Catalysis with inorganic cations. VI. The effect of chiral bis-oxazoline-magnesium perchlorate catalysts on chemo- and enantioselectivity of intramolecular Hetero Diels-Alder and ene reaction

Abstract

( E)1-acetyl-[2-(3-methyl-2-butenyloxy)benzylidene]indolin-2-one ( 1) gives competition between the intramolecular Hetero Diels-Alder (HDA) (thermal conditions) and the intramolecular ene reaction (IER) (magnesium perchlorate - MP - catalyzed conditions). Several complexes derived from MP and chiral bis-oxazolines were found to be excellent chemo- and enantioselective catalysts with a different degree of selectivity depending on the substituents on the oxazoline ring. The most chemoselective oxazolinic ligand forced the reaction to give a ratio of [HDA] : [IER] products 5 : 95. The trans-(4,5-diphenyloxazoline) - MP complex is the best enantioselective catalyst and the product of the IER of 1 can be prepared in 75% yield and 88% e.e..