Compared with α-arylated cyanoacetates and β-dicarbonyl compounds (pK a ∼ 13), 10a the pK a value of 3-alkyl-substituted oxindoles can be expected to be substantially higher, considering the pK a value of oxindole (18.2). 10b It has not been explored whether Brønsted bases could catalyze the reaction between such less active nucleophiles and azodicarboxylates. Encouraged by the Cinchona alkaloid-catalyzed direct aldol-type reaction of 3-prochiral oxindoles with ...
