PART I: ASYMMETRIC SYNTHESIS OF SIX MEMBERED N-HETEROCYCLIC RINGS MEDIATED BY BIFUNCTIONAL THIOUREA CATALYSTS PART II: EXPLORATION OF DNA CLEAVAGE ACTIVITIES BY CERTAIN SMALL ORGANIC MOLECULES

Abstract

This thesis is divided into two parts. Part I is focused on the asymmetric synthesis of six-membered N-heterocyclic rings mediated by bifunctional thiourea catalysts. In addition, Part II describes the exploration of DNA cleavage activities by certain small organic molecules. For the first part, after a brief historic overview of asymmetric organocatalysis, two types of six membered N-heterocyclic rings were synthesized. First, chiral 3-nitro-1,2-dihydroquinolines were synthesized through a cascade reaction catalyzed by cinchona alkaloids based bifunctional thiourea catalysts. The desired compounds were achieved in high yields and with up to 90% enantiomeric excesses. Secondly, chiral 2-Aryl-2,3-dihydroquinolin-4(1H)-ones were synthesized through a intramolecular cyclization reaction catalyzed by quinidine-derived tertiary amine-thiourea catalyst. The resulting compounds were obtained followed by decarboxylation and desulfonylation reaction. The yields were high and the enantioselectivity could research to 98%. For the second part, two classes of sufur-containing cyclic organic compounds, aminomethylated Beaucage¿s reagent and its sulfoxide analogue together with varacin B were successfully synthesized. All of them showed the good effectiveness in converting circular supercoiled DNA to the corresponding circular nicked form. In addition, a new selected oligonuleotide was discovered to perform a site specific cleavage reaction based on guanine quadruplex structure catalyzed by the combination of Mg2+ and histidine in a hydrolysis way.