Synthesis of novel thiazole and pyrrolothiadiazine derivatives from aldehyde thiosemicarbazones

Article abstract of DOI:10.1002/hc.20198

Substituted thiosemicarbazones 7a-e reacted with ethenetetracarbonitrile (TCNE) in ethyl acetate with formation of 5-amino-3-(substituted benzylidene - amino)- 2-phenylimino-2,3-dihydrothiazole-4-carbonitrile 8a-e 2-amino-6-phenyl-imino-1, 6-dihydropyrrol

Full text of DOI:10.1002/hc.20198

Heteroatom Chemistry
Volume 17, Number 4, 2006
Synthesis of Novel Thiazole and
Pyrrolothiadiazine Derivatives from Aldehyde
Thiosemicarbazones
Mohsen A.-M. Gomaa, Alaa A. Hassan, and Hamdy S. Shehatta
Chemistry Department, Faculty of Science, El-Minia University, El-Minia, A. R. Egypt
Received 24 May 2005; revised 10 October 2005
respectively, in the presence of either acid or
ABSTRACT: Substituted thiosemicarbazones 7a–e
alkali. The cyclization of suitable open chain
reacted with ethenetetracarbonitrile (TCNE) in ethyl
organic molecules is a classical and very pop-
acetate with formation of 5-amino-3-(substituted ben-
ular approach to the synthesis of heterocyclic
zylidene-amino)-2-phenylimino-2,3-dihydrothiazole-
derivatives, owing to both the large variety of
4-carbonitrile 8a–e 2-amino-6-phenyl-imino-1,6-
heterocycles accessible through this way, and
dihydropyrrolo[1,3,4]thiadiazine-3-carbonitrile 9, and
the large variety of conditions under which the
phenyl-(5-{substituted phenyl}-3H-[1,3,4]thiadiazole-
reaction can be performed [12–14]. Noticeably,
2-ylidene)amines 10a–e. Rationales for the observed
when more than one product can be obtained at
ꢀ
C
conversations are presented. 2006 Wiley Periodicals,
the same time in a particular reaction, good to
Inc. Heteroatom Chem 17:261–266, 2006; Published on-
excellent selectivities can often be achieved by
line in Wiley InterScience (www.interscience.wiley.com).
means of a suitable choice of the cyclizing agent and
DOI 10.1002/hc.20198
conditions [15,16]. Frequently, such cyclization re-
actions are induced by acidic media or electrophilic
reagents [15,17–20]. On other occasions, the cy-
INTRODUCTION
clization mechanism involves a plain oxidation
step—electron abstraction or dehydrogenation—
prior to the actual ring closure step [21–23]. On
the other hand, the interaction of thiosemicar-
bazide derivatives with some ꢀ-acceptors such
as ethenetetracarbonitrile (TCNE), (1,3-dioxo-
2,3-dihydro-1H-inden-2-ylidene)propanedinitrile
and benzoquinones as well as naphthoquinones
afforded thiazole, thiazine, thiadiazole, thiadiazine,
thiadiazepine, oxathiadiazole, and indazole as well
as pyridazine derivatives [22–26].
Heterocyclic compounds as 1,2,4-triazole, 1,3,4-
oxadiazole, and 1,3,4-thiadiazole derivatives can
be obtained by oxidative cyclization of suitable
open chain molecules such as semicarbazones
and thiosemicarbazones [1–5]. Oxidative cycliza-
tion of substituted aldehyde thiosemicarbazones,
induced by different metallic salts, led to 1,2,4-
triazoline derivatives [2–6]. Cyclization of 1,4-
disubstituted thiosemicarbazides may be achieved
under various conditions [7–11]. For example, 4-
substituted 1-phenylacetylthiosemicarbazides under-
go cyclization to yield 3-benzyl-ꢀ2-1,2,4-triazoline-
5-thiones and 2-amino-5-benzyl-1,3,4-thiadiazole,
Recently,
we
have
demonstrated
that
4-substituted thiosemicarbazides 1a–c and 1,4-
disubstituted thiosemicarbazides 2a–c reacted with
TCNE in ethyl acetate with admission of air to
give products 3 and 4 (with 1a–c) and 5 and 6, in
addition to the thiadiazole derivatives (with 2a–c)
(Chart 1) [27,28].
Correspondence to: Alaa A. Hassan; e-mail: alaahassan2001@
yahoo.com.
c
ꢀ
2006 Wiley Periodicals, Inc.
261

262 Gomaa, Hassan, and Shehatta
CHART 2
changes were observed any more. The reaction mix-
ture was concentrated to dryness, and the residue
was subjected to vacuum sublimation to remove
any unreacted TCNE. Chromatographic separa-
tion of the residue in each case gave numerous
zones, from which products 8–10 could be iso-
lated (Chart 2). 5-Amino-3-(benzylideneamino)-2-
phenylimino-2,3-dihydrothiazole-4-carbonitrile 8a
was obtained as characteristically colorless crystals.
Its molecular structure is supported by the following
findings:
CHART 1
Aldehyde semi- and thiosemicarbazones are
polyfunctionalyzed compounds which easily cy-
clized by action of bases [7,8], acids [9], or oxidants
[2–6]; therefore, they are useful and versatile syn-
thons for the preparation of five- or six-membered
heterocyclic compounds.
As will be outlined in detail below in this paper,
we report several heterocyclization of substituted
aldehyde thiosemicarbazones 7a–e using TCNE ei-
ther as a reaction mediator or a building block.
Ethenetetracarbonitrile is the simplest of per-
cyanoalkenes (cyanocarbons) [29]. The general
chemistry of TCNE has been reviewed with empha-
sis on molecular complexes, reactions with ketones
and amines, tricyanovinylation reactions [29], addi-
tions [29], and cycloaddition reactions [30] as well
as synthesis of heterocycles [29–34].
The gross formula C17H13N5S represents a prod-
uct from one molecule of 7a and one molecule
of TCNE with loss of C(CN)2 unit probably in the
form of H2C(CN)2.
Both NH2 (δH = 6.88 ppm) and low-field CH N
group (δH = 8.15 ppm) are present.
In its 13C NMR spectrum, thiazole C-2, C-4, and
C-5 resonate at δ = 164.34, 50.93, and 153.77 ppm,
respectively, are in accordance with the observed
trends in the δ values for C-atoms in push–pull
alkenes [35,36]. Further peaks at 117.70 ppm (CN)
and 154.36 ppm (CH N) besides the aromatic car-
bons support the assigned structure. The presence of
CH N group is also evident from the 13C DEPT NMR
spectrum exhibiting positive signals at δ = 154.36
ppm. The absence of a 13C C S signal and the pres-
ence of C N signal as well as only one CN resonance
at 117.70 ppm support structure 8a.
RESULTS AND DISCUSSION
Treatment of 7a–e with two molar equivalents of
TCNE in ethyl acetate as solvent at room temper-
ature resulted in a pink coloration of the solution
which quickly turned into brown. This behavior may
be explained as due to initial formation of unstable
charge-transfer (CT) complexes followed by chemi-
cal reaction.
Monitoring of the reaction by visible spec-
troscopy failed, since the reaction is fast and
also at lower concentration no significant color
The combination of the radicals 11 and 12
form tetracyanoethylated product 13 (Scheme 1). In-
stead of expelling one molecule of hydrogen cyanide,
however, the adduct 15 was observed which abstract
Heteroatom Chemistry DOI 10.1002/hc

Synthesis of Novel Thiazole and Pyrrolothiadiazine Derivatives from Aldehyde Thiosemicarbazones 263
SCHEME 1
a molecule of hydrogen from 7 [22,23] with succes-
sive release a molecule of malononitrile to give 8.
The decoupled carbon spectrum of 9 showed sig-
nals at δ = 165.04, 120.28, 118.49, 61.91, and 154.87
assigned to C-6, C-7ꢁ, C-3ꢁ, C-3, and C-2, respectively.
loss of HCN from 21 and hydrolysis form the tri-
cyanovinylation product 22 which cyclized to give
product 9 (Scheme 2).
The structure of thiadiazoles 10a–e was con-
firmed by comparison with authentic samples [37–
39].
1
The H NMR of 9 showed two broad singlets with
the ratio 2:1 centered at 7.22 and 10.95 ppm due to
exocyclic NH2 and pyrazole NH, in addition to five
aromatic protons.
In a fairly complex and multistep process, two
types of cyanoheterocyclic products are formed from
various substituted thiosemicarbazones 7a–e and
The alternative structures 17–19 (Chart 3) could
be ruled out on the basis of 1H NMR, 13C NMR, and
the fragment ions in the mass spectrum of 9.
Nucleophilic attack of the active nitrogen atom
of the imino group of 7 on TCNE followed by the
CHART 3
SCHEME 2
Heteroatom Chemistry DOI 10.1002/hc

264 Gomaa, Hassan, and Shehatta
TCNE. The latter has a dual functioning role as
a dehydrogenating agent and as a source of a di-
cyanomethylene units. Since the reactions reported
very likely require numerous steps, moderate yields
(based throughout on the amount of starting mate-
rials) have to be regarded as acceptable.
acetate, a solution of 1.0 mmol of 7a–e was added
dropwise, which caused a spontaneous change of
color from pink to brown. The mixture was stirred
for 5 h and left standing for 48 h at room temper-
ature. After concentration to dryness, the residues
were sublimed at 80◦C under vacuum to remove un-
reacted TCNE and then subjected to PLC using cy-
clohexane/ethyl acetate (5:1) as eluent to give numer-
ous zones, three of which (with high intensity) were
removed and extracted. The fastest moving zone
CONCLUSION
In this study three kinds of ring forming reactions
of substituted thiosemicarbazones 7a–e with TCNE
have been observed: (i) cyclization of 7a–e to 8a–e
by tetracyanoethylation of secondary amino func-
tion with loss of malononitrile (Scheme 1); (ii) the
nucleophilic attack of the active nitrogen atom of the
imino group on the TCNE followed by loss of HCN
and hydrolysis to give the tricyanovinylation which
cyclized to form the product 9 (Scheme 2); and (iii)
dehydrogenation and cyclization of 7 using TCNE as
dehydrogenating agent. Thus, TCNE may act either
as mediator or as a building block in heterocycliza-
tion of thiosemicarbazone derivatives.
(R = 0.54) contained the pyrrolothiadiazine deriva-
f
tive 9 (14–17%), the second zone (R = 0.46) con-
f
tained the thiadiazole derivatives 10 a–e (28–33%),
while the slowest moving zone (R = 0.34) contained
f
the thiazole derivatives 8a–e (41–46%). Extraction
of the zones with acetone and concentration gave
residues, which were rechromatographed to improve
purification and then recrystallized.
5-Amino-3-(benzylideneamino)-2-phenylimino-2,
3-dihydrothiazole-4-carbo-nitrile 8a. Yield (137 mg,
43%), mp 220◦C, IR (KBr) νmax 3430 (NH2), 2220
1
(CN), 1640 (C=N), 1595 (Ar-C C) cm−1. H NMR
(300.13 MHz, DMSO-d6) δ 6.88 (s, br, 2H, NH2),
7.39–7.67 (m, 10H, Ar-H), 8.15 (s, 1H, CH N). 13C
NMR (75.47 MHz, DMSO-d6) 164.34 (C-2), 154.36
(CH N), 134.63, 134.15, 130.35, 130.18, 129.94,
129.19, 128.92, 127.49, 126.50 (Ar-C), 117.70 (CN),
50.93 (C-4), 153.77 (C-5). MS (EI, 70 eV), m/z (%)
319 (M+, 100), 253 (45), 148 (21), 118 (27), 90 (40),
77 (56). C17H13N5S (319.39); Calcd C, 63.93; H, 4.10;
N, 21.93; S, 10.04%; Found: C, 64.11; H, 4.01; N,
22.09; S, 9.88%.
EXPERIMENTAL
Melting points have been determined using open
glass capillaries on a Gallenkamp melting point ap-
paratus and are uncorrected. The IR spectra were
recorded with a Shimadzu 408 instrument using
potassium bromide pellets. 1H NMR (300 MHz),
and 13C NMR (75 MHz) spectra were obtained
on a Bruker WM 300 instrument. Mass spectra
have been obtained with an AMD 604 doubly fo-
cusing instrument using electron impact ionization
(70 eV). Elemental analyses were determined by Mi-
croanalytical Center, Cairo University. Preparative
layer chromatography (PLC) was made using 48
cm × 20 cm glass plates covered with a 1 mm layer
of Merck silica gel PF254, from applied slurry and air
dried. Zones were detected by indicator fluorescence
quenching upon 254 nm excitation, removed from
plates and extracted with cold acetone.
5-Amino-3-[4-(methoxybenzylidene)amino]-2-
phenylimino-2,3-dihydro-thiazole-4-carbonitrile 8b.
Yield (161 mg, 46%), mp 236◦C. IR (KBr) νmax 3425
(NH2), 2980 (Ali-CH), 2215 (CN), 1635 (C N), 1600
1
(Ar-C C) cm−1. H NMR (300.13 MHz, DMSO-d6) δ
3.86 (s, 3H, OCH3), 6.94 (s, br, 2H, NH2), 7.36–7.65
(m, 9H, Ar-H), 8.18 (s, 1H, CH N). 13C NMR (75.47
MHz, DMSO-d6) 164.22 (C-2), 154.71 (CH N),
140.11, 133.97, 129.87, 129.16, 128.91, 128.66,
126.14, 125.18 (Ar-C), 118.18 (CN), 51.11 (C-4),
54.69 (OCH3), 152.86 (C-5). MS (EI, 70 eV), m/z (%)
349 (M+, 100), 318 (37), 283 (44), 148 (31), 90 (28),
77 (54). C18H15N5SO (349.42); Calcd C, 61.87; H,
4.33; N, 20.04; S, 9.18%; Found: C, 61.98; H, 4.18;
N, 20.19; S, 9.04%;
Starting Materials
All thiosemicarbazones 7a–e were synthesized by re-
action of 4-phenylthio-semicarbazide and the proper
aldehyde according to the published procedures
7a [40], 7b [41], 7c [42], 7d [43], and 7e [44]. Ethen-
tetracarbonitrile (TCNE, Merck) was purified by
crystallization from chlorobenzene and sublimed.
5-Amino-3-[4-(chlorobenzylidene)amino]-2-pheny-
limino-2,3-dihydrothiazole-4-carbonitrile 8c. Yield
(145 mg, 41%), mp 213◦C. IR (KBr) νmax 3440 (NH2),
2210 (CN), 1630 (C N), 1590 (Ar-C C) cm−1. 1H
NMR (300.13 MHz, DMSO-d6) δ 6.91 (s, br, 2H,
Preparation of Thiazole, Pyrrolothiadiazine, and
Thiadiazole Derivatives 8–10. To a stirred solution
of 256 mg (2.0 mmol) of TCNE in 10 mL of ethyl
Heteroatom Chemistry DOI 10.1002/hc

Synthesis of Novel Thiazole and Pyrrolothiadiazine Derivatives from Aldehyde Thiosemicarbazones 265
NH2), 7.13–7.59 (m, 9H, Ar-H), 8.17 (s, 1H, CH N).
Phenyl-(5-{4-chlorophenyl}-3H-[1,3,4]thiadia-
zole-2-ylidene)amine 10c. Yield (71 mg, 28%), mp
175◦C, lit. 175 [38].
MS (EI, 70 eV), m/z (%) 355/353 (M+, 76), 318 (36),
287 (38), 182 (24), 90 (41), 77 (100). C17H12ClN5S
(353.83); Calcd C, 57.71; H, 3.42; N, 19.79; S, 9.06%;
Found: C, 57.89; H, 3.29; N, 19. 94; S, 8.88%.
Phenyl-(5-{2-hydroxyphenyl}-3H-[1,3,4]thiadia-
zole-2-ylidene)amine 10d. Yield (78 mg, 31%), mp
203◦C, lit. 205◦C [39].
5-Amino-3-[2-(hydroxybenzylidene)amino]-2-
phenylimino-2,3-dihydro-thiazole-4-carbonitrile 8d.
Yield (147 mg, 44%), mp 208◦C. IR (KBr) νmax
3480–3375 (OH, NH2), 2220 (CN), 1635 (C N), 1600
Phenyl-(5-{4-N,N-dimethylaminophenyl}-3H-
[1,3,4]thiadiazole-2-ylidene)amine 10e. Yield (76
mg, 30%), mp 140◦C, lit. 142◦C [38].
1
(Ar-C C) cm−1. H NMR (300.13 MHz, DMSO-d6) δ
6.92 (s, br, 2H, NH2), 7.29–7.74 (m, 9H, Ar-H), 8.15
(s, 1H, CH N), 10.39 (s, br, 1H, OH). MS (EI, 70
eV): m/z (%) 335 (M+, 100), 319 (32), 269 (47), 163
(19), 90 (64), 77 (52). C17H13N5SO (335.39); Calcd C,
60.88; H, 3.91; N, 20.88; S, 9.56%; Found: C, 61.06;
H, 4.08; N, 20.71; S, 9.38%.
ACKNOWLEDGMENT
A. A. Hassan is indebted to the A. v. Humboldt-
Foundation for the donation of the Shimadzu 408
IR spectrophotometer.
5-Amino-3-[(4-N,N-dimethylaminobenzylidene)-
amino]-2-phenylimino-2,3-dihydro-thiazole-4-carbo-
nitrile 8e. Yield (152 mg, 42%), mp 256◦C. IR (KBr)
νmax 3430 (NH2), 2980–2965 (Ali-CH), 2210 (CN),
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Heteroatom Chemistry DOI 10.1002/hc

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