Novel and chemoselective one-pot synthesis of 4-arylidene-2-phenyl-5(4H)-oxazolones
1179
Scheme 3
O
Ph
O
(89%)
N
p-CH3OC6H4CH2OH
p-CH3OC6H4CH
[C14H24N4]2W10O32-[bmim]NO3
+
+
PhCONHCH2CO2H + Ac2O
80 °C, 1 h
C6H5CH2CH2CH2OH
C6H5CH2CH2CH2OH
(100%)
detected in the reaction mixture. These data are consistent
with the low yields for preparation of azlactones with ali-
phatic aldehydes as reported previously. Also, it was found
that cinnamyl alcohol or 3-phenylprop-2-yn-1-ol was
unchanged under the reaction condition, and no aldehyde
or azlactone derivatives were detected after 3 h.
were identified by comparison of their physical and spectral
data with those of authentic samples. Thin-layer chroma-
tography (TLC) was performed on ultraviolet (UV)-active
aluminum-backed plates of silica gel (TLC silica gel 60
F254). 1H and 13C nuclear magnetic resonance (NMR)
spectra were measured on a Bruker DPX 400 MHz spec-
trometer in CDCl3 or dimethyl sulfoxide (DMSO)-d6 with
chemical shifts (d) given in ppm relative to tetramethylsilane
(TMS) as internal standard. Coupling constants are given in
Hz. Low-resolution mass spectra (LRMS) were recorded on
a Bell and Howell 21-490 spectrometer. IR spectra were
taken on a FT-IR-Tensor 27 spectrometer in KBr pellets and
are reported in cm-1. Melting points were determined using
a Stuart Scientific SMP2 apparatus.
This method was also highly chemoselective. In binary
mixtures of benzyl alcohols and alkyl alcohols, the benzyl
alcohols were almost quantitatively converted to their az-
lactone, while other alcohols remained intact and were
completely recovered (Scheme 3). These observations may
be due to the higher reactivity of benzyl alcohols toward
oxidation in comparison with aliphatic alcohols.
The recovered [(C14H24N4)2W10O32] could also be
reused without any significant loss of its high catalytic
performance. The catalyst was separated by simple filtra-
tion and washed with ethanol. This catalytic system was
easily recyclable after activation at 80 °C under reduced
pressure. [(C14H24N4)2W10O32] retained its activity after
four consecutive reactions.
Synthesis of di[1,6-bis(3-methylimidazolium-1-yl)hexane]-
decatungstate ([(C14H24N4)2W10O32])
To a solution of Na2WO4ꢀ2H2O (5 mmol) in 10 cm3 H2O
was added 9 cm3 3 M aqueous HCl, following by boiling
until a clear yellow solution was obtained. An aqueous
solution of 1,6-bis(3-methylimidazolium-1-yl)hexane chlo-
ride [C6(MIm)2Cl2] (10 mmol; dried at 80 °C) was added
to this solution, and the precipitate was filtered, washed
with water, and dried overnight at 80 °C in vacuum.
In summary, we have developed a new, efficient,
chemoselective one-pot procedure for synthesis of azlac-
tones via sequential oxidation of alcohols to aldehydes and
¨
their Erlenmeyer–Plochl reaction. The present work opens
¨
up a novel one-pot synthetic route to Erlenmeyer–Plochl
General procedure for the synthesis of adducts 2
adducts starting directly from benzyl alcohols. To the best
of our knowledge, this is the first examination of
[(C14H24N4)2W10O32]-[bmim]NO3 as a new promoter in
heterocyclic synthesis. Finally, the simple experimental
procedure combined with the ease of work-up of the product
in the absence of any toxic organic solvents make this
method quite convenient and environmentally benign for
synthesis of azlactones. Further synthetic study of the
present method is underway in this laboratory.
A mixture of aryl alcohol 1 (1.00 mmol) and [bmim]NO3
(0.33 mmol) in the presence of [(C14H24N4)2W10O32]
(0.07 mmol) was stirred at 80 °C until the aldehyde was
consumed, then hippuric acid (1.2 mmol) and 2.5 cm3
acetic anhydride were added and the mixture was stirred at
the same temperature. After completion of the reaction as
indicated by TLC, 5 cm3 ethanol was added and the mix-
ture stirred for 10 min until a yellow solid precipitated. The
mixture was allowed to stand overnight, and then it was
cooled in an ice bath. The crude azlactones were obtained
after filtration and washing with hot water. Recrystalliza-
tion from ethanol/water afforded the pure product in
87-95% yield.
Experimental
All chemicals were purchased from Merck chemical com-
pany. 1,6-Bis(3-methylimidazolium-1-yl)hexane chloride
[C6(MIm)2Cl2] was synthesized according to the literature
[26]. [(C14H24N4)2W10O32] was completely characterized
by Fourier-transform infrared (FTIR), thermogravimetry
(TG), X-ray diffraction (XRD), and scanning electron
microscopy (SEM) techniques. All known organic products
(Z,Z)-4,40-(1,4-Phenylenedimethylidyne)bis(2-phenyl-5
(4H)-oxazolone) (2o, C26H16N2O4)
ꢀ
IR (KBr): m = 3,039, 1,789, 1,762, 1,653, 1,548, 1,327,
1
1,159 cm-1; H NMR (400 MHz, CDCl3): d = 7.57–7.60
(5H, m), 7.65–7.69 (2H, m), 8.21–8.27 (4H, m), 8.32 (5H, s)
123