Organic & Biomolecular Chemistry
Communication
When we treated 14 with 2 equivalents of Oxone at room to use. Our results open up a new avenue for the preparation
temperature in a mixture of 4 : 1 CH3CN–H2O, isatoic anhy- of isatoic anhydrides by using a solid, environmentally benign
dride (10a) was isolated in 78% yield. The reaction appeared to reagent.
be regioselective. Indolin-3-one (13) is unstable in air and
readily dimerizes to form indigo (12). When 12 was heated at
60 °C in a 2 : 1 mixture of CH3CN–H2O in the presence of
4 equivalents of Oxone for one hour, the resulting product
was isatoic anhydride (10a) in 76% yield. Indirubin (11), on
Notes and references
the other hand, was very stable towards the oxidation with 1 H. Hussain, I. R. Green and I. Ahmed, Chem. Rev., 2013, 113,
Oxone and was oxidized only slowly. Therefore, based on the
information available to us we hypothesize that under the reac-
3329–3371; M. C. Marcotullio, F. Epifano and M. Curini,
Trends Org. Chem., 2003, 10, 21–34.
tion conditions 13 is formed as an initial oxidation product. 2 X.-A. Gao, R.-L. Yan, X.-X. Wang, H. Yan, J. Li, H. Guo and
This compound either dimerizes to form indigo (12) or alter-
natively converts into isatin (14) directly. Further oxidation of
12 and 14 leads to isatoic anhydride (10a).
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With the optimized reaction conditions in place, we tested
the applicability of this reaction to aryl-substituted derivatives 3 H. Umemoto, M. Umemoto, C. Ohta, M. Dohshita,
of indoles (Table 2, entries 2–9). In general the yields are high
and numerous functional groups are tolerated. In the 5-posi-
tion halogens, alkyl and nitro groups can be present on the 4 G.
ring (entries 2–9); 6-flouroindole (9h) and 7-fluoroindole (9i)
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and
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parent indole 9a.
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