A stereochemical investigation of the nucleophilic addition of methyl ethyl ketone to (E)-alkyloxindolylideneacetates

The reaction of (E)-(2-oxo-1,2-dihydroindol-3-ylidene)acetic acid esters (1) with methyl ethyl ketone in the presence of morpholine as a catalyst gave 4-oxo-2-(2-oxo-2,3-dihydro-1H-indole-3-yl)hexanoic acid esters, a diastereomeric mixture of two isomers (2_A and 2_B). Using aluminium oxide as a catalyst for the above reaction led to the formation of 4-oxo-3-methyl-2-(2-oxo-2,3-dihydro–1H-indole-3-yl)-pentanoic acid esters as a mixture of three isomers 3_A, 3_B and 3_C, which upon methylation with methyl iodide in acetone and in the presence of anhydrous potassium carbonate yield the corresponding methylated products. The structural assignments of the new compounds are based on their chemical and spectroscopic properties. The stereochemical structures of 5a_A, 5b_B and 5a_C are identified by X-ray analysis.