Oxidative cyclization of dithiobiuret under basic conditions and theoretical tautomeric studies of 5-amino-2,3-dihydro-1,2,4-thiadiazole-3-thione

Article abstract of DOI:10.3987/com-03-9742

The oxidative cyclization of dithiobiuret under basic conditions (NaOH-H₂O₂ or CH₃CO₃H) afforded bis(5-amino-1,2,4-thiadiazolyl) 3,3′-disulfide (5), oxidative dimer form of 5-amino-3-mercapto-1,2,4-thiadiazole (

Full text of DOI:10.3987/com-03-9742

HETEROCYCLES, Vol. 60, No. 6, 2003
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HETEROCYCLES, Vol. 60, No. 6, 2003, pp. 1401 - 1409
Received, 12th February, 2003, Accepted, 4th April, 2003, Published online, 7th April, 2003
OXIDATIVE CYCLIZATION OF DITHIOBIURET UNDER BASIC
CONDITIONS AND THEORETICAL TAUTOMERIC STUDIES OF 5-
AMINO-2,3-DIHYDRO-1,2,4-THIADIAZOLE-3-THIONE
Nam Sook Cho,* Young Hoon Kim, Mi Sun Park, Eun Hee Kim, Sung Kwon
Kang, and Chang-moon Park
Department of Chemistry, Chungnam National University, Daejeon, 305-764,
Korea
Absract- The oxidative cyclization of dithiobiuret under basic conditions (NaOH-
H2O2 or CH3CO3H) afforded bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide (5),
oxidative dimer form of 5-amino-3-mercapto-1,2,4-thiadiazole (3b). The
theoretical tautomeric study (ab initio calculations) shows that the most stable
tautomeric form of 3 is 5-amino-3-mercapto-1,2,4-thiadiazole (3b) among the
four possible tautomers. Thus, 3b is speculated as an intermediate of the
formation of the compound (5).
INTRODUCTION
In light of potential biologically active agents, new analogues of pyrimidine bases are of interest. On this
basis we have previously studied 5-amino-2,3-dihydro-1,2,4-oxadiazol-3-one (1)1 and 5-amino-2,3-
dihydro-1,2,4-thiadiazol-3-one (2).2 These compounds are analogues of cytosine, in which the C=C bond
in heteroaromatic rings (cytosine) is replaced with either divalent oxygen or sulfur, respectively. This
analogy is a well-known subject within benzenoid hydrocarbon and heterocyclic chemistry. 5-Amino-2,3–

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dihydro-1,2,4-thiadiazole-3-thione (3) is an analogue of compound (2). The compound (2) was prepared
through oxidative cyclization of 2-thiobiuret with H2O2 under basic conditions. We tried to synthesize 5-
amino-2,3–dihydro-1,2,4-thiadiazole-3-thione (3) from dithiobiuret in a similar way to the synthesis of
compound (2). The oxidative cyclization of dithiobiuret was reported under acidic conditions (HCl-H2O2)
to produce 3,5-diimino-1,2,4-dithiazole (4).3 As an extension of our systematic studies of analogues of
pyrimidines, we report our synthetic results of oxidative cyclization of dithiobiuret under basic conditions
and theoretical calculations of 5-amino-2,3–dihydro-1,2,4-thiadiazole-3-thione (3) for tautomeric study.
O
O
S
N
N
N
H2N
H
O
H2N
H
H2N
H
S
S
3
2
1
Compound (3) can theoretically exist in equilibria of four possible tautomeric forms as cytosine (shown
as in Scheme 1). If the most sable tautomeric form might be a thiol structure, this compound is easily
oxidized to disulfide compound. The dealing with the interaction of mercapto and disulfide compounds is
particularly important in biochemical phenomena for activation or inactivation of enzymatic processes.
RESULTS AND DISCUSSION
The oxidative cyclization of dithiobiuret was tried similarly as the preparation of 5-amino-2,3–dihydro-
1,2,4-thiadiazol-3-one2 from 2-thiobiuret (NaOH-H2O2)under nitrogen atmosphere, as shown in Scheme 1.
SR
13.6
SCH3
166.8
N
S S
N
N
2.49
6a; R = Me
6b; R = Et
N
HN
NH
SH
N
H2N
S
H HCl
H2N
7.99
S
S
S
6
4
183.0
N
NH2
RX
KOH, EtOH
S S
H2N
H
163.4
N
N
S S
N
S
O
+
+
N
N
N
N
HN
H2N
NH
HCl
S
3
NH2
N
N
NH2
H2N
8.21
S
S
H2N
H
H
184.1
5
S
SH
N
H
S
H
SH
N
N
N
N
N
N
H
H2N
H2N
H
S
S
HN
HN
S
S
3a
3c
3d
3b
Scheme 1. Oxidative cyclization of dithiobiuret and alkylation of compound (5).

HETEROCYCLES, Vol. 60, No. 6, 2003
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However, the reaction gave bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide (5) in stead of compound (3),
along with 3,5-diimino-1,2,4-dithiazole (4) and 2-thiobiuret.4 The structure of compound (5) was clearly
verified as bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide by X-Ray crystallography (as shown in Figure
1).
5
6a
Figure 1. The molecular structures of compound (5) and (6a), showing the atomic numbering used for the
crystallographic analysis.
Compound (5) is an oxidative dimer of compound (3b). Even if the amount of oxidation reagent (H2O2) is
much less than required one, we could not find the expected monomer (3). We can speculate that the
oxidation rate of the intermediate (3) might be much faster than the oxidative cyclization of dithiobiuret.
When CH3CO3H, instead H2O2, was used as an oxidant, the oxidative cyclization proceeded cleanly
giving only compound (5) with much better yield (37%). 1H and 13C NMR and HRFABMS spectra also
supported the structure of 5. HRFABMS spectrum shows a molecular ion at 264.9463 (calcd 264.9459).
1
In the H NMR spectrum, the disappearance of NH2 (9.47 and 9.15 ppm) and NH (10.58 ppm) groups
(the ratio 2 : 2 :1) of dithiobiuret and appearance of NH2 (8.21 ppm) group served as an evidence for the
13
oxidative cyclization of dithiobiuret to give 5. The C NMR spectrum contains signals for the two ring
carbons at 184.1 and 163.4 ppm, respectively, instead of a signal (180.1 ppm) of dithiobiuret. To
distinguish the chemical shifts between C(3) and C(5) of compound (5), a standard compound is required
13
for the comparison of chemical shifts. C NMR spectroscopy was widely utilized to differentiate thione
from thiol. However interpretation of only the absolute chemical shifts can easily lead to erroneous
conclusions. Thus, we tried to prepare the derivative of compound (5). Sulfur-sulfur bond can be cleaved
by OH- while forming the RS- as the following pattern.5

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HETEROCYCLES, Vol. 60, No. 6, 2003
+
OH-
+
RS-
RSOH
RS SR
The alkylation of 5 was performed under basic conditions with alkyl halide utilizing the above reaction.
In case of methylated product, the structure of the product (6a) was determined by X-Ray crystallography
(as shown in Figure 1). 1H NMR spectrum indicates a methyl group at 2.49 ppm, and NH2 group at 7.99
13
ppm. In C NMR spectrum chemical shifts of 5 and 6a are almost similar. To designate the chemical
signals of 6a, HMBC experiment was performed. The methyl signal (2.49 ppm) was associated with
1
signal at 166.8 ppm C(3)in H-13C long range correlation. In the comparison of chemical shifts of 5
(163.4 and 184.1 ppm) and 6a (166.8 and 183.0 ppm), the chemical shifts at C(3) and C(5) in both
compounds are more and less identical in consideration of methyl substituent. Consequently, the chemical
shifts of C(3) and C(5)in compound (5) are designated at 163.4 and 184.1 ppm, respectively.
To speculate an intermediate of the oxidation of dithiobiuret ab initio quantum mechanical calculations
were carried out on the four possible tautomers of 5-amino-2,3-dyhydro-1,2,4-thiadiazole-3-thione (3a-d)
using the GAUSSIAN94 package6 on the SGI-O2 workstation. The results of quantum mechanical
calculations show that 5-amino-3-mercapto-1,2,4-thiadiazole (3b) is the global energy minimum structure
(Table 1). It is well-known that thiol is easily oxidized to disulfide. On the other hand, thiolactam is very
hard to be oxidized in comparison with thiol. Thus, to form 5 the oxidative cyclization of dithiobiuret.
might be proceed through intermediate (3b).
The geometries were optimized in vacuum on the levels of both Hartree-Fock (HF/6-31G**) and Density
Functional Theory (DFT/B3LYP/6-31G**).7 To obtain improved energy comparisons second-order
(MP2) and fourth-order Moller-Pleset perturbation (MP4) calculations were also carried out on the HF
optimized geometries. The results of calculations, including the information of energies and dipole
moments, are given in Table 1. The optimized structural information is shown in Scheme 2. The ring
structures are almost planar due to the presence of the partial double bond characters within the molecules.
Scheme 2 shows that the quantum mechanically optimized geometry of 3b fits very well with the results
of the X-Ray crystallography for 5 and 6a.

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Table 1. The relative energies and dipole moments of four tautomers from the various quantum
mechanical methods.
∆E (kcal/mol)a
Dipole (Debye)
Tautomer
HF
5.03
0.00
4.09
11.19
MP2b
8.67
MP4b
8.50
DFT
3.42
0.00
4.17
10.37
HF
DFT
7.19
3.66
3.70
1.23
3a
3b
3c
3d
8.44
3.80
4.56
1.39
0.00
0.00
9.64
8.97
12.87
11.92
a
The total energies for the global minimum structure of 3b are -1035.84906888 (HF),
-1036.8465350739 (MP2), -1036.9314001 (MP4), and -1038.64924262884 (DFT).
The MP2 and MP4 energies are obtained using the HF optimized structures.
b
S
S
SH
SH
6
1.3687 (1.3800)
1.6581
(1.6620)
1.3550 (1.3746)
1.3670 (1.3748)
1.3657 (1.3718)
1.7630
(1.7772)
1.7626
(1.7749)
1.6616
(1.6617)
N
HN
HN
1.3853
(1.3976)
N
4
3
1.2805
1.2915
(1.3109)
1.3818
1.3561
1.2789
(1.3096)
1.3381
1.2619
(1.289
4)
1.7042
(1.2964)
1.3386
(1.3586)
1.2485
(1.2699)
(1.3892)
1.2431
(1.2654)
1.3330
(1.3515)
N (1.3848)
N(1.3631)
H
N
N
5
2
H2N
H
HN
H2N
HN
1.7747
1.7552
1.7804
1.6793 (1.7018)
7
1.6933 (1.7291)
(1.7847) S1
1.7387
(1.7629)
(1.8107) S
(1.8026) S
1.7133 (1.7463)
S
3a
3d(1.7295)
3b
3c
Bond Length (¡Ê)
HF (DFT)
Scheme 2. Optimized bond distance at HF/6-31G** and DFT/B3LYP/6-31G** (the values in parentheses)
levels.
EXPERIMENTAL
All melting points were determined on an electrically heated Thomas-Hoover capillary melting point
apparatus and are uncorrected. The IR spectra were recorded on a JASCO Report-100 spectrophotometer.
The 1H and 13C NMR spectra were obtained using a Bruker ARX-400 spectrometer at 400 MHz and 100
MHz respectively with tetramethylsilane as the internal reference. Elemental analyses were carried out on

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an EA 1110 (CE Instrument). NMR measurements and elemental analyses were performed at the Central
Research Facilities of Chungnam National University. FABHRMS spectra were obtained on a JEOL-JMS
HX-100/110A spectrometer at Korea Basic Science Institute, Taeduk, Taejon. The progresses of reaction
and purity of products were traced with TLC.
Method A: Oxidative cyclization of dithiobiuret in NaOH-H2O2
Dithiobiuret (20.0 g, 148 mmol) was dissolved in 2N NaOH (220 mL, 440 mmol) and cooled to 0-3 ºC in
a dry ice-acetone bath. 30% H2O2 (40.0 mL, 392 mmol) was added dropwise. The reaction mixture was
stirred for 30 min and acidified to pH 3.5-4.0 with 5.8 N HCl (about 50 mL). The precipitate was filtered
off and dried to give compound (5). The crude product was purified by chromatography over silica gel
(eluted with toluene : EA : EtOH = 5 : 3 : 0.5) to give bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide (3.78
g, 19.4 %). The filtrate was concentrated under reduced pressure to obtain 2-thiobiuret and 3,5-diimino-
1,2,4-dithiazole·HCl. The crude products were separated by column chromatography over silica gel
(eluted with toluene : ethyl acetate : ethanol = 5 : 3 : 1 and acetonitrile : carbontetrachloride : 80% Formic
acid = 60 : 5 : 2) to give 2-thiobiuret (6.39 g, 36.3%) and 3,5-diimino-1,2,4-dithiazole·HCl (0.78 g, 3.1%).
Bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide (5): mp >295 ºC (from DMF), Rf; 0.29 (toluene : ethyl
acetate : ethanol = 5 : 3 : 1 ). υmax(KBr)/cm-1 3352, 3267, 3159, 3109, 2924, 2698, 2361, 2339, 1626,
1610, 1523, 1510, 1523, 1510, 1447, 1238; δH (400 MHz; DMSO-d6) 8.21 (s, NH2); δC (100 MHz;
DMSO-d6) 184.1 (C=N), 163.4 (C-S). HRFABMS Calcd for C4H4N6S4 264.9459; found 264.9463.
2-Thiobiuret: mp 187-190 ºC (from H2O), (lit.,8-10 189-191 ºC), Rf; 0.39 (toluene : ethyl acetate : ethanol
= 5 : 3 : 1 ). υmax(KBr)/cm-1; 3441, 3383, 3219, 2361, 1720, 1705, 1687, 1602, 1545, 1427; δH (400 MHz;
DMSO-d6) 9.80 (1H, br), 9.66 (1H, br), 9.46 (1H, br), 6.85(1H, br), 6.30 (1H, br); δC (100 MHz; DMSO-
d6) 181.7 (C=S), 155.1 (C=O).
3,5-Diimino-1,2,4-dithiazole·HCl (4): mp > 295 ºC (from DMSO), Rf; 0.08 (acetonitrile : carbon
tetrachloride : 80% formic acid = 60 : 5 : 2 ). υmax(KBr)/cm-1 3252, 3049, 2361, 1635, 1614, 1496, 1367,
1045; δH (400 MHz; DMSO-d6) 10.5 (br, NH2), 10.3 (br, NH); δC (100 MHz; DMSO-d6) 181.2.
HRFABMS Calcd for C2H4N3S2 133.9847; found 133.9846. Anal. Calcd for C2H3N3S2HCl: C, 14.15; H,

HETEROCYCLES, Vol. 60, No. 6, 2003
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2.38; N, 24.77. Found: C, 14.15; H, 2.58; N; 24.89.
Method B: Oxidative cyclization of dithiobiuret in NaOH-CH3CO3H
Dithiobiuret (20.0 g, 148 mmol) was dissolved in 2N NaOH (220 mL, 220 mmol) and cooled to 0-3 ºC in
a dry ice-acetone bath. 32% CH3CO3H (180 mL, 856 mmol) was added dropwise at 5-10 ºC. The reaction
mixture was stirred for 30 min at the same temperature. The precipitate was filtered off and dried. to give
5 (7.41 g, 40.7 %, purity 97.6% by HPLC). It was purified by chromatography over silica gel (eluted with
toluene : EA : EtOH = 5 : 3 : 0.5) to give bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide (6.68 g, 36.6 %).
The product was identical with the bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide obtained from method A.
5-Amino-3-methylthio-1,2,4-thiadiazole
Bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide (5) (0.5 g, 1.89 mmol) was dissolved in 85% KOH (0.32 g,
4.85 mmol)-ethanol (50 mL). Methyl iodide (1.6 g, 11.2 mmol) was added to the solution. The reaction
mixture was stirred at rt for 2 h. The solvent was evaporated under reduced pressure to afford solid
residue, which was washed with cold water (10 mL) and CHCl3 (50 mL). The crude product was purified
by chromatography over silica gel (eluted with toluene : ethyl acetate = 9 : 1) to give 5-amino-3-
methylthio-1,2,4-thiadiazole (6a) (0.19 g, 34.1%) as colorless crystal. mp 138.1 ºC (from ethanol), (lit.,11
142 ºC), Rf; 0.42 (toluene : ethyl acetate (5 : 1)). υmax(KBr)/cm-1 3321, 3277, 3107, 2926, 2719, 2361,
1630, 1529, 1448, 1433, 1255; δH (400 MHz; DMSO-d6) 7.99 (2H, br, NH2), 2.49 (3H, s, CH3); δC (100
MHz; DMSO-d6) 183.0 (C=N), 166.8 (C-S), 13.6 (SCH3). Anal. Calcd for C3H5N3S2: C, 24.47; H, 3.42;
N, 28.54. Found: C, 24.35; H, 3.41; N; 28.01.
5-Amino-3-ethylthio-1,2,4-thiadiazole
Bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide (5) (0.5 g, 1.89 mmol) was dissolved in 85% KOH (0.32 g,
4.85 mmol)-ethanol (50 mL) solution. Ethyl iodide (0.45 mL, 5.63 mmol) was added to the solution. The
reaction mixture was stirred for 2 h at rt. The solvent was evaporated under reduced pressure to afford
solid residue. The crude product was purified by chromatography over silica gel (eluted with ethyl
acetate : n-hexane = 1 : 3) to give 5-amino-3-ethylthio-1,2,4-thiadiazole (6b) (0. 24 g, 39.5%) as colorless
crystal. mp 92.9 ºC (from ethyl acetate), (lit.,11 98.2 ºC), Rf; 0.18 (ethyl acetate : n-hexane (1 : 3)).

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υ
max(KBr)/cm-1 3420, 3275, 3161, 2962, 2926, 1601, 1579, 1521, 1494, 1448, 1238; δH (400 MHz;
DMSO-d6) 6.83(2H, br, NH2), 3.14(2H, q, J 6.7, CH2), 1.39(3H, t, J 6.7, CH3); δC (100 MHz; DMSO-d6)
183.0 (C=N), 167.25 (C-S), 25.77 (CH2) , 14.55 (CH3). Anal. Calcd for C4H7N3S2: C, 29.79; H, 4.38; N,
26.06. Found: C, 30.41; H, 4.39; N; 25.51.
X-Ray crystal structure of bis(5-amino-1,2,4-thiadiazolyl) 3,3’-disulfide (5)
Compound (4) was crystallized from slow evaporation of a solution of DMF. C17H29N15O3S8: MW
748.11, triclinic, space group P-1, a = 8.6523(8), b = 8.9261(4), c = 11.1644(13) Å, α = 98.999(13), β =
103.489(9), γ = 90.299(10)o, V = 827.35(18) Å3, Z =1, Dc = 1.502 g/cm3, µ = 0.588 mm-1, F(000) = 388,
T = 293(2) K. CAD-4 diffractometer (Enraf-Nonius, 1994) collected data using graphite-mono-chromated
Mo-Kα radiation (0.71073 Å). The structure was solved by direct methods (SHELX86)12 (all non-H
atoms), followed by full-matrix least-squares refinement (SHELX97)13 on F2. Hydrogen atoms were
located from ∆F synthesis and positionally refined. All non–hydrogen atoms were anisotropically refined,
leading to a final R1 and wR2, 0.0420 and 0.0943 respectively, for 3795 unique reflections and 222 refined
parameters. S[F2] 1.039 and (∆/σ)max was 0.000. Maximum and minimum features in ∆F synthesis are
0.395 and -0.194 eÅ -3, respectively.
X-Ray crystal structure of 5-amino-3-methylthio-1,2,4-thiadiazole (6a)
Compound (6a) was crystallized from slow evaporation of a solution of ethanol. C3H5N3S2: M.W.
147.22, monoclinic, space group P 21/n, a = 6.399(3), b = 8.0279(9), c = 12.5560(17) Å, α = 90, β =
91.170(19), γ = 90o, V = 644.9(3) Å3, Z = 4, Dc = 1.516 g/cm3, µ = 0.720 m,-1 F(000) = 304, T = 293(2)
K. CAD-4 diffractometer (Enraf-Nonius, 1994) collected data using graphite-mono-chromated Mo-K
radiation (0.71073 Å). The structure was solved by direct methods (SHELX86)12 (all non-H atoms),
followed by full-matrix least-squares refinement (SHELX97)13 on F2. Hydrogen atoms were located from
∆F synthesis and positionally refined. All non–hydrogen atoms were anisotropically refined, leading to a
final R1 and wR2, 0.0295 and 0.0783 respectively, for 1135 unique reflections and 74 refined parameters.
S[F2] 1.097 and (∆/σ)max was 0.000. Maximum and minimum features in ∆F synthesis are 0.265 and -
0.200 e Å -3, respectively.

HETEROCYCLES, Vol. 60, No. 6, 2003
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ACKNOWLEDGEMENT
This work supported by grant No. (R05-2001-000-00214-0) from the Basic Research Program of the
Korea Science & Engineering Foundation.
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