Chiral bicyclic guanidine catalyzed diels-alder reactions of anthrones

Abstract

The aim of this study is to develop highly enantioselective Dielsb Alder reactions of anthrones catalyzed by a chiral bicyclic guanidine.We try to find an efficient type of catalyst, and three categories of catalysts were screened for the Dielsb Alder reaction between anthrone and N-phenylmaleimide, including bis(oxazoline) (BOX), imidazoles, guanidines. 2,3,5,6-Tetrahydro-2,6-dibenzyl-1H-imidazo[1,2-a]imidazole, a bicyclic guanidine base, was found to be the most efficient organocatalyst. A wide variety of Dielsb Alder dienes and dienophiles can participate in these reactions using 10 mol% of the chiral bicyclic guanidine. The conjugate addition between 1,8-dihydroxy-9(10H)-anthracenone (dithranol) and different dienophiles also works very well with the chiral bicyclic guanidine. These reactions are mild, fast, easy to perform, and proceed with high yields. The enantioselectivities generally range from 85-99%, with yields between 80-96%. The mechanism of both triethylamine and 2,3,5,6-tetrahydro-2,6-dibenzyl-1H-imidazo[1,2-a]imidazole catalyzed reactions of anthrones has been investigated though VT-NMR. When using triethylamine as the catalyst, it fouctions as a normal base to abstract a proton from anthrone. The anthrone works a reactive diene in Diels-Alder reaction. When chiral bicyclic guanidine was used as the catalyst, it works as a bifunctional catalyst; it activates both the diene and dienophile at defined positions simultaneously.An enantioselective synthesis of anthrone-derived N-hydroxyphthalimide (NHPI) analogues has been developed. One of these analogues, in combination with Co salts, was employed to catalyse the aerobic oxidation of benzylic compounds and diols. Exploratory studies using a racemic version of the catalyst were also conducted. Radical addition of dioxolanes or alcohols to activated alkenes with molecular oxygen as the terminal oxidant was also shown to be catalysed with NHPI-analogues.