Background: Indoles and various indolyl derivates are very common in naturally occurring biological active
compounds. Many methods are being developed for the synthesis of various bioactive indole derivatives.
Objective: Synthesis of biologically promising structurally diverse indole derivatives under mild and environmentally
Methods: Synthesis of 3-hydroxy-3-(5-(trifluoromethoxy)-1H-indol-3-yl)indolin-2-one was achieved by the reaction of
equimolar mixture of isatin and 3-(trifluoromethoxy)-1H-indol using 20 mol% of mandelic acid as catalyst in aqueous
ethanol at room temperature. Under the same optimized reaction conditions, synthesis of 3-(3-hydroxy-2-oxoindolin-3-
yl)chroman-2,4-diones was accomplished via the reactions of substituted isatins and 4-hydroxycoumarin. On the other
hand, 2-hydroxy-2-(indol-3-yl)-indene-1,3-diones and 10-hydroxy-10-(5-methoxy-1H-indol-3-yl)phenanthren-9(10H)-one
were synthesized from the reactions of indoles and ninhydrin or 9,10-phenanthrenequinone respectively using the same 20
mol% of mandelic acid as an efficient organo-catalyst in aqueous ethanol at room temperature.
Results: Mild, safe and clean reaction profiles, energy efficiency, high atom-economy, use of naturally occurring nontoxic organo-catalyst, easy isolation procedure by avoiding column chromatographic purification and gram scale
production are some the major advantages of this developed protocol.
Conclusion: A simple, straightforward and eco-friendly protocol has been developed for the efficient synthesis of
biologically promising novel 3-hydroxy-3-(5-(trifluoromethoxy)-1H-indol-3-yl)indolin-2-one, 3-(3-hydroxy-2-
oxoindolin-3-yl)chroman-2,4-diones, 2-hydroxy-2-(indol-3-yl)-indene-1,3-diones and 10-hydroxy-10-(5-methoxy-1Hindol-3-yl)phenanthren-9(10H)-one using a catalytic amount of mandelic acid in aqueous ethanol at room temperature.