Abstract
Functionalized oxindoles and pyrrolizidines form the central structural framework for numerous natural products with extensive biological and pharmacological applications. The requirement for high regio- and stereoselectivity is the main obstacle in the synthesis of such five-membered heterocycles. Multicomponent cycloaddition reactions often provide an efficient and straightforward approach for the preparation of specific regio- and stereoisomers. In this article, the regio- and stereochemistry of the polar [3 + 2]-cycloaddition (32CA) reaction of azomethine ylides prepared by the reaction of isatin derivatives and L-proline with a series of (E)-3-(2-oxo-2-(pyren-1-yl)ethylidene)indolin-2-ones was investigated by experimental and theoretical methods. Among the isatin and (E)-3-(2-oxo-2-(pyren-1-yl)ethylidene)indolin-2-one derivatives, a remarkable inversion of regioselectivity was observed in the 32CA reaction of azomethine ylide generated by the reaction of L-proline and 5-chloroisatin or N-methyl-5chloroisatin with (E)-5-chloro-3-(2-oxo-2-(pyren-1-yl)ethylidene)indolin-2-one. The regio- and stereochemical assignment of the structures of the cycloaddition products was determined by one- and two-dimensional (1D&2D) homonuclear and heteronuclear correlation nuclear magnetic resonance spectroscopy. The molecular mechanism as well as the regio- and stereoselectivity of the cycloaddition were investigated by means of global and local reactivity indices and a density functional theory (DFT) and explained in detail on the basis of the transition state stabilities of the reactants.