Enaminones in heterocyclic synthesis: A novel route to polyfunctionalized substituted thiophene, 2,3-dihydro-1,3,4-thiadiazole and naphtho[1,2-b] furan derivatives

Article abstract of DOI:10.1080/10426500210232

The enaminones 1a,b reacted with phenylisothiocyanate to afford the thioanilides 2a,b. The latter could be utilized to synthesize several new polysubstituted thiophenes 9a-f, and 1,3,4-thiadiazoles 12a-f.

Full text of DOI:10.1080/10426500210232

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Phosphorus, Sulfur, and Silicon and the Related
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Enaminones in Heterocyclic Synthesis: A Novel Route
to Polyfunctionalized Substituted Thiophene, 2,3-
Dihydro-1,3,4-Thiadiazole and Naphtho[1,2- b ] Furan
Derivatives
Zaghloul E. Kandeel a , Khadija M. Al-Zaydi a , Ebtisam A. Hafez a & Mohamed H. Elnagdi b
a Department of Chemistry, Faculty of Science , Cairo University , Giza, Egypt
b Department of Chemistry , Girls' College of Education , Jeddah, PO 50918, Jeddah, 21533,
Kingdom of Saudi Arabia
Published online: 27 Oct 2010.
To cite this article: Zaghloul E. Kandeel , Khadija M. Al-Zaydi , Ebtisam A. Hafez & Mohamed H. Elnagdi (2002) Enaminones
in Heterocyclic Synthesis: A Novel Route to Polyfunctionalized Substituted Thiophene, 2,3-Dihydro-1,3,4-Thiadiazole
and Naphtho[1,2- b ] Furan Derivatives, Phosphorus, Sulfur, and Silicon and the Related Elements, 177:1, 105-114, DOI:
10.1080/10426500210232
To link to this article: http://dx.doi.org/10.1080/10426500210232
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Phosphorus, Sulfur and Silicon, 2002, 177:105–114
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ENAMINONES IN HETEROCYCLIC SYNTHESIS:
A NOVEL ROUTE TO POLYFUNCTIONALIZED
SUBSTITUTED THIOPHENE, 2,3-DIHYDRO-1,3,
4-THIADIAZOLE AND NAPHTHO[1,2-b]
FURAN DERIVATIVES
,a
Zaghloul E. Kandeel, Khadija M. Al-Zaydi,a
Ebtisam A. Hafeza and Mohamed H. Elnagdib
Department of Chemistry, Faculty of Science, Cairo University,
Giza, Egypta and Department of Chemistry, Girls’ College
of Education, Jeddah, PO 50918 Jeddah 21533, Kingdom
of Saudi Arabiab
(Received March 24 2001.)
The enaminones 1a,b reacted with phenylisothiocyanate to afford the
thioanilides 2a,b. The latter could be utilized to synthesize several new
polysubstituted thiophenes 9a–f, and 1,3,4-thiadiazoles 12a–f.
Keywords: 1,3,4-thiadiazole; naphtho[1,2-b]benzofuran; thioanilides;
thiophenes
INTRODUCTION
Enaminones are readily obtainable reagents and their utility in het-
6
erocyclic synthesis has recently received a considerable interest.1
Although utility of the reactivity of the electron-deficient C-1 and
C-3 in enaminones toward nucleophiles has been extensively investi-
gated, little has been reported on reactivity of electron rich C-2 in these
compounds.7,8 Recently, the reaction of C-2 in enaminones with some
aromatic diazonium salts has been investigated in our laboratory.9 In
continuation with this work, we report here on the reactivity of C-2 in
enaminones toward some carbon electrophiles.
Address correspondence to Zaghloul E. Kandeel, Department of Chemistry, Faculty
of Science, Cairo University, Giza, Egypt.
105

106
Z. E. Kandeel et al.
DISCUSSION
It was found that the enaminones 1a,b reacted with phenylisothio-
cyanate in DMF in the presence of potassium hydroxide to afford after
acidification with HCl two products, the major of which were of molec-
ular formulae corresponding to structure 2a,b or its tautomeric forms
3a,b–5a,b (Scheme 1). Although Regitz et al.10 have assigned hydroxy-
methylene structure of type 3 or 4 for diketothioamides, structure 2
SCHEME 1
better agreed with the obtained spectral data for these compounds, thus
structures 3 and 4 could be ruled out. Structure 5 was also ruled out
on the basis of 13C NMR spectra of the isolated product which revealed
the absence of any sp3-hybridized carbon atoms. For example, 13C NMR

Enaminones
107
spectrum of compound 2a revealed two low field carbonyl carbons at
188.91 (benzoyl C O) and 176.5 (CHO) ppm. Moreover, 1H NMR
spectra of these products 2a,b indicated the presence of a low field
aldehydic proton in the region of 8.89. The existence of this proton at
such value compared with that expected for aromatic aldehyde is prob-
ably due to conjugation of the double bond system in these compounds
(Scheme 1).
The minor products for this reaction were assigned structure 6a,b
on the basis of the elemental analysis and spectral data of the isolated
reaction products and on comparison with an authentic sample of com-
pound 6a.11 The latter products 6a,b were assumed to be formed via
an initial conversion of a part phenyl isothiocyanate into aniline in the
employed alkaline solution which, in turn, reacted with 1a,b to afford
compounds 6a,b—though reaction of isothiocyanate with 1,3-diketones
has been extensively investigated earlier.12 To our knowledge this is the
first reported reaction of an enaminone with isothiocyanate.
Compounds 2a,b so obtained could be utilized for the synthesis
of a variety of polyfunctionally substituted heteroaromatics. For ex-
ample, reacting equimolar amounts of compounds 2a,b with some
-haloketones 7a,b and with ethyl bromoacetate (7c) in ethanol in
the presence of a catalytic amount of triethylamine led to the forma-
tion of condensation products via hydrogen halide and water elimina-
tion. Thiophene structures 9a–f were established for these products
(Scheme 2) on the basis of the elemental analyses and spectral data of
the isolated reaction products. For example, the mass spectrum of com-
pound 9a revealed a molecular ion peak at m/z 383 (M+). Also, 1H NMR
spectrum of the same product indicated the involvement of the formyl
group in this reaction as it revealed the thiophene H-4 as a singlet sig-
nal at 6.43 in addition to the aromatic protons as a multiplet at
6.91–7.52 and the NH signal as a singlet at 11.6.
Compounds 2a,b reacted also with some selected hydrazonyl halides
10a–c in the presence of a catalytic amount of triethylamine to afford
the 2,3-dihydro-1,3,4-thiadiazole derivatives 12a–f (Scheme 2) on the
basis of elemental analysis and spectral data of the isolated reaction
products. For example, the mass spectrum of compound 12a revealed a
molecular ion peak at m/z 350 (M+). Also, 1H NMR spectrum of the same
product revealed a singlet at 8.79 corresponding to the CHO function
in addition to signals of aromatic and acetyl protons. Compounds 12a–f
are assumed to be formed via the nonisolable intermediates 11a–f fol-
lowed by an aniline molecule elimination to afford the final products
12a–f as has been previously reported (Scheme 2).13
Compounds 1a,b reacted with 1,4-naphthoquinone (13) in acetic
acid at room temperature to afford in a quantitative yield products

108
Z. E. Kandeel et al.
SCHEME 2
of addition and dimethylamine elimination. These products were
formulated the naphtho[1,2-b]furan carboxyaldehyde derivatives 16a,b
on the basis of elemental analysis and spectral data of the isolated
reaction products. Compounds 16a,b are suggested to be formed via an
initial attack of the enaminone C-2 to the electron-deficient quinone
carbon to give the nonisolable intermediate 14a,b. The latter cyclizes
to afford either the nonisolable intermediates 15a,b which underwent
hydrolysis to give the formylnaphtho[1,2-b]furan derivatives 16a,b or
to afford the 3-aroyl-5-hydroxynaphtho[1,2-b]furan derivatives 17a,b
1
(Scheme 3). Structure 17 was ruled out on the basis of H NMR spec-
tra of the isolated reaction products which revealed the existance of
aldehydic groups at 8.90 and the phenolic OH group at 10.52.
The appearance of the aldehydic proton at such a higher field is
probably due to ring donation by oxygen lone pair. In addition, 13C
NMR spectrum of compound 16a indicated the presence of the alde-
hydic group at 190.80. It was thus believed that the intermediate

Enaminones
109
SCHEME 3
adducts 14a,b cyclized at room temperature at the aroyl carbon rather
than the enaminone carbon. This preference might be attributed to
thermodynamic stability of the final isolated products as a result of
the presence of an external conjugation in these products. A simi-
lar sequence for such a reaction has been recently reported from our
laboratory.14
In conclusion, C-2 in enaminones was reactive toward carbon elec-
trophiles and this reactivity was useful in synthetic heterocyclic
synthesis.
EXPERIMENTAL
Melting points were recorded on a Gallenkamp melting point apparatus
and are uncorrected. IR spectra were recorded as KBr pellets on a FT-
1
IR-8201 PC spectrophotometer (Shimadzu). H and 13C NMR spectra
were recorded in CDCl3 at 200 MHz on a Varian Gemini spectrometer
using tetramethylsilane (TMS) as an internal reference. Mass spectra
were performed on a Shimadzu GCMS-QP1000 EX mass spectrometer
at 70 eV. Elemental analyses were carried out at the Microanalytical

110
Z. E. Kandeel et al.
Data Center of Cairo University. Enaminones 1a,b,15 phenacyl bromide
19
(7a),16 and hydrazonoyl halides 10a–c17
were prepared according to
the literature.
Reaction of the Enaminones 1a,b with
Phenylisothiocyanate: Formation of 2a,b, 6a,b
To a stirred solution of potassium hydroxide (0.11 g, 2 mmol)
in DMF (20 ml) was added either 3-N,N-dimethylamino-1-phenyl-2-
propenone (1a) (0.35 g, 2 mmol) or 3-N,N-dimethylamino-1-(2-thienyl)-
2-propenone (2b) (0.36 g, 2 mmol). After stirring for 2 h, phenylisothio-
cyanate (0.27 g, 2 mmol) was added to the resulting mixture. Stirring
was continued for 6 h at room temperature, the mixture was poured
onto ice-water (100 g), and acidified with dilute HCl. The solid prod-
uct formed in each case was collected by filtration, washed with wa-
ter and dried. Recrystallization from ethanol afforded the thioanilides
2a,b.
2a: Yield (58%); mp 92–93 C; greyish crystals; max/cm 1 3450 (NH),
2950 (CH), 2590 (SH), 1623, 1613 (2C O);
(CDCl3) 2.62 (s, 1H, SH),
H
7.03–7.33 (m, 10H, 2 phenyl protons), 8.04 (s, 1H, NH), 8.80 (s, 1H,
CHO); c (CDCl3) 188.9 benzoyl CO), 176.5 (CHO), 142.3, 124.5 (olefinic
carbons), 138.6, 136.3, 131.4, 128.7, 128.1, 122.9, 119.6 (two phenyl
carbons); MS m/z (%) 283 (36) (M+), 105 (82), 77 (100); (Found: C, 67.7;
H, 4.5; N, 5.0; S, 11.2. C16H13NO2S requires C, 67.82; H, 4.62; N, 4.94;
S, 11.31%).
2b: Yield (57%); mp 105–106 C; pale yellow crystals; max/cm 1 3450
(NH), 2950 (CH), 2590 (SH), 1624, 1620 (2C O); H (CDCl3) 2.60 (s, 1H,
SH), 6.52–7.30 (m, 8H, Ph and 2-thienyl protons), 8.04 (s, 1H, NH), 8.82
(s, 1H, CHO); MS m/z (%) 289 (81) (M+), 255 (17), 77 (100); (Found: C,
58.0; H, 4.0; N, 4.6; S, 22.3. C14H11NO2S2 requires C, 58.11; H, 3.83; N,
4.84; S, 22.16%).
Upon leaving the filtrate to stand for 1 h at room temperature, an-
other solid product was formed in each case, this was collected by fil-
tration, washed with water, and dried. Recrystallization from ethanol
afforded yellow products that were identified as the 3-phenylimino-1-
phenyl-2-propenone (6a) and 3-phenylimino-1-(2-thienyl)-2-propenone
(6b), respectively.
6a: Yield (25%); mp 140–142 C; lit.11 mp 140–141 C, MS m/z (%) 223
(87) (M+), 222 (100) (M+-1), 146 (39), 77 (58).
6b: Yield (21%); mp 127–129 C; pale yellow crystals; max/cm 1 3430
(NH), 1646 (C O), 1580 (C C); H (CDCl3) 6.47 (d, 1H, J = 12.2 olefinic-
H), 6.62 (d, 1H, J = 12.2, olefinic-H), 7.66–7.08 (m, 8H, Ph and 2-thienyl
proton), 11.76 (s, 1H, NH); MS m/z (%) 229 (81) (M+), 192 (18), 77 (58);

Enaminones
111
(Found: C, 68.4; H, 4.9; N, 6.0; S, 14.0. C13H11NOS requires C, 68.10;
H, 4.84; N, 6.11; S, 13.98%).
Reaction of the Aroylthioanilides 2a,b with
-Haloketones or -Haloesters. Formation of 9a--f
General Procedure
To a solution of either the thioanilide 2a (0.283 g, 1 mmol) or 2b
(0.289 g, 1 mmol) in absolute ethanol, the appropriate -haloketones
(1 mmol) viz; phenacyl bromide (7a), -chloroacetone (7b) or ethyl bro-
moacetate (7c), and 1–2 drops of triethylamine as a catalyst were added.
The reaction mixture was heated under reflux, formation of a yellowish
solid, in each case, started to appear after 30–40 min. Heating was con-
tinued for 2 h; the solvent was removed under reduced pressure and
the solid products were collected by filtration, washed with ethanol,
and dried. Recrystallization from EtOH/DMF afforded the thiophene
derivatives 9a–f.
1
9a: Yield 98%, mp 265–267 C; yellow needles; max/cm 3418 (NH),
3050 (CH), 1640, 1635 (2C O);
(DMSO) 6.43 (s, 1H, thiophene H-4),
H
7.75–6.90 (m. 15H, aromatic protons), 11.60 (s, 1H, NH), MS m/z (%)
383 (100) (M+), 105 (83), 77 (97); (Found: C, 75.3; H, 4.4; N, 3.7; S, 8.5.
C24H17NO2S requires C, 75.17; H, 4.47; N, 3.65; S, 8.36%).
1
9b: Yield 81%, mp 205–206 C; max/cm 3418 (NH), 3055 (CH aro-
matic), 2855 (CH-aliphatic), 1650 (C O); H (DMSO) 2.67 (s, 3H, CH3),
6.45 (s, 1H, thiophene H-3), 7.50–6.88 (m, 10H, two Ph), 11.62 (s, 1H,
NH); m/z (%) 321 (71) (M+), 105 (100), 77 (63); (Found: C, 71.00; H,
4.50; N, 4.50; S, 10.00. C19H15NO2S requires C, 71.01; H, 4.70; N, 4.36;
S, 9.98%).
1
9c: Yield 64%; mp 163–165 C; max/cm 3417 (NH), 3047 (CH aro-
matic), 2947 (CH-aliphatic), 1720 (C O ester; 1636 (C O benzoyl);
H
(CDCl3) 1.13 (t, 3H, CH3), 4.12 (q, 2H, CH2), 6.46 (s, 1H, thiophene
H-4), 7.60–6.90 (m, 10H, two Ph), 11.56 (s, 1H, NH); MS m/z (%) 351
(33) (M+), 105 (100), 77 (66); (Found: C, 68.5; H, 4.6; N, 4.0; S, 9.0.
C20H17NO3S requires C, 68.36; H, 4.87; N, 3.99; S, 9.12%).
1
9d: Yield 88%, mp 197–199 C; max/cm 3420 (NH), 3050 (CH-
aromatic), 2860 (CH-aliphatic), 1650 (2C O);
(DMSO) 6.40–6.66
H
(m, 4H, thiophene-H), 6.90–7.50 (m, 10H, two Ph), 11.41 (s, 1H, NH);
MS m/z (%) 389 (6) (M+), 328 (100), 180 (63), 77 (82); (Found: C, 67.7;
H, 4.0; N, 3.6; S, 16.50. C22H15NO2S2 requires C, 67.84; H, 3.88; 3.60;
S, 16.46%).
1
9e: Yield 76%, mp 142–144 C; max/cm 3418 (NH), 3055 (CH aro-
matic), 2855 (CH-aliphatic), 1668 (C O acetyl), 1620 (2-thienoyl C O);
H (CDCl3) 2.66 (s, 3H, CH3), 7.78–6.60 (m, 9H, Ph and thienyl H), 11.48

112
Z. E. Kandeel et al.
(s, 1H, NH); MS m/z (%) 327 (100) (M+), 105 (6), 77 (76); (Found: C, 62.4;
H, 4.0; N, 4.5; S, 19.4. C17H13NO2S2 requires C, 62.37; H, 4.00; N, 4.28;
S, 19.58%).
1
9f: Yield 81%, mp 165–166 C; max/cm 3415 (NH), 3055 (CH aro-
matic), 2860 (CH-aliphatic), 1720 (C O ester), 1620 (C O);
(CDCl3)
H
1.14 (t, 3H, CH3), 4.22 (q, 2H, CH2), 7.55–6.80 (m, 9H, aromatic pro-
tons), 11.56 (s, 1H, NH), MS m/z (%) 357 (100) (M+); (Found: C, 60.0; H,
4.1; N, 4.0; S, 18.0. C18H15NO3S2 requires C, 60.48; H, 4.23; N, 3.92; S,
17.94%).
Reaction of the Thioanilides 2a,b with Hydrazonyl
Halides 10a--c. Formation of Compounds 12a--f
General Procedure
To a solution of either the thioanilide 2a (0.283 g, 1 mmol) or 2b
(0.289 g, 1 mmol) in absolute ethanol (20 ml), the appropriate hydra-
zonyl halide 10a–c (1 mmol) followed by triethylamine (0.3 ml) was
added. The mixture was heated under reflux. Formation of colored pre-
cipitates started to take place within 5–20 min. Heating was continued
for 2 h, then the reaction mixture was left to cool to room temperature.
The solid products were collected by filtration, washed with ethanol,
and dried. Recrystallization from EtOH/DMF afforded the correspond-
ing 2,3-dihydro-1,3,4-thiadiazole derivatives 12a–f.
1
12a: Yield 90%, mp 222–224 C; max/cm 3395 (NH), 3163 (CH-
aromatic), 1650 (C O), 1597 (C N); H (DMSO) 2.42 (s, 3H, CH3), 7.02–
7.55 (m, 10H, two Ph), 8.79 (s, 1H, CHO); MS m/z (%) 350 (81) (M+),
105 (91), 77 (100); (Found: C, 65.2; H, 4.0; N, 8.0; S, 9.3. C19H14N2O3S
requires C, 65.13; H, 4.03; N, 8.00; 9.15%).
12b: Yield 88%, mp 130–132 C; max/cm 1 3040 (CH-aromatic), 2985
(CH-aliphatic), 1728 (C O ester), 1636 (C O), 1598 (C N);
(CDCl3)
H
1.13 (t, 3H, CH3), 4.12 (q, 2H, CH2), 7.02–7.55 (m, 10H, two Ph), 8.80
(s, 1H, CHO); MS m/z (%) 380 (80) (M+), 105 (73), 77 (100); (Found:
C, 63.0, H, 4.2; N, 7.5; S, 8.5. C20H16N2O4S requires C, 63.15; H, 4.24;
N, 7.36; S, 8.43%).
1
12c: Yield 82%, mp 212–214 C; max/cm 3395 (NH), 3163 (CH-
aromatic), 2986 (CH-aliphatic), 1659 (C O, CHO), 1601 (C N);
H
(DMSO) 6.60–7.03 (m, 14H, aromatic), 8.83 (s, 1H, CHO), 11.94 (s, 1H,
NH); MS m/z (%) 461 (73) (M+), 105 (91), 77 (100); (Found: C, 62.5; H,
3.5; Cl, 7.9; N, 9.0; S, 7.0. C24H16ClN3O3S requires C, 62.41; H, 3.49;
Cl, 7.68; N, 9.10; S, 6.94%).
12d: Yield 80%, mp 166–168 C; max/cm 1 3040 (CH-aromatic), 2986
(CH-aliphatic), 1662, 1638 (C O CHO), 1597 (C N);
(DMSO) 2.42
H
(s, 3H, CH3), 6.50–7.45 (m, 8H, Ph, thienyl protons), 8.69 (s, 1H, CHO);

Enaminones
113
MS m/z (%) 356 (1.6) (M+), 355 (6) (M+-1), 354 (4) (M+-21), 248 (83),
247 (93), 92 (100); (Found: C, 57.4; H, 3.4; N, 7.6; S, 18.0. C17H12N2O3S2
requires C, 57.29; H, 3.39; N, 7.86; S, 17.99%).
12e: Yield 79%, mp 150–152 C; max/cm 1 3124 (CH-aromatic), 3040
(CH-aliphatic), 1728 (C O ester), 1674 (CHO, C O), 1598 (C N);
H
(CDCl3) 1.12 (t, 3H, CH3), 4.24 (q, 2H, CH2), 6.47–7.33 (m, 8H, aromatic
protons), 8.90 (s, 1H, CHO); MS m/z (%) 386 14) (M+), 385 (56) (M+-1),
354 (43) (MM+-2), 308 (71), 247 (6), 194 (39), 92 (66), 77 (100); (Found:
C, 56.0; H, 3.7; N, 7.5; S, 16.8. C18H14N2O4S2 requires C, 55.95; H, 3.65;
N, 7.25; S, 16.59%).
1
12f: Yield 88%, mp 225–226 C; max/cm 3050 (CH-aromatic), 2986
(CH-aliphatic), 1662 (C O, CHO), 1598 (C N); H (DMSO) 6.25 (d, 1H,
thiophene 3-H); 7.76–6.81 (m, 11H, aromatic protons), 11.46 (s, 1H,
NH); MS m/z (%) 467 (82) (M+), 92 (70), 77 (100); (Found: C, 56.5; H,
3.0; N, 9.0; S, 13.5; Cl, 7.5. C22H14ClN3O3S2 requires C, 56.47; H, 3.02;
N, 8.89; S, 13.70; Cl, 7.58%).
Reaction of the Enaminones 1a,b with
1,4-Naphthoquinone (13). Formation of
2-aryl-5-hydroxynaphtho[1,2-b]furan-3-carboxaldehydes
16a,b
General Procedure
To a solution of either the enaminones 1a (1.75 g, 10 mmol) or
1b (1.81 g, 10 mmol) in 96% acetic acid (20 ml), 1,4-naphthoquinone
(13) (1.58 g, 10 mmol) was added. The mixture was stirred at room
temperature for 24 h, then diluted with water. The solid prod-
uct formed, in each case, was collected by filtration, washed with
ethanol, and dried. Recrystallization from DMF afforded 2-phenyl-5-
hydroxynaphtho[1,2-b]furan-3-carboxaldehyde (16a) and 5-hydroxy-2-
(2-thienyl)naphtho[1,2-b]furan-3-carboxaldehyde (16b), respectively.
1
16a: Yield 95%, mp 275 C (dec.); max/cm 3250 (OH), 1622 (CHO);
(DMSO) 7.50-8.10 (m, 10H, aromatic H), 8.90 (s, 1H, CHO), 10.52
(s, 1H, OH);
(DMSO) 121.6 (C-3), 121.8, 122.1, 124.0, 124.4, 129.3,
c
129.6, 130.0, 139.5 (phenyl, naphthyl carbons), 145.4 (C-5), 151.7
(C-2), 190.80 (CHO); MS m/z (%), 288 (20) (M+), 212 (24), 211 (30),
105 (47), 77 (67); (Found: C, 79.0; H, 4.0. C19H12O3 requires C, 79.16; H,
4.20%).
1
16b: yield 95%, mp 251–252 C; max/cm 3250 (OH), 1632 (CHO);
(insoluble in common NMR solvents); MS m/z (%) 294 (78) (M+), 77
H
(100); (Found: C, 69.5; H, 3.3; S, 10.0. C17H10O3S requires C, 69.35; H,
3.42; S, 10.90%).

114
Z. E. Kandeel et al.
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