PAPER
A Novel Method For The Synthesis Of Oxazolocoumarin Derivatives
1375
ring, both of which resulted in the formation of the same was measured versus Ag/10–2 M Ag+ reference electrode and
–
1
scanned from –0.4 to 1.3 V at a scan rate of 0.03 Vs . Solutions
o-quinone derivative, and more quantitative electrochem-
ical studies were difficult because of secondary dimeriza-
tion reactions and filming of the electrode surface.13
However, according to our results, it seems that the inter-
molecular addition of acetonitrile to the formed o-quinone
were de-aerated by N bubbling before each experiment for 30 min.
The gas flow was maintained at low rates during the experiments.
2
Electrochemical Oxidation; General Method
In a de-aerated (N gas) stirred solution of pure MeCN (175 mL) as
2
derivative and intramolecular cyclization was faster than a solvent and lithium perchlorate (2.13 g, 20 mmol) as a supporting
other secondary dimerization reactions, leading presum- electrolyte, anodic oxidation of each 7-hydroxycoumarin deriva-
tives 1a–f (5 mmol) was performed by fixing the electrode potential
ably to the formation of 4-hydroxy-2,9-dimethyloxazo-
lo[4,5-f]coumarin (3) as a major product. The structures of
the isolated oxidation products as oxazolocoumarin deriv-
at the values indicated in Table 1 using two Pt-electrodes and the
–
2
+
reference electrode as Ag/10 M Ag electrode. During electroly-
sis, the products were checked using TLC to see the reaction path
and how many products were formed. At different time intervals,
atives 2a–e and 3 were unambiguously determined on the
1
basis of their IR, H NMR, MS spectra and correct analy- the two Pt-gauze electrodes were removed from the cell and cleaned
tical data.
using nitric acid and then DMF, distilled water, acetone and then
dried to remove the anodically formed non-conductive passive
films on the anode. After the oxidation, the MeCN was evaporated
Ph
O
using the rotatory evaporator, extracted with CHCl , washed with
3
H O and dried (Na SO ). After evaporation of CHCl the semi-solid
2
2
4
3
produced was crystallized from suitable solvent and detected as
usual.
HO
O
HN
O
O
Compound 2a
IR: 1716, 1677, 1608 cm .
–
1
CH3
4
1H NMR (DMSO-d6): = 7.80 (s, 1 H, ArH), 7.22, 7.20 (s, 2 H,
Figure 3
Table 1 Physical Data of Prepared Compoundsa
ArH), 3.40 (s, 3 H, CH ), 2.60 (s, 3 H, CH ).
3
3
+
MS: m/z = 215 (M ), 192, and the base peak at 176.
Compound 2b
IR: 1695, 1620, 1605 cm .
1H NMR (CDCl3): = 7.70–6.55 (m, 7 H, ArH), 6.10 (s, 1 H,
C=CH), 2.25 (s, 3 H, CH3).
Compd
Mp (°C)
Solvent)
Pot. (V)
vs. Ag/
1
Time
(h)
Yield
(%)
–
1
(
0
M Ag+
–
2
2
2
2
2
2
a
b
c
160
Toluene
1.75
1.70
1.70
1.80
1.80
1.80
3
33
43
34
38
39
33
+
MS: m/z = 277 (M ), 253, 239, 238.
187
Toluene
3.5
3
Compound 2c
IR: 1680, 1620, 1608 cm .
–
1
1H NMR (CDCl3): = 7.45, 7.40 (s, 2 H, ArH), 2.62–2.54 (t, 2 H,
CH CH CH ), 2.37 (s, 3 H, CH ), 2.34 (s, 3 H, CH ), 1.65–1.45 (m,
176
Toluene
2
2
3
3
3
2
H, CH CH CH ), 1.01–0.94 (t, 3 H, CH CH CH ).
2 2 3 2 2 3
d
e
200
Toluene
3.5
4
+
+
MS: m/z = 257 (M ) and the base peak at 231 (M – C H ).
2
2
Compound 2d
IR: 1720, 1690, 1618 cm .
1H NMR (DMSO-d6): = 7.95, 7.91 (s, 2 H, ArH), 6.50 (s, 1 H,
145
Benzene
–
1
3
150
EtOH (95%)
4.5
ArH), 3.90 (s, 3 H, CH ), 3.70 (s, 3 H, CH ).
3
3
+
MS: m/z = 259 (M ), 220, 192, 161.
a
All Compounds gave satisfactory microanalyses: C, H, N 0.4.
Compound 2e
IR: 3412, 1727, 1611, 1575 cm .
–
1
1
H NMR (DMSO-d ): = 7.64 (s, 1 H, OH), 7.61–7.41 (m, 5 H,
Mps are uncorrected. IR spectra (KBr discs) were recorded on a Pye
Unicam (SP-1000) spectrophotometer. H NMR spectra were re-
6
1
ArH), 6.17 (s, 1 H, C=CH), 3.32 (s, 3 H, CH3).
+
corded on a Varian Gemini 200 MHz spectrometer in DMSO-d us-
MS: m/z = 293 (M ), 279, 255, 238 and the base peak at 226.
6
ing TMS as internal standard. The EIMS were obtained with a
Varian MAT 311A instrument. Elemental analyses were performed
at the microanalytical unit of Cairo University. Compounds 1a–f
Compound 3
IR: 3423, 1729, 1640, 1607 cm .
1H NMR (DMSO-d6): = 8.42 (s, 1 H, OH), 7.66 (s, 1 H, ArH), 6.43
(s, 1 H, C=CH), 3.47
–
1
5
–7
were prepared by reported procedure.
Cyclic voltammetry of 7-hydroxycoumarin derivatives 1a–f were
recorded using MeCN as a solvent and LiClO as supporting elec-
4
(
s, 3 H, CH ), 3.43 (s, 3 H, CH ).
3 3
trolyte at 25°C. The experiments were carried out using three elec-
trodes as one compartment cell. Both working and counter
electrodes were Pt-gauze. The potential of the working electrode
+
MS: m/z = 231 (M and the base peak), 205, 192, 176, 164.
Synthesis 2003, No. 9, 1373–1376 ISSN 1234-567-89 © Thieme Stuttgart · New York