Activated Alumina Balls under Neat Conditions: A Green Catalyst for the Synthesis of Spiro-Heterocyclic Scaffolds by Ring-Opening versus Annulation of the Isatin Moiety

We report a green, efficient and straightforward approach for the regioselective synthesis of new spiro-heterocyclic scaffolds using environmentally benevolent activated alumina balls (3-5mm diameter) as a catalyst under neat reaction conditions. The methodology is advantageous because of its short reaction time, moderate to excellent yields, easy work-up, practical simplicity and high regioselectivity without any troublesome/hazardous disposal of the catalyst or toxic byproducts. The pore size of the catalyst plays a key role in the formation of the spiro derivatives. Specifically, we observed that the reaction yield was hardly affected by the surface area or pore volume, but surprisingly, the yield correlated directly with the pore size of the catalyst.