A new method for regioselective synthesis of a broad-spectrum parenteral S-3578-related cephalosporin bearing an imidazo[4,5-b]pyridinium derivative at C-3'

Article abstract of DOI:10.3987/COM-04-10097

A broad-spectrum S-3578-related cephalosporin, 7β-[2-(5-amino-1,2,4- thiadiazol-3-yl)-2(Z)-fluoromethoxyiminoacetamido]-3-[1-(3-methylaminopropyl) -1H-imidazo[4,5-b]pyridinium-4-yl]methyl-3-cephem-4-carboxylate sulfate was regioselectively synthesized in

Full text of DOI:10.3987/COM-04-10097

HETEROCYCLES, Vol. 63, No. 8, 2004, pp. 1757 - 1763
Received, 9th April, 2004, Accepted, 31st May, 2004, Published online, 1st June, 2004
A NEW METHOD FOR REGIOSELECTIVE SYNTHESIS OF A BROAD-
SPECTRUM PARENTERAL S-3578-RELATED CEPHALOSPORIN
BEARING AN IMIDAZO[4,5-b]PYRIDINIUM DERIVATIVE AT C-3’
Hidenori Yoshizawa, Katsuki Yokoo, Takashi Nomura, Takafumi Ohara,
Koji Ishikura,* and Yasuhiro Nishitani
Shionogi Research Laboratories, Shionogi & Co., Ltd., Fukushima-ku, Osaka 553, Japan
Abstract- A broad-spectrum S-3578-related cephalosporin, 7β-[2-(5-amino-1,2,4-
thiadiazol-3-yl)-2(Z)-fluoromethoxyiminoacetamido]-3-[1-(3-methylamino-
propyl)-1H-imidazo[4,5-b]pyridinium-4-yl]methyl-3-cephem-4-carboxylate
sulfate was regioselectively synthesized in a good yield using diaminopyridine
derivative bearing a dimethylformamidine group.
Methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa are nosocomial
pathogens associated with serious infections and considerable mortality. Also, the incidence of mixed
infection by MRSA and P. aeruginosa has been increasing,1 and antibacterial agents having high activity
against these pathogens are needed. Therefore, we began a search for novel parenteral C-3’ quaternary
S
N
O
H
N
ammonium cephalosporins exhibiting potent activity against both
H2N
S
N
N
N
O
N
Me
N
N
O
MRSA and P. aeruginosa.
O
H
N
R
O
S-3578 (R= Et) H2SO4 salt
1 (R= CH2F) HCl salt
Figure 1. Structures of S-3578 and 1
In our previous paper,2 we reported the synthesis and structure-

activity relationships of a series of C-3’ imidazo[4,5-b]pyridinium cephalosporins such as S-3578 and 1
(Figure 1) exhibiting potent antibacterial activity against both MRSA and P. aeruginosa.
Scheme 1 shows the earlier reported method for the synthesis of S-3578 and 1.
Quaternization of imidazo[4,5-b]pyridine derivative (3) with the bromide (2a) or iodide (2b) gave a
mixture of imidazopyridinium (4) and the undesirable regioisomer (5). The ratio of regioisomers (4) and
(5) was usually about 4~5:1 (determined by HPLC).
S
N
O
H
N
BocHN
R2
S
S
N
S
X
N
X
(quaternization)
+
X
N
N
N
O
N
O
N
R2
N
O
N
+
R2
N
N
O
O
N
N
N
R1
O
O
PMB
O
3
PMB
PMB
2a (R1= Et, X= Br)
2b (R1= CH2F, X= I)
Me
R2 =
5a (R1= Et, X= Br)
5b (R1= CH2F, X= I)
4a (R1= Et, X= Br)
4b (R1= CH2F, X= I)
N
Boc
Me
NH
S
N
O
S
H
N
O
H2N
N
H2N
N
S
NH
O
S
N
(deprotection)
N
N
N
N
O
N
Me
+
N
N
O
N
N
O
O
O
H
N
R1
N
R1
O
O
.HCl or H2SO4
.HCl or H2SO4
6a (R1= Et)
6b (R1= CH2F)
S-3578 (R1= Et)
1 (R1= CH2F)
Abbreviations; PMB: p-methoxybenzyl; Boc: tert-butoxycarbonyl
Scheme 1.
Isolation and purification of the desired quaternary imidazopyridinium salts (4) by crystallization or
chromatography was difficult. Therefore, after deprotection of the mixture of 4 and 5, the target
compound (S-3578 or 1) was purified by reversed phase chromatography, although separation of the
regioisomeric mixture was troublesome.
In order to facilitate purification of the desired regioisomer such as S-3578 or 1, we explored a method
for the highly regioselective synthesis of imidazopyridinium cephalosporin. We report herein a new
method for the synthesis of imidazopyridinium cephem (1) without the formation of undesirable
regioisomer (6b).
Our approach was based on quaternization of the diaminopyridine derivative followed by cyclization to

form an imidazopyridinium ring under deprotection conditions. We found that the diaminopyridine
derivative bearing a dimethylformamidine group is suitable and effective for this purpose.
Boc
S
N
O
H
N
R2
N
N
R2
N
Boc
N
S
R1HN
I
Me
S
N
e
Me
N
O
N
a, b, c or d
1 (H2SO4 salt)
+
I
N
N
O
O
N
O
N
N
O
NMe2
NMe2
O
PMB
F
2b (R1= Boc)
2c (R1= H)
PMB
7a (R2= PMB)
7b (R2= Boc)
8a (R1= Boc, R2= PMB)
8b (R1= Boc, R2= Boc)
8c (R1= H, R2= PMB)
8d (R1= H, R2= Boc)
Abbreviations: DMF: N,N-dimethlyformamide, DMA: N,N-dimethylacetamide
Scheme 2. Reagents and conditions; (a) 2b+7a→8a: DMF, rt, 2.5 h; (b) 2b+7b→8b: DMF, rt, 17 h; (c)
o
2c+7a→8c: DMA, rt, 3.5 h; (d) 2c + 7b→8d: DMA, rt, 17 h; (e) i) 62%H SO -AcOH, 5 C, 1 h, ii)
2
4
Purification by HP-20 chromatography, 8a→1: 26% yield from 2b; 8b→1: 28% yield from 2b; 8c→1: 49%
yield from 2c; 8d→1: 53% yield from 2c.
Scheme 2 shows a new method for the synthesis of imidazopyridinium cephem (1) using 7.
Quaternization of 7a-b with iodomethyl 2b or 2c at room temperature gave the corresponding
imidazopyridinium salts (8a-d) respectively, of which purification by crystallization or chromatography
was difficult. The crude compounds (8a-d) were treated with a mixture of 62% H2SO4 and AcOH (at
5oC for 1 h) to obtain the target material (1) in moderate to good yields (26%~53% yield from 2),3
involving both deprotection and cyclization.
Diaminopyridine derivatives (7a-b) were readily prepared from 2,3-diaminopyridine (9) in good yields
as shown in Scheme 3. Pyridine derivative (11)2 was obtained by reductive alkylation of 9 and aldehyde
10 using borane-pyridine complex as a reductant. The pyridine derivative (11) was treated with N,N-
dimethylformamide dimethyl acetal to yield the amidine derivative (12) in a quantitative yield, followed
by protection of the amino group at the 3-position of 12 to give 7a and 7b.4
R
N
Boc
N
Boc
N
H
Boc
N
H
N
Boc
N
N
c or d
NH2
NH2
Me
b
a
Me
Me
+
OHC
Me
N
N
N
N
N
N
NH2
NMe2
NMe2
7a (R= PMB)
7b (R= Boc)
9
10
12
11
o
Scheme 3. Reagents and conditions; (a) BH -Py (1 eq.), CH Cl / AcOH (1 / 1), -15 C, 69.6%; (b) N,N-
3
2
2
o
dimethylformamide dimethyl acetal (1.5 eq.), 70 C, 4 h, quant.; (c) 7a: p-methoxybenzyl bromide(1.1 eq.),
NaHCO , rt, 3.5 h, 73%; (d) 7b: Boc O (1.2 eq.), THF, 3.5 h, reflux, 82%.
3
2

In conclusion, the new method presented in this paper offers the advantages of a good yield, high
regioselectivity and easy purification. It could be applied to the synthesis of other imidazopyridinium
cephalosporins including S-3578.
REFERENCES AND NOTES
1. Matsuoka, Y. Nagatomi, K. Imanishi, M. Matsubara, I. Nakanishi, and H. Watanabe, Jpn. J.
Antibiotics,1994, 47, 1363.
2. H.Yoshizawa, H. Itani. K, Ishikura, T. Irie, K. Yokoo, T. Kubota, K. Minami, T. Iwaki, H. Miwa, and
Y. Nishitani, J. Antibiotics, 2002, 55, 975.
3. A typical procedure for the synthesis of 1 using 7a is as follows.
To a solution of iodomethyl 2b (19.1 g, 25 mmol) in DMF (40 mL) was added pyridine derivative
(7a) (14.4 g, 30 mmol), and the reaction mixture was stirred at rt for 2.5 h. The mixture was poured
into 5% NaCl. The precipitate was filtered, washed with EtOAc and dried in vacuo to give
quaternary ammonium (8a) (27.4 g) as an amorphous solid. The crude product (8a) was used
1
directly in the next reaction. A sample of the crude 8a was analyzed by H-NMR, IR and HRMS;
1H-NMR (DMSO-d6) δ 1.32 (9H, s), 1.51 (9H, s), 2.71 (3H, s), 2.89 (3H, s), 3.10 (3H, s), 3.72 (3H,
s), 3.76 (3H, s), 4.02 (2H, m), 5.10 (1H, d, J = 4.8 Hz), 5.20 (4H, m), 5.82 (2H, d, J = 54.6 Hz), 5.96
(1H, dd, J = 8.1 Hz, 4.8 Hz), 6.87 (2H, q, J = 8.4 Hz), 6.95 (2H, d, J = 8.7 Hz), 7.00 (2H, d, J = 8.4
Hz), 7.38 (2H, d, J = 8.7 Hz), 7.22 (1H, m), 7.78 (1H, m), 8.16 (1H, m), 8.43 (1H, m); IR (CHCl3)
cm-1 1775, 1720, 1695, 1640, 1555, 1520; HRMS Found: 1090.4277 ([M-I]+). Calcd for
C51H65N11O11FS2:1090.4291.
To 62% H2SO4 (42 mL) was added a solution of the crude compound (8a) (9.99 g, 8.2 mmol) in
AcOH (18 mL) at 5oC, and the reaction mixture was stirred at the same temperature for 1 h. The
reaction mixture was poured into 2-propanol. The precipitate was collected by filtration and

dissolved in H2O. The solution was chromatographed on HP-20 resin. The target product was eluted
with 3% MeCN/0.001 N H2SO4. The solution containing the target product was concentrated in
vacuo, and compound (1) was crystallized as a sulfate salt (1.90 g, 26% from 2b); mp >200oC
(decomp); 1H-NMR (D2O) δ 2.42 (2H, m), 2.73 (3H, s), 3.17 (2H, t, J = 7.6 Hz), 3.30 and 3.64 (2H,
ABq, J = 18.3 Hz), 4.62 (2H, t, J = 7.4 Hz), 5.25 (1H, d, J = 4.8 Hz), 5.70 and 5.91 (2H, ABq, J =
13.0 Hz), 5.82 (2H, d, J = 54.6 Hz), 5.86 (1H, d, J = 4.8 Hz), 7.87 (1H, dd, J = 8.2 Hz, 6.4 Hz), 8.80
13
(1H, d, J = 8.2 Hz), 8.83 (1H, d, J = 6.4 Hz); C-NMR (D2O) δ 28.6, 28.8, 35.7, 46.4, 48.7, 57.3,
60.1, 61.7, 105.6, 108.6, 117.7, 122.4, 131.7, 133.8, 136.0, 141.0, 150.8, 152.6, 153.4, 162.5, 165.1,
166.5, 169.4, 187.0; IR (KBr) cm-1 1774, 1720, 1679, 1631, 1577, 1529, 1495, 1463, 1417; MS m/z
605 ([M+H]+, 40), 604 (4), 557 (2), 531 (2), 456(2), 429 (2), 329 (5), 303 (2), 191 (100), 189 (10),
146 (8); HRMS Found: 605.1521. Calcd for C23H26N10O5FS2: 605.1513. Anal. Calcd for
.
.
C23H25N10O5FS2 1.0H2SO4 8.2H2O: C 32.48, H 5.14, N 16.47, F 2.23, S 11.31. Found: C 32.57, H
5.00, N 16.49, F 2.22, S 11.31.
4. The method for the syntheses of 7a and 7b is as follows.
[3-(2-Aminopyridin-3-ylamino)propyl]methylcarbamic acid tert-butyl ester (11)
2,3-Diaminopyridine (9) (161.5 g, 1.48 mol) was dissolved in a mixture of CH2Cl2 (1.6 L) and
AcOH (1.6 L), and the solution was cooled to –15oC. To this was added borane-pyridine complex
(150 mL, 1.48 mol), followed by addition of a solution of 3-tert-butoxycarbonylmethylamino-
propionaldehyde (10) (360.2 g, 1.92 mol) in CH2Cl2 (300 mL). The reaction mixture was stirred at
–15oC for 1 h. The organic layer was washed with 5% NaOH and brine, dried over Na2SO4, filtered
and concentrated in vacuo to give the title compound (11) (289 g, 69.6%) as a pale yellow crystal
1
which was crystallized from Et2O; mp 103-104oC; H-NMR (CDCl3) δ 1.44 (9H, s), 1.83 (2H, m),
2.85 (3H, s), 3.1 (2H, t, J = 6.3 Hz), 3.36 (2H, t, J = 6.6 Hz), 4.13 (1H, s), 4.58 (2H, bs), 6.66 (1H,
13
m), 6.75 (1H, m), 7.55 (1H, d, J = 4.3 Hz); C-NMR (CDCl3) δ 26.1, 28.3, 34.0, 40.0, 45.1, 79.6,

115.2, 116.4, 131.5, 135.6, 148.6, 156.0; MS m/z 281 ([M+H]+, 100), 280 (64), 225 (18), 224 (15),
181 (17), 122 (18), 57 (19). HRMS Found: 281.1974. Calcd for C14H25N4O2 : 281.1978. Anal. Calcd
for C14H24N4O2: C 59.98, H 8.63, N 19.98. Found: C 59.98, H 8.65, N 19.88.
{3-[2-(Dimethylaminomethyleneamino)pyridin-3-ylamino]propyl}methylcarbamic acid tert-
butyl ester (12)
A mixture of 11 (200 g, 0.713 mol) and N,N-dimethylfolmamide dimethyl acetal (142 mL, 1.06 mol)
was stirred at 70oC for 4 h. The reaction mixture was poured into a mixture of EtOAc and H2O, and
the organic layer was washed with brine, dried over Na2SO4, filtered and concentrated in vacuo to
give the title compound (12) (239 g). A portion (3.9 g) of the crude product was chromatographed
on silica gel with chloroform-methanol (9:1) as eluent to give pure 12 as a reddish oil (3.9 g,
1
quantitative yield); H-NMR (CDCl3) δ 1.45 (9H, s), 1.87 (2H, m), 2.87 (3H, s), 3.09 (6H, s), 3.13
(2H, t, J = 6.4 Hz), 3.34 (2H, t, J = 6.9 Hz), 6.74 (2H, m), 7.56 (1H, dd, J = 4.8 Hz, 1.6 Hz), 8.44
(1H, s); 13C-NMR (CDCl3) δ 27.2, 28.3, 34.2, 34.5, 40.5, 46.1, 46.8, 79.1, 114.1, 118.3, 134.0, 137.1,
149.0, 153.7, 155.5; IR (CHCl3) cm-1 1680, 1485, 1460, 1405; MS m/z 336 ([M+H]+, 87), 335 (100),
291 (8), 280 (6), 235 (12), 191 (19), 177 (15), 57 (12). HRMS Found: 335.2311. Calcd for
C17H30N5O2: 335.2321.
{3-[[2-(Dimethylaminomethyleneamino)pyridin-3-yl]-(4-methoxybenzyl)amino]propyl}methyl-
carbamic acid tert-butyl ester (7a)
To a solution of the crude compound (12) (50.8 g, 0.151 mol) in DMF (250 mL) was added
NaHCO3 (38.17 g, 0.454 mol) and subsequently p-methoxybenzyl bromide (33.5 g, 0.166 mol) at
25oC. The reaction mixture was stirred at the same temperature for 3.5 h. After addition of EtOAc,
the organic layer was washed with brine, dried over Na2SO4, filtered and concentrated in vacuo to
give a crude compound (7a) (59.46 g). A portion (12.1 g) of the crude product (7a) was
chromatographed on silica gel with EtOAc as eluent to give pure 7a as a reddish oil (11.0 g, 73%);

1H-NMR (CDCl3) δ 1.39 (9H, s), 1.65 (2H, m), 2.70 (3H, s), 3.03 (3H, s), 3.07 (3H, s), 3.10 (4H, m),
3.78 (3H, s), 4.37 (2H, s), 6.80 (3H, m), 7.03 (1H, dd, J = 7.9 Hz, 1.7 Hz), 7.23 (2H, d, J = 8.7 Hz),
13
7.87 (1H, dd, J = 4.7 Hz, 1.6 Hz), 8.33 (1H, s); C-NMR (CDCl3) δ 25.4, 28.3, 34.1, 34.6, 40.4,
47.1, 47.4, 55.0, 56.2, 78.9, 113.1, 113.5, 117.4, 129.3, 131.1, 138.5, 140.7, 153.5, 155.4, 155.9,
158.1; IR (CHCl3) cm-1 1685, 1635, 1580, 1520, 1405; MS m/z 456 ([M+H]+, 55), 334 (35), 297 (3),
283 (35), 203 (9), 191 (13), 177 (4), 121 (100), 57 (11). HRMS Found: 456.2972. Calcd for
C25H38N5O3 : 456.2975,
[3-(tert-Butoxycarbonylmethylamino)propyl]-[2-(dimethylaminomethyleneamino)pyridin-3-yl-
carbamic acid tert-butyl ester (7b)
To a solution of 12 (312 g, 0.92 mol) in THF (624 mL) was added di-tert-butyldicarbonate (243 g,
1.11 mol) and then the reaction mixture was refluxed for 3.5 h. After evaporation of the solvent,
EtOAc and 10% oxalic acid were added to the residue. The aqueous layer was alkalinized with 4 N
NaOH and subsequently extracted with EtOAc. The organic layer was washed with H2O, dried over
Na2SO4, filtered and concentrated in vacuo to give a crude compound (7b) (356.9 g). A portion (2.1
g) of the crude product (7b) was chromatographed on silica gel with EtOAc as eluent to give pure
1
7b as a reddish oil (1.99 g, 82%); H-NMR (DMSO-d6) δ 1.21 (9H, s), 1.35 (9H, s), 1.59 (2H, m),
2.70 (3H, s), 2.96 (3H, s), 3.08 (3H, s), 3.15 (2H, t, J = 7.1 Hz), 6.89 (1H, dd, J = 7.6 Hz, 4.8 Hz),
13
7.42 (1H, dd, J = 7.6 Hz, 1.6 Hz), 8.08 (1H, dd, J = 4.8 Hz, 1.6 Hz), 8.44 (1H, s); C-NMR
(DMSO-d6) δ 26.1, 27.6, 27.8, 33.5, 338, 45.9, 47.0, 78.0, 78.5, 116.8, 129.7, 136.2, 137.2, 145.7,
153.7, 154.2, 154.3, 158.0; IR (CHCl3) cm-1 1690, 1638, 1585, 1465, 1405; MS m/z 436 ([M+H]+,
100), 291 (20), 235 (11), 191 (30), 177 (12), 57 (39). HRMS Found: 436.2927. Calcd for
C22H38N5O4: 436.2924.