Discovery of a potent phenolic N1-benzylidene- pyridinecarboxamidrazone selective against Gram-positive bacteria

Article abstract of DOI:10.1016/j.bmcl.2005.11.005

As part of a study into antimycobacterial compounds a set of phenolic N¹-benzylidene-pyridinecarboxamidrazones was prepared and evaluated. This report describes the unexpected discovery of a potent compound with a pronounced selectivity for Gram-positive bacteria over Gram-negative micro-organisms. In addition, this compound is active against various drug-resistant Gram-positive bacteria.

Full text of DOI:10.1016/j.bmcl.2005.11.005

Bioorganic & Medicinal Chemistry Letters 16 (2006) 879–883
Discovery of a potent phenolic
N -benzylidene–pyridinecarboxamidrazone selective
against Gram-positive bacteria
1
*
Daniel L. Rathbone, Katy J. Parker, Michael D. Coleman,
Peter A. Lambert and David C. Billington
School of Life and Health Sciences, Aston University, Birmingham B4 7ET, UK
Received 31 August 2005; revised 30 October 2005; accepted 2 November 2005
Available online 18 November 2005
1
Abstract—As part of a study into antimycobacterial compounds a set of phenolic N -benzylidene–pyridinecarboxamidrazones was
prepared and evaluated. This report describes the unexpected discovery of a potent compound with a pronounced selectivity for
Gram-positive bacteria over Gram-negative micro-organisms. In addition, this compound is active against various drug-resistant
Gram-positive bacteria.
Ó 2005 Elsevier Ltd. All rights reserved.
1
Certain N -benzylidene–pyridinecarboxamidrazones are
known to have antimycobacterial activity and this con-
stitutes most of the published work for these compounds
pare compounds 7 and 10. The precursor to 7, 2-hy-
droxy-3,5-dimethylbenzaldehyde, was prepared from
2,4-dimethylphenol and paraformaldehyde according
1
–6
8
in the antimicrobial area.
Four compounds of this
to the method proposed by Casiraghi et al. In addition,
7
type were also published by Mamolo and Vio. These
were inactive against a panel of (non-myco)bacteria
apart from one compound, a 2,4-dimethylphenyl deriv-
ative, which exhibited activity against various Gram-po-
sitive bacteria in the MIC range 5–10 lg mL . Most of
the published compounds of this type that have been
examined for their antimicrobial activity contain benzyl-
idene moieties substituted with relatively non-polar
functionalities (halogen, alkyl and alkoxy) and very little
indeed by way of hydrogen bond donor functionality.
Therefore, in order to explore this neglected area for
possible bioactivity we prepared and screened a set of
2-hydroxy-3,5-di-tert-butylbenzaldehyde was acetylated
with acetic anhydride to give the precursor to 10. The
benzylidene compounds were obtained as solids which
usually precipitated from the reaction mixture. These
were recrystallised where appropriate. Characterisation
ꢀ
1
9
of the key compound is given.
The MIC for each compound was measured against a
panel of ten clinical isolate methicillin-resistant strains
of Staphylococcus aureus using an agar dilution meth-
1
0
od (Mueller–Hinton agar) by means of a multipoint
inoculator delivering 10 colony-forming units per spot.
4
1
phenolic N -benzylidene–heteroarylcarboxamidrazones.
The MIC was defined as the lowest concentration inhib-
iting growth after incubation at 37 °C for 18 h. For two
selected cases a second panel of Gram-positive organ-
isms was also investigated. See Table 2 for the full set
of organisms. This comprised three methicillin-sensitive
Staphylacoccus aureus strains, two Enterococcus faecium
strains, and seven strains of Enterococcus faecalis
(including five clinical isolates). A selection of eight dif-
ferent Gram-negative bacteria was also tested, to inves-
tigate the possibility of any broad-spectrum activity.
One compound was also screened against a panel of
vancomycin-resistant enterococci using the multipoint
inoculator approach described above. Seventeen of the
compounds (1–6, 11–21) were also screened against
The compounds were prepared by condensation of the
appropriate aldehyde and pyridine-, pyrazine- or quinol-
6
yl-heteroarylcarboxamidrazone (Scheme 1 and Table
1
). The latter building blocks were prepared by the ac-
tion of ethanolic hydrazine hydrate upon the corre-
sponding cyano compounds. The benzaldehydes were
all commercially available apart from those used to pre-
Keywords: Carboxamidrazone; MRSA; Antimicrobial.
*
Corresponding author. Tel.: +44 121 2044002; fax: +44 121
590733; e-mail: D.L.Rathbone@aston.ac.uk
3
0
960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.11.005

880
D. L. Rathbone et al. / Bioorg. Med. Chem. Lett. 16 (2006) 879–883
NH NH2
Aryl-CHO / EtOH
reflux
2
Heteroaryl
N
Heteroaryl
N
Heteroaryl CN
NH2
N
Aryl
NH2
NH2
1
Scheme 1. Preparation of N -benzylidene-heteroarylcarboxamidrazones.
Table 1. Structures of the compounds examined in this study and the range of MIC values obtained from a panel of 10 MRSA strains
2
R
3
Heteroaryl
N
R
N
NH2
4
6
R
R
R5
2
R
3
R
4
R
5
R
6
R
ꢀ1
a
Compound
Heteroaryl
MRSA MIC (lg mL
)
Calcd LogP
1
2
3
4
5
7
8
9
4-Pyridyl
2-Pyridyl
3-Pyridyl
2-Pyrazinyl
2-Quinolyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
4-Pyridyl
OH
OH
OH
OH
OH
OH
OH
OH
OCH
t-Butyl
t-Butyl
t-Butyl
t-Butyl
t-Butyl
H
t-Butyl
t-Butyl
t-Butyl
t-Butyl
t-Butyl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
OH
H
H
H
H
H
H
2–4
X
5.1
5.5
5.1
4.2
6.9
2.8
1.9
4.4
5.1
4.9
1.6
1.6
1.3
1.3
1.3
1.6
1.6
1.8
1.6
2.6
2.6
H
H
X
H
X
H
X
CH
H
3
H
CH
H
3
X
H
X
Iodo
t-Butyl
t-Butyl
OH
H
Iodo
t-Butyl
t-Butyl
H
10–20
X
1
1
1
1
1
1
1
1
1
1
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
H
OCOCH
OH
H
3
H
4–8
X
H
OH
OH
OH
OH
OH
OH
H
H
X
OH
OH
OH
OH
OH
OH
OH
OH
OH
H
30–40
20–30
X
H
OH
H
H
OCH
H
3
H
X
OCH
H
3
H
X
H
OCH
NO
NO
Br
2
2
20–30
X
3
H
NO
Br
2
H
X
H
NO
2
X
X, inactive.
a
LogP calculated using CAChe WorkSystem Pro version 6.1.10 (Fujitsu Ltd).
Mycobacterium fortuitum (NCTC 10394). Each com-
pound was tested for a zone of inhibition on Columbia
agar plates, supplemented with horse blood, against
M. fortuitum (reference strain NCTC 10394). In addi-
tion, the compounds were screened at a single concen-
est potency against the MRSA panel of all the com-
pounds tested. Any structural variation made to this
compound proved to be detrimental to its antimicrobial
activity. Thus, the simple isomeric variation of substitut-
ing the 4-pyridyl group of 1 for 2-pyridyl, 3-pyridyl
(compounds 2 and 3) resulted in loss of activity. Going
further and replacing the 4-pyridyl group by pyrazinyl
or 2-quinolyl (compounds 4 and 5, respectively) similar-
ly abolished the antimicrobial activity. Moving to the
benzylidene fragment, compound 6, an isomer of 1, dif-
ferent only by virtue of the position of the hydroxyl
ꢀ1
tration of 32 lg mL
in Middlebrook 7H9 broth,
supplemented with glycerol and Middlebrook ADC
enrichment. If a zone of inhibition, or activity at
ꢀ1
3
was measured in broth. Two compounds were screened
2 lg mL , was observed, the MIC for that compound
for inhibition of Mycobacterium tuberculosis H Rn
3
7
1
1
ꢀ1
(
ATCC 27294) in BACTEC 12B medium conducted
group, was effectively inactive (MIC > 256 lg mL ).
by the Tuberculosis Antimicrobial Acquisition and
Coordinating Facility (TAACF), Southern Research
Institute, 2000, 9th Ave., Birmingham, AL.
Maintaining the same substitution pattern in the benzyl-
idene portion but reducing the size of the alkyl substitu-
ents from tert-butyl to methyl (compound 7) similarly
abolished the antimicrobial activity as did removal of
the alkyl groups altogether (compound 8). Some activity
was regained, however, by replacement of the alkyl
groups by iodine atoms, which are approximately the
same size as a methyl group (compound 9, MIC
One compound in particular (1, Table 1) gave an
intriguing antimicrobial activity profile and it is from
that perspective that the results are presented. The com-
pound in question contained a 4-pyridyl moiety and a
0
ꢀ1
phenolic hydroxyl at the 2 -position as well as two bulky
lipophilic tert-butyl substituents. It exhibited the great-
10–20 lg mL ). Blocking the phenolic hydroxyl group
by conversion to the methyl ether (compound 10) negat-

D. L. Rathbone et al. / Bioorg. Med. Chem. Lett. 16 (2006) 879–883
Table 2. Expanded panel of organisms exposed to compounds 1 and 6, and vancomycin
881
Organism
Reference
1
6
Vancomycin
S. aureus
S. aureus
S. aureus
MRSA
NCTC 6571
NCTC 10788
Cowan 1
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
2–4
X
>256
X
0.05
0.12
0.25
0.12
0.12
0.12
0.12
0.12
0.12
0.12
0.12
0.12
0.12
0.12
0.12
0.25
0.12
0.25
0.25
X
Isolate 1
Isolate 2
X
MRSA
X
MRSA
MRSA
Isolate 3
Isolate 4
>256
X
MRSA
Isolate 5
Isolate 6
X
MRSA
X
MRSA
MRSA
Isolate 7
Isolate 7
Isolate 8
X
X
MRSA
X
MRSA
Isolate 10
ACTCC 10541
NCTC 7171
NCTC 5957
Isolate 1
X
E. faecium
E. faecium
E. faecalis
E. faecalis
E. faecalis
E. faecalis
E. faecalis
E. faecalis
S. epidemisis
S. epidemisis
S. haemolyticus
E. coli
X
X
X
X
Isolate 2
Isolate 3
X
X
Isolate 4
Isolate 5
X
16
X
2
8
NCTC 11047
Isolate 1
Isolate 1
X
X
6
X
8
W3110 Rꢀ
W3110 R+
Strain 327
Strain 4444
NCTC 6749
NCTC 10257
NCTC 11582
NCTC 8344
X
>16
>16
>16
>16
>16
>16
>16
>16
E. coli
X
X
K. pneumoniae
S. marcesens
P. auriginosa
S. maltophilia
E. cloacae
B. bronchiseptica
X
X
X
X
X
X
X
X
X
X
X
X
X, inactive.
ed the activity completely, indicating that the phenolic
hydroxyl is vital to antimicrobial activity. Moderate
activity was observed for compounds 14, 15, and 19,
which lacked the alkyl substituents but contained
instead a tri-hydroxy motif or a 4-nitrophenol moiety.
the alkyl groups altogether as in 8 shaves a further 0.9
units from the LogP value. In both cases, the anti-
MRSA activity vanished. The LogP of 8 is increased
to 4.4 by the introduction of iodo substituents at the
0
0
3 - and 5 -positions as in 9. This was rewarded by a re-
turn of some anti-MRSA activity. It cannot be ruled out
at this point, however, that it was the introduction of
mildly electron-withdrawing groups in place of the
hydrogens or electron-releasing methyl groups which
engendered the biological activity rather than the
change in LogP value. Blocking the phenolic group of
1 as a methyl ether (10) or an acetate (11) resulted in
only small changes to the LogP value. In the former
case, the anti-MRSA activity was abolished, whereas
for the latter the activity was only slightly diminished.
It may be, however, that 11 was simply acting as a pro-
drug of 1 under the conditions of the assay and that
there was hydrolysis of the acetyl group in vivo, thus
regenerating the potent compound 1. Though not con-
clusive, it is highly probable that a free hydroxyl in the
The dataset is too small and too polarised in terms of
activity to be able to present a computational rationali-
sation of the biological activity. Attempts were made to
construct a quantitative structure activity relationship
from these data using the program TSAR 3D (Version
3
.3, Oxford Molecular Ltd). The calculated properties
molecular volume, surface area, ellipsoidal volume, iner-
tia moments, dipole moment, dipole moment compo-
nents, LogP, lipole, lipole components, molar
refractivity and the Verloop substituent parameters were
considered along with various connectivity indices. No
meaningful equation connecting anti-MRSA activity
with calculated properties could be derived. A visual
inspection of the data did reveal some partial trends.
If the assumption can be made that the optimum LogP
value is that found for compound 1 (5.1), then certain
conclusions may be made. For example, it can be seen
from the MRSA MIC values for the isomeric com-
pounds 2, 3 and 6 that the optimum position for the pyr-
idyl nitrogen is the 4-position and that the best place for
0
2 -position is essential for the activity of compound 1.
The remaining compounds (12–22) lack alkyl substitu-
ents and are much less lipophilic than 1, falling in the
LogP range 1.3–2.6. Moderate activity was seen for
two of the polyhydroxylated compounds, 14 and 15. It
may be that these two compounds acted through a dif-
ferent mechanism, perhaps through a catechol-based
chelation of essential metal ions, though that cannot
be confirmed at this time. Moderate activity was also
0
the benzylidene hydroxyl is the 2 -position. Similarly,
0
reducing the size of the alkyl substituents as in 7 (3 ,
0
5
-dimethyl) depresses the LogP by 2.7 units. Removing

882
D. L. Rathbone et al. / Bioorg. Med. Chem. Lett. 16 (2006) 879–883
observed when the inactive phenolic compound 8 was
converted to 19 by addition of a nitro group para to
the phenolic OH. Bearing in mind the difference of
Table 3. MIC values for compound 1 and vancomycin against a panel
of vancomycin-resistant enterococci
ꢀ1
ꢀ1
)
Organism
1 MIC (lg mL
)
Vancomycin MIC (lg mL
3
.6LogP units between 19 and the most active com-
3
3
3
001562
002043
002066
2–4
2–4
2–4
2–4
2–4
2–4
2–4
4–8
4–8
4–8
>16
4–8
>16
>16
pound 1 it is probable that the activity observed for 19
is via a different mechanism. The presence of the strong-
ly electron-withdrawing nitro group para to the hydrox-
yl increases the acidity of the phenol. It is not proved,
however, that this is essential for the observed activity
since the same can be said for the inactive compounds
3005323
3005426
3005353
3
102095
20–22 where the nitro group is either ortho or para to
the hydroxyl.
of organisms, 17 of the compounds (1–6, 11–21) were
also screened against M. fortuitum (NCTC 10394). It
was noted that a compound, which produced a larger
zone of inhibition, did not necessarily give a higher
MIC reading. This suggested that different compounds
permeated the agar to different extents. Since com-
pounds of high lipophilicity may not permeate through
the hydrophilic agar, testing of substances in broth at
Compounds 1 and 6 were tested against an expanded
panel of organisms. This comprised 24 Gram-positive
and 8 Gram-negative organisms. The results are present-
ed in Table 2. Compound 6 was essentially inactive
across this range of organisms. In contrast, compound
1
exhibited good activity against all of the Gram-positive
organisms but was inactive against all of the Gram-neg-
ative strains. The structures of the two bacterial cell walls
are very different. Gram-positive bacteria, such as staph-
ylococci and enterococci, have a single, very thick cell
wall, consisting largely of peptidoglycan. Gram-negative
bacteria, such as Eschericha coli, have a very thin inner
membrane consisting of only 1–5% peptidoglycan, sur-
rounded by an outer membrane consisting of a large
amount of lipoproteins and lipopolysaccharides. It is this
difference in structure that may account for the selectiv-
ity of 1 between the two bacterial types, particularly if the
mode of action of this compound is associated with pep-
tidoglycan or its synthesis.
ꢀ
1
a single concentration of 32 lg mL , was also carried
ꢀ
1
out. If a zone of inhibition, or activity at 32 lg mL
was observed, the MIC for that compound was mea-
sured in broth. Compounds 1 and 6 were screened
against M. tuberculosis H37Rv. All members of this sub-
set of compounds were inactive against M. fortutium.
Compound 6 exhibited weak activity against M. tuber-
ꢀ
1
culosis (75% inhibition at 6.25 lg mL ). The isomeric
compound 1, however, was far more active (100% inhi-
ꢀ
1
ꢀ1
bition at 6.25 lg mL and a MIC of 6.25 lg mL .)
In light of the promising activity of compound 1 against
M. tuberculosis and all the Gram-positive bacterial test-
ed, including drug-resistant strains, this compound and
its synthetic precursors, pyridine-4-carboxamidrazone
and 3,5-di-tert-butyl-2-hydroxybenzaldehyde, were
investigated for their toxicity towards human white
blood cells following procedures previously published
It is tempting to speculate that the antimicrobial mecha-
nism of action of compound 1 might be connected with
its potential metal-chelating properties. The data, how-
ever, are inconsistent on this point. Compound 1 has a
4
-pyridyl ring. Substitution by 2-pyridyl or 3-pyridyl
1
4–16
abolishes the antimicrobial activity. This suggests that
the if the mechanism of action is anything to do with
metal chelation then it does not involve the heteroaryl
ring. This is owing to the fact that only the 2-pyridyl case
has a 1,4-disposition of nitrogen atoms suitable for the
formation of a five-membered chelate with a metal ion
involving the pyridyl nitrogen. This has been exemplified
for related compounds.
The anti-tuberculosis drug
isoniazid was included for comparison. The in vitro as-
say used human mononuclear leucocytes (MNL, white
blood cells) which were incubated with the test com-
pound for 1 h and then subsequently incubated in tox-
in-free media for 18 h; cell death was determined by
trypan blue exclusion. All test compounds were incubat-
ed at a concentration of 1 mM apart from compound 1,
which was limited to 0.5 mM owing to its lesser solubil-
ity. The results are shown in Table 4. Compound 1 was
found to be very toxic to leucocytes, causing lysis of the
cells during the course of the experiment. Compound 8,
which lacks the two tert-butyl groups, was only mildly
1
2
in a related case for copper (II). Using the remainder of
the structure it is possible to form a six-membered che-
late using the motif of the ortho-hydroxyl of the benzyl-
idene substituent of compound 1 and the adjacent chain
1
3
nitrogen. If this is the source of the antimicrobial activ-
ity for 1, then it is very surprising that no activity is seen
for 2 and 3, which are simply isomeric in the pyridyl part.
Similarly, there are a further eleven compounds in Table
1 which contain the same 1,5-O,N arrangement but
which show no antimicrobial activity.
Table 4. Results of direct leucocyte toxicity testing
Compound
Concn % leucocyte
(
mM)
death
Since 1 was active against all strains of MRSA as well as
all other Gram-positive bacteria, it was tested further
against some vancomycin-resistant enterococci. It can
be seen from Table 3 that the high level of activity was
retained. Mycobacteria do not fit perfectly into the
pigeonholes of Gram-positive and Gram-negative.
Thus, in order to investigate activity against this class
DMSO (control)
Isoniazid
Pyridine-4-carboxamidrazone
—
1
7.4 ± 1.1
12.4 ± 2.8
13.8 ± 0.4
32.7 ± 3.2
100
1
3
1
8
,5-Di-tert-butyl-2-hydroxybenzaldehyde
1
0.5
1
11.7 ± 1.2

D. L. Rathbone et al. / Bioorg. Med. Chem. Lett. 16 (2006) 879–883
883
1
toxic in comparison, indicating that the alkyl groups
may be implicated in the toxicity of 1.
9. Characterisation of compound 1, N -[3,5-di-(tert-butyl)-2-
hydroxybenzylidene]-pyridine-2-carboxamidrazone.
Recrystallised from methanol/40–60 petroleum ether to
give a yellow solid, 68% yield. R
f
[EtOAc]: 0.48 (single
spot). H NMR (DMSO-d ; 250 MHz): 1.27 (s, 9H,
In conclusion, sporadic antimicrobial activity was
encountered in the set of phenolic N -benzylidene–
1
1
6
CMe
1
3
), 1.42 (s, 9H, CMe
0
3
), 7.18 (br s, 2H, NH
H, J = 2.4 Hz, 4 H), 7.34 (d, 1H, J = 2.3 Hz, 6 H), 7.87
2
), 7.30 (d,
0
heteroarylcarboxamidrazones prepared and tested. The
most active member was compound 1. This proved to
be consistently more potent across a panel of Gram-po-
sitive bacteria, including drug-resistant strains, than any
of its structural isomers or very close structural
analogues. Compound 1 was also active against
M. tuberculosis but inactive against Gram-negative bac-
teria. Its mechanism of action is not clear at this point
and the possibility that it is via metal chelation cannot
be substantiated by the data presented. Compound 1
was also found to be a highly cytotoxic compound,
which prevented any further study of the compound as
a potential antimicrobial drug. It has, nevertheless, pro-
vided the basis for ongoing studies which are focussed
upon the preparation of potent antimicrobial com-
pounds but with better therapeutic ratios.
(
d, 2H, J = 6.1 Hz, Pyr-H3 and H5), 8.67–8.69 (overlap-
ping m, 3H, @CHAr and Pyr-H2 and H6), 11.60 (br s, 1H,
1
3
OH) ppm. C NMR (CDCl -d ): 29.4 (CMe ), 31.4
3
6
3
0
(CMe ), 34.1 (CMe ), 35.0 (CMe ), 117.4 (C1 ), 120.6 (C3
3
3
3
0
0
0
0
and C5), 126.5 (C4 ), 127.2 (C6 ), 136.3 (C3 ), 141.2 (C4),
0
1
50.3 (C2 and C6), 154.3 (C5 ), 156.1 (C2 ), 162.4 (C8)
ppm. IR (KBr disc): 3468 (mas NH ), 3282 (m NH ), 3250–
2
s
2
3
(
1
000 (m OH, overlapping m Ar-CH), 2954 (m satd CH), 2865
m satd CH), 1633 (m C@N), 1610 (m skeletal Ar or Pyr),
595 (m skeletal Ar or Pyr), 1534 (m skeletal Pyr), 1463 (m
skeletal Ar or Pyr), 1436 (m skeletal Ar or Pyr), 1374, 1247,
178, 1070 (m C–N), 997, 968, 877, 818 (c CH, 4-Pyr), 746
1
ꢀ1
(b ring, 4-Pyr), 713, 642 cm . APCI-MS (+ve) m/z: 353
+
(M+H) . Mp corrected: 156.7–158.0 °C. CHN Analysis,
% m/m (% calculated/% found): C, 71.56/71.70; H,
8.01/7.96; N, 15.89/16.01.
1
0. Onda, H.; Wagenlehner, F. M.; Lehn, N.; Naber, K. G.
Int. J. Antimicrob. Agents 2001, 18, 263.
References and notes
11. Collins, L.; Franzblau, S. G. Antimicrob. Agents Chemo-
ther. 1997, 41, 1004.
1
2
3
. Mamolo, M. G.; Vio, L.; Banfi, E.; Predominanto, M.;
Fabris, C.; Asaro, F. Farmaco 1992, 47, 1055.
. Mamolo, M. G.; Vio, L.; Banfi, E.; Predominanto, M.;
Fabris, C.; Asaro, F. Farmaco 1993, 48, 529.
. Banfi, E.; Mamolo, M. G.; Vio, L.; Predominanto, M. J.
Chemother. 1993, 5, 164.
12. Gokhale, N.; Padhye, S.; Rathbone, D.; Billington, D.;
Lowe, P.; Schwalbe, C.; Newton, C. Inorg. Chem. Com-
mun. 2001, 4, 26.
13. Sandbhor, U.; Padhye, S.; Billington, D.; Rathbone, D.;
Franzblau, S.; Anson, C. E.; Powell, A. K. J. Inorg.
Biochem. 2002, 90, 127.
4
5
. Mamolo, M. G.; Vio, L. Farmaco 1996, 51, 65.
. Billington, D. C.; Lowe, P. R.; Rathbone, D. L.; Schw-
albe, C. H. Acta Crystallogr., Sect. C 2000, 56, e211.
. Billington, D. C.; Coleman, M. D.; Ibiabuo, J.; Lambert,
P. A.; Rathbone, D. L.; Tims, K. J. Drug Des. Discov.
14. Coleman, M. D.; Rathbone, D. L.; Abberly, L.; Lambert,
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7, 59.
15. Coleman, M. D.; Rathbone, D. L.; Endersby, C. R.;
Hovey, M. C.; Tims, K. J.; Lambert, P. A.; Billington, D.
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