Catalysis with inorganic cations .5. Intramolecular hetero Diels-Alder versus ene reactions: Effect of magnesium perchlorate on chemoselectivity.

The competition between Hetero Diels-Alder (HDA) and intramolecular ene reaction (IER) was tested on (E)1-acetyl-3-arylideneindolin-2-one (5). Under thermal conditions HDA products 6a,b are obtained in a quantitative yield stressing the importance of the configuration of the alpha,beta-unsaturated carbonyl system on the reactivity. Both reactivity and chemoselectivity are strongly influenced if the reaction is carried out in the presence of magnesium perchlorate and two IER products 10a,b are identified by nmr and converted into their thermodynamically stable isomers 11a,b by silica gel. The importance of two equivalents of ketone, behaving as ligands in the complex between 5 and Mg(II), is emphasized together with its importance in shifting chemoselectivity in favour of the IER. Copyright (C) 1996 Elsevier Science Ltd.