Design, synthesis and antibacterial activities of a series of new 2-oxaisocephems

Article abstract of DOI:10.1016/j.bmcl.2012.06.040

A series of 2-oxaisocephems with a thio-substituted methyl group at the 3-position and a [2-(5-amino-1,2,4-thiadizol-3-yl)-2-(Z)-alkoxyimino]acetamido moiety at 7-position were synthesized and tested for their antibacterial activities. The analogs 17c and 17f have well-balanced potency and significantly enhanced activity as compared with the reference compound ceftazidime.

Full text of DOI:10.1016/j.bmcl.2012.06.040

Bioorganic & Medicinal Chemistry Letters 22 (2012) 5293–5296
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and antibacterial activities of a series of new 2-oxaisocephems
Jianbo Wu a, Xueying Wu b, Fan Lei a, Huajie Yuan a, Li Hai a, Yong Wu a,
⇑
a Key Laboratory of Drug-Targeting and Drug Delivery System of the Education Ministry, Department of Medicinal Chemistry West China School of Pharmacy, Sichuan University,
Chengdu 610041, China
b College of Pharmacy, Chengdu University of Traditional Chinese Medicine, Chengdu 610075, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2-oxaisocephems with a thio-substituted methyl group at the 3-position and a [2-(5-amino-
1,2,4-thiadizol-3-yl)-2-(Z)-alkoxyimino]acetamido moiety at 7-position were synthesized and tested for
their antibacterial activities. The analogs 17c and 17f have well-balanced potency and significantly
enhanced activity as compared with the reference compound ceftazidime.
Received 22 March 2012
Revised 28 May 2012
Accepted 13 June 2012
Available online 23 June 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
2-Oxaisocephems
Synthesis
Antibacteria
Over the past decades, a large variety of antibiotics have be-
come available for clinical use. Cephalosporin antimicrobial agents
continue to be an important role in the treatment of bacterial
infections for their favorable safety profile and generally bacterici-
dal mode of action. These antibiotics share a common structure
and mechanism of action by inhibiting peptidoglycan synthesis
in the cell wall. Their clinical efficacy and favorable safety profiles
have spurred the development of new products that practitioners
can now add to their armamentarium to fight infections. Unfortu-
nately, the wide-spread and indiscriminant use of cephalosporins
has accelerated the selection of resistant strains and led to a rapid
increase in the number of bacterial strains which are resistant to
these compounds.1 Furthermore, the introduction of new antibiot-
ics has been decreasing recently compared with the period from
1945 to 1970, when a variety of new antibiotics with new struc-
tures were discovered.2 Thus it is urgent to develop new agents
which can solve the problem of resistant bacteria.
2-Oxaisocephem is a special group among b-lactam antibacte-
rial agents and a class of new compounds possessing an oxygen
atom at the 2-position of the cephalosporin frame. Doyle et al.
reported the synthesis and activity of 2-oxaisocephems, but the
results showed that 2-oxaisocephems lacked comprehensive
antibacterial activity.3–11 Later, Mastalerz et al. proposed that
new orally absorbable 2-oxaisocephems were effective against
Gram-positive organisms.12 Until the application of the 2-(2-amin-
othiazol-4-y1)-2-(Z)-(methoxyimino)acetamido side chain at the
7-position, a great progress has been made in the development
of 2-oxaisocephems.13 Tsubouchi et al. further developed the side
chains at 7-position from this standpoint and presented the syn-
thesis of parenteral 2-oxaisocephems with the side chains of
third-generation cephalosporins at 7-position, which apparently
enhanced the activity against Gram-positive bacteria including
methicillin-resistant Staphylococcus aureus (MRSA) and Enterococ-
cus faecalis and broadened anti-Gram-positive spectrum while
maintaining Gram-negative activity.14–20 Whereas, there was no
extensive study about 2-oxaisocephems reported in recent dec-
ades. As the development of the antibiotics, more new side-chains
with good activity and broad antibacterial spectrum appeared,
therefore we speculated that introduction of the side-chains of
new-generation cephalosporins into the 3-position and 7-position
of 2-oxaisocehems might further enhance the activity and broaden
the antibacterial spectrum. We herein reported the synthesis and
antibacterial activity of new 2-oxaisocephems possessing thio-
substituted methyl groups at the 3-position and the 5-amino-
1,2,4-thiadizol-3-yl moiety at 7-position.
First, compound 10, a key intermediate in the synthesis of new
2-oxaisocephems, was prepared. The starting material 2 was pre-
pared in 12 steps from L-ascorbic acid 1 according to a modified lit-
erature procedure.21 Typical procedures to obtain 10 from 2 are as
follows. Protection of the alcohol group of 2 as a silyl ether 3 was
achieved by treatment with tert-butyldimethylsilyl chloride in
DMF. Removal of the sulfuryl group in the presence of trifluoroace-
tic acid gave 4 followed by hydrogenolysis and reaction with trityl
chloride to accomplish the replacement of the benzyloxycarbonyl
protecting group on 6 with a trityl group. Attachment of the allyl
acetate group to the nitrogen atom of the azetidinone 6 was
easy to achieve in the presence of strong base sodium
⇑
Corresponding author. Tel./fax: +86 28 85503666.
E-mail address: wyong@scu.edu.cn (Y. Wu).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.06.040

5294
J. Wu et al. / Bioorg. Med. Chem. Lett. 22 (2012) 5293–5296
OH
HO
H
N
H
N
H
N
O
TBS
TBS
O
Cbz
OH
HO
12 steps
Cbz
O
Cbz
O
O
a
b
N
N
NH
f
-
+
-
+
OH
O
SO3 NBu4
O
SO3 NBu4
O
4
2
1
3
H
N
TBS
(Ph)3C
H
N
H2N
TBS
O
TBS
Cl
N
c
d
g
e
O
(Ph)3C
(Ph)3C
O
NH
NH
O
O
O
O
5
6
7
H
N
H
H
N
TBS
N
O
OH
O
(Ph)3C
O
Cl
(Ph)3C
h
N
N
N
Cl
O
O
O
O
O
O
O
O
O
O
9
10
8
Scheme 1. Synthesis of key intermediate 10. Reagents and conditions: (a) TBSCl, DMAP, Et3N, DMF, rt, 1 h,; (b) TFA, THF, rt, 5 h; (c) Pd/C, H2, EA, rt, 5 h; (d) Ph3CCl, Et3N,
CH2Cl2, rt, 5 h; (e) allyl bromoacetate, NaHMDS, ꢀ50 °C, 30 min; (f) chloracetyl chloride, LiHMDS, THF, ꢀ78 °C; (g) TBAF, THF, rt; (h) PPh3, DIAD, THF, rt, 15 min.
hexamethyldisiloxane and allyl bromoacetate to afford 7. Conver-
sion of 7 to its lithium salt in the presence of a strong base made
it readily react with chloroacetyl chloride to give the a-keto ester
group by p-toluene sulfonic acid (PTSA), the generated amines
were allowed to react with 2-mercaptobenzothiazole active
ester 13a–g to provide the ally esters 14a–g and 15a–g, which
were subjected to deprotection with Pd(PPh3)4 to give the desired
2-oxaisocephems derivatives 16a–g and 17a–g (Scheme 2 and
Table 1).
All of the compounds 16a–g and 17a–g prepared in this inves-
tigation were tested for their in vitro antibacterial activity against
Gram-positive (S. aureus, and Staphylococcus epidermidis) and
Gram-negative (Escherichia coli, Pseudomonas aeruginosa, and Kleb-
siella pneumoniae) bacterial strains. The minimum inhibitory con-
centrations (MICs) of the tested compounds compared with
ceftazidime as reference compounds are displayed in Table 2.
8. Removal of the silyl protecting group furnished the 3-hydroxy-
methyl compound 9, which was subsequently subjected to a Mits-
unobu reaction to obtain the key intermediate 10 (Scheme 1).
With the key intermediate 10 in hand, next we carried out the
synthesis of the targeting 2-oxaisocephems derivatives (series 1:
16a–g, series 2: 17a–g) according to the modified literature meth-
od.22 C-30 substituents were introduced by treatment of 10 with
sodium salts of the thiol derivatives in the presence of the catalyst
tetra-n-butyl ammonium bromide (TBAB) in methylene dichloride
and H2O to afford 11a and 11b. After deprotection of the trityl
H
N
H
N
H2N
O
Ph3C
O
O
Ph3C
O
a
b
N
Cl
N
O
S
R1
N
R1
S
O
O
O
O
O
O
O
12a, 12b
10
11a, 11b
O
O
R2
O
R2
N
N
H
H
N
N
N
O
N
c
d
O
N
N
N
O
S
O
R1
N
O
S
H2N
R1
S
H2N
S
O
O
O
OH
14a-g
15a-g
16a~g
17a~g
O
O
R2
N
S
O
N
N
S
N
H2N
S
13a~g
Scheme 2. Synthesis of 2-oxaisocephems. Reagents and conditions: (a) NaSR1, TBAB, CH2Cl2/H2O, 30 °C, overnight; (b) PTSA, acetone, 0 °C, 30 h; (c) 2-mercaptobenzothiazole
active esters 13a–g, THF, 30 °C, 6 h; (d) PPh3, Pd(PPh3)4, THF, 30 °C, 4 h.

J. Wu et al. / Bioorg. Med. Chem. Lett. 22 (2012) 5293–5296
5295
Table 1
2-Oxaisocephems 16a–g and 17a–g
Compd
R1
16a
16b
16c
16d
16e
16f
16g
N
N
S
CH3
R2
H2C
Compd
17a
17b
17c
17d
17e
17f
17g
N
N
N
R1
R2
N
HO
CH3
H2C
Table 2
In vitro antibacterial activity (MICa,
hydroxyethyl)-1H-tetrazol-5-yl)thiomethyl group in combination
with the [2-(5-amino-1,2,4-thiadizol-3-yl)-2-(Z)-alkoxyimino]-
acetamido moiety, which is often used as the side-chain at the 7-
position of fourth-generation cephalosporins, contributes more to
the enhancement of the activity against Gram-positive bacteria.
Compound 17a displayed the potency fourfold more than the
reference against S. aureus and 17b showed encouraging activity
l
g mLꢀ1) of the compounds 16a–g and 17a–g
Gram-positive bacteriab Gram-negative bacteriab
Compd
S. au.
S. ep.
E. c.
P. ae.
K. pn.
16a
16b
16c
16d
16e
16f
16g
17a
17b
17c
17d
17e
64
64
32
64
128
128
128
0.5
0.5
0.5
16
4
16
128
1
64
128
8
16
16
1
0.125
0.125
0.125
0.125
64
64
128
128
128
128
128
128
2
2
0.5
8
16
4
32
128
4
16
128
16
16
16
64
8
8
128
1
against S. epidermidis, with the MIC value 0.125 l
g mLꢀ1, which
32
128
4
16
32
64
32
32
64
8
was 128-fold lower than that of ceftazidime. Against the Gram-
negative bacteria strains, the antibacterial activity of 17c was
equal or superior to that of the reference, especially against
E. coli, 17c was the most potent agent in all the new 2-oxaisocep-
hems. Compounds 17c–f were 2- to 16-fold more potent than the
reference compound against P. aeruginosa. And compound 17f, the
most potent compound in series 2, was found to be fourfold more
potent than ceftazidime in its ability to inhibit K. pneumoniae.
In conclusion, introductions of the side-chains used of fourth-
generation cephalosporins into the 7-position of the 2-oxaisoc-
ephem nucleus and the alteration of the 3-subtituents into the side
chains which were often used at 3-position of oxacephems gave
new 2-oxaisocephems with good activity and broad antibacterial
spectrum. Among them, particularly 17c and 17f had well-bal-
anced potency and were found to have excellent antibacterial
activities against Gram-positive organism while maintaining good
Gram-negative activities. Encouraged by these findings, we are
continuing to conduct the evaluation of extensive antibacterial
activity and search more effective 2-oxaisocephem antibiotics,
while the results will be reported in due course.
4
8
2
17f
17g
1
2
32
4
64
16
128
32
Ceftazidime
2
16
a
Minimum inhibitory concentrations (106 cells/mL).
Definitions of organism abbreviations: S. au. = S. aureus ATCC25923, S. ep. = S.
b
epidermidis, E. c. = E. coli ATCC25922, P. ae. = P. aeruginosa ATCC27853, K. pn. = K.
pneumoniae.
Generally speaking, compounds in the series 1 with the C-30
(1,3,4-thiadiazol-2-yl)thiomethyl group, 2-oxaisocephem deriva-
tives (16a–g), most of which was less active against the micro-
organisms including the Gram-positive and Gram-negative bacte-
rial strains, especially against the Gram-positive bacteria, than
the ones in series 2 (17a–g) possessing a (1-(2-hydroxyethyl)-
1H-tetrazol-5-yl)thiomethyl group at C-3. Among those com-
pounds in series 1, there was generally no compound more potent
against S. aureus and E. coli than the reference. The activity of 16a
against S. epidermidis was equal to that of the reference, but against
K. pneumoniae, it was fourfold more potent than that of ceftazi-
dime. Compound 16d displayed about the same activity against
P. aeruginosa and K. pneumoniae as reference compound. Com-
pounds 16b and 16e showed poor activity against all the tested
strains, whereas 16c showed significantly enhanced activity
against S. epidermidis, P. aeruginosa and K. pneumoniae as compared
with the reference. In particular, against K. pneumoniae, it was the
most potent compound in all the synthetic 2-oxaisocephems with
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.bmcl.
2012.06.040. These data include MOL files and InChiKeys of the
most important compounds described in this article.
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