Recent Advances in Transition Metal Catalyzed Synthesis of C3-Substitution-free 2-Oxindole Derivatives

2-Oxindole unit is one of the most important scaffolds found in several alkaloids, natural products, antitumor agents, pharmaceutically important compounds, etc. Molecules containing the 2-oxindole moiety were first isolated from the cat claw plant, widely distributed in the Amazon jungle. It has now been demonstrated that these molecules are present in a wide range of chemicals derived from plant sources. The capacity of 2-oxindole to be altered by various chemical groups to provide unique biological activities can be attributed to its function as a chemical framework for creating and developing biological medications. Since the development of its first synthetic methodology, several research groups have developed protocols for producing 2-oxindole core and its bioactive derivatives. These include the traditional method and the transition/non-transition metal-catalyzed pathway for the synthesis of C3-non-substituted/C3-mono-substituted/C3-di-substituted core. Among those, C3-substitution-free 2-oxindole core synthesis is quite a challenging task, as C3-centre is very reactive. Syntheses of C3-substitution-free 2-oxindole cores have been less explored compared to other substituted 2-oxindole derivatives. In this review article, we have mainly focused on showcasing the transition metal-catalyzed synthetic methodology for the synthesis of 2-oxindoles with no substitution at C3-centre.