Synthesis of 2-azido-1,3-thiazoles as 1,2,3-triazole precursors

Article abstract of DOI:10.1080/00397910902981274

By diazotization of 2-aminothiazoles and reaction with sodium azide, the derivatives of 2-azidothiazole were synthesized. Conditions of diazotization were selected according to the nature of a substituent in thiazoles. 2-Azidothiazole derivatives were studied in the base-catalysed condensation reactions with activated methylenic compounds to yield new 1-(1,3-thiazol-2-yl)- 1H-1,2,3-triazole-4-carboxylic acids.

Full text of DOI:10.1080/00397910902981274

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Synthesis of 2-Azido-1,3-thiazoles as
1,2,3-Triazole Precursors
Nazariy T. Pokhodylo a , Roman D. Savka a , Nazar I. Pidlypnyi a ,
Vasyl S. Matiychuk a & Mykola D. Obushak a
a Department of Organic Chemistry, Ivan Franko National University
of Lviv, Lviv, Ukraine
Published online: 06 Jan 2010.
To cite this article: Nazariy T. Pokhodylo , Roman D. Savka , Nazar I. Pidlypnyi , Vasyl S. Matiychuk &
Mykola D. Obushak (2010) Synthesis of 2-Azido-1,3-thiazoles as 1,2,3-Triazole Precursors, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
40:3, 391-399, DOI: 10.1080/00397910902981274
To link to this article: http://dx.doi.org/10.1080/00397910902981274
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Synthetic Communications1, 40: 391–399, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902981274
SYNTHESIS OF 2-AZIDO-1,3-THIAZOLES
AS 1,2,3-TRIAZOLE PRECURSORS
Nazariy T. Pokhodylo, Roman D. Savka, Nazar I. Pidlypnyi,
Vasyl S. Matiychuk, and Mykola D. Obushak
Department of Organic Chemistry, Ivan Franko National University of Lviv,
Lviv, Ukraine
By diazotization of 2-aminothiazoles and reaction with sodium azide, the derivatives of
2-azidothiazole were synthesized. Conditions of diazotization were selected according to
the nature of a substituent in thiazoles. 2-Azidothiazole derivatives were studied in the
base-catalysed condensation reactions with activated methylenic compounds to yield new
1-(1,3-thiazol-2-yl)-1H-1,2,3-triazole-4-carboxylic acids.
Keywords: Azides; cyclocondensation; thiazoles; 1,2,3-triazoles
Azides are convenient reagents for the synthesis of triazoles,[1–5] which are widely
used for medical and technical purposes.[4,5] The reaction of diazonium salts with
sodium azides is one of the synthetic paths for aromatic azide preparation. The reac-
tion takes place without any catalysts with the formation of azides in good yields.[1]
The diversity of available aromatic amines is the advantage of such a method. How-
ever, heterocyclic azides, yielded from the corresponding amines, are poorly described
in literature because of the insufficient study of heterocyclic diazonium salts.[6]
In the current article, some 2-amino-1,3-thiazoles, which are easily prepared by
Hantzsch cyclization from a-halocarbonylic compounds and thiourea, were studied
as starting reagents for 2-azido-1,3-thiazoles. Unfortunately, there are not many
examples of this class of compounds studied in the diazotization reaction because
of the structural peculiarities of 2-amino-1,3-thiazoles. One of the major problems
of the amino group reactivity in 2-amino-1,3-thiazoles is a tautomeric equilibrium
(Fig. 1). Literary data analysis in relation to the tautomeric equilibrium of
2-imino(amino)-thiazoline derivatives shows that predominance of the tautomeric
form depends on the aggregate state and, in solutions, on the nature of solvent.[7]
Commercially available a-halogencarbonyl compounds, as starting materials,
were used for the synthesis of 2-aminothiazoles 2a–e. However, the method
described in Ref. 8[8] was used for the preparation of compounds 2f–i. 3-Aryl-2-
chloropropanals 1f–i were prepared by the reaction of arenediazonium chlorides
with acrolein under conditions of the Meerwein reaction (Scheme 1).
Received February 13, 2009.
Address correspondence to Nazariy T. Pokhodylo, Department of Organic Chemistry, Ivan Franko
National University of Lviv, Kyryla I Mefodiya St. 6, Lviv 79005, Ukraine. E-mail: pokhodylo@gmail.com
391

392
N. T. POKHODYLO ET AL.
Figure 1. Tautomeric equilibrium.
a-Haloaldehydes 1f–i or a-haloketones reacted with thiourea and formed
2-amino-1,3-thiazoles 2a–i with good yields (Scheme 2).[8]
Herein we describe novel advantages of 2-aminothiazole derivative diazotiza-
tion. Three types of diazotization reagents [nitrosyl chloride (NaNO2 in HCI solution),
nitrosylsulfuric acid (NaNO2, in H2SO4 either 40% or 70% solutions), and nitrosylni-
tric acid (NaNO2 in the mixture HNO3–H3PO4)] were used for diazotization.
We have succeeded in diazotization of 2-aminothiazole in concentrated HCl
and synthesized azide 4a with 36% yield.[9] Unfortunately, some quantity of the 2-ami-
nothiazole remained unreacted after diazotization. Furthermore, only 2-aminothiazole
formed diazonium salt in concentrated HCl. In case of the other 2-aminothiazoles,
efficient diazotization can be achieved only by using nitrosylsulfuric acid obtained
from NaNO2 and concentrated H2SO4 (Table 1). Yield of diazonium salts in diazotiza-
tion reaction of substituted 2-aminothiazoles depended on the character of the
substituent and reaction conditions. 1,3-Thiazol-2-amines 2b,c and 2f–i reacted with
nitrosyl ion in sulfuric acid. However, diazotization in nitrosylsulfuric acid occurred
very slowly because the concentration of the free amine was exceedingly low.
4-Aryl-1,3-thiazol-2-amines 3d,e formed diazonium salt only in the mixture of phos-
phoric and nitrate acids. Compounds 4d,e were formed by treatment of diazonium salt
solution with sodium azides. Previously, compounds 4a,d,e were prepared by the
reaction of sodium azide with 2-diazonium salts of 2-aminothiazole in sulfuric acid
with approximately 15% yields.[9] A particular nitrosyl agent (NaNO2 in HNO3–
H3PO4 solution) was used as an extensive diazotizing agent for weakly basic amines.
Thus, 2-azido-1,3-thiazoles 4a–i were synthesized under optimal conditions selected
for diazotization of amines 3a–i (Tables 1 and 2).
It is well known that thiazoles and other nitrogen-bearing heterocycles[10]
(including the azido group in the a-position of nitrogen in the cycle), depending
Scheme 1. Synthesis of 3-aryl-2-chloropropanals 1f–i.
Scheme 2. Synthesis of 2-azido-1,3-thiazoles 4a–i.

1-(1,3-THIAZO-2-YL)1H-1,2,3-TRIAZOLES
Table 1. Synthesis of azides 4a–i
393
Condition of diazotization of
amines 3 and yielda (%) of azides 4
H2SO4
H2SO4
Compound
R1
R2
HCl conc. (ꢁ 40%) (ꢁ 70%) H3PO4 þ HNO3c
4a
4b
4c
4d
4e
4f
4g
4h
4i
H
H
36
—
41
—
—
—
21
—
—
14
—
68
71
—
—
52
41
56
43
20
b
ꢀ(CH2)4ꢀ
—
—
—
—
—
—
—
—
Me
Ph
4-MeC6H4
COOEt
H
H
PhCH2
23
29
H
H
H
H
3-MeC6H4CH2
4-MeC6H4CH2
4-ClC6H4CH2
aIsolated yields are based on single experiment and the yields were not optimized.
bNo reaction.
cMixture of H3PO4 (80%) and concentrated HNO3 (2.2:1 by volume).
on the aggregate state and polarity of solvent, can exist in either azido and tetrazole
forms or in the state of azido-tetrazole equilibrium. Tautomerizme of azides 4 were
studied.[9,11,12]
Such a tautomerizme is increasingly investigated by quantum chemical
methods. The transformation of 2-azidothiazole 4a into thiazolo[3,2-d]tetrazole 4a0
has been studied as a prototype reaction for the generation of tetrazoles (Fig. 2).[13,14]
In the infrared (IR) spectra of compounds 4a–i in CCl4 solutions, intensive
azide bands are present in regions 2130–2265 cmꢀ1 (nN3). These bands are most
intensive in the IR spectra, which proves the predominance of azido form in these
compounds.
Thus, the results of the spectral research show that studied compounds in solu-
tions exist mainly in azido form. It made it possible to use them in the characteristic
Table 2. Azides 4a–i
Compound
MS (CI) m=z
Temp. (ꢂC)a
IRb [n(N3) cmꢀ1
]
4a
4b
4c
4d
4e
4f
126
180
212
202
216
216
230
230
251
—
109–110
39–40
87–88
62–63
57–56
—
2135
2138
2162
2137
2140
2144
2145
2140
2144
4g
4h
4i
64–65
91–92
aAll melting points are given for crude products without purification and are
uncorrected.
bIR spectra were recorded in CCl4 solution.

394
N. T. POKHODYLO ET AL.
Figure 2. Azido-tetrazole equilibrium.
reactions of azides for triazole synthesis. For instance, 2-azido-1,3-thiazole, prepared
from 1,3-thiazolelithium and tosyl azide by the azido transfer procedure, was used
for the synthesis of 1–1,3-thiazole-4-trimethylsilyl-1,2,3-triazole by 1,3-dipolar
cycloaddition.[15]
One of the most convenient methods of triazole synthesis is the base-catalyzed
cyclization of organic azides with CH acids (Dimroth cyclization).[4] The reaction
occurred at different rates depending on the dipolarophile. However, in some cases
(especially when azides were used with an electron-withdrawing substituent),
reduction of the azido moiety to the amino group took place instead of the triazole
formation.[4]
Previously, only two 2-azidothiazole derivatives were studied in such
reactions.[12] Thus, to explore the synthetic application of azides 4 for triazole
synthesis, we have carried out some cylocondensation reactions with b-ketoesters.
It was established that base-catalyzed reactions of azides 4 with ethyl acetoace-
tate and ethyl benzoylacetate 5b occurred at room temperature. Completion of the
reaction was defined with disappearance of the absorption band of azido moiety
in IR spectrum of the reaction mixture. During the reaction, we observed exclusive
formation of triazoles 6 (Table 3). Moreover, the electron-poor 2-azidothiazoles
rapidly reacted and were converted into products in good yield.
In conclusion, 2-azidothiazoles were synthesized by diazotization of
2-aminothiazoles and subsequent treatment with sodium azides. Conditions of diazo-
tization were optimized according to the character of a substituent. This method
can be an excellent synthetic route to new (1,3-thiazol-2-yl)-1H-1,2,3-triazoles.
EXPERIMENTAL
1H NMR spectra were recorded on a Varian Mercury 400 instrument
(400 MHz for 1H). The 1H chemical shifts were reported in parts per million
(ppm) relative to tetramethylsilane (TMS) as an internal reference. Mass spectra
(MS) were run using Agilent 1100 series liquid chromatography (LC)=MS instru-
ment with atmospheric pressure ionization–electrospray=atmospheric pressure
chemical ionization (API-ES=APCI) mode. 2-Aminothiazoles 3b–e were prepared
Scheme 3. Synthesis of (1,3-thiazol-2-yl)-1H-1,2,3-triazoles 6a–j.

1-(1,3-THIAZO-2-YL)1H-1,2,3-TRIAZOLES
395
Table 3. 1-(1,3-Thiazol-2-yl)-1H-1,2,3-triazole-4-carboxylic acids 6a–h
Compound
Product
Yield (%)
6a
6b
42
71
6c
6d
64
79
6e
82
6f
75
71
6g
6h
6i
83
87
6j
79

396
N. T. POKHODYLO ET AL.
from the commercially available a-haloketones, their constants are described in the
literature.[7]
Synthesis of 2-Azido-1,3-thiazole 4a
2-Amino-1,3-thiazole 3a (0.05 mol) was dissolved in concentrated HCl (19 mL).
The solution was cooled to 0ꢂC, and sodium nitrite (3.45 g, 0.05 mol) in a minimal
quantity of water was added dropwise while keeping the temperature below 5ꢂC.
The solution of sodium azide (3.25 g, 0.05 mol) in 10 mL of water was added slowly
with intensive stirring. The temperature was kept below 7ꢂC. After sodium azide was
added, the mixture was left for 2 h at room temperature, and azide was extracted by
diethyl ether (3 ꢃ 10 mL). Ether was evaporated in vacuo. Azide 4a was used without
subsequent purification (mp 94ꢂC, lit.[15] 94–96ꢂC).
Synthesis of 2-Azido-1,3-thiazoles 4b,c,f–i
An appropriate 2-amino-5-(R-benzyl)-1,3-thiazole 3b,c,f–i (0.02 mol) was
dissolved in the mixture of concentrated sulfuric acid (5 mL) and water (5 mL). When
the mixture was cooled to 0ꢂC, saturated sodium nitrite (1.73 g; 0.025 mol) aqueous
solution was added while keeping the temperature below 5ꢂC. After 10 min, resinous
sediment (if it was formed) was filtered. To the filtrate solution of the diazonium salt,
sodium azide (1.3 g; 0.02 mol) in 5 mL of water was added dropwise. The solution
was left for 15 min at room temperature, and azide was extracted by diethyl ether
(3 ꢃ 15 mL). Azides were used without the subsequent purification.
Synthesis of 2-Azido-4-(R-phenyl)-1,3-thiazoles 4d,e
Appropriate 2-amino-4-aryl-thiazole 3d,e (0.012 mol) was dissolved in 4 mL of
phosphoric acid (80%). The solution was cooled with ice, and 1.8 mL of concentrated
nitric acid was added dropwise during 5 min while keeping the temperature below
5ꢂC. Then the solution of sodium nitrite (0.90 g; 0.013 mol) in 1.5 mL of water was
added, and the solution of diazonium salt was left for 30 min. The solution of sodium
azide (0.72 g; 0.02 mol) in 5 mL of water was added. Azide was extracted by diethyl
ether (3 ꢃ 10 mL). Ether was evaporated in vacuo. Azides were used without the
subsequent purification.
Synthesis of 1-(1,3-Thiazol-2-yl)-1H-1,2,3-triazole-4-carboxylic
Acids 6a–j
Sodium (0.23 g, 0.01 mol) was added to 20 mL of absolute methanol. b-Ketoester
5 (0.01 mol) and the appropriate azide 4 (0.01 mol) was slowly added (cooling by ice
water) to the obtained sodium methylate solution. The mixture was kept in an
ice-water bath for 30 min and then slowly heated under reflux for 1 h. The solid sedi-
mented. Hot water was added to dissolve the sediment (50 mL); if necessary, a solution
of sodium hydroxide was added to increase pH to 11–12 and heated under reflux for
1 h. Hot solution was poured to a 10 mL of concentrated HCl and left to crystallize.
The obtained solid was filtered, washed with water twice, and crystallized.

1-(1,3-THIAZO-2-YL)1H-1,2,3-TRIAZOLES
397
Data
5-Methyl-1-(1,3-thiazol-2-yl)-1H-1,2,3-triazole-4-carboxylic
acid
6a.
Yield: 42%; brown solid; mp 167–168ꢂC (CH2Cl2). 1H NMR: d ¼ 2.92 (s, 3H,
3
3
CH3), 7.74 (d, 1 H, J ¼ 3.9 Hz, HTz), 7.80 (d, 1 H, J ¼ 3.9 Hz, HTz). MS (CI):
m=z ¼ 211 [Mþ þ 1]. Anal. calcd. for C7H6N4O2S: C, 39.99; H, 2.88; N, 26.65.
Found: C, 39.72; H, 2.94; N, 26.58.
5-Methyl-1-(4,5,6,7-tetrahydro-1,3-benzothiazol-2-yl)-1H-1,2,3-triazole-
1
4-carboxylic acid 6b. Yield: 71%; white crystals; mp 154–155ꢂC (EtOH–H2O). H
NMR: d ¼ 1.85–1.94 (m, 4H, CH2), 2.72–2.76 (m, 2H, CH2), 2.81–2.85 (m, 2H,
CH2), 2.86 (s, 3H, Me). MS (CI): m=z ¼ 265 [Mþ þ 1]. Anal. calcd. for C11H12N4O2S:
C, 49.99; H, 4.58; N, 21.20. Found: C, 49.85; H, 4.73; N, 21.40.
1-(5-Carboxy-4-methyl-1,3-thiazol-2-yl)-5-methyl-1H-1,2,3-triazole-4-
1
carboxylic acid 6c. Yield: 64%; white crystals; mp 244–245ꢂC (EtOH–H2O). H
NMR: d ¼ 2.67 (s, 3H, CH3), 2.93 (s, 3H;, CH3). MS (CI): m=z ¼ 269 [Mþ þ 1]. Anal.
calcd. for C9H8N4O4S: C, 40.30; H, 3.01; N, 20.89. Found: C, 40.70; H, 3.21; N,
21.00.
5-Methyl-1-(4-phenyl-1,3-thiazol-2-yl)-1H-1,2,3-triazole-4-carboxylic
acid 6d. Yield: 79%; white crystals; mp 175–176ꢂC (EtOH–H2O). 1H NMR:
d ¼ 3.02 (s, 3H, CH3), 7.33 (t, 1H, J ¼ 7.8 Hz, 4-HPh), 7.44 (t, 2H, J ¼ 7.8 Hz,
3,5-HPh), 7.95 (d, 2H, J ¼ 7.8 Hz, 2,6-HPh), 8.06 (s, 1H, HTz). MS (CI): m=z ¼ 287
[Mþ þ 1]. Anal. calcd. for C13H10N4O2S: C, 54.53; H, 3.52; N, 19.57. Found: C,
54.25; H, 3.40; N, 19.70.
5-Methyl-1-[4-(4-methylphenyl)-1,3-thiazol-2-yl]-1H-1,2,3-triazole-4-
1
carboxylic acid 6e. Yield: 82%; white crystals; mp 172–173ꢂC (EtOH–H2O). H
NMR: d ¼ 2.37 (s, 3H, CH3), 3.02 (s, 3H, CH3), 7.23 (d, 2H, J ¼ 8.0 Hz, 3,5-HAr),
7.82 (d, 2H, J ¼ 8.0 Hz, 2,6-HAr), 7.99 (s, 1H, HTz). MS (CI): m=z ¼ 301 [Mþ þ 1].
Anal. calcd. for C14H12N4O2S: C, 55.99; H, 4.03; N, 18.65. Found: C, 56.11; H,
4.23; N, 18.61.
1-(5-Benzyl-1,3-thiazol-2-yl)-5-methyl-1H-1,2,3-triazole-4-carboxylic
acid 6f. Yield: 75%; white crystals; mp 157–158ꢂC (EtOH–H2O). 1H NMR: d ¼ 2.88
(s, 3H, CH3), 4.24 (s, 2H, CH2), 7.22–7.36 (m, 5H, HPh), 7.60 (s, 1H, HTz), 13.15
(br.s, 1H, COOH). MS (CI): m=z ¼ 301 [Mþ þ 1]. Anal. calcd. for C14H12N4O2S:
C, 55.99; H, 4.03; N, 18.65. Found: C, 55.81; H, 3.79; N, 18.74.
5-Methyl-1-[5-(3-methylbenzyl)-1,3-thiazol-2-yl]-1H-1,2,3-triazole-4-
1
carboxylic acid 6g. Yield: 71%; white crystals; mp 124–125ꢂC (EtOH–H2O). H
NMR: d ¼ 2.32 (s, 3H, CH3), 2.88 (s, 3H, CH3), 4.18 (s, 2H, CH2), 7.04 (d, 1H,
J ¼ 7.2 Hz, 4-HAr), 7.09 (d, 1H, J ¼ 7.2 Hz, 6-HAr), 7.10 (s, 1H, 2-HAr), 7.20 (t,
1H, J ¼ 7.2 Hz, 5-HAr), 7.60 (s, 1H, HTz), 13.16 (br.s, 1H, COOH). MS (CI):
m=z ¼ 315 [Mþ þ 1]. Anal. calcd. for C15H14N4O2S: C, 57.31; H, 4.49; N, 17.82.
Found: C, 57.52; H, 4.29; N, 17.94.
5-Methyl-1-[5-(4-methylbenzyl)-1,3-thiazol-2-yl]-1H-1,2,3-triazole-4-
1
carboxylic acid 6h. Yield: 83%; white crystals; mp 158–159ꢂC (EtOH–H2O). H
NMR: d ¼ 2.32 (s, 3H, CH3), 2.89 (s, 3H, CH3), 4.19 (s, 2H, CH2), 7.12 (d, 2H,

398
N. T. POKHODYLO ET AL.
J ¼ 8.0 Hz, 3,5-HAr), 7.19 (d, 2H, J ¼ 8.0 Hz, 2,6-HAr), 7.57 (s, 1H, HTz), 13.13 (br.s,
1H, COOH). MS (CI): m=z ¼ 315 [Mþ þ 1]. Anal. calcd. for C15H14N4O2S: C, 57.31;
H, 4.49; N, 17.82. Found: C, 57.16; H, 4.38; N, 17.75.
1-[5-(4-Chlorobenzyl)-1,3-thiazol-2-yl]-5-methyl-1H-1,2,3-triazole-4-
1
carboxylic 6i. Yield: 87%; white crystals; mp 183–184ꢂC (EtOH–H2O). H NMR:
d ¼ 2.80 (s, 3H, CH3), 4.23 (s, 2H, CH2), 7.33 (s, 4H, HAr), 7.59 (s, 1H, HTz). MS
(CI): m=z ¼ 335 [Mþ þ 1]. Anal. calcd. for C14H11ClN4O2S: C, 50.23; H, 3.31; N,
16.74. Found: C, 50.09; H, 3.26; N, 16.55.
1-(5-Benzyl-1,3-thiazol-2-yl)-5-phenyl-1H-1,2,3-triazole-4-carboxylic
1
acid 6j. Yield: 79%; white crystals; mp 100–101ꢂC (EtOH). H NMR: d ¼ 4.18 (s,
2H, CH2), 7.20–7.34 (m, 5H, HPh), 7.39–7.47 (m, 6H, HPh þ HTz). MS (CI):
m=z ¼ 363 [Mþ þ 1]. Anal. calcd. for C19H14N4O2S: C, 62.97; H, 3.89; N, 15.46.
Found: C, 62.78; H, 3.99; N, 15.54.
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