Stereoselective synthesis and antibacterial evaluation of 4-amido-isothiazolidinone oxides

Article abstract of DOI:10.1016/S0960-894X(01)00409-7

Two well-defined oxidative chlorination-cyclization processes have been developed for the stereoselective synthesis of a variety of 4-amido-isothiazolidinone oxide derivatives. The stereochemistry of the cyclization products was confirmed by X-ray crystallography. These new compounds were designed as bacterial serine protease inhibitors. In tests, some of them showed weak antibacterial activity.

Full text of DOI:10.1016/S0960-894X(01)00409-7

Bioorganic & Medicinal ChemistryLetters 11 (2001) 2111–2115
Stereoselective Synthesis and Antibacterial Evaluation of
4-Amido-isothiazolidinone Oxides
Zhuoliang Chen,* Thomas P. Demuth, Jr. and Fred C. Wireko
Procter & Gamble Pharmaceuticals, 8700 Mason-Montgomery Road, Mason, OH 45040, USA
Received 15 January2001; accepted 18 May2001
Abstract—Two well-defined oxidative chlorination–cyclization processes have been developed for the stereoselective synthesis of a
varietyof 4-amido-isothiazolidinone oxide derivatives. The stereochemistryof the cyclization products was confirmed byX-ray
crystallography. These new compounds were designed as bacterial serine protease inhibitors. In tests, some of them showed weak
antibacterial activity. # 2001 Elsevier Science Ltd. All rights reserved.
Several classes of serine proteases are covalentlyinhib-
ited by b-lactam compounds.1 Penicillin binding pro-
teins (PBPs) are bacterial serine transpeptidases/
carboxypeptidases involved in bacterial cell wall synth-
esis. Inhibition of PBPs by b-lactams produces the anti-
biotic action of these drugs. Bacterial b-lactamases
hydrolyze a variety of b-lactam antibiotics, but can be
inhibited with appropriatelydesigned b-lactam deriva-
tives. Human leukocyte elastase (HLE), on the other
hand, is a mammalian serine protease known to be
inhibited byvarious b-lactam compounds, such as
cephalosporins.2 The inhibition of these enzymes by b-
lactams involves acylation of the active site serine residue
of protein bythe b-lactam amide.
to resemble the corresponding substituents in known
PBP inhibitors, such as monocyclic b-lactams III. These
substituents should impart the structural specificity
required bybacterial PBPs. In this communication, we
report new stereoselective synthetic methods to
compounds I and their biological testing results.
There are onlyfew literature reports describing the
synthesis of 4-amino-isothiazolidinone derivatives: a
This common mechanism of protease inhibition sug-
gests a general strategyfor drug design involving the
selective deliveryof a reactive warhead to react with the
catalytic serine of the target enzyme by exploiting its
unique structural, mechanistic, and substrate pre-
ferences. In an effort to develop a new class of anti-
bacterial agent, we designed 4-amido-isothiazolidinone
oxides I as potential PBP inhibitors. We based our
design on known inhibitors of HLE, including various
derivatives of saccharin II (benzisothiazolidinone).3
Their inhibition mechanism involves covalent acylation
of HLE bythe activated amide carbonyl of the benz-
isothiazolidinone derivatives. The new isothiazolidinone
targets I incorporate 4-amido and R2 groups designed
cycloaddition protocol4 and a halogenation–cyclization
5
strategystarting from csytine derivatives.
Although
this latter strategywas ultimatelyuseful for us, the
reported halogenation–cyclization procedures were
initiallynot reproducible in our hands. We speculated
that our problems with these procedures stemmed from
the absence of water when the halogenation reactions
were run in halogenated solvents, or from insufficient
solubilityof the substrates when the reactions were per-
formed in water or acetic acid. Based on the reaction’s
mechanism, we developed a well-defined halogenation–
cyclization process. Thus, reaction of l-cystine ester 1
with 7–8 equiv Cl2 and 4 equiv H2O in ethyl acetate at
À78 ꢀC provided sulfonyl chloride 2, which upon expo-
sure to saturated ammonium hydroxide, formed sulfon-
amide 3 in 57% yield from 1. The extra 2 equiv Cl2 was
consumed in the undesired, but inconsequential, chlor-
ination of the activated phenoxygroup. Efficient
*Corresponding author. Fax: +1-513-622-0085; e-mail: chen.jz@pg.
com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00409-7

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Z. Chen et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2111–2115
cyclization of sulfonamide 3 to isothiazolidinone 4a was
effected bytreatment with sodium methoxide followed by
acidification with Dowex 50WX2-100 resin (Scheme 1).
occurred during the alkylation step. In addition, dioxide
4a was also converted to sulfamic acid salt 12 following
a known method for monocyclic b-lactams II.8
Following the same sequence, an N-benzyloxycarbonyl
(Z) protected isothiazolidinone dioxide 6 was prepared
from l-Z-cystine ester 5 (Scheme 2). Deprotection via
hydrogenolysis provided the parent isothiazolidinone
dioxide 7.6 Acylation of the 4-amino group, followed by
alkylation at the sulfonamido position, and acidic
deprotection, formed isothiazolidinone carboxylic acids
117 as stereoisomeric mixtures (60/40 dr for 11b and 3%
ee for 11c) at C-4 and at C-6 as determined by 1H NMR
and chiral HPLC. The stereoisomerization probably
The above oxidative chlorination–cyclization process
involved amide formation in the cyclization step. A
more general strategyfor the chlorination–cyclization
process involved in situ sulfonamide formation to effect
the cyclization and provided N-alkylated iso-
thiazolidinone oxides in one pot. When dipeptide 13,
prepared from l-Z-cystine and d-valine-OBut, was
titrated with the respective amount of chlorine and
water, as indicated in Scheme 3, sulfenyl chloride 14a,
sulfinyl chloride 14b, or sulfonyl chloride 14c was
formed selectivelyand cleanlyas observed by 1H NMR.
Upon addition of pyridine, isothiazolidinone 15a was
formed from 14a in low yield; and the isothiazolidinone
mono-oxide 15b9 was formed from 14b in 56% yield
with >95/5 dr. The formation of isothiazolidinone
dioxide 15c from sulfonyl chloride 14c on treatment
with pyridine was not observed, presumably due to
steric hindrance. Following a similar process, diaster-
eomeric isothiazolidinone mono-oxide 17b was obtained
in 62% yield as a single stereoisomer from dipeptide 16,
an epimer of dipeptide 13. Both 15b and 17b had the S-
configuration on sulfur as determined byX-raycrystal-
lographyanalysis of 15b (Scheme 3) and a derivative of
17b (vide infra). The observed stereoselectivityseemed
to be controlled primarilybythe Z-protected 4-amino
group, which mayprefer to be oriented trans to the
oxygen of the evolving sulfinamide group, thus avoiding
a 1,3 interaction during the cyclization step.
Scheme 1. (a) 7–8 equiv Cl2, 4 equiv H2O, EtOAc, À78 to 0 ꢀC; (b)
NH4OH, 67% from 1; (c) NaOMe, then Dowex resin, 100%.
Scheme 2. (a), (b), (c) as in Scheme 1 48–65% from 5; (d) Pd–C, H2, MeOH, 57–71%; (e) 8, Py , DMF, 0ꢀC to rt, 43–60%; (f) 9, i-Pr2NEt, DMF, rt
or 90 ꢀC, 39–63%; (g) TFA, 48–83%; (h) (1) NaH; (2) SO3Py, n-Bu4NHSO4, 45%.

Z. Chen et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2111–2115
2113
Scheme 3. (a) EtOAc, À78 to 0 ꢀC, 1–3 equiv Cl2 for 14a; 3.5 equiv Cl2, 2 equiv H2O for 14b; 7–8 equiv Cl2, 4 equiv H2O for 14c; (b) Py, À78 ꢀC to
rt, 56%, dr >95/5 for 15b, 62%, dr 100/0 for 17b.
A mild, one-pot procedure was developed to transform
intermediates 15b or 17b into isothiazolidinone acids
19b,c or 20b,c as shown in Scheme 4. Both the Z and the
BOC groups of compound 15b were removed success-
fullybytreatment with a 1:5 mixture of methanesulfonic
acid and CH2Cl2. This new method removed the pro-
tecting groups cleanlywithout ring-opening side reac-
tions.10 The amine, generated from amine salt 18b, was
acylated in the presence of 2-6-di-tert-butylpyridine to
produce acid 19b after aqueous workup. Partial epi-
merization at the carbon a to the carboxylic acid was
observed when pyridine was used in the reaction. Oxi-
dation of compound 15b with MCPBA formed dioxide
15c. Following the same reaction sequences, compounds
15c and 17b were converted to the corresponding acids
19c11 and 20b,c in 61–92% overall yield. Also shown in
Scheme 4 is the X-raystructure of mono-oxide 20b.
pounds and three reference monocyclic b-lactams 21–
23.8,13 With the exception of compound 19c, no anti-
bacterial activitywas observed for the isothiazolidinone
acids, whereas antibacterial activitywas observed for
the reference b-lactams. Weak antibacterial activity,
however, was observed for acid 19c and all iso-
thiazolidinone esters (24–2614 and 15b), mostlyagainst
Gram-negative bacteria (Enterobacter cloacae, Morax-
ella catarrhalis, and/or Klebsiella pneumoniae). In a
standard PBP binding assay,15 all new compounds
exibited IC50 values of >250 mg/mL for PBP1, 2 or 3
from Staphylococcus aureus. These compounds, espe-
ciallythe mono-oxides 19b and 20b, had much shorter
half lives (see last column of Table 1) than their b-lac-
tam counterparts in pH 7.4 phosphate buffer. The
chemical instabilitycould potentiallycontribute to the
poor antibacterial activityobserved.
The prepared isothiazolidinone oxides were evaluated
for in vitro antibacterial activity. Table 1 summarizes
the range of minimum inhibition concentration (MIC)
against multiple strains of bacteria12 of these new com-
In summary, we have developed stereoselective methods
for the preparation of
a class of 4-amido-iso-
thiazolidinone oxides through two well defined oxida-
tive chlorination–cyclization processes. Some of these
Scheme 4. (a) CH3SO3H/CH2Cl2 (1:5), anisole; (b) (1) CF3CON(CH3)TMS, CH2Cl2; (2) PhOCH2COCl, 2,6-di-tert-Bu-Py, 0 ꢀC, 61–92%; (c)
MCPBA, NaHCO3, CH2Cl2, 88%.

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Z. Chen et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2111–2115
Table 1. Antibacterial activityand stabilityof isothiazolidinone oxides
Compd
Structure
MIC range (mg/mL)a
1.6–>100
t1/2 (h)b
Core
R1
—
R2
—
R3
—
NT
218
2213
2313
—
—
Pri
H
—
—
2–>32
62.5–>1000
>48
>32
12
—
—
—
—
—
—
>64
>64
<0.4
11d
5.6
11c
11b
19c(6R)
PhOCH2
p-Cl-C6H4O
PhOCH2
H
CH3
Pri
—
—
—
>100
125–>1000
16–>1000
11.9
15.7
22
19b(6R)
20b(6S)
—
—
—
—
—
—
>1000
>1000
<1.4
<3.3
2414
25
p-Cl-C6H4O
p-Cl-C6H4O
PhCH2O
H
CH3
Pri
Bn
But
But
15.6–>1000
31.3–>1000
31.3–>1000
5.1
NT
15.8
15b(6R)
2614
—
—
—
62.6–>1000
<0.5
aDetermined for 24 strains of Gram+ and GramÀ bacteria.12 Low end MIC of b-lactams 21–23 is for Staphylococcus aureus, Klebsiella pneumoniae
and/or Streptococcus pneumoniae, and of the active isothiazolidinone oxides is for Enterobacter cloacae, Moraxella catarrhalis and/or Klebsiella
pneumoniae.
bMeasured in pH 7.4 phosphate buffer; NT, not tested.
2. Doherty, J. B.; Ashe, B. M.; Argenbright, L. W.; Barker,
new compounds exhibited weak antibacterial activity,
P. L.; Bonney, R. J.; Chandler, G. O.; Dahlgren, M. E.; Dorn,
but are not PBP inhibitors.
C. P., Jr.; Finke, P. E.; Firestone, R. A. Nature 1986, 322, 192.
3. (a) Kuang, R.; Epp, J. B.; Ruan, S.; Chong, L. S.; Venka-
taraman, R.; Tu, J.; He, S.; Truong, T. M.; Groutas, W. C.
Bioorg. Med. Chem. 2000, 8, 1005. (b) Hlasta, D. J.; Acker-
man, J. H.; Court, J. J.; Farrel, R. P.; Johnson, J. A.; Kofron,
Acknowledgements
The authors wish to thank Drs. P. Koenigs and W.-P.
Lu, Misses S. Paule, and H. McKeever for conducting
the MIC and PBP binding assays. We are also thankful
to Dr. P. Zoutendam and Miss R. Imbus for performing
the stabilitytests. F.C.W. thanks Mr. M. Mootz for
experimental assistance in the X-raystructure determi-
nation.
J. L.; Robinson, D. T.; Talomie, T. G. J. Med. Chem. 1995,
38, 4687.
4. (a) Beaudengnis, R.; Ghosez, L. Tetrahedron: Asymmetry
1994, 5, 557. (b) Burri, K. F. Chimia 1992, 46, 335. (c) Wadner,
A. Tetrahedron Lett. 1989, 30, 3061 and references cited therein.
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Chem. Soc., Chem. Commun. 1973, 790. (c) Easton, C. J. J.
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References and Notes
6. Compound 7: mp 228 ꢀC (dec.); [a]D20 +1ꢀ (c 0.65, CH3OH/
1
H2O 1:1); H NMR (CD3OD, 300 MHz) d 3.12 (dd, J=13.0,
8.0 Hz, 1H), 3.66 (dd, J=13.0, 8.0 Hz, 1H), 4.07 (dd, J=8.0,
1. Mascaretti, O. A.; Boschetti, C. E.; Danelon, G. O.; Mata,
E. G.; Roveri, O. A. Curr. Med. Chem. 1995, 1, 441.
8.0 Hz, 1H); 13C NMR (CD3OD, 75 MHz) d 57.27, 57.90,

Z. Chen et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2111–2115
2115
182.19; MS m/z (ESI, negative): 149 (MÀH+); FTIR (cmÀ1
,
1.82 mmol) was added methanesulfonic acid (1.18 mL,
18.2 mmol). The pink solution was stirred at room tempera-
ture for 18 h. Anhydrous diethyl ether (60 mL) was added and
the mixture was stirred for 10 min. The resulting white slurry
was filtered and the residue was washed with a 1/10 mixture of
dichloromethane and ether (16.5 mL) twice and dried in
vacuo. To the resulting white powder was added anhydrous
dichloromethane (12 mL) and N-methyl-N-(trimethylsilyl)tri-
fluoroacetamide (0.673 mL, 3.63 mmol). The mixture was stir-
red for 15 min and then cooled to 0 ꢀC, treated with 2,6-di-tert-
butylpyridine (0.254 mL, 1.09 mmol), and phenoxyacetyl chlo-
ride (0.125 mL, 0.908 mmol). The mixture was stirred at 0 ꢀC
for 2 h and concentrated. The resulting residue was treated at
0 ꢀC with 0.1 N NaHCO3 (100 mL) and brine (20 mL), and
was extracted with ethyl acetate three times. The aqueous
solution was further treated at 0 ꢀC with 1 N HCl (ca. 10 mL
until aqueous pH 2–3), and was extracted again with ethyl
acetate three times. The combined organic extract from the
second extraction was washed with brine two times and dried
over anhydrous Na2SO4. Removal of the organic solvent via
rotaryevaporation provided compound 19c as a white solid
(321 mg, 92% yield). Mp 63–65 ꢀC; [a]D20 +71ꢀ (c 0.47,
CH2Cl2); 1H NMR (CDCl3, 300 MHz) d 0.97 (d, J=7.0 Hz,
3H), 1.14 (d, J=7.0 Hz, 3H), 2.74 (m, 1H), 3.86 (dd, J=10.5,
12.5 Hz, 1H), 4.16 (d, J=8.5 Hz, 1H), 4.23 (m, 1H), 4.59 (m,
2H), 5.24 (m, 1H), 6.94–7.09 (m, 3H), 7.30–7.37 (m, 2H), 7.84
(br. d, 1H); 13C NMR (CDCl3, 75 MHz) d 19.91, 20.91, 27.69,
49.13, 51.91, 60.40, 67.12, 115.04, 122.58, 130.04, 157.14,
166.24, 170.61, 172.16; MS m/z (ESI, positive): 402
(M+NH+), 4385 (M+H+); HR-MS: Anal. calcd for
C16H21N2O7S: 385.1069. Found: 385.1073; FTIR (cmÀ1, KBr
pellet): 3370, 1752, 1673, 1535, 1496, 1350, 1241, 1168.
KBr pellet): 3400 (br.), 1634 (br.), 972. When reacted with (R)-
phenethylisocyanate, 7 formed onlyone urea diastereomer (by
1H NMR and HPLC).
7. Alternatively, 11 was prepared from 6 via the following
sequence: alkylation with bromide 9, removal of the Z group
using CH3SO3H/CH2Cl2 (1:5) as in Scheme 4, and acylation
with compound 8 followed byacidic deprotection.
8. Cimarusti, C. M.; Sykes, R. B. In Medicinal Research
Reviews; John Wiley: New York, 1984; Vol. 4 (1), pp 1–24.
9. Compound 15b: to disulfide 13 (5.328 g, 6.505 mmol) in
anhydrous ethyl acetate (65 mL) and water (0.234 mL,
13.0 mmol) at À78 ꢀC was added chlorine in carbon tetra-
chloride (0.825 M in CCl4, 27.6 mL, 22.8 mmol). The mixture
was stirred at 0 ꢀC for 30 min. At À78 ꢀC pyridine (4.2 mL,
52.0 mmol) was added. The resulting white slurrywas stirred
at 0 ꢀC for 15 min and at room temperature for 18 h, filtered,
washed with ethyl acetate twice. The filtrate was con-
centrated, and the residue was purified via silica gel chroma-
tography(30% and 40% EtOAc in hexane) to provide 15b as
a white solid (3.018 g, 54.7%): mp 115–116 ꢀC; [a]D20 +131ꢀ (c
0.49, CH2Cl2); 1H NMR (CDCl3, 300 MHz) d 0.92 (d,
J=6.5 Hz, 3H), 1.06 (d, J=6.5 Hz, 3H), 1.52 (s, 9H), 2.35 (m,
1H), 3.34 (m, 1H), 3.58 (dd, J=12.5, 6.5 Hz, 1H), 4.51 (d,
J=9.0 Hz, 1H), 4.98 (m, 1H), 5.15 (s, 2H), 5.46 (m, 1H),
7.30–7.40 (m, 5H); 13C NMR (CDCl3, 75 MHz): d 19.21,
19.29, 27.82, 30.50, 44.92, 49.82, 53.31, 57.31, 63.57, 67.44,
83.12, 128.19, 128.40, 128.60, 135.64, 155.52, 167.85, 173.35;
MS m/z (ESI, positive) 447 (M+Na+), 442 (M+NH+), 425
(M+H+), 369. Anal. calcd for C20H28N2O6S: C, 56.59; H,
6.65; N, 6.60. Found: C, 56.7; H, 6.63; N, 6.46; FTIR (cmÀ1
,
KBr pellet): 3283, 1737, 1712, 1554, 1146, 1090. For a success-
ful reaction, it is essential to control the amounts of water and
chlorine and to convert the relativelyless stable sulfonyl chlo-
ride 14b directly, without isolation, to the mono-oxide 15b.
10. The use of TfOH (Yajima, H.; Fuji, N.; Ogawa, H.;
Kawatani, H. J. Chem. Soc., Chem. Commun. 1974, 107)
resulted in ring-opening products.
12. National Committee for Clinical LaboratoryStandards.
Method for Dilution Antimicrobial Susceptibility Test for Bac-
teria that Grow Aerobically. DOC. M7-A4, 1997.
13. Wei, C. C.; Weigele, M. Synthesis 1983, 287.
14. Compound 24 prepared in 25% from 11a with
BrCH2CO2Bn/Hunig’s base/DMF; compound 26 prepared in
29% from 19b with BnOH/WSC/DMF.
11. Compound 19c: To dioxide 15c (400 mg, 0.908 mmol) in
anhydrous dichloromethane (6.0 mL) and anisole (0.197 mL,
15. Spratt, B. G. Eur. J. Biochem. 1977, 72, 341.