Asymmetric Cyclization of 2′‐Hydroxychalcones to Flavanones: Catalysis by Chiral Brønsted Acids and Bases

摘要: The asymmetric cyclization of 2′-hydroxychalcones to flavanones is a basic, enzyme-catalyzed step in the biosynthesis flavonoid natural products, but poses long-standing problem for catalysis with small molecule catalysts. Earlier claims concerning realization an flavanone synthesis by means camphorsulfonic acid as chiral Bronsted catalysts were reinvestigated using accurate HPLC methods quantification enantiomer ratios. previous induction thus shown be untenable. On other hand, cinchona alkaloids serve base mediators either 6′-substituted or 2′,6′-dihydroxychalcones. 2′,6′-Dihydroxy-4,4′-dimethoxychalcone, instance, cyclized give naturally occurring naringenin-4′,7-dimethyl ether up 64 % ee at 81 % conversion. shows marked dependency enantiomeric excess product on catalyst, solvent and reactant concentration. Based these successful examples cyclizations flavanones, requirements more active can defined.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)