3- Aminooxindoles, isatins

Dihalogenation of oxindole followed by alkaline hydrolysis of the 3,3-dihalooxindole has been applied to the synthesis of some isa-tins.66,126,136 137b A number of oxindoles have been treated with nitrous acid to give isatin-3-oximes.100,138 Reduction of the oximes to 3-aminooxindoles followed by ferric chloride oxidation gave isatins.100,138 When this sequence was applied to 7-azaoxindole, the azaisatin 33 was... 

Reduction of isatin-3-oximes to the corresponding 3-aminooxindole has been carried out using stannous chloride in acetic acid51 or in hydrochloric acid.92 3-Aminooxindole has also been obtained by electrochemical reduction288 and catalytic hydrogenation.169 Reduction... 

Recently, the Chi group disclosed an NHC-promoted asymmetric inter-molecular cross-aza-benzoin reaction of enals with isatin-derived ketimines to afford chiral quaternary 3-aminooxindoles (18 examples, with up to 76% yield, 96% ee). The chemoselectivity is controlled by the NHC catalysts, with electron-deficient and sterically non-congested carbene catalysts favoring the enal acyl anion reaction pathway and aza-benzoin, allowing convenient access to products of different scaffolds by starting from the same set of readily available substrates (Scheme 7.16). 

Recently, Chi and coworkers disclosed the enantioselective cross-aza benzoin reaction of enals with isatin-derived ketimines, affording the 3-aminooxindoles 29 bearing a quaternary stereogenic centre with high enantioselectivities. The electron-deficient and sterically noncongested car-bene catalyst H2 was shown to favour the pathway of enal acyl anion leading to an aza-benzoin reaction (Scheme 20.14). 

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