Oxidative azacyclization of 1-monosubstituted thioureas in reaction with [bis(acyloxy)iodo]arenes to form 1,2,4-thiadiazole derivatives

Article abstract of DOI:10.1016/S0040-4020(03)01176-1

For the first time, derivatives of 1,2,4-thiadiazoles have been obtained by the reaction of [bis(acyloxy)iodo]arenes with 1-monosubstituted thioureas. 1-Acetylthiourea is subject to intermolecular azacyclization to form 3,5-bis-(acetylamino)-1,2,4-thiadia

Full text of DOI:10.1016/S0040-4020(03)01176-1

Tetrahedron 59 (2003) 7521–7525
Oxidative azacyclization of 1-monosubstituted thioureas in
reaction with [bis(acyloxy)iodo]arenes to form
1,2,4-thiadiazole derivatives
*
Elena A. Mamaeva and Abdigali A. Bakibaev
Department of Physical and Analytical Chemistry, Tomsk Polytechnic University, 30 Lenin avenue, 634050 Tomsk, Russian Federation
Received 20 February 2003; revised 30 June 2003; accepted 25 July 2003
Abstract—For the first time, derivatives of 1,2,4-thiadiazoles have been obtained by the reaction of [bis(acyloxy)iodo]arenes with
1-monosubstituted thioureas. 1-Acetylthiourea is subject to intermolecular azacyclization to form 3,5-bis-(acetylamino)-1,2,4-thiadiazole in
reaction with [bis(acyloxy)iodo]benzene. 1-Phenylthiourea forms 3,5-bis-(phenylamino)-1,2,4-thiadiazole in a single-stage reaction with
(diacetoxyiodo)benzene. The reaction of 1-phenylthiourea with [bis(trifluoroacetoxy)iodo]benzene leads to the formation of 5-imino-4-
phenyl-3-phenylamino-4H-1,2,4-thiadiazoline.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
ing to form alkylcyanamides and hydrogen sulfide, while
1,3-disubstituted thioureas (R¼Alk, Ar) under similar
conditions are oxidized to the appropriate 1,3-disubstituted
ureas.9
As structural fragments, ureas are present in many pharma-
ceuticals with antispasmodic, antihypoxic and other pharma-
cologically valuable properties. Taking into account that the
search for new pharmaceuticals affecting the central nervous
system has been a problem for many years, and that ureas have
a wide spectrum of valuable properties,1 the investigation of
urea functionalization in reaction with [bis(acyloxy)iodo]
arenes (BIA) is an area of interest. It should be mentioned that
the only references on transformations of ureas in the reaction
with BIA concerns their use as secondary substrates in
azaheterocyclizationreactions2–7 orindimerizationprocesses
in methanol solution.8
Oxidation reactions of substituted thioureas have been
known for a long time. As a rule, peroxide compounds or
molecular bromine are used as traditional oxidizing
agents.10–13 Among other oxidants, TsNvSO reagent has
been used with acylthioureas.14 Moreover, 2-chlorobenzo-
thiazole,15 tert-butylhypochloride,16 dioxandibromide,17
and others18 are used in reactions with arylthioureas.
It should be especially noted that the data about the
structures of thiadiazolidines formed from thiourea and
its 1-substituted derivatives on oxidation are sufficiently
inconsistent, since most of the synthesized compounds
can isomerize due to interconversion in solution.
Besides, the structure of thioureas oxidation products
often depends on the oxidant being used. For the first
time these compounds were described by the German
chemists about 100 years ago19,20 and later, these
substances have been studied by various chemical and
physical methods. Among the compounds mentioned, an
especially great number of disputes surrounded the so-
called Hector’s and Dost’s bases. Thus, 5-imino-4-
phenyl-3-phenylamino-4H-1,2,4-thiadiazoline was found
to be oxidation product of 1-phenylthiourea with
hydrogen peroxide (Hector’s base—HB) in 1978.21
The Dost’s base (DB) obtained by heating the Hector’s
base in ammonia alcohol solution was identified as
3,5-dianilino-1,2,4-thiadiazole.20
Thiourea is widely used in the production of some
pharmaceuticals (sulfothiazoles, thiobarbiturates) and as
an additive to some plastic materials.9 Thiourea shows more
basic properties than urea and can be protonated with strong
acids at the sulfur atom. Moreover, thiourea is sensitive to
the effect of strong oxidants. For instance, when the
oxidation of thiourea is run in neutral (with potassium
permanganate) or acid solution (or with many other
oxidants), the typical oxidation product—formamidine
disulfide9 is produced. Silver, mercury, and lead oxides in
water at room temperature eliminate hydrogen sulfide from
the thiourea molecule giving cyanamide.9 1-Alkylthioureas
are oxidized with mercury oxide and water under heat-
Keywords: oxidation; polyvalent iodine; thioureas; 1,2,4-thiadiazoles.
*
Corresponding author. Tel.: þ7-3822-415-832; fax: þ7-3822-415-235;
e-mail: eam@anchem.chtd.tpu.ru
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)01176-1

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E. A. Mamaeva, A. A. Bakibaev / Tetrahedron 59 (2003) 7521–7525
Scheme 1. Azacyclization of 1-acetylthiourea 1a in reaction with DIB and BTI.
The appropriate 3,5-bis-(acylamino)-1,2,4-thiadiazoles
were found to be the products of oxidation of 1-monoacyl
derivatives thioureas.12–14 A more complicated scheme has
been observed with 1-arylthiourea.10,11,22–24 In this case the
dimeric compounds are isolated as intermediates, and then
they are subject to oxidation and are cyclized with formation
of HB-type bases.10,11,23 The latter are subject to isomer-
ization to form 3,5-diarylamino-1,2,4-thiadiazoles—DB-
type bases.22,24 We have carried out the generalization of
For further research we found that 1-phenylthiourea (PTU)
(1b) was subject to azacyclization forming 3,5-dianilino-
1,2,4-thiadiazole (2b), so-called Dost’s base—DB in 55%
yield only in the presence of DIB (Scheme 2).
For comparison, we considered other known ways of
obtaining compound 2b. Thus, 1,2,4-thiadiazole 2b is
obtained in 48% yield as a result of isomerization by
heating a suspension of 5-imino-4-phenyl-3-phenylamino-
4H-1,2,4-thiadiazoline (HB) in ammonia alcohol solution in
an autoclave at 135–1408C for 2.5 h.24 The appropriate HB
has been obtained by oxidation of 1-phenyl-1-phenyl-
amidinothiourea bromohydrate with molecular bromine in
methanol solution in 73% yield,11 or by oxidation of PTU
1b with hydrogen peroxide (70%),25,26 or with tert-
butylhypochloride (46%),16 or with dioxandibromide
(85%),17 or with diaryltelluroxide (97%).18 The compound
2b has also been synthesized by the reaction of 3,5-dichloro-
1,2,4-thiadiazole with aniline (boiling, 100 h) in 63%
yield.27
oxidation–reduction transformations of 1-arylthioureas.10,
11,15–17,22–25
Due to the fact that the choice of the available oxidants for
such transformations is sufficiently restricted,10–18 we took
interest in the prospect of using BIA in similar azacycliza-
tion processes because of the absence of similar investi-
gations. Taking into account the known and unique ability
of BIA to oxidize organic substrates, we supposed that in
reactions with 1-monosustituted thiourea, BIA would work
in a more specific way than traditional oxidizers.
In comparison with these well-known methods of obtaining
1,2,4-thiadiazole 2b, needing preliminary, often labour-
intensive synthesis of precursors, our method produces the
desired azaheterocycle 2b starting from available PTU
under normal conditions (at room temperature) by a single-
stage reaction. It should be noted that direct methods for the
synthesis of compound 2b by oxidation of PTU are not
known. Our process using DIB as an oxidizing reagent in
reaction with PTU is a step forward in this field.
2. Results and discussion
Thus, 1-acetylthiourea (ATU) (1a) in the presence of
equimolar quantities of (diacetoxyiodo)benzene (DIB) in
chloroform (Method A) or in the presence of equimolar
quantities of [bis(trifluoroacetoxy)iodo]benzene (BTI) in
acetonitrile (Method B), was subject to oxidative cycliza-
tion to form 3,5-diacetylamino-1,2,4-thiadiazole (2a) in
yields of 41 and 37% (Scheme 1: Methods A and B,
respectively).
The structure of compound 2b was confirmed by IR-, 1H and
13C NMR-spectroscopy, and mass-spectrometry and also by
comparison of the melting point of the substance 2b with the
literature data.24,27
It should be noted that compound 2a has been obtained
earlier by the oxidation of ATU with hydrogen peroxide in
alcohol solution at 0–258C in 23% yield12 or with 1-
sulfinyl-tosylamide in 38% yield.14 In our method the yields
of compound 2a were better using DIB or almost equal
when using BTI to the literature reports.
In our opinion, the 1,2,4-thiadiazoles 2a,b formation using
BIA proceeds according to Scheme 3. At first, 1-substituted
thioureas 1a,b by the reaction with BIA dimerize to form
1-R-3-R-amidinothioureas B via the polyvalent iodine
compound A with elimination of the sulfur atom. Then,
substances B are easily subject to the oxidative azacycliza-
tion initiated by BIA to form the appropriate bases 2a,b of
DB-type.
1
The structure of compound 2a was confirmed by IR-, H
NMR-spectroscopy, mass-spectrometry, and also by the
comparison of the melting point of compound 2a with the
literature data.12,14
Scheme 2. Azacyclization of 1-phenylthiourea 1b in reaction with DIB.

E. A. Mamaeva, A. A. Bakibaev / Tetrahedron 59 (2003) 7521–7525
7523
Scheme 3. Suggested scheme of synthesis of 3,5-bis-(R-amino)-1,2,4-thiadiazoles 2a,b.
In contrast to PTU conversion in the reaction with DIB,
the same reaction initiated by BTI (Scheme 4) results
in trifluoroacetate formation in situ, the latter
without preliminary isolation, after addition of Na2CO3
water solution to give 5-imino-4-phenyl-3-phenylamino-
4H-1,2,4-thiadiazoline (3) or HB with well-known
properties.10,11,16–18,23,28,29
phenylamidinothiourea B (Scheme 3). It determines the
direction of the oxidation reaction. Besides, the direction of
azacyclization also depends on the electrophilicity of
iodosyl center of BIA molecules. When alkaline is added
to trifluoroacetate D solution, free base 3 of HB-type is
easily formed. In the case of the reaction of ATU 1a with
BTI the protonation of the appropriate base is evidently
complicated due to the acetyl-groups in the structure, which
decrease the electron density of the nitrogen atom of the
amide group. Therefore, the presence of strong TFA in
small amounts (the weak acetic acid in reaction of ATU 1a
with DIB) does not sufficiently affect the oxidation
processes, which are favourable for the synthesis of
compound 2a. As follows from the above, the specific
behavior of BTI in respect to PTU 1b results from the basic
properties of the intermediates, appearing during the
oxidation reaction with the BTI reagent. The ability of
strong TFA to protonate intermediates plays a special role,
inhibiting the processes of 1,2,4-thiadiazole 2b (isomer of
compound 3) formation. Known methods of PTU oxidation
involve using the addition of strong mineral acid or acid-
generating reagents. It is not make possible to earlier obtain
the compounds of 2b-type starting from 1-arylthioureas.
This fact limited the application of traditional methods.
Using DIB as a ‘weak-acid’ oxidant allows preparative
possibilities of 3,5-disubstituted-1,2,4-thiadiazoles
synthesis.
The structure of compound 3 was confirmed by IR-, 1H and
13C NMR-spectroscopy, and mass-spectrometry. Com-
pound 3 is identical to the product of PTU oxidation with
hydrogen peroxide in acidified alcoholic solution. Data
about HB structure have been published.21,30–32 It should be
noted that the melting point of substance 3, obtained by
different methods (existing and suggested by us), is lower
than that of HB, by about 138C.11
As our contribution, we suggest the process of formation of
the intermediate polyvalent iodine compound A, initially
appearing as a result of BTI and PTU 1b reaction (Scheme
5). The intermediate A after iodobenzene elimination
dimerizes to form 1,6-diphenyl-dithioformamidine B. The
latter easily converts into 1-phenyl-1-phenylamidino-
thiourea C by sulfur atom elimination, which with BTI-
initiation cyclizes to form trifluoroacetate D. The free base 3
of HB-type is easily formed by the treatment of salt D
solution with alkaline reagents.
Thus, the results of our research have showed that, in
comparison in the ATU 1a conversion in reaction with BIA,
the reaction of PTU 1b with BTI finishes differently to the
same reaction with DIB. In this connection the strong
trifluoroacetic acid (TFA) generated gives the main effect
on the formation of the final product 3. Obviously, the TFA
leads to the intermediate bases protonation, resulting in
adduct D formation instead of its isomer 2b. The key
moment in Scheme 5 is 1-phenyl-1-phenylamidinothiourea
C formation in the presence of TFA instead of 1-phenyl-3-
3. Conclusion
Summarizing the results of our research it is possible to
conclude that we, using BIA as oxidizing reagents,
discovered that 1-monosubstituted thioureas treated with
BIA are subject to azaheterocyclization processes to form
1,2,4-thiadiazole derivatives. Analysis of the literature data
has shown that our methods of desired nitrogen-containing
heterocycles synthesis are more attractive both in simplicity
Scheme 4. Azacyclization of 1-phenylthiourea 1b in reaction with BTI.

7524
E. A. Mamaeva, A. A. Bakibaev / Tetrahedron 59 (2003) 7521–7525
Scheme 5. Suggested scheme of synthesis of 5-imino-4-phenyl-3-phenylamino-4H-1,2,4-thiadiazoline 3.
of realization and in efficiency of the used oxidative agents.
The difference in DIB and BTI behaviour in reactions with
PTU was found. Using DIB we, for the first time, succeeded
in a single-stage reaction to obtain 3,5-dianilino-1,2,4-
thiadiazole starting from PTU. Taking into account that its
precursor 3,5-diamino-1,2,4-thiadiazole shows hypo-
glycemic activity along with low toxicity,33 the direct method
of its derivatives synthesis becomes especially valuable.
Method B. A mixture of ATU 1a (0.275 g, 2.33 mmol), BTI
(1.000 g, 2.33 mmol) and acetonitrile (7 mL) was stirred at
room temperature for 40 min. The precipitate was filtered,
dried, and recrystallized from ethanol to form the title
compound 2a (0.086 g, 37%) as white crystals; mp 342–
3438C (lit.,12 mp 336–3388C; lit.,14 mp 325–3308C
(AcOH)); nmax(KBr) 3449, 3201, 3109, 3000, 1693, 1665,
1579, 1545, 1529, 1332, 1289, 1237, 585 cm21; dH
(100 MHz, DMSO-d6) 11.00 (2H, s, 2NH), 2.42 (3H, s,
Me), 2.32 (3H, s, Me); m/z (CI, N2) 200 (35 Mþ), 158 (87),
116 (100), 74 (17), 43 (86), 28 (7%).
4. Experimental
4.1. General
4.1.2. 3,5-Dianilino-1,2,4-thiadiazole 2b. A mixture of
PTU 1b (0.472 g, 3.10 mmol), DIB (1.000 g, 3.10 mmol)
and acetonitrile (10 mL) was stirred at room temperature for
30 min; the precipitated sulfur was filtered off. The filtrate
after water (20 mL) addition was extracted with chloroform
(60 mL). Acetonitrile (5 mL) and then water (15 mL) were
added to the residue, which boiled down to get rid of
solvents and PhI. The resulting precipitate was filtered off,
dried, and recrystallized from acetonitrile to form the title
compound 2b (0.231 g, 55%) as white crystals; mp 200–
2020C (lit.,24,27 mp 200–2028C (EtOH)); nmax (KBr) 3472,
3411, 3379, 1665, 1592, 1535, 1482, 1441, 1398, 1340,
697 cm21; dH (100 MHz, DMSO-d6) 8.66 (1H, s, NH),
7.88–6.60 (10H, m, 2Ph), 4.98 (1H, s, NH); dC (25 MHz,
DMSO-d6) 173.2, 157.6, 144.2, 143.5, 139.5, 135.8, 128.0,
127.5, 126.0, 121.0, 117.9, 114.4; m/z (CI, N2) 268 (100
Mþ), 220 (35), 176 (7), 150 (42), 118 (48), 91 (32), 77 (45),
65 (23), 51 (22%).
Reaction and compound identification were achieved by
thin-layer chromatography (TLC) Merck DC-Alufolien
Kieselgel 60 F254 (mobile phase: benzene–methanol¼8:2).
1H and 13C NMR spectra were recorded on a Tesla BS-497
NMR spectrometer (at 100 and 25 MHz, respectively) in
DMSO-d6 or CDCl3. Chemical shifts were registered
relatively to Me4Si (in ppm). IR-spectra were recorded on
a FT-IR spectrometer (Nicolett) in KBr tablets. Accurate
values of molecular ions masses were determined by high-
resolution mass-spectrometry on a MAT-8200 spectrometer
(from Finnigan), supplied with direct input of the sample
into source, at ionizing emission energy of 70 eV.
The identification of the synthesized compounds structures
and the spectral signals were made by the comparison of the
chemical shifts of NMR spectra with the data obtained from
computer programs Chem Office 2000 Ultra, ACDH, and
ACDC.
4.1.3. 5-Imino-4-phenyl-3-phenylamino-4H-1,2,4-thia-
diazoline 3. A mixture of PTU 1b (0.708 g, 4.65 mmol),
BTI (2.000 g, 4.65 mmol) and acetonitrile (10 mL) was
stirred at room temperature for 2 h. The resulting yellow
sulfur precipitate was filtered off. Obtaining solution was
left to evaporate at room temperature for one week. 1 mL of
isopropyl alcohol was added to the residue, mixed up to
dissolution, then water (70 mL) was added and precipitate
was filtered. Aqueous Na2CO3 solution (10%) was added;
up to pH 8. The precipitate was filtered, washed out well
with water and dried to form the title compound 3 (0.468 g,
75%) as white, lightly yellowish crystals; mp 172–1748C
BTI and DIB were synthesized according to the literature
procedure.34
4.1.1. 3,5-Diacetylamino-1,2,4-thiadiazole 2a. Method A.
A mixture of ATU 1a (0.366 g, 3.10 mmol), DIB (1.000 g,
3.10 mmol) and chloroform (10 mL) was stirred at room
temperature for 4 h. The precipitate was filtered, dried and
recrystallized from ethanol to form the title compound 2a
(0.128 g, 41%).

E. A. Mamaeva, A. A. Bakibaev / Tetrahedron 59 (2003) 7521–7525
7525
(C6H6 or acetone/spirit) (lit.,11 mp 185–1868C; lit.,28 mp
179–1808C; lit.,26 mp 183–1848C; lit.,35 mp 1828C); nmax
(KBr) 3472, 3411, 3379, 1665, 1592, 1535, 1482, 1441,
1398, 1340, 697 cm21; dH (100 MHz, DMSO-d6) 8.22 (2H, s,
2NH), 8.03–7.10 (10H, m, 2Ph); dC (25 MHz, DMSO-d6)
163.9, 146.3, 137.5, 132.1, 128.5–125.7, 120.5, 118.40; m/z
(CI, N2) 268 (100 Mþ), 195 (28), 150 (49), 119 (51), 92
(21), 77 (25), 65 (17), 51 (12), 32 (8), 28 (38%).
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