3.17. Indan-2-yl chloromethylcarbonate (31)
The title compound was prepared from 11a (1.4 g, 5.5 mmol,
1.0 equiv) according to procedure C to afford a pale yellow oil
(1.8 g, 95%): Rf = 0.53 (20% EtOAc–hexanes); 1H NMR (600 MHz,
CDCl3) d 1.62–1.77 (m, 2H), 1.84–1.92 (m, 1H), 1.94–2.01 (m,
1H), 2.02–2.12 (m, 2H), 2.72–2.79 (m, 1H), 2.97–3.04 (m, 1H),
5.86 (d, J = 8.4 Hz, 1H), 5.95 (d, J = 4.8 Hz, 1H), 5.99 (d, J = 4.8 Hz,
1H), 7.14 (d, J = 6.0 Hz, 1H), 7.17–7.24 (m, 2H), 7.31 (d, J = 6.0 Hz,
1H); 13C NMR (150 MHz, CDCl3) d 26.7, 27.3, 32.9, 33.9, 35.4,
81.5, 125.9 (2C), 127.9, 129.7, 138.5, 141.1, 152.8.
Synthesized from 2-indanol 28 (971 mg, 7.2 mmol, 1.0 equiv)
according to general procedure B. Purification by flash chromatog-
raphy (10% EtOAc–hexanes with 1% formic acid) afforded the title
compound (1.53 g, 94%) as a white solid: Rf = 0.57 (10% EtOAc–hex-
anes with 1% formic acid); 1H NMR (600 MHz, CDCl3) d 3.14 (br s,
1H), 3.17 (br s, 1H), 3.37 (d, J = 6.6 Hz, 1H), 3.40 (d, J = 6.6 Hz, 1H),
5.54 (br s, 1H), 5.74 (s, 2H), 7.21–7.23 (m, 2H), 7.25–7.28 (m, 2H);
13C NMR (150 MHz, CDCl3) d 39.3, 72.1, 80.5, 124.6, 127.0, 139.6,
153.1.
4.3.25. (R)-Benzosuber-1-yl iodomethylcarbonate (15b)
This compound was prepared analogously to 15a, but using 11b
as the substrate to afford a yellow oil (1.5 g, 93%).
4.3.18. Tetral-2-yl chloromethylcarbonate (32)
Synthesized from 2-tetralol 29 (1.0 g, 6.8 mmol, 1.0 equiv)
according to general procedure B. Purification by flash chromatog-
raphy (hexane to 20% EtOAc–hexanes) afforded the title compound
(1.14 g, 70% over two steps) as a colorless solid: Rf = 0.39 (20%
EtOAc–hexanes); 1H NMR (600 MHz, CDCl3) d 2.06 (m, 1H), 2.14
(m, 1H), 2.87 (dt, J = 16.8, 7.2 Hz, 1H), 2.97 (t, J = 6.0 Hz, 1H), 2.99
(dd, J = 16.2, 6.6 Hz, 1H), 3.20 (dd, J = 16.7, 5.0 Hz, 1H), 5.18 (m,
1H), 5.73 (d, J = 6.6 Hz, 1H), 5.75 (d, J = 6.6 Hz, 1H), 7.08 (m, 1H),
7.11 (m, 1H), 7.15 (m, 2H); 13C NMR (150 MHz, CDCl3) d 26.2,
27.6, 34.3, 72.1, 75.3, 126.1, 126.3, 128.6, 129.3, 132.7, 135.1, 152.9.
4.3.26. (1S, 10R/S)-Benzosuber-1-yl iodoethylcarbonate (16a)
Synthesized from 12a (1.57 g, 5.84 mmol, 1.0 equiv) according
to procedure C. 1H NMR showed only 30% conversion to the title
compound. 1H NMR (600 MHz, CDCl3) d 1.61 (m, 0.5H), 1.69 (m,
1H), 1.73 (m, 0.5H), 1.87 (m, 1H), 1.97 (m, 1.5H), 2.06 (m, 1.5H),
2.24 (d, J = 5.9 Hz, 1.5H), 2.27 (d, J = 5.9 Hz, 1.5H), 2.75 (m, 1H),
3.00 (m, 1H), 5.84 (d, J = 9.6 Hz, 0.5H), 5.86 (d, J = 9.6 Hz, 0.5H),
6.76 (q, J = 5.9 Hz, 0.5H), 6.80 (q, J = 5.9 Hz, 0.5H), 7.13 (m, 1H),
7.19 (m, 2H), 7.32 (m, 1H).
4.3.19. Benzosuber-2-yl chloromethylcarbonate (33)
Synthesized from (S)-2-benzosuberol 30 (62 mg, 0.38 mmol,
1 equiv) according to general procedure B. Purification by flash
chromatography (hexane to 20% EtOAc–hexanes) afforded the title
compound (90 mg, 92%) as a colorless solid: Rf = 0.42 (20% EtOAc–
hexanes); 1H NMR (600 MHz, CDCl3) d 1.57 (m, 1H), 1.98 (m, 2H),
2.24 (m, 1H), 2.80 (t, J = 4.2 Hz, 2H), 3.07 (d, J = 13.5 Hz, 1H), 3.25
(dd, J = 13.8, 10.3 Hz, 1H), 4.76 (t, J = 9.4 Hz, 1H), 5.72 (s, 2H),
7.12 (d, J = 7.0 Hz, 1H), 7.17 (m, 3H); 13C NMR (150 MHz, CDCl3)
d 24.3, 35.2, 36.7, 41.0, 72.0, 77.6, 126.5, 127.1, 128.9, 130.4,
135.1, 142.9, 152.5.
4.3.27. Indan-2-yl iodomethylcarbonate (34)
The title compound was prepared from 31 (1.50 g, 6.65 mmol,
1.0 equiv) according to general procedure C to afford a white solid
with a yellow hue (1.47 g, 70%): Rf = 0.52 (20% EtOAc–hexanes); 1H
NMR (600 MHz, CDCl3) d 3.12 (br s, 1H), 3.15 (br s, 1H), 3.36 (d,
J = 6.6 Hz, 1H), 3.39 (d, J = 6.6 Hz, 1H), 5.53 (br s, 1H), 5.95 (s,
2H), 7.20–7.22 (m, 2H), 7.24–7.27 (m, 2H); 13C NMR (150 MHz,
CDCl3) d 33.9, 39.4, 80.5, 124.6, 127.0, 139.6, 152.9.
4.3.28. Tetral-2-yl iodomethylcarbonate (35)
The title compound was prepared from 32 (1.14 g, 4.74 mmol,
1.0 equiv) according to general procedure C to afford a colorless so-
lid (1.09 g, 69%): Rf = 0.53 (20% EtOAc–hexanes); 1H NMR
(600 MHz, CDCl3) d 2.10 (m, 1H), 2.16 (m, 1H), 2.90 (dt, J = 16.8,
7.2 Hz, 1H), 2.98 (t, J = 6.6 Hz, 1H), 3.02 (dd, J = 16.8, 7.8 Hz, 1H),
3.23 (dd, J = 16.4, 4.7 Hz, 1H), 5.13 (m, 1H), 5.96 (d, J = 4.8 Hz,
1H), 5.98 (d, J = 4.8 Hz, 1H), 7.13 (m, 1H), 7.15 (m, 1H), 7.19 (m,
2H); 13C NMR (150 MHz, CDCl3) d 25.9, 27.3, 34.0, 34.2, 75.0,
125.8, 126.0, 128.3, 129.0, 132.4, 134.7, 152.3.
4.3.20. (S)-Indan-1-yl iodomethylcarbonate (13a)
The title compound was prepared from 9a (1.6 g, 7.19 mmol,
1.0 equiv) according to general procedure C to afford a yellow oil
(1.73 g, 76%): Rf = 0.61 (20% EtOAc–hexanes); 1H NMR (600 MHz,
CDCl3) d 2.25 (m, 1H), 2.51 (m, 1H), 2.89 (ddd, J = 16.1, 8.5,
3.5 Hz, 1H), 3.15 (m, 1H), 5.71 (d, J = 6.0 Hz, 1H), 5.75 (d,
J = 6.0 Hz, 1H), 6.15 (dd, J = 6.7, 2.6 Hz, 1H), 7.24 (m, 1H), 7.28 (d,
J = 7.2 Hz, 1H), 7.32 (t, J = 7.2 Hz, 1H), 7.49 (d, J = 7.6 Hz, 1H); 13C
NMR (150 MHz, CDCl3) d 30.1, 32.1, 72.1, 83.6, 124.9, 125.8,
126.8, 129.6, 139.5, 144.7, 153.3.
4.3.29. Benzosuber-2-yl iodomethylcarbonate (36)
The title compound was prepared from 33 (0.55 g, 2.14 mmol,
1.0 equiv) according to general procedure C to afford a colorless so-
lid (0.57 g, 77%): Rf = 0.55 (20% EtOAc–hexanes); 1H NMR
(600 MHz, CDCl3) d 1.57 (m, 1H), 1.96 (m, 2H), 2.22 (m, 1H), 2.79
(t, J = 5.4 Hz, 2H), 3.05 (d, J = 14.1 Hz, 1H), 3.24 (dd, J = 14.1,
10.0 Hz, 1H), 4.75 (t, J = 9.4 Hz, 1H), 5.94 (s, 2H), 7.10 (d,
J = 7.0 Hz, 1H), 7.16 (m, 3H); 13C NMR (150 MHz, CDCl3) d 24.3,
34.0, 35.2, 36.8, 41.0, 77.6, 126.5, 127.1, 128.9, 130.5, 135.1,
142.9, 152.3.
4.3.21. (R)-Indan-1-yl iodomethylcarbonate (13b)
This compound was prepared analogously to 13a, but using 9b
as the substrate to afford a yellow oil (1.72 g, 75%).
4.3.22. (S)-Tetral-1-yl iodomethylcarbonate (14a)
The title compound was prepared from 10a (1.6 g, 6.7 mmol,
1.0 equiv) according to general procedure C to afford a yellow oil
(1.8 g, 85%): Rf = 0.64 (20% EtOAc–hexanes); 1H NMR (600 MHz,

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4.3.30. Isopropoxycarbonyloxymethyl (1R,5S,6S)-2-{[(2S,4S)-2-
(N,N-dimethylcarbamoyl)pyrrolidin-4-yl]thio}-6-[(1R)-1-
hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylate (2)
A solution of meropenem (100 mg, 0.26 mmol, 1.0 equiv) and
Cs2CO3 (367 mg, 1.04 mmol, 4.0 equiv) in DMF (0.5 mL) was stirred
at 0 °C for 10 min. Next, 1-iodoethyl isopropyl carbonate (87 mg,
0.34 mmol, 1.3 equiv) was added dropwise and the reaction mix-
ture was stirred for 30 min at 0 °C. The reaction mixture was sub-
sequently evaporated to dryness under reduced pressure.
Purification by flash chromatography (10% MeOH–CH2Cl2 with 1%
NH4OH) afforded the title compound (75 mg, 64%) as an off-white
foam as a 1:1 mixture of diastereomers. The analytical data is for
the mixture: Rf = 0.23 (diastereomer 1) and 0.25 (diastereomer 2)
(10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR (600 MHz, CDCl3)
d 1.26 (d, J = 7.2 Hz, 3H), 1.28 (app t, J = 5.9 Hz, 3H), 1.29 (d,
J = 6.0 Hz, 3H), 1.32 (d, J = 6.0 Hz, 3H), 1.55 (app dd, J = 13.5,
5.3 Hz, 3H), 1.60 (m, 1H), 2.44 (m, 2H), 2.60 (dt, J = 13.6, 8.1 Hz,
1H), 2.95 (s, 3H), 2.99 (s, 3H), 3.06 (td, J = 11.3, 4.4 Hz, 1H), 3.24
(d, J = 6.5 Hz, 1H), 3.24 (m, 1H), 3.42 (m, 1H), 3.74 (m, 1H), 3.99
(m, 1H), 4.23 (m, 2H), 4.88 (hept, J = 6.2 Hz, 1H), 6.83 (q,
J = 5.4 Hz, 1H), 6.84 (q, J = 5.4 Hz); 13C NMR (150 MHz, CDCl3) d
17.3, 17.4, 19.9, 20.0, 21.9 (2C), 21.97, 22.00, 22.04, 22.1, 36.1,
36.5, 36.6, 37.0, 43.9, 44.0, 44.56, 44.63, 56.07, 56.12, 56.59,
56.65, 58.70, 58.72, 60.41, 60.45, 66.07, 66.12, 73.28, 73.31, 91.9,
92.3, 124.9, 125.0, 152.3, 152.4, 152.9, 153.0, 159.2, 159.3, 172.0,
172.1, 173.1, 173.2; HRMS (ESI+) calcd for C23H36N3O8S [M+H]+
514.2218, found: 514.2219 (error 0.2 ppm).
for C28H36N3O8S [M+H]+ 574.2218, found: 574.2221 (error
0.5 ppm).
4.3.33. (S)-Tetral-1-yloxycarbonyloxymethyl (1R,5S,6S)-2-
{[(2S,4S)-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-yl]thio}-6-
[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylate
(18)
Synthesized according to general procedure D employing 14a to
afford the title compound (87 mg, 57%) as a white solid: Rf = 0.35
(10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR (600 MHz, CDCl3)
d 1.24 (d, J = 7.6 Hz, 3H), 1.27 (d, J = 6.5 Hz, 3H), 1.54 (dt, J = 13.6,
7.0 Hz, 1H), 1.80 (m, 1H), 1.93 (m, 1H), 2.01 (td, J = 11.4, 2.4 Hz,
1H), 2.13 (m, 1H), 2.58 (dt, J = 13.8, 8.4 Hz, 1H), 2.72 (ddd,
J = 16.8, 9.0, 6.0 Hz, 1H), 2.84 (m, 1H), 2.92 (s, 3H), 2.96 (s, 3H),
3.05 (dd, J = 11.7, 3.5 Hz, 1H), 3.06 (m, 2H), 3.20 (dd, J = 12.0,
5.6 Hz, 1H), 3.23 (dd, J = 6.6, 1.8 Hz, 1H), 3.42 (m, 1H), 3.73 (m,
1H), 3.93 (t, J = 8.2 Hz, 1H), 4.19 (m, 1H), 4.24 (d, J = 9.4 Hz, 1H),
5.80 (d, J = 5.9 Hz, 1H), 5.85 (t, J = 4.2 Hz, 1H), 5.92 (d, J = 5.3 Hz,
1H), 7.11 (d, J = 7.2 Hz, 1H), 7.17 (t, J = 7.2 Hz, 1H), 7.22 (t,
J = 7.2 Hz, 1H), 7.37 (d, J = 7.2 Hz, 1H); 13C NMR (150 MHz, CDCl3)
d 17.3, 19.0, 22.1, 29.2, 29.3, 36.1, 36.6, 36.9, 44.2, 44.6, 56.1,
56.6, 58.7, 60.4, 65.8, 75.8, 82.6, 124.4, 126.6, 129.0, 129.5, 130.2,
133.8, 138.6, 153.7, 154.2, 159.7, 172.2, 173.5; HRMS (ESI+) calcd
for
C
29H38N3O8S [M+H]+ 588.2374, found: 588.2379 (error
0.9 ppm).
4.3.34. (R)-Tetral-1-yloxycarbonyloxymethyl (1R,5S,6S)-2-
{[(2S,4S)-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-yl]thio}-6-
[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylate
(21)
4.3.31. (S)-Indan-1-yloxycarbonyloxymethyl (1R,5S,6S)-2-
{[(2S,4S)-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-yl]thio}-6-
[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylate
(17)
Synthesized according to general procedure D employing 14b to
afford the title compound (124 mg, 82%) as a white solid foam:
Rf = 0.29 (10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR
(600 MHz, CDCl3) d 1.23 (d, J = 7.0 Hz, 3H), 1.28 (d, J = 6.5 Hz, 3H),
1.55 (dt, J = 13.6, 7.0 Hz, 1H), 1.80 (m, 1H), 1.93 (m, 1H), 2.00 (m,
1H), 2.13 (m, 1H), 2.63 (dt, J = 13.8, 8.4 Hz, 1H), 2.72 (ddd,
J = 16.8, 9.0, 6.0 Hz, 1H), 2.84 (m, 1H), 2.92 (s, 3H), 2.96 (s, 3H),
3.05 (dd, J = 12.0, 3.8 Hz, 1H), 3.07 (m, 2H), 3.22 (dd, J = 6.6,
2.4 Hz, 1H), 3.28 (dd, J = 11.7, 5.9 Hz, 1H), 3.46 (q, J = 7.8 Hz, 1H),
3.76 (m, 1H), 4.03 (t, J = 8.2 Hz, 1H), 4.18 (m, 1H), 4.27 (dd,
J = 9.4, 1.2 Hz, 1H), 5.81 (d, J = 5.9 Hz, 1H), 5.85 (m, 1H), 5.91 (d,
J = 5.9 Hz, 1H), 7.11 (d, J = 7.6 Hz, 1H), 7.16 (t, J = 7.5 Hz, 1H), 7.22
(t, J = 7.2 Hz, 1H), 7.35 (d, J = 7.6 Hz, 1H); 13C NMR (150 MHz,
CDCl3) d 17.4, 19.0, 22.0, 29.2, 29.3, 36.1, 36.4, 36.9, 43.9, 44.6,
56.0, 56.7, 58.6, 60.5, 66.0, 75.9, 82.7, 124.5, 126.5, 129.0, 129.6,
130.2, 133.8, 138.7, 153.6, 154.1, 159.8, 171.9, 173.5; HRMS
(ESI+) calcd for C29H38N3O8S [M+H]+ 588.2374, found: 588.2378
(error 0.7 ppm).
Synthesized according to general procedure D employing 13a to
afford the title compound (112 mg, 75%) as a white solid: Rf = 0.19
(10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR (600 MHz, CDCl3) d
1.23 (d, J = 7.0 Hz, 3H), 1.29 (d, J = 5.9 Hz, 3H), 1.59 (m, 1H), 2.20
(m, 1H), 2.48 (td, J = 15.0, 7.2 Hz, 1H), 2.71 (dt, J = 13.2, 7.8 Hz,
1H), 2.87 (m, 1H), 2.92 (s, 3H), 2.97 (s, 3H), 3.10 (m, 2H), 3.24
(dd, J = 6.6, 1.8 Hz, 1H), 3.49 (dd, J = 11.4, 5.6 Hz, 1H), 3.57 (m,
1H), 3.83 (m, 1H), 4.19 (t, J = 6.6 Hz, 1H), 4.27 (t, J = 8.2 Hz, 1H),
4.35 (d, J = 9.4 Hz, 1H), 5.79 (d, J = 5.9 Hz, 1H), 5.90 (d, J = 5.9 Hz,
1H), 6.11 (dd, J = 6.5, 2.9 Hz, 1H), 7.21 (t, J = 6.9 Hz, 1H), 7.29 (m,
2H), 7.47 (d, J = 7.6 Hz, 1H); 13C NMR (150 MHz, CDCl3) d 17.5,
22.0, 30.6, 32.5, 36.0, 36.2, 37.1, 43.2, 44.6, 55.5, 56.9, 58.5, 60.5,
66.1, 82.7, 83.7, 124.6, 125.3, 126.3, 127.2, 129.9, 140.3, 145.3,
153.2, 154.2, 159.9, 171.1, 173.5; HRMS (ESI+) calcd for C28H36N3-
O8S [M+H]+ 574.2218, found: 574.2224 (error 1.0 ppm).
4.3.32. (R)-Indan-1-yloxycarbonyloxymethyl (1R,5S,6S)-2-
{[(2S,4S)-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-yl]thio}-6-
[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylate
(20)
4.3.35. (S)-Benzosuber-1-yloxycarbonyloxymethyl (1R,5S,6S)-2-
{[(2S,4S)-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-yl]thio}-6-
[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylate
(19)
Synthesized according to general procedure D employing 13b to
afford the title compound (108 mg, 72%) as a white foam: Rf = 0.24
(10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR (600 MHz, CDCl3) d
1.22 (d, J = 7.0 Hz, 3H), 1.29 (d, J = 5.9 Hz, 3H), 1.59 (dt, J = 13.8,
7.2 Hz, 1H), 2.21 (m, 1H), 2.48 (m, 1H), 2.71 (dt, J = 13.2, 8.1 Hz,
1H), 2.85 (m, 1H), 2.91 (s, 3H), 2.97 (s, 3H), 3.09 (m, 2H), 3.23
(dd, J = 6.0, 2.4 Hz, 1H), 3.46 (dd, J = 11.4, 5.6 Hz, 1H), 3.56 (m,
1H), 3.82 (m, 1H), 3.98 (m, 2H), 4.19 (m, 1H), 4.25 (t, J = 8.2 Hz,
1H), 4.34 (dd, J = 9.1, 1.5 Hz, 1H), 5.82 (d, J = 5.9 Hz, 1H), 5.88 (d,
J = 5.9 Hz, 1H), 6.10 (dd, J = 6.5, 2.9 Hz, 1H), 7.21 (t, J = 6.9 Hz,
1H), 7.28 (m, 2H), 7.46 (d, J = 7.6 Hz, 1H); 13C NMR (150 MHz,
CDCl3) d 17.4, 22.0, 30.6, 32.6, 36.1, 36.2, 37.1, 43.3, 44.6, 55.6,
56.9, 58.5, 60.5, 66.1, 82.7, 83.7, 124.6, 125.4, 126.2, 127.2, 129.9,
140.3, 145.4, 153.3, 154.2, 159.9, 171.2, 173.5; HRMS (ESI+) calcd
Synthesized according to general procedure D employing 15a to
afford the title compound (131 mg, 84%) as a white foam: Rf = 0.32
(10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR (600 MHz, CDCl3) d
1.25 (d, J = 7.0 Hz, 3H), 1.30 (d, J = 6.5 Hz, 3H), 1.58 (dt, J = 13.6,
7.0 Hz, 2H), 1.68 (m, 1H), 1.83 (m, 1H), 1.96 (m, 1H), 2.01 (m,
2H), 2.62 (m, 1H), 2.73 (dd, J = 13.8, 9.6 Hz, 1H), 2.94 (s, 3H), 2.95
(m, 1H), 2.98 (s, 3H), 3.04 (dd, J = 11.7, 4.1 Hz, 1H), 3.20 (m, 2H),
3.23 (d, J = 6.5 Hz, 1H), 3.30 (dd, J = 11.4, 5.6 Hz, 1H), 3.47 (m,
1H), 3.75 (m, 1H), 4.06 (t, J = 7.5 Hz, 1H), 4.21 (m, 1H), 4.28 (d,
J = 9.4 Hz, 1H), 5.80 (m, 2H), 5.89 (d, J = 5.9 Hz, 1H), 7.11 (d,
J = 7.6 Hz, 1H), 7.16 (m, 2H), 7.29 (m, 1H); 13C NMR (150 MHz,
CDCl3) d 17.4, 22.0, 27.5, 28.1, 33.7, 36.0, 36.1, 36.3, 37.0, 43.7,
44.6, 55.8, 56.8, 58.5, 60.4, 65.9, 81.6, 82.8, 124.4, 126.5, 128.4,

A. M. Teitelbaum et al. / Bioorg. Med. Chem. 21 (2013) 5605–5617
5615
130.3, 139.7, 141.8, 153.6, 153.7, 159.8, 171.7, 173.6 (missing 1
aryl C); HRMS (ESI+) calcd for C30H40N3O8S [M+H]+ 602.2531,
found 602.2537 (error 1.0 ppm).
3H), 1.60 (m, 1H), 1.68 (m, 1H), 1.80 (s, 1H), 1.84 (m, 2H), 1.96
(m, 1H), 2.01 (s, 3H), 2.01 (m, 1H), 2.67 (dt, J = 14.4, 7.8 Hz, 1H),
2.72 (dd, J = 13.5, 10.6 Hz, 1H), 2.90 (s, 3H), 2.96 (t, J = 11.7 Hz,
1H), 3.07 (s, 3H), 3.20 (m, 1H), 3.24 (dd, J = 6.5, 2.3 Hz, 1H), 3.42
(m, 1H), 3.51 (t, J = 10.0 Hz, 1H), 3.70 (m, 1H), 3.98 (dd, J = 10.0,
7.6 Hz, 1H), 4.17 (m, 1H), 4.25 (dd, J = 9.0, 1.8 Hz, 1H), 4.78 (t,
J = 8.2 Hz, 1H), 5.79 (d, J = 6.5 Hz, 1H), 5.82 (d, J = 5.9 Hz, 1H),
5.89 (d, J = 5.9 Hz, 1H), 7.11 (m, 1H), 7.16 (m, 2H), 7.28 (m, 1H);
13C NMR (150 MHz, CDCl3) d 17.5, 22.1, 22.7, 27.5, 28.1, 33.7,
35.5, 36.0, 36.3, 37.4, 41.6, 44.6, 56.1, 56.6, 58.1, 60.7, 66.1, 81.7,
82.8, 125.2, 126.6, 128.4, 130.3, 139.7, 142.0, 151.4, 153.6, 159.7,
168.9, 171.3, 173.6 (missing 1 aryl C); HRMS (ESI+) calcd for
4.3.36. (R)-Benzosuber-1-yloxycarbonyloxymethyl (1R,5S,6S)-2-
{[(2S,4S)-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-yl]thio}-6-
[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylate
(22)
Synthesized according to general procedure D employing 15b to
afford the title compound (123 mg, 79%) as a white solid foam:
Rf = 0.24 (10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR
(600 MHz, CDCl3) d 1.23 (d, J = 7.0 Hz, 3H), 1.27 (d, J = 5.9 Hz, 3H),
1.55 (m, 2H), 1.66 (m, 1H), 1.83 (m, 1H), 1.93 (m, 1H), 2.00 (m,
2H), 2.66 (m, 1H), 2.72 (dd, J = 13.8, 9.6 Hz, 1H), 2.92 (s, 3H), 2.97
(s, 3H), 3.05 (dd, J = 12.0, 4.4 Hz, 1H), 3.22 (d, J = 5.3 Hz, 1H), 3.35
(dd, J = 11.4, 5.6 Hz, 1H), 3.52 (m, 1H), 3.78 (m, 1H), 3.89 (m, 3H),
4.12 (t, J = 8.2 Hz, 1H), 4.17 (t, J = 6.0 Hz, 1H), 4.30 (d, J = 8.8 Hz,
1H), 5.80 (d, J = 8.2 Hz, 1H), 5.85 (s, 2H), 7.10 (d, J = 6.5 Hz, 1H),
7.15 (m, 2H), 7.28 (d, J = 7.0 Hz, 1H); 13C NMR (150 MHz, CDCl3)
d 17.4, 22.0, 27.4, 28.1, 33.6, 36.0, 36.1, 36.3, 37.0, 43.6, 44.6,
55.8, 56.8, 58.5, 60.4, 66.0, 81.7, 82.6, 124.4, 126.5, 128.4, 130.3,
139.6, 142.0, 153.6, 153.7, 159.8, 171.7, 173.6 (missing 1 aryl C);
HRMS (ESI+) calcd for C30H40N3O8S [M+H]+ 602.2531, found
602.2529 (error 0.3 ppm).
C
32H42N3O9S [M+H]+ 644.2636, found 644.2633 (error 0.5 ppm).
4.3.39. Indan-2-yloxycarbonyloxymethyl (1R,5S,6S)-2-{[(2S,4S)-
2-(N,N-dimethylcarbamoyl)pyrrolidin-4-yl]thio}-6-[(1R)-1-
hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylate (37)
Synthesized according to general procedure D employing 34 to
afford the title compound (107 mg, 71%) as an off-white foam:
Rf = 0.26 (10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR
(600 MHz, CD2Cl2) d 1.25 (d, J = 7.6 Hz, 3H), 1.29 (d, J = 5.9 Hz,
3H), 1.55 (dt, J = 13.5, 6.7 Hz, 1H), 2.58 (dt, J = 14.4, 7.2 Hz, 1H),
2.94 (s, 3H), 2.96 (m, 1H), 2.97 (s, 3H), 3.06 (m, 2H), 3.13 (m,
2H), 3.19 (dd, J = 11.7, 5.3 Hz, 1H), 3.24 (d, J = 5.9 Hz, 1H), 3.33
(m, 2H), 3.41 (m, 1H), 3.74 (m, 1H), 3.92 (t, J = 7.9 Hz, 1H), 4.20
(t, J = 6.2 Hz, 1H), 4.24 (d, J = 9.4 Hz, 1H), 5.47 (m, 1H), 5.79 (d,
J = 5.9 Hz, 1H), 5.88 (d, J = 5.9 Hz, 1H), 7.17 (m, 2H), 7.24 (m,
2H))H); 13C NMR (150 MHz, CD2Cl2) d 17.3, 22.1, 36.1, 36.7, 36.9,
39.8, 44.3, 44.7, 56.3, 56.6, 58.8, 60.4, 65.9, 80.7, 82.5, 124.4,
125.1, 127.3, 140.5, 153.8, 154.1, 159.7, 172.2, 173.4; HRMS
(ESI+) calcd for C28H36N3O8S [M+H]+ 574.2218, found: 574.2214
(error 0.7 ppm).
4.3.37. (1S,10S/R)-Benzosuber-1-yloxycarbonyloxyethyl
(1R,5S,6S)-2-{[(2S,4S)-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-
yl]thio}-6-[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-
carboxylate (23a)
Synthesized according to general procedure D employing 16a
to afford the title compound (69 mg, 43%) as a 1:1 mixture of 2
diastereomers: Rf = 0.29 and 0.32 (10% MeOH–CH2Cl2 with 1%
NH4OH); 1H NMR (600 MHz, CDCl3) d 1.22 (d, J = 7.0 Hz, 1.5H),
1.25 (d, J = 7.0 Hz, 1.5H), 12.7 (d, J = 7.2 Hz, 1.5H), 1.29 (d,
J = 7.2 Hz, 1.5H), 1.53 (m, 1H), 1.54 (d, J = 5.3 Hz, 1.5H), 1.60 (d,
J = 5.3 Hz, 1.5H), 1.70 (m, 1H), 1.83 (m, 1H), 1.97 (m, 2H), 2.01
(m, 1H), 2.56 (m, 1H), 2.73 (m, 2H), 2.79 (m, 2H), 2.93 (d,
J = 4.1 Hz, 3H), 2.95 (m, 0.5H), 2.96 (d, J = 5.3 Hz, 3H), 3.03 (td,
J = 11.4, 4.2 Hz, 1H), 3.06 (m, 0.5H), 3.17 (dt, J = 6.0, 5.4 Hz, 1H),
3.21 (ddd, J = 15.6, 6.0, 2.4 Hz, 1H), 3.38 (m, 1H), 3.72 (m, 1H),
3.91 (m, 1H), 4.18 (m, 1H), 4.21 (td, J = 9.6, 2.4 Hz, 1H), 5.78 (m,
1H), 6.84 (m, 1H), 7.10 (m, 1H), 7.16 (m, 2H), 7.28 (m, 1H); 13C
NMR (150 MHz, CDCl3) d 17.3, 19.9, 20.0, 22.1, 27.7, 28.17,
28.23, 33.8, 33.9, 36.06, 36.11, 36.7, 36.8, 36.9, 44.27, 44.29,
44.6, 44.7, 56.3, 56.4, 56.51, 56.53, 58.9, 60.3, 60.4, 66.1, 81.2,
81.3, 92.1, 92.5, 124.7, 124.9, 126.5, 126.7, 128.3, 128.4, 130.3,
152.5, 152.6, 152.8, 153.0, 159.1, 159.2, 172.2, 172.3, 173.0,
173.1; HRMS (ESI+) calcd for C31H42N3O8S [M+H]+ 616.2687, found
616.2691 (error 0.7 ppm).
4.3.40. (2S/R)-Tetral-2-yloxycarbonyloxymethyl (1R,5S,6S)-2-
{[(2S,4S)-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-yl]thio}-6-
[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-carboxylate
(38)
Synthesized according to general procedure D employing 35 to
afford the title compound (127 mg, 82%) as an off-white foam as a
mixture of diastereomers (epimeric at the C-2 indanyl carbon):
Rf = 0.25 and 0.30 (10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR
(600 MHz, CD2Cl2) d 1.25 (d, J = 7.0 Hz, 3H), 1.28 (d, J = 6.5 Hz,
3H), 1.55 (dt, J = 13.5, 6.7 Hz, 1H), 2.03 (m, 1H), 2.10 (m, 1H),
2.61 (dt, J = 13.8, 8.1 Hz, 1H), 2.84 (m, 1H), 2.93 (s, 3H), 2.95 (m,
2H), 2.97 (s, 3H), 3.06 (dd, J = 11.7, 2.9 Hz, 1H), 3.19 (m, 3H), 3.44
(m, 1H), 3.54 (m, 2H), 3.75 (m, 1H), 3.95 (t, J = 7.9 Hz, 1H), 4.18
(t, J = 6.0 Hz, 1H), 4.25 (d, J = 9.4 Hz, 1H), 5.13 (m, 1H), 5.80 (t,
J = 5.3 Hz, 1H), 5.89 (t, J = 4.7 Hz, 1H), 7.10 (m, 4H); 13C NMR
(150 MHz, CD2Cl2) d 17.3, 22.1, 26.6, 28.0, 34.7, 36.0, 36.6, 36.8,
44.1, 44.6, 54.4, 56.1, 56.5, 58.6, 60.4, 65.7, 75.3, 82.5, 124.3,
126.3, 126.5, 129.0, 129.7, 133.6, 135.8, 153.8, 159.7, 172.1,
173.6; HRMS (ESI+) calcd for C29H38N3O8S [M+H]+ 588.2374,
found: 588.2369 (error 0.9 ppm).
4.3.38. (S)-Benzosuber-1-yloxycarbonyloxymethyl (1R,5S,6S)-2-
{[(2S,4S)-1-acetyl-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-
yl]thio}-6-[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-
carboxylate (24a)
A solution of meropenem (100 mg, 1.0 equiv) and Cs2CO3
(367 mg, 1.04 mmol 4.0 equiv) in DMF (1.3 mL) was stirred at
0 °C for 10 min. Next, acetic anhydride (30 mg, 0.29 mmol,
1.1 equiv) was added and the reaction mixture was stirred for
1 h until starting material was consumed by TLC. Subsequently,
(S)-1-benzosuberol iodomethylcarbonate 15a (200 mg, 1.73 mmol,
1.57 equiv) in DMF (1.0 mL) was added dropwise to the reaction
mixture. After 1 h the reaction was concentrated in vacuo and
the residue was purified by flash chromatography (10% MeOH–
CH2Cl2 with 1% NH4OH) to afford the title compound (104 mg,
62%): Rf = 0.25 (10% MeOH–CH2Cl2 with 1% NH4OH); 1H NMR
(600 MHz, CD2Cl2) d 1.25 (d, J = 7.0 Hz, 3H), 1.28 (d, J = 5.9 Hz,
4.3.41. (2S/R)-Benzosuber-2-yloxycarbonyloxymethyl
(1R,5S,6S)-2-{[(2S,4S)-2-(N,N-dimethylcarbamoyl)pyrrolidin-4-
yl]thio}-6-[(1R)-1-hydroxyethyl]-1-methylcarbapen-2-em-3-
carboxylate (39)
Synthesized according to general procedure D employing 36 to
afford the title compound (110 mg, 65%) as an off-white foam as a
mixture of diastereomers (epimeric at the C-2 benzosuberyl car-
bon): Rf = 0.25 and 0.27 (10% MeOH–CH2Cl2 with NH4OH); 1H
NMR (600 MHz, CD2Cl2) d 1.26 (d, J = 7.0 Hz, 3H), 1.28 (app dd,
J = 6.2, 2.6 Hz, 3H), 1.54 (m, 2H), 1.93 (m, 2H), 2.18 (m, 1H), 2.59
(dt, J = 13.5, 8.2 Hz, 1H), 2.77 (m, 2H), 2.94 (s, 3H), 2.97 (s, 3H),
3.06 (m, 4H), 3.19 (m, 2H), 3.24 (dd, J = 6.5, 1.8 Hz, 1H), 3.41 (dt,

5616
A. M. Teitelbaum et al. / Bioorg. Med. Chem. 21 (2013) 5605–5617
J = 16.2, 7.5 Hz, 1H), 3.74 (m, 1H), 3.92 (t, J = 7.9 Hz, 1H), 4.20 (pent,
J = 6.5, 1.8 Hz, 1H), 4.24 (dd, J = 9.1, 2.1 Hz, 1H), 4.68 (t, J = 8.5 Hz,
1H), 5.78 (app dd, J = 7.0, 5.9 Hz, 1H), 5.87 (app dd, J = 8.2, 5.9 Hz,
1H), 7.13 (m, 4H); 13C NMR (150 MHz, CD2Cl2) d 17.4, 22.1, 25.1,
35.7, 36.1, 36.7, 36.9, 37.3, 41.6, 44.2, 44.7, 56.3, 56.7, 58.8, 60.4,
66.0, 77.7, 82.5, 124.5, 126.9, 127.4, 129.4, 130.9, 136.3, 143.8,
153.6, 153.7, 159.8, 172.2, 173.4; HRMS (ESI+) calcd for C30H40N3-
O8S [M+H]+ 602.2531, found 602.2530 (error 0.2 ppm).
4.6. Plasma stability
The plasma stability of selected prodrugs was investigated with
Dunkin Hartley female pooled guinea pig plasma (BioChemed,
Winchester, VA). In triplicate, plasma (1.0 mL) was pre-incubated
for 2 min at 37 °C followed by the addition of 20
stock solution of prodrug. Aliquots (100 L) were withdrawn
lL of a 5.0 mM
l
at 0, 2.5, 5, 10, 15, and 30 min and immediately quenched
with an equal volume of acetonitrile to precipitate the proteins.
4.4. High performance liquid chromatography
The samples were filtered through nylon 0.2
lM spin filters
(Chrom Tech, Inc., Apple Valley, Minnesota) and 100
l
L of the
Solvents and reagents for liquid chromatography included HPLC
grade methanol, isopropanol, and n-heptanes, (Sigma–Aldrich, St.
Louis, MO), ammonium acetate (Fisher Scientific, Pittsburgh, PA),
and acetic acid (Pharmco, Brookfield, CT).
filtrate was injected onto the HPLC. The stability of the prodrugs
was determined by calculating the half-lives of the parent
compounds.
Acknowledgments
4.4.1. Reversed-phase HPLC
The HPLC system was composed of an Agilent 1100 Liquid Chro-
matograph (Agilent Technologies, Santa Clara, CA), a Phenomenex
The research was supported by the National Institute of Health
grant R21AI090147 to R.P.R. We thank Kathryn Nelson for carefully
proofreading this manuscript.
Gemini-NX 150 ꢂ 4.6 mm, 5
lm reversed-phase column (Phenom-
enex, Torrence, CA), and a Shimadzu UV/Vis detector (Shimadzu,
Kyoto, Japan) set at 300 nm. Ammonium acetate (25 mM, pH 4.8)
and methanol were the aqueous (A) and organic (B) components
of the mobile phase. Meropenem, and prodrugs eluted at 4.3, 5.7,
and 7.5–8.5 min, respectively, with the following step gradient elu-
tion: 15% B over 5.0 min, 95% B from 5.0 to 8.5 min, and re-equili-
bration at 15% B from 8.5 to 10.0 min prior to the next injection.
The detection wavelength was set at 300 nm and the flow rate
was 1.0 mL/min.
References and notes
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4.5. Prodrug stability
Reagents for prodrug stability included hydrochloric acid
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20 L of a 5.0 mM DMSO stock solution of meropenem or prodrug.
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removed from the incubation solution at 0, 5, 10, 15, 30, 45, 60, and
120 min, and immediately injected (85 L) onto the HPLC. Stability
lM) were incubated at
l
l
l
l
was determined by calculating the half-life of the parent com-
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