Substituted 2,5-diazabicyclo[4.1.0]heptanes and their application as general piperazine surrogates: synthesis and biological activity of a Ciprofloxacin analogue

Article abstract of DOI:10.1016/j.tet.2010.02.046

Piperazines and modified piperazines, such as homopiperazines and 2-methylpiperazines, are found in a wide range of pharmaceutical substances and biologically active molecules. In this study 2,5-diazabicyclo[4.1.0]heptanes, in which a cyclopropane ring is

Full text of DOI:10.1016/j.tet.2010.02.046

Tetrahedron 66 (2010) 3370–3377
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Substituted 2,5-diazabicyclo[4.1.0]heptanes and their application as general
piperazine surrogates: synthesis and biological activity of a Ciprofloxacin
analogue
Rivka R.R. Taylor a, Heather C. Twin a, Wendy W. Wen a, Rebecca J. Mallot b, Alan J. Lough a,
,
Scott D. Gray-Owen b, Robert A. Bateya
*
a Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5R 3H6
b Medical Sciences Building, University of Toronto, 1 King’s College Circle, Toronto, Ontario, Canada M5S 1A8
a r t i c l e i n f o
a b s t r a c t
Article history:
Piperazines and modified piperazines, such as homopiperazines and 2-methylpiperazines, are found in
a wide range of pharmaceutical substances and biologically active molecules. In this study 2,5-
diazabicyclo[4.1.0]heptanes, in which a cyclopropane ring is fused onto a piperazine ring, are described as
modified piperazine analogues. Differentially N,N0-disubstituted and N-monosubstituted compounds can
be readily prepared from 2-ketopiperazine in a few steps, using a Simmons–Smith reaction of 1,2,3,4-
tetrahydropyrazines with diethylzinc and diiodomethane for the key cyclopropane ring formation. An
analogue of the fluoroquinolone antibacterial Ciprofloxacin was synthesized using a palladium-catalyzed
Buchwald–Hartwig cross-coupling to attach the diazabicyclo[4.1.0]heptane core to the 7-position of the
fluoroquinolone core. The resultant analogue was demonstrated to have similar antibacterial activity
to the parent drug Ciprofloxacin. X-ray crystallographic analysis of this analogue reveals a distorted
piperazine ring in the diazabicyclo[4.1.0]heptane core. The pKa of the conjugate acid of N-Cbz-monop-
rotected 2,5-diazabicyclo[4.1.0]heptane was determined to be 6.74ꢀ0.05, which is 1.3 pKa units lower
than the corresponding N-Cbz-monoprotected piperazine compound. The lower basicity of dia-
zabicyclo[4.1.0]heptanes is due to the electron-withdrawing character of the adjacent cyclopropane
rings. The modified physicochemical and structural properties of diazabicyclo[4.1.0]heptanes relative to
piperazines are expected to lead to interesting changes in the pharmacokinetic and biological activity
profile of these molecules.
Received 23 December 2009
Received in revised form 7 February 2010
Accepted 8 February 2010
Available online 13 February 2010
Keywords:
Piperazine heterocycle analogues
2,5-Diazabicyclo[4.1.0]heptanes
Cyclopropanation
Enamide
Quinolone antibiotic
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
must not add significantly to the MWof the system or introduce other
structural features that might lead to physicochemical or ADMET
Nitrogen heterocycles are found in the majority of clinically
approved small molecule pharmaceuticals. A few privileged hetero-
cyclic scaffolds, such as pyridines, piperidines, piperazines,
problems. This notion is reinforced by well-known practical medic-
inal chemistry guidelines such as Lipinski’s rules.3
One important pharmacologically relevant heterocycle is the
piperazine ring system 1, which is found in roughly 7% of all small
molecule therapeutics approved by the FDA between the years 1998
and 2009 (Fig. 1).4 Of these piperazine-containing therapeutics,
roughly 25% contained modified piperazine cores. The piperazine
ring is used as both a rigid three-dimensional diamine core struc-
ture, as well as a modifying group to introduce an amino residue in
pyrrolidines, imidazoles, pyrazoles, indoles, b-lactams, etc., dominate
in most cases. There is general recognition of the need for the de-
velopment of both new heterocyclic scaffolds,1 as well as approaches
to modify existing scaffolds in novel ways to confer desirable bi-
ological and pharmacological properties. The goal of modifying
existing heterocyclic scaffolds is particularly challenging, however,
since most modifications add molecular weight (MW), which often
results in concomitant undesirable changes in physicochemical and
ADMET behaviour.2 Therefore for a novel structurally modified het-
erocyclic scaffold to have utility in a medicinal chemistry program, it
* Corresponding author. Tel.: þ1 416 978 5059; fax: þ1 416 978 6033; e-mail
address: rbatey@chem.utoronto.ca.
Figure 1. Piperazine and piperazine analogue core structures.
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.02.046

R.R.R. Taylor et al. / Tetrahedron 66 (2010) 3370–3377
3371
molecules. The latter approach is often taken in order to improve
solubility or achieve changes in the ionization state of a potential
drug. Structurally modified versions of the piperazine core structure
found in pharmaceuticals are mainly limited to the ring-expanded
homopiperazine ring system 2 and the 2-methylpiperazine ring
system 3. We now report the synthesis and use of 2,5-diaza-
bicyclo[4.1.0]heptanes 4, which are minimally modified analogues
of the piperazine ring system possessing an embedded fused
cyclopropane ring. In addition, we demonstrate the utility of this
piperazine surrogate in an analogue of the antibiotic Ciprofloxacin.
Introduction of a fused cyclopropane ring onto the piperazine
ring, as for 2,5-diazabicyclo[4.1.0]heptanes 4, provides a novel pi-
perazine core with only a modest increase in molecular weight. The
presence of the cyclopropane ring in 4 would be expected to im-
pose additional conformational constraints, relative to 1, albeit with
the introduction of two new chiral centres (for R1sR2). Amino-
cyclopropanes are known to possess rigid structures, and have
found extensive application in the synthesis of conformationally
restricted polyamines and peptidomimetics.5 More specifically 4
incorporates a cis-substituted 1,2-diaminocyclopropane function-
ality, a structural motif that is found in a number of biologically
active molecules.6 Thus we envisaged that the 2,5-diazabicyclo-
[4.1.0]heptane ring system could be an attractive medicinal chemistry
target, combining several biologically active elements within a highly
compact, low molecular weight core.
differentially protected 2,5-diazabicyclo[4.1.0]heptane 10 in 70%
yield. Typical procedures for Fmoc deprotection of 10, involving
treatment of the protected compound with an excess of piperidine in
a polar solvent such as DMF, complicated the isolation of the desired
free base 5 from the reaction mixture. Instead, deprotection of 10
using catalytic DBU with octanethiol as a scavenging agent,11
followed by rapid column chromatography afforded 5 in 80% iso-
lated yield. Generally this compound was freshly prepared from the
stable, fully protected precursor 10 immediately prior to its use, as it
degraded slowly under ambient conditions.
Beyond this intriguing combination of structural features, the
presence of a fused cyclopropane ring in 4 would be further
expected to alter physicochemical and pharmacokinetic behaviour
relative to the corresponding piperazines. In particular, the en-
hanced s-character of the peripheral cyclopropane bonds should
reduce electron density at the ring-nitrogen atoms in 4 through
inductive effects, leading to decreased basicity or lower pKa’s of the
conjugate acids (ammonium salts). The additional aliphatic sub-
stitution imparted by the cyclopropane ring would be expected to
modestly increase lipophilicity. Since the acid dissociation constant
and lipophilicity figure strongly in determining a drug’s pharma-
cokinetic properties, analogues based on 4 could have altered
membrane permeability, distribution behaviour, etc.
Scheme 1. Synthesis of Boc-protected 2,5-diazabicyclo[4.1.0]heptane 5.
A similar protocol was used for the synthesis of the Cbz-pro-
tected compound. Thus, reaction of 2-ketopiperazine with benzyl
chloroformate and potassium carbonate using biphasic conditions
afforded 11 in 93% yield (Scheme 2). Installation of a Boc group
using triethylamine and DMAP in the presence of Boc anhydride
gave 12 in 89% yield. DIBAL reduction proceeded smoothly to give
lactamol 13, which was used in the subsequent reaction without
further purification. As for the reduction of 7 to 8 above, an expe-
dient quench and workup was necessary to minimize
decomposition of lactamol 13.
2. Results and discussion
The approach chosen for the synthesis of the 2,5-diaza-
bicyclo[4.1.0]heptane core7,8 was based upon a cyclopropanation of
a 1,2,3,4-tetrahydropyrazine, as exemplified in the preparation of the
mono-protected compound 5 (Scheme 1). Reaction of 2-ketopiper-
azine9 with Fmoc-o-succinamide and potassium carbonate under
biphasic conditions furnished Fmoc-2-ketopiperazine 6 in 94% yield.
Subsequent treatment with Boc anhydride and LHMDS at ꢁ78 ꢂC
furnished the diprotected compound 7 in 88% yield. Reaction at low
temperature using a strong sterically hindered base was required to
prevent premature Fmoc cleavage. Due to the presence of the Fmoc
protecting group, the remainder of the synthesis was achieved
without the use of bases. Reduction of 7 with DIBAL at ꢁ78 ꢂC
proceeded smoothly to give the protected hemiaminal 8 in near
quantitative yield. At low temperatures 7 had diminished solubility in
dichloromethane, and the best yields were attained when a 9:1
dichloromethane/THF solvent system was used. Standard workup
conditions for DIBAL reductions, which involve extended stirring
with Rochelle’s salt, were unsatisfactory for the isolation of 8, and an
expedient quench and workup were required to prevent
decomposition of the product. Treatment of 8 with trifluoroacetic
anhydride in THF at 0 ꢂC for 20 min furnished the desired protected
enamine 9 in 74% yield, via in situ formation of the activated tri-
fluoroacetate ester of 8, and subsequent trifluoroacetate induced
elimination. Finally, cyclopropanation of 9 using a Simmons–Smith
procedure with diethylzinc and diiodomethane,10 afforded the
Scheme 2. Synthesis of Cbz-protected 2,5-diazabicyclo[4.1.0]heptane 16.

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The elimination step proved to be more challenging for this
antibiotics, Ciprofloxacin enters the cerebrospinal fluid (CSF) via
a passive transport mechanism,25 and the distribution of such
drugs into the CSF is mediated largely by lipophilicity and pKa.
Therefore modifications to Ciprofloxacin that would increase its
lipophilicity and lower its pKa, could lead to an increase in its CSF
concentration, and perhaps improve its clinical efficacy in the
treatment of cerebrospinal infections such as meningitis.
The synthesis of 17 was achieved from commercially available 7-
chloro-1-cyclopropyl-6-fluoro-4-oxo-1,4,-dihydroquinoline-3-car-
boxylic acid 18 using an amination protocol as the key step (Scheme
3). Previous methods employed in the synthesis of Ciprofloxacin
from 18 involve SNAr conditions, and afford approximately 10% of the
undesired 6-substituted side product.26 The use of a boron chelate of
18 has been shown to afford the correct 7-isomer in higher yields
and regioselectivity.27 However, in our experience, this procedure
afforded trace amounts of the incorrect 6-substituted product as an
inseparable impurity. This impurity was unacceptable given the
necessity of the 7-fluoro substituent for good antibacterial activity.
substrate. The use of the previous conditions, treatment of 13 with
TFAA in THF, gave the desired enamine 14 in only 50% yield, and
attempts to improve the yields were unsuccessful (e.g., by altering
reaction temperature or the addition of bases such as triethylamine,
DIPEA, 2,6-lutidine and 2,5-lutidine). The best conditions that were
identified involved treatment of 13 with 4 equiv each of acetyl
chloride and Hu¨nig’s base in refluxing benzene for 1.5 h, to give an
improved 63% yield of 14. Cyclopropanation of 14 proceeded
smoothly to give 15 in 76% yield. The improved yield for this trans-
formation compared to the analogous one for 10 is perhaps as a re-
sult of the improved base stability of the Cbz compared to the Fmoc
protecting groups. Finally the Boc group was removed by treatment
of 15 with 4 equiv of anhydrous HCl to afford 16 as the hydrochloride
salt in 94% yield. One limitation with the current route is that the
Simmons–Smith reaction is not enantioselective, leading to racemic
products 10 and 15. Although there is considerable interest in the
development of chiral drugs as single enantiomers,12 in the current
study a racemic synthesis was sufficient in order to demonstrate the
utility of 2,5-diazabicyclo[4.1.0]heptanes as piperazine surrogates.
With an approach to 2,5-diazabicyclo[4.1.0]heptanes estab-
lished, we next wished to show their application in an analogue of
a known drug. The second-generation fluoroquinolone13 antibiotic
Ciprofloxacin14 was chosen as an appropriate target since it bears
the piperazine motif, and modifications of this residue are known
to be tolerated, as for example, in the analogues Trovafloxacin and
Enoxacin (Fig. 2). Ciprofloxacin shows activity against a variety of
Gram-positive and Gram-negative strains,15 and through time, has
maintained excellent activity against Gram-negative bacteria,16
although resistance has developed in a number of Gram-positive
strains.17 Ciprofloxacin is commonly prescribed for the treatment
of urinary tract infections, prostatitis and enteric typhoid fever,18
and it is currently the most effective treatment for anthrax in-
fections.19 It has also been used to treat difficult cases of neonatal
meningitis.20 Like other fluoroquinolones, the accepted mechanism
by which Ciprofloxacin exhibits antibacterial activity is by in-
teraction with DNA gyrase and topoisomerase.21 Structure–activity
studies have demonstrated that variation at the 7-position of flu-
oroquinolones influences potency and spectrum,22 and many ana-
logues have been prepared with different substituents at this site.
SAR studies have also demonstrated that the 6-fluoro substituent
greatly enhances antibacterial activity,23 while 8-aza analogues
display enhanced pharmacokinetics, although their QSAR is sub-
stantially different from that of true quinolones.24 The continued
appearance of antibiotic resistant bacteria, as well as the impor-
tance of modulating pKa and lipophilicity on pharmacological
properties, suggested that the Ciprofloxacin analogue 17 would be
a clinically relevant target. For example, like the other quinolone
Scheme 3. Synthesis of Ciprofloxacin analogue 17.
A further undesirable aspect of standard SNAr conditions is that
a large excess of the amine component is invariably required. In
contrast, metal catalyzed Buchwald–Hartwig cross-coupling re-
actions28 are selective for chlorinated sites, and 1 equiv of amine
is often sufficient. A palladium-catalyzed route was therefore
investigated for the key amine–quinolone coupling step. In-
terestingly, there is only one literature example of a Buchwald–
Hartwig coupling using 18 as a substrate,29 with a Pd2(dba)3/
BINAP catalyst system. For our studies the ethyl ester 19 was
employed due to its increased solubility. It was prepared in 70%
yield by treatment of 18 with iodoethane and potassium carbon-
ate (Scheme 3).
In an effort to improve the efficiency of the palladium-catalyzed
coupling step, optimization studies were performed between 19
and a slight excess of N-Boc piperazine (1.2 equiv). A number of
ligands including QUINAP, Dppf, PCy3, DavePhos, dpePhos and
Xphos were examined. Although dpePhos showed similar activity
to BINAP, no ligand assayed offered any significant improvement
over the literature procedure. Thus, reaction of N-Boc piperazine
(1.2 equiv) with 19 using Cs2CO3 (2 equiv), Pd2(dba)3 (2 mol %) and
BINAP (3 mol %) in DMF for 3 h gave the N-Boc protected ethyl ester
of Ciprofloxacin, in 73% yield. The reaction conditions were such
that there was no difference in yield using 1.2 and 3.0 equiv of the
piperazine nucleophile, and none of the undesired 6-substituted
side product was observed by 1H NMR spectra analysis of the crude
reaction mixtures, indicating excellent regioselectivity for
Figure 2. Representative fluoroquinolone antibiotics and 2,5-diazabicyclo[4.1.0]-
heptane analogue 17.

R.R.R. Taylor et al. / Tetrahedron 66 (2010) 3370–3377
3373
substitution at the 7-position. Application of these conditions to the
reaction of 19 with 5 at 140 ꢂC led to the desired product 20 in only
20% yield. 1H NMR analysis of the crude reaction mixture revealed
complete consumption of amine 5. Suspecting decomposition to be
the culprit for the diminished yield, the reaction was repeated at
a lower temperature. At 115 ꢂC the reaction took considerably
longer but after 16 h, 20 was obtained in 57% yield. Further low-
ering of the temperature did not improve the yield of 20, but in-
creased the reaction time substantially. Having accomplished the
key amination step, all that remained was to deprotect the ester
and Boc groups. Refluxing 20 in 2 M HCl for 2 h, achieved global
deprotection to give 17 as its hydrochloride salt in 80% yield.
The antibacterial activity of analogue 17 and Ciprofloxacin were
compared as minimum inhibitory concentration (MIC) and mini-
mum bactericidal concentration (MBC),30 against a number of
clinical isolates, representing both Gram-negative and Gram-
positive strains (Fig. 3).31 All of the strains tested were sensitive to
17 at an MIC equal to or slightly higher than Ciprofloxacin.
Streptococcus pneumoniae and Escherichia coli were the only species
that required up to four times the concentration of 17 in order to
inhibit growth, otherwise MICs were the same such as for
Staphylococcus aureus, or only two-times higher as for Neisseria
gonorrhoeae, Neisseria meningitidis and Haemophilus influenzae. For
bactericidal effects, higher concentrations of both Ciprofloxacin and
17 were required for S. pneumoniae, N. meningitidis and H. influenzae.
The MBC’s of 17 and Ciprofloxacin were very similar for S. aureus,
while the remainder of the species tested exhibited MBC’s for 17 that
were two to four times those of Ciprofloxacin. Even though the MIC
and MBC values for 17 were somewhat higher than for Ciprofloxacin,
they are within the reported range of MIC values of Ciprofloxacin
for the various species.32,33 These results indicate that 17 holds
a chiral heterocycle at position-7 show differences in activity, par-
ticularly when the chiral centre occurs alpha to a heteroatom.34 It is
therefore possible that the reduced antibacterial activity of racemic
17 compared to the parent Ciprofloxacin may be the result of dif-
ferences in activity between the enantiomeric forms of 17.
A more thorough knowledge of the structural and physico-
chemical parameters of the diazabicyclo[4.1.0]heptane system
would be useful for it to have general medicinal chemistry utility.
Representative structural data for this system were obtained from
an X-ray crystallographic analysis of 17 (Fig. 4). The 2,5-diaza-
bicyclo[4.1.0]heptane ring of 17 adopts a twist-like conformation,
where the cyclopropane ring is somewhat distorted, with the
exocyclic C–C bond proximal to the quaternary nitrogen being
shortened (1.46 Å) relative to the other exocyclic and endocyclic
bonds (1.50 Å). The presence of the cyclopropane forces the two
piperazine nitrogens to within a distance of 2.77 Å, which is shorter
than that for Ciprofloxacin monohydrate35 for which the piperazine
ring adopts a chair conformation with a distance of 2.86 Å between
the nitrogen atoms.
potential as an antimicrobial agent. Since
a racemic cyclo-
propanation strategy was used in the synthesis of the 2,5-diaza-
bicyclo[4.1.0]heptane core, the product 17 is racemic. It has been
demonstrated previously that enantiomeric quinolones possessing
Figure 4. X-ray crystallographic structural analysis of 17 with 50% thermal ellipsoids.
As expected, the hydrochloride salts of 2,5-diaza-
bicyclo[4.1.0]heptanes were more stable and easily stored than the
corresponding free bases. The pKa of 16 was determined to be
6.74ꢀ0.05 and that of N-Cbz piperazine$HCl to be 8.09ꢀ0.02,36
using a multicomponent NMR titration method described by
Perrin and Fabian.37,38 A pKa of 8.09ꢀ0.02 obtained for N-Cbz pi-
perazine is in accordance with the reported pKa of N-carbethoxy-
piperazine (8.28) and that of N-benzoylpiperazine (7.78).39 Our
results demonstrate that 16 is less basic than N-Cbz piperazine by
approximately 1.3 pKa units. This reduction in pKa can be attributed
both to the closer proximity of the ring-nitrogen atoms in 16, which
would lead to a greater electron-withdrawing through-space in-
teraction, and the electron-withdrawing effects of the cyclopro-
pane ring, mediated by the greater p-character of the cyclopropane
C–C bonds. The latter effect has been noted previously by Roberts
and Chambers,40 who observed a 1.15 pKa unit difference between
cyclopropylamine (pKa¼8.66) and cyclohexylamine (pKa¼9.81). A
similar reduction in pKa was noted for a cyclopropylamine, when
compared to an alkylamine situated within the same molecule.41
Another relevant comparison can be made with the pKa’s of Tro-
vafloxacin (pKa¼8.09),42 Enoxacin (pKa¼8.50) and Ciprofloxacin
(pKa¼8.62)43 where the proximal cyclopropane results in a reduced
pKa for Trovafloxacin.44 At physiological pH¼7.4,45 compound 16
would be expected to be approximately 83% in the neutral form,
whereas only 17% of N-Cbz piperazine would be in the neutral form
under these conditions. Given that passive transport across mem-
branes relies heavily on the charge state of a given substance, with
neutral species passing most easily, such alterations to the piper-
azine ring could be expected to display altered pharmacokinetic
behaviour.46
Figure 3. Minimum inhibitory concentrations (solid blocks), and minimum bacteri-
cidal concentrations (checkered box) for 17 and Ciprofloxacin against a number of
bacterial strains. Results are the consensus of three trials.

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R.R.R. Taylor et al. / Tetrahedron 66 (2010) 3370–3377
3. Conclusions
and K2CO3 (31.0 g, 239 mmol) in 420 mL of ethyl acetate/water
(2:1). The reaction mixture was stirred at room temperature for
4 h, until complete consumption of 2-ketopiperazine. The organic
phase was collected and the aqueous portion extracted with
ethyl acetate. After washing with brine, the combined organic
extracts were dried over MgSO4 and concentrated at reduced
pressure. Purification by silica gel column chromatography elut-
ing with ethyl acetate afforded 8.38 g (26.0 mmol, 94%) of 6 as
a white solid: mp¼150–151 ꢂC from ethyl acetate. IR (chloro-
form): 3222, 2549, 2889, 1700, 1680, 1476, 1447, 1362, 1333, 1281,
This study has demonstrated the utility of the 2,5-
diazabicyclo[4.1.0]heptane core as a piperazine surrogate, through
an examination of a Ciprofloxacin analogue 17, which shows com-
parable antibacterial activity to Ciprofloxacin. The formation of 17
required the use of differentially protected 2,5-diazabicyclo
[4.1.0]heptanes, which could be synthesized using a Simmons–
Smith cyclopropanation of a 1,2,3,4-tetrahydropyrazine as the key
step. A palladium-catalyzed Buchwald–Hartwig cross-coupling was
demonstrated to achieve regioselective attachment of the 2,5-
diazabicyclo[4.1.0]heptane ring to the fluoroquinolone core.
Further studies will be necessary to validate the generality of the
2,5-diazabicyclo[4.1.0]heptane core as a piperazine surrogate in
medicinal chemistry. It can be envisioned that this approach could
be employed in order to modulate physicochemical or pharmaco-
kinetic properties of piperazine-based systems. Notably the pKa of
the conjugate acid of 2,5-diazabicyclo[4.1.0]heptanes is lower than
comparable piperazines by about 1 pKa unit, an alteration achieved
with only a modest increase in MW. Further studies will be
required to develop an asymmetric route to the 2,5-diaza-
bicyclo[4.1.0]heptane core, such as through an enantioselective
cylopropanation strategy. Moreover the possibility of incorporating
additional substitution on the 2,5-diazabicyclo[4.1.0]heptane scaf-
fold, particularly at the exocyclic cyclopropane methylene position,
may be attractive given the subtly different ring conformation
expected relative to standard piperazines.
1232, 1128, 980, 761, 739, 621 cmꢁ1 1H NMR (400 MHz, CDCl3):
.
d
7.77 (2H, d, J¼7.5 Hz), 7.56 (2H, d, J¼7.5 Hz), 7.41 (2H, t,
J¼7.5 Hz), 7.41 (2H, dt, J¼7.5 Hz, J¼1.0 Hz), 6.77 (1H, br s), 4.49
(2H, br s), 4.25 (1H, t, J¼6.5 Hz), 4.10 (2H, s), 3.68–3.54 (2H, br
m), 3.38–3.27 (2H, br m). 13C NMR (100 MHz, CDCl3):
d 168.4,
168.0, 154.7, 143.8, 141.4, 127.9, 127.2, 125.0, 120.1, 67.9, 67.6, 47.3,
40.9, 40.3. MS (EI) m/z 322 (1, Mþ), 178 (100), 176 (14), 152 (8), 99
(3); HRMS (EI) calculated for [M]þ C19H18N2O3 322.1317;
observed 322.1314.
4.3. 4-(9H-Fluoren-9-yl)methyl 1-tert-butyl 2-oxopiperazine-
1,4-dicarboxylate 7
LHMDS (7.0 mL of 1.0 M solution in THF, 7.0 mmol) was added
to a solution of 6 (2.27 g, 7.02 mmol) in 100 mL of THF cooled to
ꢁ78 ꢂC. The reaction mixture was allowed to stir at ꢁ78 ꢂC for
20 min and Boc anhydride (2.30 g, 10.5 mmol) was added. After
1 h, TLC analysis indicated consumption of 6 and the reaction was
quenched with distilled water and warmed to room temperature.
The reaction mixture was extracted with dichloromethane,
washed with brine and dried over MgSO4. Evaporation of the
solvent and purification by silica gel column chromatography
eluting with 25% ethyl acetate in hexanes afforded 2.59 g
(6.13 mmol, 88%) of 7 as a white solid: mp¼109–110 ꢂC from
hexanes/ethyl acetate. IR (chloroform): 2987, 2891, 1778, 1712,
1472, 1446, 1424, 1365, 1297, 1237, 1153, 1118, 1049, 1029, 1004,
4. Experimental section
4.1. General
THF, ether and benzene were distilled over sodium under
nitrogen. Dichloromethane and ethylenediamine were distilled
over calcium hydride under nitrogen. All other solvents were
obtained from Sigma–Aldrich and were ACS grade or better. All
other reagents were obtained from Sigma–Aldrich and were used
as received. Melting points were determined on a Fisher-Johns
melting point apparatus and were uncorrected. Analytical TLC
was performed on silica coated aluminium plates (Alugram SIL, G/
UV254), Silicycle Inc. Spots were visualized with UV254 or ninhydrin.
Flash chromatography was performed using SiO2 60 Å, 230–400
mesh low acidity silica gel and was obtained from Silicycle Inc.
Eluting solvents were reagent grade and obtained from Fischer or
Caledon. IR spectra were determined on a Perkin Elmer Spectrum
1000 FT-IR spectrometer, and samples were prepared as either
a thin film on NaCl plates or as a KBr dispersion disc. NMR spectra
were determined on a Varian Mercury 400 MHz spectrometer.
Peaks were referenced to solvent residual peaks: CDCl3 7.27 ppm
proton, 77.23 ppm carbon; D2O 4.80 ppm proton, DMSO 2.50 ppm
proton, 39.51 ppm carbon, dioxane 66.5 ppm carbon, TMS
0 ppm carbon, proton or TSP 0 ppm carbon. High-resolution mass
spectra were obtained with an AB/Sciex QStar mass spectrometer.
X-ray crystal structures were obtained using a Nonius KAPPA-CCD
system. Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC 749764. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
þ44 (0)1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).
966, 943, 854, 761, 738 cmꢁ1 1H NMR (400 MHz, CDCl3):
. d 7.77
(2H, d, J¼7.5 Hz), 7.56 (2H, d, J¼7.0 Hz), 7.41 (2H, t, J¼7.5 Hz), 7.34
(2H, dt, J¼7.5, 1.0 Hz), 4.50 (2H, d, J¼5.5 Hz), 4.23 (1H, t, J¼7.0 Hz),
4.18–1.09 (2H, br m), 3.75–3.53 (4H, m), 1.55 (9H, s). 13C NMR
(100 MHz, CDCl3):
d 165.9, 165.6, 154.4, 151.4, 143.7, 141.4, 127.9,
127.2, 124.9, 120.2, 84.2, 67.9, 67.7, 49.0, 47.3, 44.2, 43.8, 41.8, 41.3,
28.2. MS (EI) m/z 423 (4), 422 (35, Mþ), 322 (4), 321 (3), 178 (100),
152 (3); HRMS (EI) calculated for [M]þ C24H26N2O5 422.1842;
observed 422.1842.
4.4. 4-(9H-Fluoren-9-yl)methyl 1-tert-butyl
2-hydroxypiperazine-1,4-dicarboxylate 8
DIBAL (10.0 mL of 1.0 M in hexanes, 10.0 mmol) was added
slowly to a solution of 7 (3.62 g, 8.57 mmol) in 90 mL CH2Cl2 and
10 mL of THF at ꢁ78 ꢂC. The reaction mixture was stirred at
ꢁ78 ꢂC for 2 h when an additional portion of DIBAL (2.6 mL of
1.0 M in hexanes, 2.6 mmol) was added. After an additional 2 h,
the reaction was quenched with saturated aqueous Rochelle’s salt
(100 mL) and warmed slowly to room temperature. The resulting
gel was filtered over a pad of Celite and washed with diethyl
ether. The organic phase was collected, and the aqueous phase
extracted with ether. The combined organic extracts were
washed with water and brine and then dried over MgSO4. After
concentration to a volume of 10 mL, the crude extracts were
passed through silica on Celite, eluting with ether. Evaporation of
the solvent afforded 3.59 g (8.47 mmol, 99%) of 8 as a white solid.
The product was used in the next step without further
purification.
4.2. (9H-Fluoren-9-yl)methyl 3-oxopiperazine-
1-carboxylate 6
Fmoc-OSu (11.3 g, 33.4 mmol) was added to a vigorously
stirred biphasic mixture of 2-ketopiperazine (2.78 g, 27.8 mmol)

R.R.R. Taylor et al. / Tetrahedron 66 (2010) 3370–3377
3375
4.5. 1-(9H-Fluoren-9-yl)methyl 4-tert-butyl
2,3-dihydropyrazine-1,4-dicarboxylate 9
28.5, 26.6, 26.3, 12.8. MS (EI) m/z 198 (2, Mþ), 141 (100), 125 (11), 97
(36), 86 (9), 84 (15), 70 (8), 68 (14), 57 (20); HRMS (EI) calculated for
[Mþ] C10H18N2O2 198.1368; observed 198.1360.
TFAA (1.2 mL, 8.8 mmol) was added to a solution of 8 (3.39 g,
8.00 mmol) in 90 mL THF all ready at 0 ꢂC. After stirring at 0 ꢂC for
20 min, the reaction was quenched with saturated aqueous NaHCO3
and warmed to room temperature. The resulting mixture was di-
luted with distilled water and dichloromethane, and extracted with
dichloromethane. The combined organic extracts were washed
with brine and then dried over MgSO4. Evaporation of the solvent
and purification by column chromatography eluting with 10% ethyl
acetate in hexanes afforded 2.40 g (5.93 mmol, 74%) of 9 as a white
solid: mp¼105–106 ꢂC from hexanes. IR (chloroform): 3063, 2977,
2932, 2891, 1702, 1674, 1449, 1419, 1381, 1365, 1348, 1277, 1226,
4.8. Benzyl 3-oxopiperazine-1-carboxylate 11
Benzyl chloroformate (9.73 g, 57.1 mmol) was added to 2-keto-
piperazine (4.80 g, 47.9 mmol) dissolved in 400 mL ethyl acetate and
200 mL water. The reaction mixture was stirred vigorously at room
temperature for 16 h. The organic phase was separated and collected,
and the aqueous phase extracted with ethyl acetate. The combined
organic extracts were washed with water and brine, and dried over
magnesium sulfate. Purification by column chromatography eluting
with 5% methanol in ethyl acetate afforded 10.5 g (44.8 mmol, 93%)
of 11 as a white solid: mp¼119–120 ꢂC from ethyl acetate. IR (chlo-
roform): 3478, 3235, 3063, 2942, 2881, 1704, 1671, 1431, 1335, 1234,
1171, 1115, 994, 867, 761, 738 cmꢁ1 1H NMR (400 MHz, CDCl3):
.
d
7.77 (2H, d, J¼7.5 Hz), 7.60–7.55 (2H, m), 7.41 (2H, t, J¼7.0 Hz), 7.32
(2H, dt, J¼7.5, 1.0 Hz), 6.38–6.10 (2H, m), 4.51–4.46 (2H, m), 4.28
1127, 977, 757, 696 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d 7.65–7.40 (1H,
(1H, t, J¼7.0 Hz), 3.74–3.65 (4H, m), 1.51 (9H, s). 13C NMR (100 MHz,
br s), 7.38–7.30 (5H, m), 5.16 (2H, s), 4.14 (2H, s), 3.68 (2H, t,
CDCl3):
d
152.3, 151.4, 143.7, 141.3, 127.8, 127.1, 125.0, 124.8, 120.1,
J¼5.0 Hz), 3.37 (2H, br s). 13C NMR (100 MHz, CDCl3):
d 168.3, 154.8,
109.7, 109.2, 107.6, 106.6, 81.3, 68.0, 67.8, 47.2, 41.8, 41.6, 41.1, 40.8,
40.4, 40.1, 28.3. MS (ESI) m/z 407 ([MþH]þ, 7%), 373 (5), 351 (11),
179 (100), 173 (9), 29(4); HRMS (ESI) calculated for [MþH]þ
C24H27N2O4 407.1965; observed 407.1967.
136.3, 128.8, 128.5, 128.3, 67.9, 47.5, 41.1, 40.4. MS (ESI) m/z 234 (16
(M)þ), 143 (3), 99þ(10), 91 (100), 77 (2), 65 (7), 56 (2); HRMS (ESI)
calculated for [M] C19H18N2O3 322.1317; observed 322.1314.
4.9. 4-Benzyl 1-tert-butyl 2-oxopiperazine-1,4-
4.6. ( )-2-(9H-Fluoren-9-yl)methyl-5-tert-butyl-2,5-
dicarboxylate 12
diazabicyclo[4.1.0]heptane-2,5-dicarboxylate 10
Boc anhydride (4.41 g, 20.2 mmol) was added to a solution of 11
(3.15 g, 13.5 mmol), triethylamine (1.36 g, 13.5 mmol) and DMAP
(1.65 g, 13.5 mmol) in 135 mL dichloromethane. The reaction mix-
ture was stirred at room temperature for 16 h, and then diluted
with dichloromethane and washed with saturated sodium bi-
carbonate. The organic phase was washed with water and brine and
dried over magnesium sulfate. Purification by column chromatog-
raphy eluting with 40% ethyl acetate in hexanes afforded 4.0 g
(12.0 mmol, 89%) of 12 as a colourless oil. IR (thin film): 2976, 2935,
To a solution of ZnEt2 (13.7 mL of 1.0 M in hexanes, 13.7 mmol)
in 25 mL ether was added a solution of 9 (1.32 g, 3.25 mmol) in
40 mL ether via cannula. Diiodomethane (3.16 g, 11.79 mmol) was
added, and the reaction mixture was stirred for 18 h at room
temperature. After quenching with saturated aqueous NH4Cl, the
reaction mixture was filtered over a pad of Celite and washed with
ether. The aqueous portion was extracted with ether. The combined
organic extracts were washed with water and brine, dried over
MgSO4 and concentrated at reduced pressure to afford the crude
product. Purification by silica gel column chromatography eluting
with 20% ethyl acetate in hexanes afforded 0.953 g (2.27 mmol,
70%) of 10 as a white solid: mp¼52–56 ꢂC from hexanes. IR (chlo-
roform): 3063, 2977, 2876, 1699, 1449, 1406, 1358, 1255, 1229, 1171,
1118, 1057, 1006, 986, 961, 860, 759, 741 cmꢁ1. 1H NMR (400 MHz,
2894, 1776, 1708, 1419, 1295, 1233, 1154, 9947, 849, 766, 698 cmꢁ1
.
1H NMR (400 MHz, CDCl3):
d 7.37–7.35 (5H, m), 5.15 (2H, s), 4.24
(2H, s), 3.81 (2H, t, J¼5.0 Hz), 3.69 (2H, t, J¼4.9 Hz), 1.54 (9H, s). 13C
NMR (100 MHz, CDCl3):
d 165.9, 154.4, 151.3, 136.1, 128.7, 128.4,
128.2, 84.2, 67.8, 49.1, 44.1, 41.8, 41.4, 28.0. MS (ESI) m/z 357 (50
(MþNa)þ), 257 (36), 235 (19), 91 (100); HRMS (ESI) calculated for
[MþNa]þ C17H22N2O5Na 357.1420; observed 357.1426.
CDCl3):
d
7.77 (2H, d, J¼7.5 Hz), 7.65–7.55 (2H, m), 7.39 (2H, t,
J¼7.5 Hz), 7.30 (2H, t, J¼7.0 Hz), 4.51–4.42 (2H, m), 4.26 (1H, t,
J¼6.5 Hz), 3.54–2.94 (6H, m), 1.49 (9H, s), 1.18–0.88 (1H, br s), 0.47
4.10. 4-Benzyl 1-tert-butyl 2-hydroxypiperazine-1,4-
dicarboxylate 13
(1H, br s). 13C NMR (100 MHz, CDCl3):
d
156.8, 156.0, 144.0, 141.4,
127.7, 127.0, 125.0, 124.9, 120.0, 80.4, 80.2, 67.4, 47.3, 42.2, 41.7, 40.8,
40.5, 28.4, 13.8. MS (EI) m/z 420 (1, Mþ), 363 (27), 320 (5), 185 (17),
178 (100),165 (30),152 (25),141 (77),126 (7), 97 (20), 89(1), 68 (14),
57 (39); HRMS (EI) calculated for [M]þ C25H28N2O4 420.2049; ob-
served 420.2051.
A solution of 12 (10.7 g, 32.1 mmol) in 150 mL dichloromethane
was cooled to ꢁ78 ꢂC. DIBAL (48.0 mmol, 1.0 M in hexanes) was
added slowly and the reaction mixture was stirred at ꢁ78 ꢂC for 5 h,
when all 12 had been consumed as judged by TLC (dichloro-
methane). The reaction was quenched with 150 mL of saturated
aqueous Rochelle’s salt and allowed to warm to room temperature.
The resulting gel was filtered over Celite and rinsed with ether. The
organic phase was collected and the aqueous phase extracted with
ether. The combined organic extracts were washed with brine and
dried over sodium sulfate. After concentration to a volume of 50 mL,
the crude extracts were passed through silica on Celite, eluting with
ether to afford 10.19 g of 13 (30.3 mmol, 94%) as a colourless oil,
which was used in the next step without further purification.
4.7. ( )-tert-Butyl 2,5-diazabicyclo[4.1.0]heptane-2-
carboxylate 5
To a solution of 10 (0.533 g, 1.27 mmol) in 15 mL of diethyl ether
was added octanethiol (1.85 g, 12.7 mmol). The reaction mixture was
allowed to stir for 10 min at room temperature, and then DBU
(0.077 g, 0.507 mmol) was added. After 4 h the starting material was
consumed and diethyl ether and excess octanethiol were removed at
reduced pressure. Purification by column chromatography eluting
with 5% methanol in ethyl acetate afforded 0.201 g (1.02 mmol, 80%)
of 5 as a colourless oil. IR (thin film): 3349, 3086, 2993, 2932, 2878,
1688,1535,1404,1367,1263,1191,1132,1057,1003, 864, 772 cmꢁ1. 1H
4.11. 1-Benzyl 4-tert-butyl 2,3-dihydropyrazine-1,4-
dicarboxylate 14
NMR (400 MHz, CDCl3):
(1H, s), 1.49 (9H, s), 0.88–0.76 (1H, m), 0.53–0.48 (1H, m). 13C NMR
(100 MHz, CDCl3): 156.8,156.3, 79.7, 79.4, 42.5, 41.6, 40.9, 30.9, 30.7,
d
3.54–3.35 (1H, m), 3.02–2.61 (5H, m), 1.62
A solution of 13 (9.08 g, 27.0 mmol), acetyl chloride (10.6 g,
135 mmol) and Hu¨nig’s base (17.4 g, 135 mmol) in 150 mL benzene
was heated to reflux for 1.5 h. The reaction mixture was cooled to
d

3376
R.R.R. Taylor et al. / Tetrahedron 66 (2010) 3370–3377
room temperature, diluted with dichloromethane and washed with
saturated sodium bicarbonate, water and brine. Purification by col-
umn chromatography eluting with 10% ethyl acetate in hexanes
afforded 5.41 g (17.0 mmol, 63%) of 14 as a colourless oil. IR (thin
film): 3139, 3033, 2977, 2891, 1707, 1451,1416,1376,1348,1279, 1224,
1176, 1117, 996, 867, 759, 728, 696 cmꢁ1. 1H NMR (400 MHz, CDCl3):
0.002 mmol), BINAP (0.0020 g, 0.004 mmol) and 1.0 mL DMF was
placed in a recovery flask fitted with a reflux condenser and purged
with argon. The reaction mixture was heated to 115 ꢂC for 16 h
under a nitrogen atmosphere. The reaction mixture was cooled to
room temperature, and DMF removed at reduced pressure. The
residue was taken up in dichloromethane, and filtered over Celite.
Evaporation of the solvent and purification by column chroma-
tography eluting with 20% ethyl acetate in hexanes afforded 0.035 g
(0.073 mmol, 57%) of 20 as a white solid: mp¼205–210 ꢂC from
ethyl acetate/methanol. IR (chloroform): 3084, 2978, 1721, 1619,
1508, 1366, 1238, 1163, 1062, 895, 827, 800, 775, 732 cmꢁ1. 1H NMR
d
7.38–7.31 (5H, m), 6.41–6.13 (2H, br m), 5.19 (2H, s), 3.72–3.68 (4H,
br m), 1.49 (9H, s). 13C NMR (100 MHz, CDCl3):
d
152.5, 151.6, 136.2,
128.7, 128.4, 128.2, 109.8, 109.0, 107.8, 107.0, 81.4, 67.9, 41.8, 41.3, 41.0,
40.5, 40.3, 28.5. MS (ESI) m/z 341 (84 (MþNa)þ), 319 (55 (MþH)þ),
285 (16), 263 (81), 219 (56), 175 (23), 128 (5), 91 (100) HRMS (ESI)
calculated for [M]þ C17H23N2O4 319.1652; observed 319.1650.
(400 MHz, CDCl3):
d
8.49 (1H, s), 7.99 (1H, d, J¼14.5 Hz), 7.39 (1H, br
m), 4.38 (2H, q, J¼7.0 Hz), 3.77–3.05 (7H, br m), 1.50 (9H, s), 1.41
(3H, t, J¼7.0 Hz),1.29–1.28 (2H, m),1.16–1.19 (3H, m), 0.66–0.54 (1H,
4.12. ( )-2-Benzyl 5-tert-butyl 2,5-diazabicyclo[4.1.0]heptane-
2,5-dicarboxylate 15
br m). 13C NMR (100 MHz, CDCl3):
d 173.1, 166.0, 156.2, 152.7, 152.5,
150.3, 150.0, 148.1, 141.9, 141.6, 138.2, 121.2, 113.9, 113.6, 110.3, 102.4,
80.5, 80.2, 60.8, 45.9, 43.3, 41.6, 34.4, 33.4, 33.2, 29.5, 28.5,14.5,14.0,
8.2, 8.0. MS (EI) m/z 471 (3, Mþ), 414 (100), 398 (3), 370 (9), 343
(24), 324 (12), 303 (8), 257 (6), 243 (2), 229 (2), 202 (2); HRMS (EI)
calculated for [M]þ C25H30FN3O5 471.2169; observed 471.2168.
A solution of 14 (2.52 g, 7.95 mmol) in 60 mL of dry ether was
added slowly via cannula to a solution of diethylzinc (32.6 mmol,
1.0 M in hexanes) in 50 mL ether under nitrogen. The reaction
mixture was stirred at room temperature for 10 min, and diiodo-
methane (7.65 g, 28.6 mmol) was added. The reaction mixture was
stirred at room temperature for 18 h, and then cooled to 0 ꢂC.
Saturated ammonium chloride was added, and the resulting mix-
ture was filtered over Celite. The Celite pad was washed with water
and ether. After extraction with ether, the combined organic ex-
tracts were washed with water and brine, and then dried over
magnesium sulfate. Evaporation of the solvent and purification by
column chromatography eluting with 20% ethyl acetate in hexanes
afforded 2.02 g (6.07 mmol, 76%) of 15 as a colourless oil. IR (thin
film): 3588, 3518, 3387, 2978, 2932, 2878, 1702, 1454, 1399, 1354,
1295, 1237, 1178, 1119, 1057, 1003, 964, 866, 772, 702 cmꢁ1. 1H NMR
4.15. ( )-7-(2,5-Diazabicyclo[4.1.0]heptan-2-yl)-1-
cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-
carboxylic acid hydrochloride salt 17
A mixture of 20 (0.018 g, 0.038 mmol) in 1 mL of 2 M HCl was
heated to reflux for 2 h under a nitrogen atmosphere. The reaction
mixture was cooled to room temperature a small amount of ethanol
was added. The mixture was allowed to crystallize at ꢁ12 ꢂC. The
solid was filtered and washed with dichloromethane and ice cold
ethanol to afford 0.011 g (0.030 mmol, 80%) of 17 as a white solid:
mp¼241–242 ꢂC (decomp.). IR (KBr disc): 3413, 3038, 2916, 2759,
2653, 1714, 1628, 1507, 1378, 1307, 1274, 1113, 1034, 958, 893, 827,
(400 MHz, CDCl3):
2.99 (6H, br m), 1.48 (9H, s), 1.18–0.96 (1H, br m), 0.54 (1H, br s). 13C
NMR (100 MHz, CDCl3): 156.9, 156.2, 136.9, 136.6, 128.7, 128.2,
d 7.37–7.31 (5H, m), 5.20–5.12 (2H, br m), 3.53–
802, 766, 741 cmꢁ1 1H NMR (400 MHz, D2O):
. d 8.41 (1H, s), 7.41
d
127.8, 80.6, 80.3, 67.4, 42.3, 41.9, 40.9, 40.7, 28.9, 28.6, 28.3, 14.0. MS
(ESI) m/z 355 (100 (MþNa)þ), 333 (5), 315 (4), 299 (22), 277 (12),
255 (10), 233 (15); HRMS (ESI) calculated for [M]þ C18H24N2O4Na
355.1628; observed 355.1629.
(1H, br d, J¼6.5 Hz), 7.26 (1H, d, J¼14.5 Hz), 4.00 (1H, m), 3.98–3.14
(6H, br m),1.59 (1H, m),1.41–1.18 (5H, m). 13C NMR (100 MHz, D2O):
d
177.8, 171.8, 155.3, 152.8, 150.6, 145.7, 141.8, 118.4, 113.7, 113.5,
108.0, 105.9, 44.5, 43.3, 38.8, 33.1, 32.1, 12.3, 10.4, 10.2. MS (ESI) m/z
366 (5 (MþNa)þ), 344 (100 (MꢁClꢁ)þ), 326 (15), 306 (5), 285 (6),
215 (3),197 (3),179 (4), 158 (5), 149 (25), 141 (5), 113 (19),105 (8), 95
(7); HRMS (ESI) calculated for [MꢁClꢁ]þ C18H19FN3O3 344.1404;
observed 344.1417.
4.13. ( )-5-(Benzyloxycarbonyl)-2,5-
diazabicyclo[4.1.0]heptane hydrochloride 16
Anhydrous hydrochloric acid (29.1 mmol, 4.0 M in dioxane) was
added to a solution of 15 (1.61 g, 4.86 mmol) in 20 mL of ether. The
reaction mixture was stirred at room temperature for 18 h, and the
resulting white precipitate was collected by filtration and rinsed
with ether. The combined ether rinses were concentrated to dry-
ness, and the residue triturated with ethyl acetate. The combined
solids were recrystallized from ethyl acetate/methanol to afford
1.19 g (4.42 mmol, 94%) of 16 as a white solid: mp¼165–167 ꢂC
(decomp.). IR (KBr disc): 3384, 2857, 2672, 2420, 2258, 2028, 1975,
1706, 1582, 1413, 1359, 1331, 1200, 1152, 1074, 1029, 951, 827, 766,
Acknowledgements
The Natural Science and Engineering Research Council (NSERC)
of Canada and a CIHR operating grant MOP-15499 funded this
research. R.R.R.T. acknowledges receipt of an NSERC PGS M schol-
arship. R.J.M. acknowledges receipt of an OHTN fellowship. We
thank Dr. Alex Young for obtaining high-resolution mass spectra.
We also acknowledge Arlette Darfeuille-Michaud of Clermont-
Ferrand, France and Dylan Pillai of the Ontario Agency for Health
Protection and Promotion for providing bacterial strains.
741, 697, 677, 607 cmꢁ1. 1H NMR (400 MHz, DMSO-d6):
d 10.24 (2H,
br s), 7.36–7.29 (5H, m), 5.15–5.10 (2H, m), 3.73–3.55 (1H, m), 3.22–
2.97 (5H, m), 1.21–1.07 (2H, m). 13C NMR (100 MHz, DMSO-d6):
Supplementary data
d
155.6, 155.1, 136.7, 136.4, 128.4, 128.2, 127.8, 127.6, 127.3, 66.5, 37.6,
37.1, 27.0, 26.9, 26.1, 25.7, 9.0, 8.6. MS (ESI) m/z 233 (15 (M)þ), 189
(26), 142 (19), 91 (100); HRMS (ESI) calculated for [M]þ C13H17N2O2
233.1284; observed 233.1296.
Additional experimental data, synthetic procedures, NMR
titration procedures, copies of 1H and 13C NMR spectra for all new
compounds, crystallographic information file (CIF) for compound
17. Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2010.02.046.
4.14. ( )-Ethyl 7-(5-(tert-butoxycarbonyl)-2,5-diazabicyclo-
[4.1.0]heptan-2-yl)-1-cyclopropyl-6-fluoro-4-oxo-1,4-
dihydroquinoline-3-carboxylate 20
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growth, and MBC was defined as the lowest concentration of compound that
prevents visible growth.
31. Gram-positive: S. aureus ATCC 29213 (clinical isolate from a wound), S. pneu-
moniae ATCC 49619 (clinical isolate from sputum). Gram-negative: E. coli LF82
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a double Curtius rearrangement
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44. In this case, the outcome is somewhat complicated by the effects of ring strain
and the additional aliphatic substitution which would result in an increase of
the pKa of Enoxacin relative to Trovafloxacin, whereas the closer proximity of
the ring-nitrogens in Enoxacin would be expected to reduce their pKa’s relative
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46. C log P values for piperazine and 4 (R1 and R2¼H) were calculated as ꢁ1.48 and
ꢁ1.27, respectively. C log P values for N-Cbz piperazine and 16 were calculated
as 2.49 and 2.71, respectively. C log P values for Ciprofloxacin and 17 were
calculated as ꢁ0.73 and ꢁ0.51, respectively (Chem. Draw Ultra 11.0.1).