A simple and efficient method for the synthesis of 4-tosyloxazoles from tosylmethyl isocyanide with a-ketoimidoyl chlorides

Article abstract of DOI:10.24820/ark.5550190.p010.556

An efficient one-pot method for the synthesis of potentially biologically active 4-tosyloxazole derivatives is described via reaction of imidoyl chloride adducts generated in situ from the reaction of isocyanide and acyl chlorides with p-toluenesulfonylmethyl isocyanide (TosMIC). This reaction was carried out in the presence of sodium hydride as a base and in THF to produce 4-tosyloxazoles in excellent yield.

Full text of DOI:10.24820/ark.5550190.p010.556

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A simple and efficient method for the synthesis of 4-tosyloxazoles from
tosylmethyl isocyanide with α-ketoimidoyl chlorides
a
a
a
Mahdiyeh Talebizadeh, Ali Darehkordi, * and Mohammad Anary-Abbasinejad
a
Department of Chemistry, Faculty of Science, Vali-e-Asr University of Rafsanjan, Rafsanjan 77176, Iran
E-mail: darehkordi@vru.ac.ir adarehkordi@yahoo.com
Received 03-07-2018
Accepted 05-21-2018
Published on line 06-17-2018
Abstract
An efficient one-pot method for the synthesis of potentially biologically active 4-tosyloxazole derivatives is
described via reaction of imidoyl chloride adducts generated in situ from the reaction of isocyanide and acyl
chlorides with p-toluenesulfonylmethyl isocyanide (TosMIC). This reaction was carried out in the presence of
sodium hydride as a base and in THF to produce 4-tosyloxazoles in excellent yield.
Keywords: 4-Tosyloxazoles, α-ketoimidoyl chloride, tosylmethyl isocyanide, oxazole
©
DOI: https://doi.org/10.24820/ark.5550190.p010.556
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Introduction
Oxazole compounds have attracted much attention due to their emergence as subunits of interesting
1
,2
3,4
5,6
biological and pharmacological compounds with anticancer , antibacterial , anti-inflammatory and
7-10
antitubercular activities.
transformations.
cyclodehydration of ketoamides.
Also, these compounds are used as valuable precursors in many useful synthetic
Oxazoles are commonly synthesized using the Hantzsch reaction
Synthesis via dehydration of 2-acylaminoketones is an important method
for the preparation of a wide range of 2,5-disubstituted and 2,4,5-trisubstituted oxazole derivatives.
11-13
14,15
or by
16-19
2
0,21
Alkyl
isocyanoacetates, methyl isocyanoacetate were exploited by different groups towards the synthesis of 5-
22-26
substituted oxazole derivatives as well as 5-(aminomethyl) oxazoles.
Sasaki prepared bisoxazole
derivatives which were substituted at the 4-position and dioxazolo metacyclophane derivatives using
2
7
TosMIC. Oxazole derivatives with substitution at the 5-position were synthesized by cyclocondensation of
2
8-39
aldehydes with monosubstituted tosylmethyl isocyanide in moderate yield
and by reaction of aryl
40-44
aldehydes with TosMIC and also metalated methyl isocyanides.
Although there are many methods for the
few approaches are available for the preparation of 4,5-disubstituted oxazoles.
45-48
28-39
synthesis of oxazoles,
Reaction of α-diazo-β-keto- carboxylates and -phosphonates with arene carboxamides gives 2-aryloxazole-4-
carboxylates and 4-phosphonates by carbene N–H insertion, followed by cyclodehydration results in the
4
9
formation of oxazole-5-carboxylates and 5-phosphonates. NIS-mediated iodocyclization of N-sulfonyl
propargylamides is developed for the synthesis of various oxazolidines and iodoalkylidenedihydrooxazoles via
46
50,29
a 5-exo-dig process. According to this investigation and literature reports
reactions directly arising from
acid chlorides are useful and high yielding methods for synthesis of functionalized oxazoles.
Results and Discussion
A literature search revealed that only 5-phenyl-4-tosyloxazole has been synthesized from reaction of benzoyl
o
chloride and TosMIC in the presence of K CO as base in dimethoxyethane at 20 C, but the yield of this
reaction is low (50-55%) (Scheme 1).
2
3
39
Scheme 1. Methods for the synthesis of 4-tosyloxazoles.
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Due to the importance of oxazoles in medicinal chemistry and drug discovery, and also, in continuation
48
of our work to develop syntheses of heterocyclic compounds from imidoyl chlorides, we were looking to
synthesize 4-tosyloxazole derivatives by condensation of aromatic and aliphatic α-ketoimidoyl chlorides with
cyclohexyl isocyanide. In this work, we report a simple method for the synthesis of 4-tosyloxazole derivatives
by condensation of aromatic and aliphatic α-ketoimidoyl chlorides with TosMIC in THF as solvent in excellent
yields without using any catalyst. The Nef isocyanide adducts 3 obtained from addition of isocyanide 1 with
aromatic or aliphatic acyl chloride 2 was reacted at ambient temperature with TosMIC to afford 5-phenyl-4-
tosyloxazole (5a, Scheme 2).
In search for selective and efficient conditions, different solvent such as THF, CH
room temperature and reflux conditions) and also a variety of bases, including K CO , NaH, and Et
studied. As mentioned in Table 1, in the presence of Et N or K CO (in THF solvent at room temperature and
reflux conditions) and also in the presence of K CO (in CH Cl solvent at room temperature), we did not obtain
any products (Table 1, entries 1–5). But when NaH was used as a base in THF or CH CN at room temperature,
3
CN and CH
2
Cl
2
(at
2
3
3
N were
3
2
3
2
3
2
2
3
the corresponding product was produced in excellent yields (entry 6-7). Therefore, all of the reactions were
performed in THF in the presence of NaH at room temperature. To extend the scope of the reaction, the
optimized protocol was applied for the synthesis of 4-tosyloxazoles by reaction with different imidoyl
chlorides (Table 2). α-Ketoimidoyl chlorides 1a-i bearing various electron-donating functional groups at various
positions reacted with TosMIC (2) smoothly and the corresponding compounds (5a-i) were obtained in
excellent yield (Table 2). But α-ketoimidoyl chlorides with electron-withdrawing functional groups such as 4-
NO -C H , 3-NO -C H , pyridin-4-yl, EtO CCO and MeO CCO in the same reaction conditions, did not produce
2
6
4
2
6
4
2
2
any products. The results are summarized in Table 2.
Table 1. Model reaction, conditions and yields
Entry
1
Solvent
THF
Base
Conditions
25 °C, 24 h
Yield (%)
-
NEt
3
4
5
-
THF
K2CO3
CO
Reflux, 24 h
25 °C, 45min
-
CH
THF
CH CN
2
Cl
2
K
2
3
25 °C, 24 h
6
NaH
NaH
25 °C, 45min
89
9
0
7
3
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Table 2. Synthesis of 4-tosyloxazole derivatives from tosylmethyl isocyanide and α- ketoimidoyl chlorides
Entry
R
Product
5a
Melting point (°C)
Isolated Yield (%)
1
2
3
4
5
6
7
8
9
C
6
H
5
134-135
153-155
214-216
193-195
260-262
281-282
270-272
200-212
180-182
-
90
98
96
96
97
96
95
98
98
-
4-BrC
6
H
4
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
4-FC
3-FC
6
H
H
4
6
4
3-ClC6H4
4-MeC6H4
3-MeC
CH OCH
6
H
4
3
2
Cl(CH3)
2
CCHCl
10
11
12
13
14
4-NO -C H ,
2
6 4
3-NO
2
-C
6
H
4
-
-
-
-
-
-
-
-
pyridin-4-yl
EtO CCO
5m
5n
2
MeO
2
CCO
A possible mechanism is proposed in Scheme 2. The formation of 4-tosyloxazole 5 by reaction of
cyclohexyl isocyanide, acyl chloride and TosMIC can be rationalized by initial formation of the ketoimidoyl
chloride adducts 3 by the reaction of cyclohexyl isocyanide 1 with acyl chlorides 2. Then, the intermediate 3 is
attacked by TosMIC 4 to form the intermediate A, which then undergoes an intramolecular cyclization
reaction to afford product 5 (Scheme 2).
Scheme 2. Plausible mechanism for the synthesis of 4-tosyloxazole.
According to the mechanism reaction, the second step (intramolecular cyclization reaction of
intermediate A to product 5) is suggested to be very important. So, the electron-donating groups on the α-
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ketoimidoyl chlorides increase the reaction rate and the electron-withdrawing groups on the α-ketoimidoyl
chlorides decrease the reaction rate. The structures of compounds 5a-i were deduced by the IR, NMR spectral
1
and elemental analytical data. For example the H-NMR spectrum of 5a exhibited a singlet at δ= 8.90 due to
3
the oxazole proton. Multiplet at δ= 7.89-7.91, doublet at δ= 7.84, JHH= 8.0 Hz, multiplet at δ= 7.59-7.61 and
3
doublets at δ= 7.46, JHH= 8.2 Hz correspond to nine aromatic protons present in the molecule. The signal a δ=
13
2
1
.41 was assigned to the methyl protons. The C-NMR spectrum of 5a displayed a downfield signal at δ=
52.22 for the carbon C2 of the oxazole ring and at δ= 21.60 for methyl carbon.
Conclusions
In summary, the reaction of ketoimidoyl chlorides with TosMIC in THF in the presence of sodium hydride (NaH)
as base at room temperature provides a facile and efficient route for the synthesis of 4-tosyloxazole
derivatives in excellent yields. The advantages of this method are high yield, readily available starting
materials, simple procedure and a straightforward purification of the products.
Experimental Section
1
General. H-NMR spectra were recorded in DMSO-d
6
on a Bruker model DRX-400 AVANCE spectrometer (400
1
3
MHz) with TMS as internal standard C-NMR spectra were taken a by a Bruker model DRX-400 AVANCE (100
MHz) spectrometer. Chemical shifts are given in ppm relative to TMS, the coupling constants J are given in Hz.
Melting points were determined on a Melt-Tem II melting point apparatus and are uncorrected. IR spectra
were obtained on a Thermo scientific, Nicolet is10 FT-IR spectrometer. Peaks are reported in wave numbers
-
1
(
cm ). Element analyses (CHN) were performed with a EUROVECTOR EuroEA3000 CHNSO analyzer.
Lenochka, d has to be italic (not 6). Could you please change in all Experimental Section. Thanks.
General procedure for the synthesis of compounds 5a-i
In a typical experimental procedure, a dry, two-necked, 50 mL round bottomed flask was charged with 1.0
o
mmol of acyl chloride derivatives and 1.0 mmol of cyclohexyl isocyanide and heated at 60 C for 1 h. Then a
mixture of TosMIC (1.0 mmol) and sodium hydride (60% w/w) (1.0 mmol) in THF (10 ml) was added. The
mixture was stirred at room temperature for 45 min. After completion of the reaction, as indicated by TLC
(ethyl acetate : n-hexane = 1 : 3) , the solvent was evaporated at reduced pressure; the precipitate was
washed with diethyl ether (10 ml) and was recrystallized from 95% ethanol to afford pure products 5a-i.
-
1
5
1
7
-Phenyl-4-tosyloxazole (5a). Light yellow solid, mp 134-135°C (Yield: 98%). IR (KBr) (ῡmax, cm ); 1595 (C=N),
1
3
314, 1144 (S=O). H-NMR (400 MHz, DMSO-d
6
): δ (ppm) 2.41 (3H, s, CH
3
), 7.46 (2H, d, JHH 8.1 Hz, HAr), 7.59-
.61 (3H, m, HAr), 7.84 (2H, d, J 8.1 Hz, HAr), 7.89-7.91 (2H, m, HAr), 8.90 (1H, s, oxazole proton). C-NMR
3
13
HH
(
100 MHz, DMSO-d
37.19, 145.56, 152.22 (C=N). Calcd. For (C16
.38; N, 4.68; S, 10.71%.
-(4-Bromophenyl)-4-tosyloxazole (5b). Colorless crystals, mp 153-155 °C (Yield: 90%). IR (KBr) ῡmax, cm ;
594 (C=N), 1332, 1149 (S=O). H-NMR (400 MHz, DMSO-d ): δ (ppm) 2.40 (3H, S, CH ), 7.47 (2H, d, J 8.1 Hz,
6
): δ (ppm) 21.60 (CH
3
), 125.74, 125.98, 128.53, 128.71, 129.16, 129.42, 130.55, 131.49,
S): C, 64.18; H, 4.39; N, 4.69; S, 10.70%. Found: C, 64.20; H,
1
4
5
1
H
13NO
3
–1
1
3
6
3
HH
1
3
HAr), 7.83-7.85 (6H, m, HAr). 8.68 (1H, s, oxazole proton). C-NMR (100 MHz, DMSO-d
6
): δ (ppm) 21.60 (CH ),
3
124.95, 125.24, 128.32, 130.58, 131.39, 132.25, 135.45, 136.98, 145.68, 151.29, 152.44 (C=N). Calcd. for
(
C
16
H
12BrNO
3
S): C, 50.83; H, 30.19; N, 3.71; S, 8.49%. Found: C, 50.81; H, 30.20; N, 3.70; S, 8.48%.
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-
1
5
-(4-Fluorophenyl)-4-tosyloxazole (5c). Yellow solid, mp 214 -216°C (Yield: 98%). IR (KBr) (ῡmax, cm ); 1609
1
(
(
C=N) 1329, 1149 (S=O). H- NMR (400 MHz, DMSO-d
2H, d, JHH 8.1 Hz, HAr), 7.96 (2H, dd, JHF 8.2 Hz, JHH 4 Hz, HAr), 8.66 (1H, s, oxazole proton); C NMR (100
6
): δ (ppm) 2.41 (3H, S, CH
3
), 7.43-7.48 (4H, m, HAr), 7.84
3
3
3
13
2
3
MHz, DMSO-d
6
): δ (ppm) 21.59 (CH
3
), 116.36 (d, JCF 22.0 Hz), 122.31, 128.27, 130.55, 132.10 (d, JCF 9.1 Hz),
1
134.94, 137.10, 145.60, 151.50, 152.22(C=N), 162.75(d JCF 248 Hz) Calcd. For (C16
H
12FNO S): C, 60.55; H, 3.83;
3
N, 4.40; S, 10.12%. Found: C, 60.56; H, 3.81; N, 4.41; S, 10.10%.
-
1
5
1
7
-(3-Fluorophenyl)-4-tosyloxazole (5d). Light yellow solid, mp 193-195 °C (Yield: 98%). IR (KBr) (ῡmax, cm );
1
590 (C=N) 1330, 1150 (S=O). H-NMR (400 MHz, DMSO-d
6
): δ (ppm) 2.41(3H, S, CH
3
), 7.47-7.48 (3H, m, HAr),
3
3
3
.66 (1H, dd, JHF 12.1 Hz, JHH 8.0 Hz, HAr), 7.76-7.77 (2H, m, HAr), 7.85 (2H, d, JHH 8.0 Hz, HAr), 8.70 (1H, s,
13
2
2
oxazole proton). C-NMR (100 MHz, DMSO-d
Hz), 125.71, 127.69 (d, JCF 8.0 Hz), 128.34, 130.59, 131.43 (d JCF 8.0 Hz), 135.77, 136.92, 145.73, 150.76,
6
): δ (ppm) 21.60 (CH
3
), 116.25 (d, JCF 24 Hz), 118.44 (d, JCF 20
3
3
1
1
52.50 (C=N), 162.14 (d, JCF 242.0 Hz). Calcd. For (C16
H
12FNO S): C, 60.55; H, 3.83; N, 4.40; S, 10.12%. Found:
3
C, 60.56; H, 3.81; N, 4.41; S, 10.10%.
–
1
5
-(3-Chlorophenyl)-4-tosyloxazole (5e). Colorless crystals, mp 260-262 °C (Yield: 90%). IR (KBr) ῡmax, cm ;
1
3
1609 (C=N) 1329, 1149 (S=O). H-NMR (400 MHz, DMSO-d
6
): δ (ppm) 2.40 (3H, S, CH
3
), 7.42 (2H, d, JHH= 8.0
3
Hz, HAr), 7.44-7.47 (3H, m, HAr), 7.60 (1H, s, HAr), 7.84 (2H, d, JHH 8.0 Hz, HAr), 8.60 (1H, s, oxazole proton).
13
C-NMR (100 MHz, DMSO-d
6
): δ (ppm) 21.26 (CH
3
), 124.79 125.97, 127.94, 128.55, 128.74, 129.24, 129.52,
1
9
5
1
32.52, 137.04, 138.12, 143.55, 146.15, 157.71(C=N) Calcd. for (C H ClNO S): C, 57.56; H, 30.64; N, 4.19; S,
16
12
3
.60%. Found: C, 57.57; H, 30.62; N, 4.20; S, 9.61%.
-(p-Tolyl)-4-tosyloxazole (5f). Dark brown solid, mp 281-282°C (Yield: 98%). IR (KBr) (ῡmax, cm ); 1597 (C=N)
-
1
1
3
316, 1146 (S=O). H-NMR (400 MHz, DMSO-d ): δ (ppm) 2.40, 2.41(6H, s, 2CH ), 7.40 (2H,d, J 8.1 Hz, HAr
6
3
HH
3
13
)
,7.46 (2H, d, JHH 8.1 Hz, HAr ), 7.78-7.83 (4H, m, HAr), 8.84 (1H, s, oxazole proton);
DMSO-d ) 21.38, 21.57 (2CH
38.45, 145.51, 152.07, 152.44 (C=N). Calcd. For (C H NO S): C, 65.15; H, 4.82; N, 4.49; S, 10.25%. Found: C,
5.16; H, 4.82; N, 4.47; S, 10.23%.
-(m-Tolyl)-4-tosyloxazole (5g). Dark brown solid, mp 270-272°C (Yield: 98%). IR (KBr) (ῡmax, cm ); 1594 (C=N)
C-NMR (100 MHz,
6
3
), 125.67, 126.65, 128.25, 129.04, 129.67, 130.52, 132.11, 135.00, 137.25,
1
6
5
1
7
17
15
3
-1
1
3
313, 1143 (S=O). H-NMR (400 MHz, DMSO-d ): δ (ppm) 2.40 (6H, S, 2CH ), 7.40 (1H,d, J 8.1 Hz, HAr), 7.45-
6
3
HH
3
3
.47 (3H, m, HAr), 7.68 (1H, s, HAr), 7.71 (1H, d, JHH 8.1 Hz, HAr), 7.84 (2H ,d, JHH 8.0 Hz, HAr), 8.64 (1H, s,
1
3
oxazole proton). C-NMR (100 MHz, DMSO-d
6
): δ (ppm) 21.38, 21.57 (2CH
3
), 125.67, 126.65, 128.25, 129.04,
1
6
5
1
29.67, 130.52, 132.11, 135.00, 137.25, 138.45, 145.51, 152.07, 152.44(C=N). Calcd. For (C H NO S): C,
17
15
3
5.14; H, 4.84; N, 4.48; S, 10.20%. Found: C, 65.16; H, 4.82; N, 4.47; S, 10.23%.
-
1
-(Methoxymethyl)-4-tosyloxazole (5h). Brown powder, mp 200-202 °C (Yield: 98%). IR (KBr) (ῡmax, cm );
1
595 (C=N) 1334, 1152 (S=O). H-NMR (400 MHz DMSO-d ): δ (ppm) 2.41 (3H, s, CH ), 3.33 (3H, s, OCH ), 4.82
6
3
3
13
3
3
(3H, s, OCH
2
), 7.48 (2H, d, JHH 8.1 Hz, HAr), 7.87 (2H, d, JHH 8.1 Hz, HAr) 8.61 (1H, s, oxazole proton). C-NMR
(100 MHz, DMSO-d
6 3
): δ (ppm) 21.58 (CH ), 58.59 (OCH
3
), 62.60 (OCH ), 128.23, 128.60, 130.64, 137.58,
2
1
4
5
45.71, 152.18, 153.39 (C=N). Calcd. For (C H NO S): C, 53.90; H, 4.91; N, 5.26; S, 11.99%. Found: C, 53.92; H,
.90; N, 5.24; S, 12.00%.
12
13
4
-(1,2-Dichloro-2-methylpropyl)-4-tosyloxazole (5i). Light brown powder, mp 180-182 °C (Yield: 98%). IR (KBr)
-
1
1
(
ῡ
max, cm ); 1595 (C=N) 1326, 1148 (S=O). H- NMR (400 MHz, DMSO-d
6
): δ (ppm) 1.21, 1.49, (6H, s, 2CH
3
),
3
3
2
.49, (3H, s, CH
3
), 4.40 (1H, s, CH), 7.50 (2H, d, JHH 8.1 Hz, HAr), 7.90 (2H, d, JHH 8.1 Hz, HAr) 8.57 (1H, s,
13
oxazole proton); C-NMR (100 MHz, DMSO-d ): δ (ppm) 19.41, 21.61, 24.05 (3CH ), 55.26, 62.64, 128.21,
6
3
1
30.71, 136.90, 138.24, 145.81, 151.30, 152.95(C=N) .Calcd. For (C14
H
15Cl
2 3
NO S): C, 48.29; H, 4.32; N, 4.01; S,
9.23%. Found: C, 48.28; H, 4.34; N, 4.02; S, 9.21%.
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Acknowledgements
We gratefully acknowledge, the Vail-e-Asr University of Rafsanjan Faculty Research Grant for financial support.
Supplementary Material
1
13
Copies of FT-IR, H and C NMR spectras for compounds 5a-I.
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