Nitropyridines. 9*. Synthesis of substituted 3-amino-5-nitropyridines

Article abstract of DOI:10.1007/s10593-011-0660-0

Substituted 3-amino-5-nitropyridines have been synthesized by the Curtius, Schmidt, and Hofmann reactions.

Full text of DOI:10.1007/s10593-011-0660-0

Chemistry of Heterocyclic Compounds, Vol. 46, No. 10, 2011
NITROPYRIDINES. 9*. SYNTHESIS OF SUBSTITUTED
3-AMINO-5-NITROPYRIDINES
G. P. Sagitullina1***, A. K. Garkushenko1, L. V. Glyzdinskaya1,
F. A. Uldashev1, M. A. Vorontsova1, and R. S. Sagitullin1**
Substituted 3-amino-5-nitropyridines have been synthesized by the Curtius, Schmidt, and Hofmann
reactions.
Keywords: 3-acetyl-5-nitropyridine, nitronicotinic acid, nitronicotinamide, 5-nitropyridines, 3-pyridyl-
carbamates, the Curtius, Schmidt, and Hofmann reactions and their modifications.
The discovery of bacteriostatic activity of Sulfidine was the stimulus for the study of the chemistry of
isomeric aminopyridines with the objective of converting them into promising sulfamide preparations [2].
Aminopyridines with nitrogroups on the pyridine nucleus have been insufficiently well studied, which is
explained by their low accessibility. Exceptions are 2-amino-3-nitro- and 5-nitropyridines and their derivatives,
which are accessible thanks to the activity of 2-aminopyridine to electrophilic substitution [3]. The number of
publications connected with the preparation of substituted 3-amino-5-nitropyridines is limited to those
describing the synthesis of 3-amino-5-nitropyridine [4-7], 3-amino-2-methyl-5-nitropyridine [8], and 3-amino-
5-nitrocollidine [9].
The objective of the present work was the synthesis of the previously unknown 3-amino-5-nitro-
pyridines 2a,b, 4, 6b,c, and 9a-c based on the available nitropyridines 1a,b, 3a-c, and 7a-c which contain acetyl,
amide, and carboxyl groups at position 3 of the pyridine nucleus, The choice of the variant of the sextet
rearrangement in the synthesis of 3-amino-5-nitropyridines was determined by the availability of the
corresponding 5-nitropyridines. 3-Amino-5-nitropyridines 2a,b were obtained by the Schmidt reaction starting
from 3-acetyl-5-nitropyridines 1a,b by a one-stage synthesis which we had developed earlier [10, 11]:
O2N
R
Ac
O2N
R
NH2
Me
1. NaN3, H2SO4
2. H2O, 
N
Me
N
2a,b
1a,b
1,2 a R = Me, b R = Ph
_______
* For Communication 8, see [1].
** Deceased.
*** To whom correspondence should be addressed, e-mail: Sagitullina@orgchem.univer.omsk.su.
1Department of Organic Chemistry, F. M. Dostoevsky Omsk State University, Omsk 644077, Russia.
__________________________________________________________________________________________
Translated from Khimya Geterotsiklicheskikh Soedinenii, No. 10, 1551-1558, October, 2010. Original
article received December 11, 2009.
0009-3122/11/4610-1255©2011 Springer Science+Business Media, Inc.
1255

In the synthesis of 3-amino-4,6-dimethyl-5-nitropyridine (4) from the previously known nicotinamide
3a we successfully used the classical Hofmann reaction (NaOBr) [12]. Under these conditions the
nitronicotinamides 3b,c and 7a,c, which are insoluble in aqueous alkali, did not rearrange to the corresponding
3-amino-5-nitropyridines. In this connection we turned to a known modification of the Hofmann reaction in
which a variety of oxidizing reagents were used (MeOBr, Pd(OAc)4, NBS-Hg(OAc)2, NBS-DBU,
PhI(OCOCF3)2, PhI(OAc)2, PhIO, PhI(OTs)OH, BnN+Me3Br3 , Bu4N+Br3 ) [13,19].
-
-
To prepare the 3-amino-5-pyridines 6b,c from the corresponding amides 3b,c a two-stage modification
of the Hofmann reaction was used (a solution of MeOBr in methanol).The splitting of the 3-pyridylcarbamates
5b,c was carried out with the help of an aqueous-ethanolic solution of sodium hydroxide on heating.
R1
O
Me
O2N
R
O2N
Me
NH2
NaOH, Br2
H2O, 75°C
MeONa, Br2
MeOH, 60°C
NH2
N
R2
N
3a–c
4
R1
R1
O2N
NHCOOMe
R2
O2N
R
NH2
R2
NaOH, EtOH
70°C
R
N
N
5b,c
6b,c
3 a R = R1 = Me, R2 = H; 3,5,6 b R = R1 = Ph, R2 = Me; c R = Ph, R1 = p-MeOC6H4, R2 = Me
In the synthesis of the 3-pyridylcarbamates 8a-c by the Hofmann reaction the most effective reagent for
the oxidative rearrangement of the amides 7a-c was diacetoxyiodobenzene. The carbamate 8a was stable on
heating in aqueous alkali but it was successfully hydrolyzed to the 3-aminopyridine 9a on prolonged heating in a
mixture of acetic and sulfuric acids.
TABLE 1. Spectral Characteristics of the Compounds Synthesized
Com-
pound
IR spectrum,
1Н NMR spectrum, δ, ppm (J, Hz)*
ν, cm-1
1
2
3
2a
1346, 1520 (NO2),
3420, 3510 (NH2)
2.35 (3Н, s, 2-СН3); 2.55 (3H, s, 6-СH3); 5.56 (2H, s, NH2);
7.55 (1Н, s, Н-4)
2b
4
1325, 1527 (NO2),
3400, 3505 (NH2)
2.51 (3H, s, 2-СH3); 3.96 (2H, s, NH2); 7.34 (1Н, s, Н-4);
7.36-7.49 (5Н, m, C6H5)
1370, 1530 (NO2),
3395, 3500 (NH2)
1.92 (3Н, s, 4-СН3); 2.20 (3H, s, 6-СH3); 5.55 (2H, s, NH2);
7.95 (1Н, s, Н-2)
5b
5c
1340, 1540 (NO2),
1740 (CO), 3410 (NH) 7.45-7.58 (8Н, m, 4,6-C6H5); 9.13 (1H, br. s, NH)
2.56 (3H, s, 2-СH3); 3.47 (3H, s, CO2CH3); 7.19-7.26 (2Н, m, 4,6-C6H5);
1362, 1514 (NO2), 2.54 (3H, s, 2-CH3); 3.50 (3H, s, CO2CH3); 3.79 (3H, s, OCH3);
1699 (CO), 3274 (NH) 7.00-7.05 (2H, m, JAX = 8.6, 4-C6H4, AA'XX');
7.12-7.17 (2H, m, JAX = 8.6, 4-C6H4, AA'XX');
7.47-7.57 (5H, m, 6-C6H5); 9.08 (1H, br. s, NH)
6b
1360, 1520 (NO2),
3400, 3490 (NH2)
2.54 (3H, s, 2-СH3); 3.69 (2H, s, NH2); 7.36-7.55 (10Н, m, 4,6-C6H5)
6с
1361, 1525 (NO2),
3382, 3479 (NH2)
2.54 (3H, s, 2-CH3); 3.79 (2H, s, NH2); 3.84 (3H, s, OCH3);
6.99-7.03 (2H, m, JAX = 8.8, 4-C6H4, AA'XX');
7.24-7.27 (2H, m, JAX = 8.8, 4-C6H4, AA'XX');
7.35-7.42 (3H, m, 6-C6H5); 7.53-7.57 (2H, m, 6-C6H5)
8a
1335, 1528 (NO2),
1.31 (3H, t, J = 7.2, CH2CH3); 2.85 (3H, s, 6-СH3);
1719 (CO), 3217 (NH) 4.25 (2H, q, J = 7.2, CH2CH3); 6.87 (1H, s, NH);
7.53-7.61 (5Н, m, C6H5); 9.18 (1Н, s, Н-4)
1256

TABLE 1 (continued)
1
2
3
8b
1340, 1530 (NO2),
1.31 (3H, t, J = 7.2, CH2CH3); 4.26 (2H, q, J = 7.2, CH2CH3);
1725 (CO), 3440 (NH) 7.05 (1H, s, NH); 7.57-7.63 (5Н, m, C6H5); 9.17 (1Н, d, 4J = 2.4, Н-6);
9.38 (1H, d, 4J = 2.4, H-4)
8c
1357, 1532 (NO2),
1.03 (3Н, br. s, СН2СН3); 2.64 (3H, s, 6-CH3); 3.92 (2H, br. s, СН2СН3);
6.33 (1H, br. s, NH); 6.57 (1H, dd, 3J4,3 = 3.3, 3J4,5 = 1.9, Н-4 Fur);
6.82 (1H, d, 3J3,4 = 3.3, Н-3 Fur); 7.43-7.51 (3H, m, С6H5);
7.60-7.66 (3Н, m, С6H5, Н-5 Fur)
1715 (CO), 3207 (NH)
9a
9b
9c
1360, 1540 (NO2),
3380, 3460 (NH2)
2.77 (3H, s, 6-СH3); 4.03 (2H, s, NH2); 7.43-7.53 (3Н, m, C6H5);
7.66-7.70 (3Н, m, C6H5, Н-4)
1347, 1530 (NO2),
3395, 3500 (NH2)
4.28 (2H, s, NH2); 7.31-7.78 (6Н, m, C6H5, H-4); 9.17 (1Н, s, Н-6)
1351, 1529 (NO2),
3320, 3398 (NH2)
2.53 (3H, s, 6-CH3); 4.46 (2H, s, NH2);
6.60 (1H, dd, 3J4,3 = 3.5, 3J4,5 = 1.7, Н-4 Fur);
6.79 (1H, dd, 3J3,4 = 3.5, 4J3,5 = 0.5, Н-3 Fur); 7.46-7.57 (3H, m, С6H5);
7.61 (1H, dd, 3J5,4 = 1.7, 4J5,3 = 0.5, H-5 Fur); 7.65-7.69 (2Н, m, С6H5)
10
1531, 1366 (NO2),
1263, 1721, 3067,
3447 (COOH)
2.68 (3H, s, 2-CH3); 7.26-7.33 (2H, m, 4,6-C6H5);
7.43-7.50 (6H, m, 4,6-C6H5); 7.53-7.59 (2H, m, 4,6-C6H5);
13.55 (1H, br. s, COOH)
_______
* 1H NMR spectra were taken in DMSO-d6 (compounds 2a, 4, 5b,c, 8a-c, and 10)
and in CDCl3 (compounds 2b, 6b,c, and 9a-c).
As an alternative route to the synthesis of the 3-aminopyridine 6b we investigated the possibility of
obtaining it via the Curtius reaction. The hydrazide of the pyridinecarboxylic acid was not obtained by reaction
of its ester with hydrazine hydrate (even on heating in an ampoule), nor by the reaction of the corresponding
acid chloride with hydrazine hydrate. In this connection we chose the modified Curtius reaction using an azide –
diphenylphosphorylazide – for the synthesis of the azide of acid 10 [20-25]. It should be noted that in this
reaction the 3-aminopyridine 6b was obtained in low yield (25%) and required chromatographic purification.
TABLE 2. Mass Spectra of Compounds 2a,b, 4, 5b, 8b, and 9b
Com-
pound
m/z (I, %)*
2a
2b
168 [M+1]+• (9), 167 [М]+• (100), 151 (10), 150 (45), 122 (29), 121 (88), 120 (20), 119 (17),
107 (11), 106 (24), 105 (22), 104 (17), 95 (23), 94 (46), 93 (13), 80 (19), 79 (16), 78 (23),
77 (23), 68 (18), 67 (42), 54 (15), 53 (31), 52 (37), 51 (25), 44 (17), 42 (21), 40 (17)
230 [M+1]+• (16), 229 [М]+• (100), 199 (23), 184 (24), 172 (12), 171 (17), 168 (26), 156 (12),
141 (11), 140 (12), 129 (15), 128 (12), 115 (50), 114 (24), 113 (10), 81 (39), 77 (13), 70 (10),
68 (12), 42 (17), 28 (11)
168 [M+1]+• (9), 167 [М]+• (100), 150 (29), 121 (68), 120 (13), 119 (10), 95 (15), 94 (23), 93 (11),
4
81 (12), 80 (29), 67 (20), 66 (15), 65 (15), 54 (15), 53 (27), 42 (12), 39 (13)
5b
364 [M+1]+• (23), 363 [М]+• (100), 332 (19), 331 (70), 314 (26), 303 (11), 302 (32), 301 (43),
300 (11), 286 (22), 285 (28), 275 (14), 274 (53), 273 (16), 272 (11), 260 (15), 258 (24), 257 (14),
256 (15), 255 (12), 242 (19), 231 (13), 230 (32), 216 (18), 215 (11), 214 (19), 202 (11), 190 (19),
189 (48), 188 (12), 128 (11), 127 (13), 115 (12), 81 (24), 77 (13)
8b
9b
288 [M+1]+• (16), 287 [М]+• (100), 286 (33), 242 (13), 241 (14), 228 (10), 214 (54), 213 (18),
198 (11), 182 (17), 169 (16), 168 (68), 167 (22), 141 (13), 140 (24), 115 (11), 104 (12), 77 (15)
216 [M+1]+• (11), 215 [М]+• (92), 214 (100), 185 (24), 184 (26), 169 (13), 168 (40), 115 (33),
104 (16), 77 (15), 66 (15), 44 (10)
_______
* Peaks cited with I > 10%
1257

R1
R1
O
O2N
R
NHCO2Et
Ph
O2N
R
O2N
Me
NH2
Ph
PhI(OAc)2
AcOH, H2SO4
80°C
NH2
Ph
EtOH, Δ
N
N
N
8a–c
7a–c
9a
NaOH, EtOH
70°C
R1
O2N
NH2
R
N
Ph
9b,c
7,8 a R = Me, R1 = H; 7–9 b R = R1 = H; c R = Me, R1 = 2-Fur
O
Ph
O
Ph
Ph
N
Ph
N
C
O2N
Ph
O2N
NH2
Me
N
O2N
Ph
CON3
Me
O2N
Ph
H2O
(PhO)2P(O)N3
NEt3, dioxane
OH
N
Me
Ph
N
Me
6b
10
Spectral data for the compounds synthesized for the first time 2a,b, 4, 5b,c, 8a-c, 9a-c, and 10 are cited
in Tables 1 and 2, elemental analyses are cited in the experimental section.
EXPERIMENTAL
The IR spectra of compounds 5c, 6c, 8a, 8c, and 9c were recorded on an Infralum FT-801 (with an
attachment for a single-pass breakdown of the internal refraction), compound 10 on a Simex FT-801 (in a KBr
tablet) and the remaining compounds on a Specord IR-75 in CHCl3. 1H NMR spectra of compounds 2a,b, 4, 6b,
8b, and 9b were recorded on a Bruker AC-200 (200 MHz, TMS), compounds 5b,c and 8a,c on a Bruker AC-400
(400 MHz, using the solvent as internal standard), and compounds 6c, 9a,c, and 10 on a Bruker Advance DRX-
400 (400 MHz, using the solvent as internal standard). Mass spectra were recorded with an Agilent 5973N mass
spectrometer (ionizing energy 70eV, evaporator temperature 230-250°C). Elemental analyses were carried out
with a Perkin-Elmer C,H,N-analyzer. Silica gel Merck 60A, 0.060-0.200 mm, was used for column
chromatography. Monitoring of the course of reactions and purity of the compounds obtained was by TLC on
Silufol UV-254 plates.
3-Aminopyridines 2a,b (General Method). Sodium azide (0.43g, 6.6 mmol) was added by portions
with stirring to a mixture of the corresponding 3-acetylpyridine (6 mmol) [10,11] in 3 ml conc. H2SO4. After all
of the NaN3 had been added the reaction mixture was stirred at room temperature for 12 h, then ice (12 g) was
added and the mixture was heated for 12 h, cooled. Neutralized with aqueous ammonia, and the precipitate was
filtered off.
3-Amino-2,6-dimethyl-5-nitropyridine (2a). Yield 66%; mp 137-138° (toluene). Found, %: C 50.70;
H 5.28; N 24.98. C7H9N3O2. Calculated, %: C 50.29; H 5.43; N 25.14.
3-Amino-2-methyl-5-nitro-6-phenylpyridine (2b). Yield 70%; mp 95-96°C (heptane). Found, %:
C 62.75; H 4.82; N 18.47. C12H11N3O2. Calculated, %: C 62.87; H 4.84; N 18.33.
1258

3-Amino-4,6-dimethyl-5-nitropyridine (4). Bromine (0.6 ml, 12 mmol) was added dropwise to a
solution of NaOH (1.5 g, 37.5 mmol) in water (16 ml) at -5°C, then nicotinamide 3a [1] (1.5 g, 7.7 mmol) was
added, stirred with cooling for 30 min, then slowly heated to 75°C and kept at this temperature for 2.5 h. The
precipitate formed was filtered off, washed with 10% HCl (20 ml), boiled with activated charcoal, cooled,
neutralized with aqueous ammonia, and the residue was recrystallized from toluene. Yield 63%; mp 173-174°C
(toluene). Found, %: C 50.12; H 5.32; N 24.98. C7H9N3O2. Calculated, %: C 50.29, H 5.43, N 25.14.
Carbamates 5b,c (General Method). To a mixture of the corresponding nicotinamide 3b,c [1]
(3 mmol) and a solution of sodium methoxide (prepared from 0.23 g (10 mmol) Na and 17.5 ml absolute
methanol) at 0°C, Br2 (0.23 ml, 4.5 mmol) was added dropwise. The reaction mixture was stirred for 1 h at room
temperature and 4 h at 60°C, neutralized with 50% AcOH, the precipitate formed was filtered off and
recrystallized from ethanol.
Methyl-(2-methyl-5-nitro-4,6-diphenylpyridin-3-yl)carbamate (5b). Yield 89%; mp 222-224°C.
Found, %: C 66.10, H 4.65, N 11.52. C20H17N3O4. Calculated, %: C 66.11; H 4.72; N 11.56.
Methyl-[2-methyl-4-(4-methoxyphenyl)-5-nitro-6-phenylpyridin-3-yl]carbamate (5c). Yield 87%;
mp 244-246°C. Found, %: C 64.08; H 4.91; N 10.72. C21H19N3O5. Calculated, %: C 64.12; H 4.87; N 10.68.
Carbamates 8a-c (General Method). A mixture of the corresponding nicotinamide 7a-c [1] (2.5 mmol)
and PhI(OAc)2 [14] (0.93 g, 2.9 mmol) was boiled in absolute ethanol (15 ml) (the reaction time is shown
below), cooled, the precipitate formed was filtered off and recrystallized from ethanol.
Ethyl-(6-methyl-5-nitro-2-phenylpyridin-3-yl)carbamate (8a). Reaction time 3 h. Yield 76%; mp
147-148°C. Found, %: C 59.88; H 5.00; N 14.13. C15H15N3O4. Calculated, %: C 59.79; H 5.02; N 13.95.
Ethyl-(5-nitro-2-phenylpyridin-3-yl)carbamate (8b). Reaction time 4 h. Yield 71%; mp 113-114°C.
Found, %: C 58.75; H 4.61; N 14.73. C14H13N3O4. Calculated, %: C 58.53; H 4.56; N 14.63.
Ethyl-[4-(2-furyl)-6-methyl-5-nitro-2-phenylpyridin-3-yl]carbamate (8c). Reaction time 6 h. Yield
71%; mp 194-195°. Found, %: C 62.08; H 4.60; N 11.45. C19H17N3O5. Calculated, %: C 62.12; H 4.66; N 11.44.
3-Aminopyridines 6b,c and 9b,c (General Method). To a suspension of the corresponding carbamate
5b,c, 8b,c (0.6 mmol) in ethanol (1.4 ml) a solution of NaOH (0.18 g, 4.5 mmol) in water (0.4 ml) was added.
The reaction mixture was heated at 70°C (reactions times are given below), cooled, diluted with water and the
crystals formed were filtered off.
3-Amino-2-methyl-5-nitro-4,6-diphenylpyridine (6b). Reaction time 6 h. Yield 91%; mp 206-207°C
(heptane). Found, %: C 70.50; H 4.85; N 13.62. Calculated; %: C 70.81; H 4.95; N 13.76.
3-Amino-4-(4-methoxyphenyl)-2-methyl-5-nitro-6-phenylpyridine (6c) was obtained by heating for
24 h. After cooling, the crystals which precipitated were filtered off and washed with water. The filtrate was
diluted with water and the starting carbamate 5c was separated. The yield of aminopyridine 6c was 78%; mp
225-227°C (ethanol). Found, %: C 68.04; H 5.21; N 12.48. C19H17N3O3. Calculated, %: C 68.05; H 5.11;
N 12.53.
3-Amino-5-nitro-2-phenylpyridine (9b). Reaction time 2 h. Yield 62%; mp 216-217°C (heptane).
Found, %: C 61.52; H 4.33; N 20.01. C11H9N3O2. Calculated, %: C 61.39; H 4.22; N 19.53.
3-Amino-4-(2-furyl)-6-methyl-5-nitro-2-phenylpyridine (9c). Reaction time 24 h. Yield 91%; mp
127-128°C (2-propanol). Found, %: 64.80; H 4.33; N 14.01. C16H13N3O3. Calculated, %: C 65.08; H 4.44;
N 14.23.
3-Amino-6-methyl-5-nitro-2-phenylpyridine (9a). A solution of carbamate 7a (0.5 g, 1.7 mmol) in a
mixture of glacial acetic acid (1.4 ml) and conc. H2SO4 (0.8 ml) was heated at 80° for 84 h, water (8 ml) was
added and the mixture was stirred for 8 h at room temperature. The reaction mixture was filtered, the filtrate was
neutralized with aqueous ammonia, the crystals which precipitated were filtered off and recrystallized. Yield
0.38 g (94%); mp 105-106°C (heptane). If the reaction was carried out at 100°C for 36 h or boiled for 2 h, the
yield of aminopyridine was reduced to 70-65%. In these cases the reaction product needed to be purified by
chromatography. Found, %: C 62.75; H 4.82; N 18.47. C12H11N3O2. Calculated, %: C 62.87; H 4.84; N 18.33.
1259

2-Methyl-5-nitro-4,6-diphenylnicotinic acid (10). A mixture of ethyl 2-methyl-5-nitro-4,6-diphenyl-
nicotinate [26] (1.45 g, 4 mmol), ethanol (10 ml), KOH (1.12 g, 20 mmol) in water (2 ml) was boiled for 3-4 h,
diluted with water and acidified with conc. HCl to pH 3, the precipitate was filtered off and recrystallized. Yield
1.27 g (95%); mp 274-275°C (ethanol). Found, %: C 68.29; H 4.17; N 8.43. C19H14N2O4. Calculated, %: C
68.26; H 4.22; N 8.38.
3-Amino-2-methyl-5-nitro-4,6-diphenylpyridine (6b) (modified Curtius reaction). A mixture of
diphenylchlorophosphate (11.1 g, 44 mmol), NaN3 (3.2 g, 49 mmol) and absolute dioxane (90 ml) was stirred at
room temperature for 24 h. Nicotinic acid 10 (3.7 g, 11 mmol) and absolute Et3N (6 ml) were then added and the
mixture was boiled for 24 h. After addition of water (10 ml), the reaction mixture was boiled for another 1 h,
then made basic with 10% KOH solution and most of the solvent was removed in vacuum. The residue was
diluted with water, extracted with benzene, the extract was dried over MgSO4, and after removing the solvent it
was purified by column chromatography using 9:1 chloroform–ethyl acetate as eluent. Yield 0.84 g (25%).
This work was supported financially by the Russian Fund for Fundamental Research (grant 07-03-00783-a).
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