JJC
Jordan Journal of Chemistry Vol. 2 No.3, 2007, pp. 211-218
Oxidative Cyclization of Arylidene Carboxyhydrazides:
Synthesis of Substituted Hydroxydiphenylmethyl-1,3,4-Oxadiazoles
Kayed A. Abu Safieha, Ahmad M. S. Al-Titia, Jalal A. Zahrab, and Mikdad T. Ayouba*.
a Chemistry Department, Faculty of Science, The Hashemite University, Zarqa, Jordan.
b Chemistry Department, Faculty of Science, University of Jordan, Amman, Jordan.
Received on June 17, 2007
Accepted on Nov. 15, 2007
Abstract
The
synthesis
and
characterization
of
a
series
of
5-aryl-1,3,4-oxadiazol-2-yl-
diphenylmethanols (4) was achieved by oxidative cyclization of N'-(3-aryl)methylene]-2-hydroxy2,2-diphenylacetohydrazides (3) with lead(IV) acetate. The S-{5-[hydroxy (diphenyl)methyl]-1,3,4oxadiazol-2-yl}-aryl carbothioates (7) were prepared by the reaction of (5-mercapto-1,3,4oxadiazol-2-yl)(diphenyl)methanol (5) with the respective acid chlorides.
Keywords:1,3,4-Oxadiazoles; Lead(IV) acetate; Oxidative cyclization;
Carbothionates.
Introduction
Within the framework of our systematic efforts focusing on the synthesis of new
heterocyclic compounds as potential pharmaceutical agents
oxadiazol-2-yl-diphenylmethanols
(4)
and
[1]
, a series of 5-aryl-1,3,4-
S-{5-[hydroxy(diphenyl)-methyl]-1,3,4-
oxadiazol-2-yl}-substituted benzene carbothioates (7) were synthesized . These 1,3,4oxadiazoles are known to possess peculiar biological activities and are important
intermediates and building blocks in the construction of a variety of biologically active
compounds as bactericidal, fungicidal, insecticidal, and anti-inflammatory agents
[2-4]
.
At the same time benzilic acid skeleton is commonly found in molecules with great
value in medicinal chemistry as antispasmodic
[5]
and selective anti-muscarinc
[6-8]
agent.
A number of methods have been described in the literature for the preparation of
substituted 1,3,4-oxadiazoles
[9]
. The most suitable methods for this synthesis apply
either acid-catalyzed cyclization of 1,2-diacylhydrazines [10-12] or oxidative cyclization of
acylhydrazones
[13-15]
. The reaction is promoted by heat and anhydrous reagents
including thionyl chloride[16], phosphorous oxychloride[17], phosphorous pentoxide[18],
triphenylphosphine[19], and triflic anhydride[20]. Alternative synthetic methods comprise
reaction of carboxylic hydrazides with keteneylidene triphenylphosphorane[21] or basepromoted cyclization reaction of trichloroacetic acid hydrazones[22].
* Corresponding author. E-mail: mik@hu.edu.jo
Despite the availability of these methods for the synthesis of these heterocycles,
relatively few papers have been published describing the preparation of oxadiazoles
having hydroxydiphenylmethyl functional group attached either to C-3 or C-5 of the
1,3,4-oxadiazole ring[23]. For this purpose, a new series of substituted 1,3,4oxadiazoles is described.
Experimental
Melting points were determined on an SMP2 Stuart apparatus and are are
uncorrected.
IR
spectra
were
recorded
1
spectrophotometer as KBr pellets. H and
on
Nicolet
Magna-560
FT-IR
13
C NMR spectra were performed in DMSO-
d6 containing ca. 1% TMS as internal standard on Bruker WM-400 and Bruker DPX300 spectrometers. Mass spectra were measured on Finnigan MAT-731 at 70 eV.
Microanalyses were performed at the Microanalytical Laboratory, Chemistry
Department, The Hashemite University, Jordan. TLC was carried out on pre-coated
silica gel plates (E. Merck Kiesegel 60 F254, layer thickness 0.25 mm). Benzilic acid,
hydrazine monohydrate, p-tolualdehyde, p-halobenzaldehydes, furfural, thiophene-2carbaldehyde, pyridine-2-carbaldehyde, furoyl chloride, 2-thienoyl chloride, and
picolinoyl chloride hydrochloride were purchased from Aldrich.
2-Hydroxy-2,2-diphenylacetohydrazide (1, C14H14N2O2)
This compound was prepared according to reference [17]. White crystals; mp 169170 oC; yield 93% ; IR(KBr):
= 3253 ( OH), 3191 (NH), 1650 (C=O)cm-1 ; 1H NMR
(300 MHz, DMSO-d6): δ =3.10 (bs, NHNH2), 3.50 (s, OH), 7.30-7.50 (m, ArH) ppm;
13
C NMR (75 MHz, DMSO-d6): δ = 79.1 (C-OH), 172.0 (C=O), 125.7; 125.8; 125.9;
141.8 (ArC) ppm.
N'-(3-aryl)methylene-2-hydroxy-2,2-diphenylacetohydrazides (3a-3e)
General procedure
A mixture of 1 (0.24 g, 1 mmol) and the appropriate aldehyde 2 (1.1 mmol) in
propanol (25 mL) was refluxed with stirring for 3 h. The arylidenehydrazides (3) were
isolated, washed with 20 mL cold methanol, 10 mL diethyl ether, and dried. The
products were used without further purification in the next step.
2-Hydroxy-2,2-diphenyl-N'-phenylmethyleneacetohydrazide (3a)
Solid; mp 201-202°C; Yield 96%; IR(KBr):
= 3260 (NH), 1655 (C=O), 1541
(C=N) cm-1; 1H NMR (300 MHz, DMSO-d6): δ = 3.40 (s, OH), 9.70 (s, CONH), 8.17
(s,CH=), 7.30-7.70 (m, ArH) ppm; EIMS: m/z = 330 (M+) (C21H18N2O2)
N'-(3-Fluorophenyl)methylene-2-hydroxy-2,2-diphenylacetohydrazide (3b)
Solid; mp 210-211°C; Yield 88%; IR(KBr):
= 3236 (NH), 1643 (C=O), 1550
-1 1
(C=N) cm ; H NMR (300 MHz, DMSO-d6): δ = 3.40 (s, OH), 11.70 (s, CONH), 8.60
(s,CH=), 7.30-7.60 (m, ArH) ppm;
13
C NMR (75 MHz, DMSO-d6): δ = 80.8 (C-OH),
212
160.0 (C=N), 169.9 (CONH), 113.0; 113.3; 116.9; 117.0; 123.8; 127.7; 127.8; 128.0;
ppm; EIMS: m/z = 348 (M+)
131.2; 131.3; 137.0; 137.3; 144.0; 147.6 (ArC)
(C21H17FN2O2).
N'-(4-Chlorophenyl)methylene-2-hydroxy-2,2-diphenylaceto-hydrazide (3c)
= 3233 (NH), 1657 (C=O), 1562
Solid; mp 249-250°C; Yield 96%; IR(KBr):
-1 1
(C=N) cm ; H NMR (300 MHz, DMSO-d6): δ = 3.40 (s, OH), 11.50 (s, CONH), 8.50
(s,CH=), 7.00-7.80 (m, ArH) ppm;
13
C NMR (75 MHz, DMSO-d6): δ = 80.5 (C-OH),
150.0 (C=N), 170.0 (CONH), 127.4; 127.5; 127.7; 128.7; 128.9; 133.3; 143.7 (ArC)
ppm; EIMS: m/z = 364 (M+) (C21H17ClN2O2)
N'-(4-Bromophenyl)methylene-2-hydroxy-2,2-diphenylacetohydrazide (3d)
= 3228 (NH), 1650 (C=O), 1591
Solid; mp 255-257°C; Yield 82%; IR(KBr):
-1
(C=N) cm ;
1
H NMR (300 MHz, DMSO-d6): δ = 3.30 (s, OH), 11.60 (s, CONH), 8.50
(s,CH=), 7.20-7.60 (m, ArH) ppm;
13
C NMR (75 MHz, DMSO-d6): δ = 81.4 (C-OH),
148.8 (C=N), 170.4 (CONH), 128.3; 128.4; 128.6; 129.8; 132.7; 134.5; 144.6 (ArC)
ppm; EIMS: m/z = 408 (M+) (C21H17BrN2O2)
2-Hydroxy-N'-(4-methylphenyl)methylene-2,2-diphenylaceto-hydrazide (3e)
= 3228 (NH), 1646 (C=O), 1610
Solid; mp 234-236°C; Yield 90%; IR(KBr):
(C=N)
cm-1; 1H
NMR (300 MHz, DMSO-d6): δ = 2.20 (s,CH3), 3.50 (s, OH), 9.40 (s,
CONH), 8.00 (s,CH=), 7.00-7.50 (m, ArH) ppm;
13
C NMR (75 MHz, DMSO-d6): δ =
21.5 (CH3), 81.5 (C-OH), 149.9 (C=N), 169.9 (CONH), 127.6; 127.8; 128.4; 129.4;
130.5; 141.2; 142.3 (ArC) ppm; EIMS: m/z = 344 (M+) (C22H20N2O2)
5-Aryl-1,3,4-oxadiazol-2-yl-diphenylmethanols (4a-4e)
General procedure
A suspension of the appropriate acetohydrazide 3 (1 mmol) and lead (IV)
acetate (0.45 g, 1 mmol) in glacial acetic acid (50 mL) was refluxed for 12 h. The
reaction mixture was cooled and poured into water (100 mL). The organic layer was
basified with aqueous 10% sodium carbonate (50 mL) and extracted with chloroform (2
x 100 mL). The organic layer was dried over magnesium sulphate, filtered, and the
solvent evaporated in vacuo, to give the title compounds.
Diphenyl-(5-phenyl-1,3,4-oxadiazol-2-yl)methanol (4a)
Solid; mp 153-155°C (Lit. 151-153 °C)[23] ; Yield 71%; IR(KBr):
-1
1
= 3204 (OH),
1553 (C=N) cm ; H NMR (300 MHz, DMSO-d6): δ = 3.33 (s, OH), 7.21-7.53 (m, ArH)
ppm; EIMS: m/z = 328 (M+) (C21H16N2O2).
213
[5-(3-Fluorophenyl)-1,3,4-oxadiazol-2-yl](diphenyl)methanol (4b)
Solid; mp 128-129°C; Yield 29%; IR(KBr):
= 3259 (OH), 1596 (C=N) cm-1; 1H
NMR (300 MHz, DMSO-d6): δ = 3.36 (s, OH), 7.40-7.66 (m, ArH) ppm;
13
C NMR (75
MHz, DMSO-d6): δ = 76.5 (C-OH), 113.5; 113.8; 119.4; 119.7; 123.2; 123.3; 127.0;
127.4; 127.5; 128.1; 128.4; 128.9; 132.2; 132.4; 143.7 (ArC), 164.0 (C-2), 170.0 (C-5)
ppm; EIMS: m/z = 346 (M+) (C21H15 FN2O2).
[5-(4-Chlorophenyl)-1,3,4-oxadiazol-2-yl](diphenyl)methanol (4c)
Solid; mp 189-190°C ; Yield 50%; IR(KBr):
= 3259 (OH), 1602 (C=N) cm-1; 1H
NMR (300 MHz, DMSO-d6): δ = 3.35 (s, OH), 7.32-7.67 (m, ArH) ppm; 13C NMR (75
MHz, DMSO-d6): δ = 76.45 (C-OH), 122.4; 126.9; 128.1; 128.4; 128.7; 130.0; 137.2;
143.7 (ArC), 164.0 (C-2), 169.0 (C-5) ppm; EIMS: m/z = 362 (M+) (C21H15 ClN2O2).
[5-(4-Bromophenyl)-1,3,4-oxadiazol-2-yl](diphenyl)methanol (4d)
Solid; mp 182-183°C; Yield 33%; IR (KBr):
= 3256 (OH), 1600 (C=N) cm-1; 1H
NMR (300 MHz, DMSO-d6): δ = 3.41 (s, OH), 7.40-7.70 (m, ArH) ppm; EIMS: m/z =
407 (M+)(C21H15 BrN2O2)
[5-(4-Methylphenyl)-1,3,4-oxadiazol-2-yl](diphenyl)methanol (4e)
Solid; mp 164-166°C ; Yield 55%; IR(KBr):
= 3217 (OH), 1614 (C=N) cm-1; 1H
NMR (300 MHz, DMSO-d6): δ = 2.37 (s, CH3), 3.35 (s, OH), 7.31-7.82 (m, ArH) ppm;
13
C NMR (75 MHz, DMSO-d6): δ = 21.5 (CH3), 76.0 (C-OH), 120.8; 126.9; 128.1;
128.4; 130.4; 142.6; 143.8 (ArC), 165.0 (C-2), 169.2 (C-5) ppm; EIMS: m/z = 342 (M+)
(C22H18N2O2).
(5-Mercapto-1,3,4-oxadiazol-2-yl)(diphenyl)methanol (5)
Carbon disulfide (0.15 g, 2 mmol) was added to a stirred solution of 1 (4.8 g , 2
mmol) in 30 mL ethanol containing potassium hydroxide (0.11g, 2 mmol). The reaction
mixture was heated at reflux for 20 hrs, cooled, concentrated under reduced pressure
and the resulting precipitate was dissolved in water, acidified with acetic acid, and
extracted with chloroform (4 x 30 mL). The combined organic extracts were dried over
MgSO4, filtered and concentrated under reduced pressure to give compound (5), as
white fluffy crystals. Solid; mp 140-141°C Yield 96%; IR(KBr):
= 3310 (NH), 3125
1
(OH), 2960 (SH), 1608 (C=N) cm-1; H NMR (300 MHz, DMSO-d6): δ = 3.40 (s, OH),
7.30-7.50 (m, ArH), 14.10 (bs, SH) ppm;
13
C NMR (75 MHz, DMSO-d6): δ = 76.1 (C-
OH), 126.9; 128.3; 128.4; 142.7 (ArC), 166.2 (C-2), 178.4 (C-5) ppm; EIMS: m/z =
284 (M+).
214
S-{5-[Hydroxy(diphenyl)methyl]-1,3,4-oxadiazol-2-yl}-substituted benzene
carbothioates (7a-7d)
General Procedure
Triethylamine (0.15 g, 1 mmol) was added slowly, with stirring, to an ice-cooled
suspension of 5 (2.8 g, 1 mmol) in 25 mL chloroform. The reaction mixture was treated
with the appropriate acyl chloride 6 (1.1 mmol) and further stirred for 15 min. The
residue was added to crushed ice and the mixture was extracted with chloroform (3 x
50 mL). The combined organic extracts were dried over MgSO4, filtered and
concentrated under reduced pressure. The residue was dried and crystallized from
chloroform to give the respective title compounds (7a-7d).
S-{5-[Hydroxy(diphenyl)methyl]-1,3,4-oxadiazol-2-yl} benzene carbothioate
(7a, C22H16N2O3S)
Solid; mp 138-140°C ; Yield 98%; IR(KBr):
-1
= 3323 (OH), 1703 (C=O), 1593
1
(C=N) cm ; H NMR (300 MHz, DMSO-d6): δ = 3.42 (s, OH), 7.30-7.50 (m, ArH) ppm;
13
C NMR (75 MHz, DMSO-d6): δ = 76.2 (C-OH), 127.1; 128.4; 128.6; 129.1; 129.8;
130.9; 131.2; 133.4; 143.5 (ArC), 162.6 (C-5), 167.9 (C-2), 172.7 (C=O) ppm; Anal
Calcd for C22H16N2O3S (388.45): (%), C, 68.03; H,
4.15; N, 7.21; S, 8.25. Found
(%),C, 68.32; H, 4.39; N, 6.92; S, 8.50.
S-{5-[Hydroxy(diphenyl)methyl]-1,3,4-oxadiazol-2-yl}4-chlorobenzene-carbothioate
(7b, C22H15ClN2O3S)
Solid; mp 145-146°C ; Yield 97%; IR(KBr):
= 3331 (OH), 1689 (C=O), 1583
-1 1
(C=N) cm ; H NMR (300 MHz, DMSO-d6): δ = 3.36 (s, OH), 7.30-7.50 (m, ArH) ppm;
13
C NMR (75 MHz, DMSO-d6): δ = 76.9 (C-OH), 127.1; 128.4; 128.6; 129.2; 130.1;
131.6; 138.3; 142.8; 143.5 (ArC), 162.8 (C-5), 167.0 (C-2), 178.6 (C=O) ppm; Anal
Calcd for C22H15ClN2O3S (422.89): (%), C, 62.49; H,
3.58; Cl, 8.38; N, 6.62; S,
7.58 . Found (%),C, 62.71; H, 3.75; Cl, 8.69; N, 6.38; S, 7.23.
S-{5-[Hydroxy(diphenyl)methyl]-1,3,4-oxadiazol-2-yl}(2-furyl)benzene carbothioate
(7c, C20H14N2O4S)
Solid; mp 131-132°C ; Yield 95%; IR(KBr):
= 3370 (OH), 1701 (C=O), 1563
-1 1
(C=N) cm ; H NMR (300 MHz, DMSO-d6): δ = 3.35 (s, OH), 7.20-7.40 (m, ArH), 7.708.20 (m, R) ppm; 13C NMR (75 MHz, DMSO-d6 ): δ = 76.8 (C-OH), 112.6; 114.6;
118.2 ; 121.2; 127.1; 128.4; 128.6; 143.5; 145.4; 147.6; 151.2 (ArC), 159.8 (C-5),
162.6 (C-2), 172.6 (C=O) ppm; Anal Calcd for C20H14N2O4S (378.41): (%), C, 63.48;
H, 3.73; N, 7.40; S, 8.47 . Found (%), C, 63.12; H, 3.93; N, 7.65; S, 8.62.
215
S-{5-[Hydroxy(diphenyl)methyl]-1,3,4-oxadiazol-2-yl}(2-thienyl)benzene-carbothioate
(7d, C20H14N2O3S2)
Solid; mp 119-121°C ; Yield 96%; IR:
= 3374 (OH), 1683 (C=O), 1540 (C=N)
-1 1
cm ; H NMR (300 MHz, DMSO-d6): δ = 3.40 (s, OH), 7.30-7.40 (m, ArH), 7.00-7.80
(m, R) ppm;
13
C NMR (75 MHz, DMSO-d6): δ = 76.2 (C-OH), 127.1; 128.4; 128.6;
128.7; 133.7; 133.8; 142.8 (ArC), 162.6 (C-5), 167.9 (C-2), 172.6 (C=O) ppm; Anal
Calcd for C20H14N2O3S2 (394.47 ): (%), C, 60.90; H,
3.58;
N, 7.10; S, 16.26.
Found (%),C, 61.20; H, 3.84; N, 6.95; S, 15.97.
Results and Discussion
Methyl 2-hydroxy-2,2-diphenyl acetate (methyl benzilate), prepared by acidcatalyzed esterification of 2-hydroxy-2,2-diphenylacetic acid (bezilic acid), was
converted into 2-hydroxy-2,2-diphenylacetohydrazide (1) upon reaction with hydrazine
hydrate in boiling ethanol according to the literature [24]. Two approaches were used for
the synthesis of the target compounds: in the first approach (Scheme 1), N'-(3aryl)methylene-2-hydroxy-2,2-diphenylaceto-hydrazides (3), prepared by refluxing (1)
and aldehydes (2) in propanol, were cyclized to 5-aryl-1,3,4-oxadiazol-2-yldiphenylmethanols (4) using lead tetracetate
previously for some acylhydrazones
Ph
HO
Ph
NH-NH2
[26]
[25]
.
ArCHO 2
HO
Ph
n-PrOH / reflux-3h
O
. Similarly to what was described
Ph
O 3
1
Pb(OAc)4, AcOH,
reflux-12 h
a
2-4
NH-N=CHAr
Ar
d
c
b
e
N N
HO
F
Br
Cl
Ph
Me
O
4
Ph
Ar
Scheme 1
In the second approach (Scheme 2), 1 was reacted with carbon disulfide and
potassium hydroxide in boiling ethanol to give the (5-mercapto-1,3,4-oxadiazol-2yl)(diphenyl)methanol (5) through potassium hydroxy-diphenylmethyl dithiocarbazate
as
intermediate
according
to
literature
procedure
[26]
.
The
S-{5-[hydroxy
(diphenyl)methyl]-1,3,4-oxadiazol-2-yl}-aryl carbothioates (7a-7d) were prepared by
reaction of compound (5) with acid chlorides (6) (Scheme 2).
216
N N
HO
(1)
CS2
KOH
HO
SH
O
Ph
Ph
ArCOCl / CHCl3
6
H
N N
Ph
S
O
Ph
Ph
HO
N N
O
Ph
S-COAr
7
5
6/7
a
b
Ar
c
d
O
S
Cl
Scheme 2
1H and 13C NMR spectra of all prepared compounds are in total agreement with
the suggested structures; DEPT experiments were employed to differentiate secondary
and quaternary carbons from primary and tertiary carbons. 1H NMR spectra of the
newly prepared compounds indicate the presence of α-hydroxyl proton in the range of
δ 3.30-3.50 (singlet) and the thiol group proton in compound 5 appear as singlet at δ
14.10 ppm, while 13C NMR spectra of this compound showed that the thione carbon of
the keto tautomer resonated at δ 169.2 ppm.
Compounds containing C=N were
characterized by the presence of the signals in the range of δ 148.8-160.0 ppm in their
13
C NMR spectra. The presence of the thiol forms in compound 5 was confirmed also
by the formation of a white precipitate after adding a solution of silver nitrate to a DMF
solution of this compound.
In the IR spectra of compounds 4, 5 and 7, the C=N band exhibited a weak
absorption in the range 1540-1608 cm-1. Compounds 3 and 5 exhibited N-H stretching
absorption bands in the region of 3228-3310 cm-1. The absorption band due to SH
group in (5) was observed at 2960 cm-1 which indicates the existence of both
tautomers (thiol and thione). This absorption band disappeared in compound 7.
Additional support of the proposed structures comes from mass spectrometry;
mass spectra of the prepared compounds show the correct molecular ions, M+·, as
suggested by their molecular formulas. Analyses of the molecular ions and the
fragmentation pattern are used in the identification and characterization of these
compounds.
217
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