3
aminophenol 4 (1 mmol) in THF (3 mL) for 2 h at rt. After
formation of the corresponding o-phenolic thiourea 1a11d was
completed (as indicated by TLC), TBAI (1 mol%) and 30% aq.
H2O2 (2 equiv.) were added and the reaction mixture was stirred
CSIR, New Delhi, for the award of a Junior Research
Fellowship (Ref. No: 22/06/2014 (i) EU-V). V.P.S. is grateful
to the Department of Science and Technology (DST) Govt. of
India, for the award of a DST-Inspire Faculty position (Ref.
IFA-11CH-08) and financial support.
at rt for 1 h. The product 2a was isolated in an excellent yield
(80%) by usual column chromatography.15
References and notes
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TBAI (1 mol%)
H2O2 (2 mmol)
C
o-aminophenol
4
HN
HN
HN
N
(1 mmol)
N
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(d) Uyanik M, Ishihara K. ChemCatChem. 2012; 4: 177;
S
THF
rt, 1 h
THF, rt, 2 h
OH
O
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(1 mmol)
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3
2a
1a
Scheme 2. One-pot sequential synthesis of 2-
aminobenzoxazole
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On the basis of the above observations and the literature
reports,1a,d,14 a plausible mechanism for the formation of 2-
aminobenzoxazoles 2 from o-phenolic thioureas 1 is depicted in
Scheme 3. According to the proposed mechanism,
tetrabutylammonium iodide reacts with hydrogen peroxide to
give hypoiodite and tetrabutylamonium hydroxide. The
hypoiodite reacts with the thiolic form of thiourea 1′ to form
intermediate 5, which affords the desired product 2 through 6
and 7 with the loss of H2O and elemental sulfur along with
regeneration of the catalyst TBAI.
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Scheme 3.
A
plausible mechanism for the formation of 2-
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In summary, we have developed a highly efficient, metal-
and base-free, operationally convenient strategy for the
synthesis of 2-aminobenzoxazoles at rt. The protocol involves
oxidative intramolecular cyclodesulfurization of o-phenolic
thioureas using TBAI as a catalyst and H2O2 as a green oxidant.
This methodology could also be exemplified for a one-pot
synthesis of N-phenylaminobenzoxazole starting directly from
o-aminophenol and phenyl isothiocyanate in 80% yield.
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Acknowledgments
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We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra. V.K.Y. is grateful to the