Synthesis of some new pyridine-2,6-carboxamide-derived schiff bases as potential antimicrobial agents

Article abstract of DOI:10.3390/molecules15074711

A series of pyridine-bridged 2,6-bis-carboxamide Schiff's bases has been prepared starting from 2,6-pyridinedicarbonyl dichloride (1) and L-alanine or 2-methylalanine methyl ester. The coupling of acid chloride 1 with L-alanine methyl ester hydrochloride -or 2-methylalanine methyl ester hydrochloride gave the corresponding 2,6- bis-carboxamide pyridine methyl esters 2a,b. Hydrazonolysis of 2 with hydrazine hydrate afforded the corresponding bis-hydrazides 3a,b. Treatment of 3a,b with appropriate aromatic or heterocyclic aldehydes afforded the corresponding pyridine- bridged 2,6-biscarboxamide Schiff's bases 4a-f and 5a-f, respectively. The newly synthesized compounds 2-5 were screened for their bactericidal and fungicidal activities. Many of the obtained compounds exhibited significant antimicrobial activity, comparable to streptomycin and fusidic acid, which were used as reference antibiotic drugs.

Full text of DOI:10.3390/molecules15074711

Molecules 2010, 15, 4711-4721; doi:10.3390/molecules15074711
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis of Some New Pyridine-2,6-carboxamide-derived Schiff
Bases as Potential Antimicrobial Agents
Mohamed A. Al-Omar and Abd El-Galil E. Amr *
Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, Riyadh 11451,
Saudi Arabia
* Author to whom correspondence should be addressed; E-Mail: aamr1963@yahoo.com.
Received: 13 June 2010; in revised form: 30 June 2010 / Accepted: 1 July 2010 /
Published: 6 July 2010
Abstract: A series of pyridine-bridged 2,6-bis-carboxamide Schiff's bases has been
prepared starting from 2,6-pyridinedicarbonyl dichloride (1) and L-alanine or 2-methyl-
alanine methyl ester. The coupling of acid chloride 1 with L-alanine methyl ester
hydrochloride -or 2-methylalanine methyl ester hydrochloride gave the corresponding 2,6-
bis-carboxamide pyridine methyl esters 2a,b. Hydrazonolysis of 2 with hydrazine hydrate
afforded the corresponding bis-hydrazides 3a,b. Treatment of 3a,b with appropriate
aromatic or heterocyclic aldehydes afforded the corresponding pyridine- bridged 2,6-bis-
carboxamide Schiff's bases 4a-f and 5a-f, respectively. The newly synthesized compounds
2-5 were screened for their bactericidal and fungicidal activities. Many of the obtained
compounds exhibited significant antimicrobial activity, comparable to streptomycin and
fusidic acid, which were used as reference antibiotic drugs.
Keywords: 2,6-pyridinedicarbonyl dichloride; pyridine-2,6-carboxamides; Schiff's base;
antimicrobial activity
1. Introduction
In our previous work, we have reported that certain of substituted pyridine and Schiff base
derivatives as antimicrobial, anti-inflammatory and anticancer agents [1-6]. Also, Schiff base and other
heterocyclic derivatives were reported to possess diverse biological activities, such as antibacterial

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[7-10] and anti-inflammatory [11-13] properties. In addition, several substituted pyridines and their
derivatives were reported to exhibit significant antimicrobial [14], anti-inflammatory [15] and
anticancer activities [16]. In continuation of our interest in the chemical and pharmacological
properties of disubstituted pyridine derivatives [17-20], we report herein the synthesis and
antimicrobial activities of a new series of hydrazides and their corresponding N2,N6-bis(1-oxo-1-(2-
(substituted-benzylidene)-hydrazinyl)propan-2-yl)pyridine-2,6-di-carboxamide derivatives (Schiff's
bases).
2. Results and Discussion
2.1. Chemistry
L-Alanine and/or 2-methylalanine methyl esters were initially coupled with 2,6-pyridinedicarbonyl
dichloride (1) (acid chloride method) [21] to give the corresponding 2,6-bis-carboxamide pyridine
methyl esters 2a,b. Treatment of 2,6-bis-esters 2a,b with hydrazine hydrate in absolute ethanol
afforded the corresponding 2,6-bis-hydrazides 3a,b (Scheme 1). Some physical properties of these
compounds are listed in Table 1.
Scheme 1. Synthetic Pathway for Compounds 2a,b and 3a,b.
O
O
N
CH3
NH2.HCl
H3C
R
NH
HN
O
TEA
+
O
O
N
R
O
H3C
COOMe
R
Cl
Cl
O
O
1
CH3
CH3
2a,b
O
O
N
NH2NH2.H2O
EtOH
CH3
H3C
R
NH
HN
HN
R
O
O
NH
NH2
NH2
3a,b
2a,R=H; 2b,R=CH3; 3a,R=H; 3b,R=CH3

Molecules 2010, 15
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Table 1. Melting points, crystallization solvents, yields, molecular formulae and molecular
weights of compounds 2a,b and 3a,b.
Comp.
No.
2a
2b
3a
R
Mp (ºC)
Cryst. Solv.
Yield (%)
[α]30
Molecular Formula (Mol. Wt.)
D
H
CH3
H
182-184
196-198
252-254
246-248
EtOH
EtOH
AcOH/H2O
AcOH/H2O
75
68
82
85
C15H19N3O6 (337.33)
C17H23N3O6 (365.38)
C13H19N7O4 (337.33)
C15H23N7O4 (365.39)
+15 (DMF)
-
+56 (DMF)
CH3
-
3b
Compounds 4a-f and 5a-f are new, and were synthesized via simple condensation of the hydrazides
3a,b with appropriate aromatic or heterocyclic aldehydes, namely, benzaldehyde, p-methoxy-
benzaldehyde, 3,4,5-trimethoxybenzaldehyde, p-chlorobenzaldehyde, 2-chloro-6-flourobenzaldehyde,
and/or 2-thiophenealdehyde in refluxing absolute ethanol giving the corresponding N2,N6-bis(1-(2-
(substituted benzylidene)hydrazinyl)-1-oxopropan-2-yl)pyridine-2,6-dicarboxamides 4a-f and N2,N6-
bis(1-(2-(substituted
benzylidene)hydrazinyl)-2-methyl-1-oxopropan-2-yl)pyridine-2,6-
dicarboxamides 5a-f (Scheme 2).
Scheme 2. Synthetic Pathway for Compounds 4a-f and 5a-f.
O
O
N
O
O
CH3
N
H3C
R
NH
HN
CH3
H3C
R
NH
HN
HN
ArCHO
EtOH
R
O
R
O
O
NH
N
HN
N
O
NH
NH2
NH2
3a,b
b,
Ar
Ar
4a-f, R = H
a,
R = H, R = CH3
5a-f, R = CH3
Table 2. Melting points, crystallization solvents, yields, specific rotation, molecular
formulae and molecular weights of compounds 4a-h and 5a-h.
Comp.
No.
4a
Ar
Mp
(ºC)
122-124
Cryst.
Solv.
EtOH/
Yield
(%)
68
[α]30
(DMF)
+ 18
Molecular Formula
(Mol. Wt.)
D
C27H27N7O4
n-hexane
(513.55)
210-212
148-150
AcOH/H2O
AcOH
75
80
+ 32
+ 24
C29H31N7O6
(573.60)
4b
4c
OCH3
OCH3
OCH3
OCH3
C33H39N7O10
(693.70)

Molecules 2010, 15
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Table 2. Cont.
205-207
168-170
EtOH/
n-hexane
65
72
+ 54
+ 12
C27H25Cl2N7O4
(582.44)
4d
Cl
Cl
AcOH/H2O
C27H23Cl2F2N7O4
(618.42)
4e
F
185-187
EtOH/
n-hexane
C23H23N7O4S2
(525.60)
4f
60
70
+ 16
-
S
240-242
120-122
EtOH
C29H31N7O4
(541.60)
5a
Dioxane
75
66
-
-
C31H35N7O6
(601.65)
5b
5c
OCH3
OCH3
OCH3
OCH3
135-137
AcOH/H2O
C35H43N7O10
(721.76)
155-157
213-215
AcOH/H2O
AcOH/H2O
78
86
-
-
C29H29Cl2N7O4
(610.49)
5d
Cl
Cl
C29H27Cl2F2N7O4
(646.47)
5e
F
220-222
EtOH
60
-
C25H27N7O4S2
(553.66)
5f
S
The structures of all the newly synthesized compounds 2a,b, 3a,b, 4a-f and 5a-f were confirmed by
their IR, 1H-NMR, 13C-NMR and mass spectra.
2.2. Antimicrobial testing
Preliminary biological activity screening of the synthesized compounds has been performed at 50
μg/mL against microorganisms representing Gram-positive bacteria (Bacillus subtilis and
Staphylococcus aureus), Gram-negative bacteria (Escherichia coli) and fungi (Candida albicans and
Aspergillus niger), using the bioassay technique for antibiotics [22] specified in the US Pharmacopeia.
From Table 3 it appears that the Schiff's bases 4b-f and 5b-f have significant antimicrobial activities.
Among these Schiff's bases, the 4-methoxy- 4b,5b, 3,4,5-trimethoxy- 4c,5c, 4-chloro-4d,5d, 2-chloro-
6-flouro-4e,5e and 2-thienyl- derivatives 4f,5f have antimicrobial activities higher those of 4a,5a with
an unsubstituted phenyl group. The hydrazides 3a,b were found to have lower antimicrobial activities,
while the esters 2a,b didn’t show any antifungal activity. Streptomycin and fusidic acid were used as
antibacterial and antifungal reference drugs, respectively.

Molecules 2010, 15
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Table 3. Antimicrobial activities of the new synthesized compounds 2a,b, 3a,b, 4a-f and 5a-f.
Inhibition zones (cm)
Gram-positive
Gram-negative
Escherichia
Fungi
Comp. No.
Bacillus
Staphylococcus
Candida albicans Aspergillus niger
subtilis
15
14
1
aureus
18
13
12
14
15
19
18
19
20
18
15
18
20
17
20
19
21
-
coli
16
15
13
15
16
19
20
21
20
18
17
20
19
18
20
21
22
-
-
-
-
-
2a
2b
3a
3b
4a
4b
4c
4d
4e
4f
5a
5b
5c
5d
5e
14
12
14
16
17
17
18
17
12
14
16
16
17
16
-
10
11
12
15
16
14
16
16
10
15
16
14
14
15
-
12
14
20
21
20
22
21
16
21
19
20
22
20
22
-
5f
Streptomycin
Fusidic acid
18
17
3. Experimental
3.1. General
Melting points (ºC) were measured in open glass capillaries using a Barnstead 9001 Electrothermal
melting point apparatus and are uncorrected. NMR spectra were obtained on a Bruker AC 500 Ultra
1
Shield NMR spectrometer (Bruker, Fällanden, Switzerland) operating at 500.13 MHz for H and
13
125.76 MHz for C; the chemical shifts are expressed in δ (ppm) downfield from tetramethylsilane
(TMS) used as internal standard. Electrospray ionization mass spectra (ESI-MS) were recorded on a
Waters QuatroMicro triple quadrupole tandem mass spectrometer at 4.0 and 3.5 kV for positive and
negative ions, respectively. Elemental analyses (C, H, N, Cl, S) were in full agreement with the
proposed structures within ± 0.4% of the theoretical values. Monitoring of reactions and checking of
purity of the final products were carried out by thin layer chromatography (TLC) using silica gel
precoated aluminum sheets (60 F254, Merck) and visualization with ultraviolet light (UV) at 365 and
254 nm.
3.2. Chemistry
3.2.1. N2,N6-Bis(1-methoxy-oxopropan-2-yl)pyridine-2,6-dicarboxamides 2a,b
To a solution of L-alanine and/or 2-methylalanine methyl esters (2 mmol), 2,6-pyridinedicarboyl
dichloride 1 (0.204 g, 1 mmol) in dichloromethane (15 mL) was added at -10 ºC with stirring.
Triethylamine was added dropwise to the reaction mixture in order to keep the reaction mixture
slightly basic (pH ~ 8). Stirring was continued for 3 h more at -15 ºC and then 12 h at r.t. The reaction
mixture was then washed with water, 1N hydrochloric acid, 1N sodium bicarbonate and finally with

Molecules 2010, 15
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water and dried over anhydrous calcium chloride. The solvent was evaporated under reduced pressure
to dryness and the obtained solid was crystallized from the appropriate solvent indicated in Table 1 to
give the corresponding bis-esters 2a,b.
N2,N6-Bis(1-methoxy-1-oxopropan-2-yl)pyridine-2,6-dicarboxamide (2a). IR (KBr, cm-1): ν 3352-3268
1
(NH), 1745 (C=O, ester), 1678 (C=O, amide); H-NMR (DMSO-d6): δ 1.32 (d, 6H, 2 CH3), 3.62 (s,
6H, 2OCH3), 4.25 (m, 2H, 2CH), 8.18-8.26 (m, 3H, pyridine-H), 8.62 (s, 2H, 2NH exchangeable with
13
D2O); C-NMR: 17.32 (2C, 2CH3), 47.88 (2C, 2CH), 56.15 (2C, 2OCH3), 123.12, 138.54, 149.65
(5C, pyridine-C), 159.96, 171.86 (4C, 4C=O); MS, m/z (%): 337 (M+, 5), 306 (15), 275 (100), 219
(12), 163 (26), 133 (75), 105 (14), 77 (65).
N2,N6-Bis(1-methoxy-2-methyl-1-oxopropan-2-yl)pyridine-2,6-dicarboxamide (2b). IR (KBr, cm-1): ν
1
3332-3278 (NH), 1747 (C=O, ester), 1676 (C=O, amide); H-NMR (DMSO-d6): δ 1.46 (s, 12H, 4
CH3), 3.56 (s, 6H, 2OCH3), 8.15-8.28 (m, 3H, pyridine-H), 8.42 (s, 2H, 2NH exchangeable with D2O);
13C-NMR: 23.78 (4C, 4CH3), 55.18 (2C, NH-C(CH3)2CO), 123.18, 139.05, 149.72 (5C, pyr-C),
160.12, 172.65 (4C, 4C=O); MS, m/z (%): 365 (M+, 8), 334 (25), 303 (80), 218 (100), 148 (6), 133
(42), 105 (34), 77 (78).
3.2.2. N2,N6-Bis(1-hydrazinyl)pyridine-2,6-dicarboxamides 3a,b
A mixture of bis-esters 2a or 2b (1 mmol) and hydrazine hydrate (0.8 mL, 16 mmol) in absolute
ethanol (50 mL) was refluxed for 6 h. Excess solvent was evaporated under reduced pressure to
dryness, the obtained residue was triturated with ethanol and the resulting solid was crystallized from
the appropriate solvent to give bis-hydrazide derivatives 3a,b, respectively (Table 1).
N2,N6-Bis(1-hydrazinyl-1-oxopropan-2-yl)pyridine-2,6-dicarboxamide (3a). IR (KBr, cm-1): ν
3465-3228 (NH, NH2), 1680, 1675 (2C=O, amide); 1H-NMR (DMSO-d6): δ 1.36 (d, 6H, 2 CH3), 4.12
(s, 4H, 2NH2 exchangeable with D2O), 4.62 (m, 2H, 2CH), 8.12-8.24 (m, 3H, pyridine-H), 8.68, 9.05
(2s, 4H, 4NH exchangeable with D2O); 13C-NMR: 18.05 (2C, 2CH3), 50.12 (2C, 2CH), 123.34,
139.05, 149.78 (5C, pyridine-C), 160.08, 171.24 (4C, 4C=O); MS, m/z (%): 337 (M+, 15), 321 (8), 305
(5), 275 (10), 219 (12), 176 (16), 133 (100), 105 (24), 77 (45).
N2,N6-Bis(1-hydrazinyl-2-methyl-1-oxopropan-2-yl)pyridine-2,6-dicarboxamide (3b). IR (KBr, cm-1):
1
ν 3470-3218 (NH, NH2), 1678, 1672 (2C=O, amide); H-NMR (DMSO-d6): δ 1.35 (s, 12H, 4 CH3),
4.15 (s, 4H, 2NH2 exchangeable with D2O), 8.18-8.26 (m, 3H, pyridine-H), 8.18, 8.98 (2s, 4H, 4NH
13
exchangeable with D2O); C-NMR: 25.86 (4C, 4CH3), 59.64 (2C, NH-C(CH3)2CO), 123.42, 139.00,
149.88 (5C, pyridine-C), 160.08, 178.65 (4C, 4C=O); MS, m/z (%): 365 (M+, 4), 333 (5), 233 (100),
148 (65), 133 (48), 105 (56), 77 (76).

Molecules 2010, 15
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3.2.3. General procedure for the synthesis of N2,N6-bis(1-(substituted)pyridine-2,6-dicarboxamides 4a-
f and 5a-f
A mixture of the hydrazide derivative 3a or 3b (1 mmol) and the appropriate aldehyde, namely
benzaldehyde, p-methoxybenzaldehyde, 3,4,5-trimethoxybenzaldehyde, p-chlorobenzaldehyde,
2 chloro-6-flourobenzaldehyde, and/or 2-thiophenealdehyde (2 mmol) in absolute ethanol (25 mL) was
heated under reflux for 4-6 h. The excess solvent was evaporated under reduced pressure, the residue
was washed with n-hexane and triturated with diethyl ether. The obtained solid was filtered off,
washed with ether, and crystallized from the appropriate solvent (see Table 2) to give the
corresponding dicarboxamide derivatives 4a-f and 5a-f, respectively.
N2,N6-Bis(1-(2-benzylidenehydrazinyl)-1-oxopropan-2-yl)pyridine-2,6-dicarboxamide (4a). IR (KBr,
1
cm-1): ν 3356-3198 (NH), 1675, 1674 (2C=O, amide); H-NMR (DMSO-d6): δ 1.32 (d, 6H, 2 CH3),
4.56 (m, 2H, 2CH), 7.15-7.76 (m, 10H, 2Ph-H), 8.16-8.35 (m, 5H, pyridine-H + 2CH=N), 8.72, 10.54
(2s, 4H, 4NH exchangeable with D2O); 13C-NMR: 17.76 (2C, 2CH3), 50.85 (2C, 2CH), 127.16,
128.60, 130.48, 132.98 (12C, 2Ph), 123.36, 138.70, 149.56 (5C, pyridine-C), 143.75 (2C, 2 CH=N),
160.28, 176.86 (4C, 4C=O); MS, m/z (%): 514 (M++1, 12), 436 (4), 359 (16), 317 (24), 275 (100), 245
(65), 216 (32), 176 (46), 133 (90), 105 (64), 77 (52).
N2,N6-Bis(1-(2-(4-methoxybenzylidene)hydrazinyl)-1-oxopropan-2-yl)pyridine-2,6-dicarboxamide
(4b). IR (KBr, cm-1): ν 3348-3210 (NH), 1680, 1676 (2C=O, amide); 1H-NMR (DMSO-d6): δ 1.34 (d,
6H, 2 CH3), 3.72 (s, 6H, 2OCH3), 4.52 (m, 2H, 2CH), 7.18 (d, 4H, Ar-H), 7.78 (d, 4H, Ar-H), 8.18 (s,
2H, 2CH=N), 8.23-8.32 (m, 3H, pyridine-H), 8.68, 10.62 (2s, 4H, 4NH exchangeable with D2O); 13C-
NMR: 17.72 (2C, 2CH3), 51.05 (2C, 2CH), 54.66 (2C, 2OCH3), 114.05, 125.60, 129.78, 162.56 (12C,
2Ar-C), 123.30, 138.75, 149.50 (5C, pyridine-C), 144.00 (2C, 2 CH=N), 161.02, 176.82 (4C, 4C=O);
MS, m/z (%): 574 (M++1, 6), 542 (12), 511 (6), 353 (24), 322 (18), 220 (100), 204 (65), 189 (13), 133
(76), 118 (46), 105 (46), 77 (44).
N2,N6-Bis(1-oxo-1-(2-(3,4,5-trimethoxybenzylidene)hydrazinyl)propan-2-yl)pyridine-2,6-dicarbox-
1
amide (4c). IR (KBr, cm-1): ν 3354-3218 (NH), 1677, 1674 (2C=O, amide); H-NMR (DMSO-d6): δ
1.28 (d, 6H, 2 CH3), 3.76 (s, 18H, 6OCH3), 4.48 (m, 2H, 2CH), 7.12 (s, 4H, Ar-H), 8.14 (s, 2H,
13
2CH=N), 8.24-8.36 (m, 3H, pyridine-H), 8.72, 10.76 (2s, 4H, 4NH exchangeable with D2O); C-
NMR: 18.05 (2C, 2CH3), 51.55 (2C, 2CH), 55.10 (4C, 4OCH3), 58.72 (2C, 2OCH3), 104.52, 127.65,
140.68, 152.82 (12C, 2Ar-C), 123.45, 139.06, 149.65 (5C, pyridine-C), 145.18 (2C, 2 CH=N), 161.25,
176.88 (4C, 4C=O); MS, m/z (%): 694 (M++1, 4), 662 (8), 631 (12), 526 (18), 412 (28), 280 (100), 204
(45), 189 (8), 167 (34), 133 (82), 105 (66), 77 (56).
N2,N6-Bis(1-(2-(4-chlorobenzylidene)hydrazinyl)-1-oxopropan-2-yl)pyridine-2,6-dicarboxamide (4d).
IR (KBr, cm-1): ν 3352-3198 (NH), 1677, 1675 (2C=O, amide); 1H-NMR (DMSO-d6): δ 1.28 (d, 6H, 2
CH3), 4.48 (m, 2H, 2CH), 7.42 (d, 4H, Ar-H), 7.65 (d, 4H, Ar-H), 8.16-8.35 (m, 5H, pyridine-H +
2CH=N), 8.72, 10.82 (2s, 4H, 4NH exchangeable with D2O); 13C-NMR: 18.04 (2C, 2CH3), 50.95 (2C,
2CH), 127.82, 128.62, 130.88, 135.76 (12C, 2Ar-C), 123.45, 139.08, 149.45 (5C, pyridine-C), 143.86

Molecules 2010, 15
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(2C, 2 CH=N), 160.94, 176.55 (4C, 4C=O); MS, m/z (%): 582 (M+, 6), 584 (M++2, 2), 548 (12), 546
(4), 511 (15), 435 (18), 359 (16), 204 (100), 133 (65), 105 (32), 77 (78).
N2,N6-Bis(1-(2-(2-chloro-6-fluorobenzylidene)hydrazinyl)-1-oxopropan-2-yl)pyridine-2,6-dicarbox-
1
amide (4e). IR (KBr, cm-1): ν 3410-3235 (NH), 1682, 1674 (2C=O, amide); H-NMR (DMSO-d6): δ
1.34 (d, 6H, 2 CH3), 4.55 (m, 2H, 2CH), 7.24-7.72 (m, 6H, Ar-H), 8.15-8.38 (m, 5H, pyridine-H +
2CH=N), 8.80, 10.74 (2s, 4H, 4NH exchangeable with D2O); 13C-NMR: 17.88 (2C, 2CH3), 51.10 (2C,
2CH), 112.68, 117.78, 124.86, 133.56, 134.48, 160.65 (12C, 2Ar-C), 124.00, 139.24, 149.56 (5C,
pyridine-C), 142.94 (2C, 2 CH=N), 160.76, 176.55 (4C, 4C=O); MS, m/z (%): 618 (M+, 15), 620
(M++2, 6), 488 (13), 490 (4), 359 (100), 318 (66), 275 (88), 204 (96), 133 (45), 105 (86), 77 (84).
N2,N6-Bis(1-oxo-1-(2-(thiophen-2-ylmethylene)hydrazinyl)propan-2-yl)pyridine-2,6-dicarboxamide
(4f). IR (KBr, cm-1): ν 3390-3212 (NH), 1680, 1675 (2C=O, amide); 1H-NMR (DMSO-d6): δ 1.42 (d,
6H, 2 CH3), 4.46 (m, 2H, 2CH), 7.10-7.65 (m, 6H, thiophene-H), 8.22-8.35 (m, 5H, pyridine-H +
2CH=N), 8.76, 10.65 (2s, 4H, 4NH exchangeable with D2O); 13C-NMR: 17.92 (2C, 2CH3), 51.08 (2C,
2CH), 126.76, 127.65, 139.32, 143.84 (8C, 2 thiophene-C), 124.05, 139.32, 149.48 (5C, pyridine-C),
132.88 (2C, 2 CH=N), 160.65, 176.62 (4C, 4C=O); MS, m/z (%): 525 (M+, 6), 442 (14), 400 (32), 329
(10), 317 (4), 196 (75), 204 (86), 133 (100), 105 (68), 77 (72).
N2,N6-Bis(1-(2-benzylidenehydrazinyl)-2-methyl-1-oxopropan-2-yl)pyridine-2,6-dicarboxamide (5a).
IR (KBr, cm-1): ν 3360-3210 (NH), 1678, 1674 (2C=O, amide); 1H-NMR (DMSO-d6): δ 1.42 (s, 12H,
4 CH3), 7.25-7.82 (m, 10H, 2Ph-H), 8.12-8.25 (m, 3H, pyridine-H), 8.28 (s, 2H, 2CH=N), 8.45, 10.65
13
(2s, 4H, 4NH exchangeable with D2O); C-NMR: 25.48 (4C, 4CH3), 59.78 (2C, 2 NHC(CH3)2CO),
128.10, 128.90, 130.48, 133.05 (12C, 2Ph-C), 124.12, 139.01, 149.48 (5C, pyridine-C), 143.65 (2C, 2
CH=N), 160.12, 178.86 (4C, 4C=O); MS, m/z (%): 541 (M+, 6), 464 (4), 387 (12), 345 (24), 303 (100),
218 (45), 133 (65), 105 (74), 77 (48).
N2,N6-Bis(1-(2-(4-methoxybenzylidene)hydrazinyl)-2-methyl-1-oxopropan-2-yl)pyridine-2,6-dicarbox-
1
amide (5b). IR (KBr, cm-1): ν 3354-3212 (NH), 1680, 1678 (2C=O, amide); H-NMR (DMSO-d6): δ
1.30 (s, 12H, 4 CH3), 3.68 (s, 6H, 2OCH3), 7.08 (d, 4H, Ar-H), 7.72 (d, 4H, Ar-H), 8.16 (s, 2H,
13
2CH=N), 8.18-8.30 (m, 3H, pyridine-H), 8.42, 10.68 (2s, 4H, 4NH exchangeable with D2O); C-
NMR: 25.36 (4C, 4CH3), 59.64 (2C, 2 NHC(CH3)2CO), 54.55 (2C, 2OCH3), 113.98, 125.56, 129.88,
162.34 (12C, 2Ar-C), 123.48, 139.05, 149.35 (5C, pyridine-C), 143.89 (2C, 2 CH=N), 160.75, 178.85
(4C, 4C=O); MS, m/z (%): 602 (M++1, 16), 570 (22), 539 (18), 336 (100), 218 (10), 203 (67), 205
(54), 133 (18), 118 (45), 105 (42), 77 (32).
N2,N6-Bis(2-methyl-1-oxo-1-(2-(3,4,5-trimethoxybenzylidene)hydrazinyl)propan-2-yl)pyridine-2,6-
1
dicarboxamide (5c). IR (KBr, cm-1): ν 3362-3210 (NH), 1679, 1675 (2C=O, amide); H-NMR
(DMSO-d6): δ 1.34 (s, 12H, 4 CH3), 3.72 (s, 18H, 6OCH3), 7.10 (s, 4H, Ar-H), 8.22 (s, 2H, 2CH=N),
13
8.18-8.28 (m, 3H, pyridine-H), 8.62, 10.48 (2s, 4H, 4NH exchangeable with D2O); C-NMR: 25.67
(4C, 4CH3), 55.82 (4C, 4OCH3), 59.72 (2C, 2 NHC(CH3)2CO), 59.88 (2C, 2OCH3), 104.05, 127.64,
141.08, 153.28 (12C, 2Ar-C), 123.75, 139.12, 149.70 (5C, pyridine-C), 145.22 (2C, CH=N), 160.98,

Molecules 2010, 15
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179.12 (4C, 4C=O); MS, m/z (%): 722 (M++1, 14), 690 (18), 660 (6), 524 (18), 388 (12), 345 (10), 303
(100), 218 (45), 133 (56), 77 (66).
N2,N6-Bis(1-(2-(4-chlorobenzylidene)hydrazinyl)-2-methyl-1-oxopropan-2-yl)pyridine-2,6-dicarbox-
1
amide (5d). IR (KBr, cm-1): ν 3360-3205 (NH), 1678, 1674 (2C=O, amide); H-NMR (DMSO-d6): δ
1.32 (s, 12H, 4 CH3), 7.44 (d, 4H, Ar-H), 7.66 (d, 4H, Ar-H), 8.22-8.37 (m, 5H, pyridine-H +
2CH=N), 8.64, 10.75 (2s, 4H, 4NH exchangeable with D2O); 13C-NMR: 26.01 (4C, 4CH3), 59.68 (2C,
2 NHC(CH3)2CO), 127.66, 128.88, 130.84, 135.92 (12C, 2Ar-C), 123.65, 139.10, 149.34 (5C,
pyridine-C), 143.90 (2C, 2 CH=N), 160.86, 179.55 (4C, 4C=O); MS, m/z (%): 610 (M+, 10), 612
(M++2, 3), 498 (18), 500 (5), 463 (6), 387 (24), 345 (35), 303 (78), 218 (100), 133 (25), 105 (30),
77 (82).
N2,N6-Bis(1-(2-(2-chloro-6-fluorobenzylidene)hydrazinyl)-2-methyl-1-oxopropan-2-yl)pyridine-2,6-di-
carboxamide (5e). IR (KBr, cm-1): ν 3410-3232 (NH), 1680, 1676 (2C=O, amide); 1H-NMR (DMSO-
d6): δ 1.35 (s, 12H, 4 CH3), 7.28-7.78 (m, 6H, Ar-H), 8.16-8.36 (m, 5H, pyridine-H + 2CH=N), 8.72,
10.70 (2s, 4H, 4NH exchangeable with D2O); 13C-NMR: 26.01 (4C, 4CH3), 59.56 (2C, 2
NHC(CH3)2CO), 112.62, 117.66, 124.72, 133.46, 134.52, 160.60 (12C, 2Ar-C), 123.96, 139.12,
149.50 (5C, pyridine-C), 142.67 (2C, 2 CH=N), 160.66, 179.55 (4C, 4C=O); MS, m/z (%): 646 (M+,
6), 648 (M++2, 2), 516 (18), 518 (6), 387 (45), 345 (100), 303 (16), 133 (75), 105 (66), 77 (80).
N2,N6-Bis(2-methyl-1-oxo-1-((E)-2-(thiophen-2-ylmethylene)hydrazinyl)propan-2-yl)pyridine-2,6-di-
1
carboxamide (5f). IR (KBr, cm-1): ν 3392-3208 (NH), 1680, 1676 (2C=O, amide); H-NMR (DMSO-
d6): δ 1.35 (s, 12H, 4 CH3), 7.14-7.62 (m, 6H, thiophene-H), 8.24-8.38 (m, 5H, pyridine-H + 2CH=N),
13
8.36, 10.15 (2s, 4H, 4NH exchangeable with D2O); C-NMR: 26.02 (4C, 4CH3), 59.52 (2C, 2
NHC(CH3)2CO), 126.64, 127.55, 139.28, 143.92 (8C, 2 thiophene-C), 123.98, 139.36, 149.42 (5C,
pyridine-C), 128.10 (2C, 2 CH=N), 160.55, 179.86 (4C, 4C=O); MS, m/z (%): 553 (M+, 16), 470 (24),
387 (13), 345 (12), 303 (24), 218 (100), 133 (860), 105 (64), 77 (54).
4. Conclusions
A series of pyridine-bridged 2,6-bis-carboxamide Schiff's bases was prepared starting from
2,6-pyridinedicarbonyl dichloride (1) and L-alanine or 2-methylalanine methyl ester. The structural
assignments of the new compounds were based on chemical and spectroscopic evidence. The newly
synthesized compounds 2-5 have been screened for their bactericidal and fungicidal activities, and the
Schiff's bases 4b-f and 5b-f have significant antimicrobial activities compared to streptomycin and
fusidic acid which were used as antibacterial and antifungal reference drugs, respectively. The
substituted 4-methoxy- 4b,5b, 3,4,5-trimethoxy-4c,5c, 4-chloro-4d,5d, 2-chloro-6-flouro-4e,5e and 2-
thienyl- derivatives 4f,5f have antimicrobial activities higher than that of 4a,5a with an unsubstituted
phenyl group.

Molecules 2010, 15
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Acknowledgements
The financial support of the Research Center of the College of Pharmacy, King Saud University is
greatly appreciated. The kind help of El-Sayed E. Mostafa, Department of Microbial Chemistry,
National Research Center, Cairo, Egypt, for carrying out the antimicrobial screening is
highly appreciated.
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Sample Availability: Samples of the compounds are available from the authors.
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